{"id":11279,"date":"2026-09-27T14:51:39","date_gmt":"2026-09-27T13:51:39","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp4-rp6-identifying-inorganic-ions-and-organic-functional-groups\/"},"modified":"2026-10-03T08:38:11","modified_gmt":"2026-10-03T07:38:11","slug":"rp4-rp6-identifying-inorganic-ions-and-organic-functional-groups","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp4-rp6-identifying-inorganic-ions-and-organic-functional-groups\/","title":{"rendered":"Required Practicals 4 and 6: Identifying Ions and Functional Groups"},"content":{"rendered":"<script src=\"https:\/\/cdnjs.cloudflare.com\/ajax\/libs\/three.js\/r128\/three.min.js\"><\/script>\n\n<section class=\"ols-revision-page ols-core-practical-page\">\n  <style>\n    .ols-revision-page {\n      --navy: #1C244B;\n      --blue: #2563eb;\n      --soft-blue: #eef4ff;\n      --soft-red: #fff7f7;\n      --soft-purple: #f7f0ff;\n      --soft-green: #f0f7f1;\n      --soft-orange: #fff7ed;\n      --grey-text: #667085;\n      --body-text: #1f2937;\n      --border: rgba(28, 36, 75, 0.14);\n      --shadow: 0 18px 45px rgba(28, 36, 75, 0.10);\n      --inner-shadow: 0 10px 26px rgba(28, 36, 75, 0.08);\n      font-family: Poppins, Arial, sans-serif;\n      color: var(--navy);\n      background: #ffffff;\n    }\n\n    .ols-revision-page * { box-sizing: border-box; }\n    .ols-main { min-width: 0; max-width: 970px; }\n    .ols-breadcrumbs { display: flex; flex-wrap: wrap; gap: 8px; margin: 10px 0 22px; font-size: 15px; color: var(--grey-text); }\n    .ols-breadcrumbs a, .ols-breadcrumbs span { color: var(--grey-text); text-decoration: none; font-weight: 600; }\n    .ols-breadcrumbs a:hover { color: var(--blue); text-decoration: underline; text-underline-offset: 3px; }\n    .ols-title-card, .ols-note-card, .ols-h5p-card, .ols-related-card, .ols-faq-card { background: #ffffff; border: 1px solid var(--border); border-radius: 28px; box-shadow: var(--shadow); padding: 34px; margin-bottom: 24px; overflow: hidden; }\n    .ols-title-card { background: linear-gradient(135deg, #ffffff 0%, #f8fbff 100%); }\n    .ols-title-card h1 { margin: 0 0 18px; font-size: clamp(36px, 5vw, 58px); line-height: 1.08; font-weight: 800; letter-spacing: -0.04em; color: #111827; }\n    .ols-page-intro { font-size: 19px; line-height: 1.7; font-weight: 300; color: var(--body-text); margin: 0 0 22px; }\n    .ols-badges { display: flex; flex-direction: column; align-items: flex-start; gap: 12px; margin-bottom: 22px; }\n    .ols-badge { display: inline-flex; align-items: center; padding: 9px 14px; border-radius: 999px; background: #ffffff; border: 1px solid var(--border); font-size: 15px; font-weight: 600; color: var(--navy); }\n    .ols-author { display: flex; align-items: center; gap: 20px; padding: 28px; border-radius: 28px; background: linear-gradient(135deg, #ffffff 0%, #f8fbff 100%); border: 1px solid rgba(28,36,75,0.12); box-shadow: 0 18px 45px rgba(28,36,75,0.10); margin-top: 22px; }\n    .ols-author-avatar-img { width: 92px; height: 92px; border-radius: 50%; object-fit: cover; object-position: center; flex-shrink: 0; border: 4px solid #ffffff; box-shadow: 0 14px 32px rgba(28,36,75,0.18), 0 0 0 1px rgba(28,36,75,0.10); transition: transform 0.25s ease, box-shadow 0.25s ease; }\n    .ols-author:hover .ols-author-avatar-img { transform: scale(1.04); box-shadow: 0 20px 42px rgba(28,36,75,0.22), 0 0 0 1px rgba(28,36,75,0.10); }\n    .ols-author-content { min-width: 0; }\n    .ols-author-title { margin: 0 0 4px; font-size: clamp(24px,3vw,34px); line-height: 1.15; font-weight: 700; color: var(--navy); letter-spacing: -0.03em; }\n    .ols-author-description { margin: 0; font-size: 17px; line-height: 1.7; font-weight: 300; color: var(--grey-text); }\n    .ols-linkedin-pill { display: inline-flex; align-items: center; justify-content: center; gap: 10px; margin-top: 16px; padding: 11px 18px; border-radius: 999px; background: linear-gradient(135deg,#0A66C2,#004182); color: #ffffff; text-decoration: none; font-size: 14px; line-height: 1; font-weight: 700; letter-spacing: 0.01em; box-shadow: 0 10px 22px rgba(10,102,194,0.24); position: relative; overflow: hidden; transition: transform 0.22s ease, box-shadow 0.22s ease; }\n    .ols-linkedin-pill::before { content: \"\"; position: absolute; top: 0; left: -120%; width: 100%; height: 100%; background: linear-gradient(120deg, rgba(255,255,255,0) 0%, rgba(255,255,255,0.28) 50%, rgba(255,255,255,0) 100%); transition: left 0.7s ease; }\n    .ols-linkedin-pill:hover { transform: translateY(-3px) scale(1.03); box-shadow: 0 16px 34px rgba(10,102,194,0.34), 0 0 0 6px rgba(10,102,194,0.10); color: #ffffff; }\n    .ols-linkedin-pill:hover::before { left: 120%; }\n    .ols-linkedin-icon { width: 18px; height: 18px; display: block; flex-shrink: 0; }\n\n    .ols-note-card.soft { background: linear-gradient(135deg, #ffffff 0%, var(--soft-red) 100%); }\n    .ols-note-card.purple { background: linear-gradient(135deg, #ffffff 0%, var(--soft-purple) 100%); }\n    .ols-note-card.orange { background: linear-gradient(135deg, #ffffff 0%, var(--soft-orange) 100%); }\n    .ols-note-title { display: flex; align-items: center; gap: 14px; margin-bottom: 20px; }\n    .ols-note-icon { width: 52px; height: 52px; border-radius: 16px; background: var(--navy); color: #ffffff; display: grid; place-items: center; font-size: 24px; font-weight: 800; flex-shrink: 0; }\n    .ols-note-title h2, .ols-h5p-card h2, .ols-related-card h2, .ols-faq-card h2 { margin: 0; font-size: clamp(28px, 3.5vw, 42px); line-height: 1.15; font-weight: 800; color: #111827; letter-spacing: -0.03em; }\n    .ols-h5p-card h2, .ols-related-card h2, .ols-faq-card h2 { margin-bottom: 14px; }\n    .ols-note-card p, .ols-note-card li { font-size: clamp(15px, 1.25vw, 17px); line-height: 1.65; font-weight: 300; color: var(--body-text); }\n    .ols-h5p-card p, .ols-related-card p, .ols-faq-card p { font-size: clamp(15px, 1.3vw, 18px); line-height: 1.65; font-weight: 300; color: var(--body-text); }\n    .ols-note-card strong, .ols-h5p-card strong, .ols-related-card strong, .ols-faq-card strong { font-weight: 700; color: var(--navy); }\n    .ols-note-card ul, .ols-note-card ol { margin: 0; padding-left: 22px; }\n\n    .ols-key-box { margin-top: 20px; background: var(--soft-green); border: 1px solid rgba(63, 143, 70, 0.25); border-radius: 20px; padding: 18px 20px; }\n    .ols-key-box p { margin: 0; font-weight: 400; color: var(--navy); }\n    .ols-definition-box { background: linear-gradient(135deg, #ffffff, var(--soft-purple)); border: 2px dashed rgba(122, 62, 157, 0.35); border-radius: 24px; padding: 22px 24px; margin-top: 22px; }\n    .ols-definition-box p { margin: 0; color: var(--navy); font-weight: 400; }\n\n    .ols-rule-list { display: grid; gap: 14px; margin-top: 18px; }\n    .ols-rule-item { background: #ffffff; border: 1px solid var(--border); border-left: 5px solid var(--blue); border-radius: 18px; padding: 18px; box-shadow: var(--inner-shadow); }\n    .ols-rule-item h3 { margin: 0 0 8px; font-size: 20px; line-height: 1.25; color: var(--navy); }\n    .ols-rule-item p { margin: 0; font-size: 15px; line-height: 1.6; }\n\n    .ols-figure-card { margin: 26px auto 4px; border: 1px solid var(--border); border-radius: 26px; overflow: hidden; background: #ffffff; box-shadow: var(--inner-shadow); max-width: 100%; }\n    .ols-figure-card.medium { max-width: 820px; }\n    .ols-figure-card.compact { max-width: 700px; }\n    .ols-figure-image { width: 100%; min-height: 260px; display: flex; align-items: center; justify-content: center; background: #ffffff; padding: 20px; }\n    .ols-figure-image img { max-width: 100%; height: auto; display: block; }\n    .ols-figure-caption { padding: 18px 24px 22px; background: linear-gradient(135deg, #ffffff, #f8fbff); border-top: 1px solid var(--border); }\n    .ols-figure-caption p { margin: 0; color: #5f6b85; font-size: clamp(15px, 1.3vw, 17px); line-height: 1.6; font-weight: 300; font-style: italic; }\n    .ols-zoom-card, .ols-zoom-card * { box-sizing: border-box; }\n    .ols-zoom-card { display: block !important; margin: 26px auto 34px !important; border: 1px solid rgba(28, 36, 75, 0.14) !important; border-radius: 26px !important; background: #ffffff !important; box-shadow: 0 18px 45px rgba(28, 36, 75, 0.10) !important; overflow: visible !important; position: relative !important; z-index: 1 !important; isolation: isolate !important; }\n    .ols-zoom-card.medium { max-width: 820px; }\n    .ols-zoom-card.compact { max-width: 700px; }\n    .ols-zoom-card.wide { max-width: 940px; }\n    .ols-zoom-card.slim { max-width: 600px; }\n    .ols-zoom-card:hover { z-index: 50 !important; }\n    .ols-zoom-card-image { display: flex !important; align-items: center !important; justify-content: center !important; width: 100% !important; min-height: 240px !important; padding: 20px !important; background: #ffffff !important; overflow: visible !important; border-top-left-radius: 26px !important; border-top-right-radius: 26px !important; position: relative !important; z-index: 2 !important; }\n    .ols-zoom-card img.ols-zoomable-img { display: block !important; max-width: 100% !important; height: auto !important; border-radius: 18px !important; cursor: zoom-in !important; pointer-events: auto !important; user-select: none !important; -webkit-user-drag: none !important; transform: translateZ(0) scale(1) !important; transform-origin: center center !important; transition: transform 0.32s ease, box-shadow 0.32s ease, filter 0.32s ease !important; position: relative !important; z-index: 2 !important; }\n    @media (hover: hover) and (pointer: fine) { .ols-zoom-card img.ols-zoomable-img:hover { transform: translateZ(0) scale(1.35) !important; box-shadow: 0 28px 70px rgba(28, 36, 75, 0.34) !important; filter: saturate(1.02) contrast(1.01) !important; z-index: 100 !important; } }\n    .ols-zoom-card-caption { padding: 18px 22px 20px !important; background: linear-gradient(135deg, #ffffff, #f8fbff) !important; border-bottom-left-radius: 26px !important; border-bottom-right-radius: 26px !important; position: relative !important; z-index: 1 !important; }\n    .ols-zoom-card-caption p { margin: 0 !important; color: #5f6b85 !important; font-size: 15px !important; line-height: 1.65 !important; font-weight: 300 !important; font-style: italic !important; font-family: Poppins, Arial, sans-serif !important; }\n    .ols-image-lightbox { position: fixed; inset: 0; z-index: 999999; display: none; align-items: center; justify-content: center; padding: 34px; background: rgba(10, 15, 35, 0.86); backdrop-filter: blur(8px); -webkit-backdrop-filter: blur(8px); }\n    .ols-image-lightbox.is-open { display: flex; }\n    .ols-image-lightbox-inner { position: relative; width: min(96vw, 1500px); max-height: 92vh; display: flex; align-items: center; justify-content: center; }\n    .ols-image-lightbox-img { display: block; max-width: 100%; max-height: 92vh; height: auto; width: auto; border-radius: 22px; background: #ffffff; box-shadow: 0 32px 90px rgba(0, 0, 0, 0.45); object-fit: contain; }\n    .ols-image-lightbox-close { position: absolute; top: -18px; right: -18px; width: 46px; height: 46px; border: 0; border-radius: 50%; background: #ffffff; color: var(--navy); font-family: Poppins, Arial, sans-serif; font-size: 28px; line-height: 1; font-weight: 700; cursor: pointer; box-shadow: 0 16px 34px rgba(0, 0, 0, 0.28); display: flex; align-items: center; justify-content: center; transition: transform 0.2s ease, background 0.2s ease, color 0.2s ease; }\n    .ols-image-lightbox-close:hover { transform: scale(1.08); background: var(--blue); color: #ffffff; }\n    @media (max-width: 760px) { .ols-zoom-card { border-radius: 22px !important; overflow: hidden !important; } .ols-zoom-card-image { min-height: auto !important; padding: 12px !important; overflow: hidden !important; border-top-left-radius: 22px !important; border-top-right-radius: 22px !important; } .ols-zoom-card img.ols-zoomable-img, .ols-zoom-card img.ols-zoomable-img:hover { transform: none !important; box-shadow: none !important; } .ols-zoom-card-caption { border-bottom-left-radius: 22px !important; border-bottom-right-radius: 22px !important; } .ols-image-lightbox { padding: 16px; } .ols-image-lightbox-inner { width: 100%; max-height: 88vh; } .ols-image-lightbox-img { max-height: 88vh; border-radius: 16px; } .ols-image-lightbox-close { top: 10px; right: 10px; width: 42px; height: 42px; font-size: 26px; } }\n\n\n    .ols-table-wrap { overflow: hidden; border-radius: 22px; border: 1px solid var(--border); background: #ffffff; margin-top: 18px; }\n    .ols-table { width: 100%; border-collapse: collapse; table-layout: fixed; }\n    .ols-table th { background: var(--navy); color: #ffffff; padding: 16px; text-align: left; font-size: clamp(14px, 1.2vw, 17px); font-weight: 600; overflow-wrap: anywhere; }\n    .ols-table td { padding: 16px; border-bottom: 1px solid var(--border); font-size: clamp(14px, 1.15vw, 16px); line-height: 1.45; color: var(--body-text); vertical-align: top; overflow-wrap: anywhere; }\n    .ols-table tr:last-child td { border-bottom: none; }\n\n    .ols-h5p-card { background: linear-gradient(135deg, #ffffff 0%, #f7f0ff 100%); }\n    .ols-h5p-frame { margin-top: 22px; padding: 18px; border-radius: 24px; background: #ffffff; border: 1px solid var(--border); box-shadow: inset 0 0 0 1px rgba(28, 36, 75, 0.03); }\n\n    .ols-faq-list { display: grid; gap: 14px; margin-top: 18px; }\n    .ols-faq-item { background: #ffffff; border: 1px solid var(--border); border-radius: 18px; padding: 18px 20px; box-shadow: var(--inner-shadow); }\n    .ols-faq-item h3 { margin: 0 0 8px; font-size: 20px; line-height: 1.3; color: var(--navy); }\n    .ols-faq-item p { margin: 0; font-size: 16px; line-height: 1.65; color: var(--body-text); }\n\n    .ols-related-grid { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 16px; margin-top: 18px; }\n    .ols-related-item { border: 1px solid var(--border); border-radius: 18px; padding: 18px; background: #ffffff; color: var(--navy); text-decoration: none; font-weight: 600; line-height: 1.5; transition: transform 0.2s ease, box-shadow 0.2s ease; }\n    .ols-related-item:hover { transform: translateY(-2px); box-shadow: 0 10px 22px rgba(28, 36, 75, 0.08); }\n\n    .ols-course-cta-covalent {\n      width: 100%;\n      margin: 30px 0 24px;\n      font-family: Poppins, Arial, sans-serif;\n    }\n    .ols-course-cta-covalent, .ols-course-cta-covalent * { box-sizing: border-box; }\n    .ols-course-cta-card {\n      overflow: hidden;\n      border-radius: 30px;\n      border: 1px solid var(--border);\n      background: radial-gradient(circle at top left, rgba(37, 99, 235, 0.16), transparent 34%), linear-gradient(135deg, #ffffff 0%, #f8fbff 100%);\n      box-shadow: var(--shadow);\n      padding: 30px;\n    }\n    .ols-course-cta-top {\n      display: grid;\n      grid-template-columns: minmax(260px, 0.9fr) minmax(0, 1.1fr);\n      gap: 28px;\n      align-items: center;\n      margin-bottom: 24px;\n    }\n    .ols-course-cta-image-link { display: block; text-decoration: none; border-radius: 24px; }\n    .ols-course-cta-image {\n      width: 100%;\n      border-radius: 24px;\n      overflow: hidden;\n      border: 1px solid var(--border);\n      background: #ffffff;\n      box-shadow: var(--inner-shadow);\n      transition: transform 0.35s ease, box-shadow 0.35s ease;\n    }\n    .ols-course-cta-image img { width: 100%; height: auto; display: block; transition: transform 0.45s ease; }\n    .ols-course-cta-image-link:hover .ols-course-cta-image { transform: translateY(-8px) scale(1.015); box-shadow: 0 26px 60px rgba(28, 36, 75, 0.18), 0 0 0 1px rgba(37, 99, 235, 0.12); }\n    .ols-course-cta-image-link:hover .ols-course-cta-image img { transform: scale(1.03); }\n    .ols-course-cta-header-row { display: flex; flex-wrap: wrap; align-items: center; justify-content: space-between; gap: 14px; margin-bottom: 18px; }\n    .ols-course-cta-kicker { display: inline-flex; align-items: center; padding: 8px 14px; border-radius: 999px; background: #ffffff; border: 1px solid rgba(37, 99, 235, 0.18); color: var(--blue); font-size: 14px; line-height: 1.2; font-weight: 700; box-shadow: var(--inner-shadow); }\n    .ols-course-cta-covalent h2 { margin: 0 0 14px; font-size: clamp(28px, 3.5vw, 42px); line-height: 1.15; font-weight: 800; letter-spacing: -0.03em; color: #111827; }\n    .ols-course-cta-intro { margin: 0 0 24px; color: var(--body-text); font-size: clamp(16px, 1.4vw, 18px); line-height: 1.7; font-weight: 300; }\n    .ols-course-cta-features { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 14px; margin: 0 0 30px; }\n    .ols-course-feature { background: rgba(255, 255, 255, 0.78); border: 1px solid var(--border); border-radius: 18px; padding: 16px 18px; }\n    .ols-course-feature h3 { margin: 0 0 6px; color: var(--navy); font-size: 18px; line-height: 1.3; font-weight: 800; }\n    .ols-course-feature p { margin: 0; color: var(--body-text); font-size: 15px; line-height: 1.6; font-weight: 300; }\n    .ols-course-cta-bottom { display: flex; justify-content: center; padding-top: 22px; border-top: 1px solid var(--border); }\n    .ols-course-button { display: inline-flex; align-items: center; justify-content: center; padding: 15px 28px; border-radius: 999px; background: var(--navy); color: #ffffff; text-decoration: none; font-size: 16px; line-height: 1.2; font-weight: 800; box-shadow: 0 12px 26px rgba(28, 36, 75, 0.18); transition: transform 0.2s ease, background 0.2s ease, box-shadow 0.2s ease; }\n    .ols-course-button:hover { transform: translateY(-2px); background: var(--blue); color: #ffffff; box-shadow: 0 16px 32px rgba(37, 99, 235, 0.24); }\n    .ols-course-button-top { flex-shrink: 0; padding: 12px 22px; font-size: 15px; }\n\n    .ols-attribution-card {\n      background: linear-gradient(135deg, #ffffff, #f8fbff);\n      border: 1px solid rgba(28, 36, 75, 0.14);\n      border-radius: 28px;\n      box-shadow: 0 18px 45px rgba(28, 36, 75, 0.10);\n      padding: 34px;\n      margin-bottom: 24px;\n      overflow: hidden;\n      font-family: Poppins, Arial, sans-serif;\n      box-sizing: border-box;\n    }\n    .ols-attribution-card, .ols-attribution-card * { box-sizing: border-box; }\n    .ols-attribution-card p { margin: 0; font-size: 14px; line-height: 1.65; font-weight: 300; color: #667085; }\n    .ols-attribution-card strong { font-weight: 700; color: #1C244B; }\n\n    @media (max-width: 1050px) {\n      .ols-main { max-width: none; }\n      .ols-related-grid { grid-template-columns: repeat(2, minmax(0, 1fr)); }\n      .ols-course-cta-top { grid-template-columns: 1fr; }\n      .ols-course-cta-image-link { max-width: 520px; margin: 0 auto; }\n    }\n\n    @media (max-width: 760px) {\n      .ols-title-card, .ols-note-card, .ols-h5p-card, .ols-related-card, .ols-faq-card, .ols-attribution-card { padding: 24px 18px; border-radius: 22px; }\n      .ols-note-title { align-items: flex-start; }\n      .ols-related-grid, .ols-course-cta-features { grid-template-columns: 1fr; }\n      .ols-figure-card { border-radius: 22px; }\n      .ols-figure-image { min-height: 210px; padding: 14px; }\n      .ols-figure-caption { padding: 16px; }\n      .ols-table-wrap { border: none; 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}\n      .ols-course-cta-card { padding: 20px; border-radius: 24px; }\n      .ols-course-cta-header-row { align-items: stretch; }\n      .ols-course-button-top, .ols-course-button { width: 100%; }\n    }\n  \n    .ols-figure-placeholder .ols-placeholder-box { border: 2px dashed #c9973a; background: #fffaf0; border-radius: 16px; padding: 26px 22px; font-weight: 700; color: #7a4b12; text-align: center; line-height: 1.6; }\n    .ols-figure-placeholder .ols-figure-image { background: transparent; }\n    .ols-figure-placeholder { background: #ffffff; border: 1px solid rgba(28, 36, 75, 0.12); border-radius: 22px; padding: 14px; margin: 18px 0 6px; }\n    .ols-figure-placeholder .ols-figure-caption p { margin: 10px 0 0; font-size: 14px; color: #667085; text-align: center; }\n\n    \/* inline checks (H5P re-flow, Sep 2026) *\/\n    .ols-h5p-card.ols-h5p-inline { padding: 26px 28px; border-left: 6px solid #7c3aed; }\n    .ols-h5p-card.ols-h5p-inline h2 { font-size: clamp(20px, 2.2vw, 27px); letter-spacing: -0.02em; }\n    .ols-h5p-card.ols-h5p-inline > p { margin: 8px 0 0; }\n    .ols-h5p-card.ols-h5p-inline .ols-h5p-frame { margin-top: 16px; padding: 14px; border-radius: 20px; }\n    .ols-h5p-kicker { display: inline-block; margin-bottom: 10px; padding: 5px 12px; border-radius: 999px; background: #ede9fe; color: #5b21b6; font-size: 12px; font-weight: 700; letter-spacing: 0.06em; text-transform: uppercase; }\n    .ols-h5p-card.ols-h5p-recap { border-left-color: #c9973a; background: linear-gradient(135deg, #ffffff 0%, #fff8e8 100%); }\n    .ols-h5p-recap .ols-h5p-kicker { background: #fdf0d2; color: #8a5a00; }\n    @media (max-width: 760px) { .ols-h5p-card.ols-h5p-inline { padding: 20px 16px; } }\n\n    \/* core practical pages (Sep 2026) *\/\n    .ols-note-card h3 { margin: 22px 0 8px; font-size: 18px; line-height: 1.25; font-weight: 500; color: var(--navy); }\n    .ols-table td { overflow-wrap: anywhere; }\n<\/style>\n\n  <aside class=\"ols-sidebar\">\n  <div class=\"ols-sidebar-header\">\n    <h3>Revision Notes<\/h3>\n    <p>AQA A Level Chemistry<\/p>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Required Practicals<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/molar-volume-of-a-gas\/\">Gas volumes: Molar Volume of a Gas<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp2-measurement-of-an-enthalpy-change\/\">RP2: Enthalpy Change via Hess's Law<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp1-acid-base-titration\/\">RP1: Concentration of HCl by Titration<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp1-making-a-volumetric-solution\/\">RP1: Preparation of a Standard Solution<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rates-of-hydrolysis-of-halogenoalkanes\/\">Hydrolysis rates: Hydrolysis of Halogenoalkanes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/preparing-an-organic-liquid-chlorination-of-2-methylpropan-2-ol\/\">Organic prep: Chlorination of 2-methylpropan-2-ol<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp5-distillation-of-a-product-oxidation-of-ethanol\/\">RP5: Oxidation of an Alcohol<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp4-rp6-identifying-inorganic-ions-and-organic-functional-groups\/\">RP4 and RP6: Analysis of Inorganic and Organic Unknowns<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp3-effect-of-temperature-on-rate-of-reaction\/\">RP3: Rates of Reaction<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/gravimetric-analysis-water-of-crystallisation\/\">Gravimetric: Gravimetric and Composition Analysis<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Useful Links<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/\">AQA Chemistry<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/\">Required Practicals<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\n\n<script>\ndocument.addEventListener('DOMContentLoaded', function () {\n  var currentPath = window.location.pathname.replace(\/\\\/$\/, '');\n  var sidebarLinks = document.querySelectorAll('.ols-sidebar .ols-topic-list a');\n  sidebarLinks.forEach(function (link) {\n    var linkPath = new URL(link.href, window.location.origin).pathname.replace(\/\\\/$\/, '');\n    if (linkPath === currentPath) { link.closest('li').classList.add('active'); } else { link.closest('li').classList.remove('active'); }\n  });\n});\n<\/script>\n\n  <main class=\"ols-main\">\n      <nav class=\"ols-breadcrumbs\" aria-label=\"Breadcrumb\">\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/\">Revision Notes<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/\">A Level Chemistry<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/\">AQA<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/\">Required Practicals<\/a> \/\n<span>Required Practicals 4 and 6: Identifying Ions and Functional Groups<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Required Practicals 4 and 6: Identifying Ions and Functional Groups<\/h1>\n        <p class=\"ols-page-intro\">Required Practicals 4 and 6 identifies unknown organic liquids and inorganic solids from a fixed menu of test-tube reactions: bromine water, warm acidified dichromate(VI), Fehling&#8217;s solution, hydrolysis then silver nitrate, and the acid, barium chloride, limewater and chlorine water tests, with every observation written in the words an examiner credits.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Paper 1, 2 and 3<\/div>\n<div class=\"ols-badge\">AQA<\/div>\n<div class=\"ols-badge\">Required Practicals 4 and 6<\/div>\n<div class=\"ols-badge\">7405<\/div>\n<\/div>\n\n        <div class=\"ols-author\">\n\n    <img decoding=\"async\"\n      class=\"ols-author-avatar-img\"\n      src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\"\n      alt=\"Dr. Mohammed Al-Fatah\"\n    >\n\n    <div class=\"ols-author-content\">\n\n      <h2 class=\"ols-author-title\">\n        Written by:<br><span>Dr. Mohammed Al-Fatah<\/span>\n      <\/h2>\n\n      <p class=\"ols-author-description\">Chemistry specialist revision notes for AQA A Level Chemistry.<\/p>\n\n      <a class=\"ols-linkedin-pill\" href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\" target=\"_blank\" rel=\"noopener noreferrer\">\n        <svg class=\"ols-linkedin-icon\" viewBox=\"0 0 24 24\" fill=\"currentColor\" aria-hidden=\"true\">\n          <path d=\"M4.98 3.5C4.98 4.88 3.86 6 2.48 6S0 4.88 0 3.5 1.12 1 2.48 1s2.5 1.12 2.5 2.5zM.5 8h4V24h-4V8zm7 0h3.8v2.2h.1c.5-.9 1.8-2.2 3.9-2.2 4.2 0 5 2.8 5 6.4V24h-4v-7.6c0-1.8 0-4.2-2.6-4.2s-3 2-3 4v7.8h-4V8z\"\/>\n        <\/svg>\n        View LinkedIn Profile\n      <\/a>\n\n    <\/div>\n\n  <\/div>\n      <\/header>\n\n      <section class=\"ols-h5p-card ols-h5p-inline ols-h5p-recap\">\n<span class=\"ols-h5p-kicker\">Before you start<\/span>\n<h2>GCSE Recap: Testing for Ions<\/h2>\n<p>Four quick questions on the GCSE tests this practical builds on: flame colours, the carbonate test with limewater, the sulfate test and the silver nitrate colours.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1032\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>What This Practical Is Testing<\/h2>\n<\/div>\n<p>Required Practicals 4 and 6 asks you to identify several unknown substances, typically colourless organic liquids (here A, B and C) and white inorganic solids (here X, Y and Z), from a fixed menu of test-tube reactions.<\/p><p>Nothing here needs new theory: the tests come from the Group 7 chemistry, the tests for alkenes, alcohols, aldehydes and halogenoalkanes, and the ion tests you first met at GCSE.<\/p><p>What is new is the <strong>discipline<\/strong>: a small portion of the unknown for each test, the correct reagent in the correct order, and the right conditions (warming when the test needs it).<\/p><p>The last part of that discipline is an observation written in words an examiner will credit before any inference is made.<\/p>\n<p>The logic is an <strong>evidence trail<\/strong>: unknown \u2192 test \u2192 observation \u2192 inference \u2192 identity. Negative results are part of the trail.<\/p>\n<p>A liquid that turns acidified dichromate(VI) green but leaves Fehling&#8217;s solution blue is a primary or secondary alcohol; the negative Fehling&#8217;s result is what rules out an aldehyde, which dichromate(VI) would also have oxidised.<\/p>\n<p>A solid that fizzes with acid is not shown to be a carbonate until the gas has turned limewater cloudy.<\/p>\n<p>Students lose marks by jumping from one positive test to an identity and by describing a solution with no colour as &#8220;clear&#8221;.<\/p>\n<p>The written papers (at least 15% of the marks test practical skills) and the practical endorsement.<\/p><p>For this practical that means naming reagents and conditions precisely (warm in a water bath, dilute nitric acid before silver nitrate), giving observations in full (colour before and after, precipitate colour, gas test) and writing ionic equations with state symbols.<\/p><p>It also means explaining why a step is there: why the acid, why the warming, why the solid is dissolved first, why a second reagent is needed to confirm.<\/p>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-ana-trail.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-ana-trail.jpg\" alt=\"Evidence trail for the six unknowns: unknown, test, observation, inference, identity\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The evidence trail for the six unknowns on this page: every test, every observation and the inference each one supports.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> An observation is what you see (orange to colourless, cream precipitate, effervescence). An inference is what it means (C=C present, bromide ions, carbonate ions). Write them separately and never skip the observation.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>Safety and Apparatus<\/h2>\n<\/div>\n<p>Everything is done on a test-tube scale: a few drops of the liquid or a spatula tip of the solid. Small quantities are a safety measure and a practical one, because a tube crowded with solid gives a cloudy mixture in which a precipitate cannot be seen.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Apparatus<\/th><th>What it is for<\/th><th>Note<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Test tubes and rack<\/strong><\/td><td>one clean tube per test<\/td><td>a dirty tube contaminates the next test<\/td><\/tr>\n<tr><td><strong>Boiling tube, bung and delivery tube<\/strong><\/td><td>carbonate test: gas led into limewater<\/td><td>fit the bung immediately after adding the acid<\/td><\/tr>\n<tr><td><strong>Water bath (250 cm\u00b3 beaker on a hotplate) and thermometer<\/strong><\/td><td>warming the dichromate(VI), Fehling&#8217;s and hydrolysis tubes at about 60 \u00b0C<\/td><td>no naked flame near ethanol<\/td><\/tr>\n<tr><td><strong>Dropping pipettes<\/strong><\/td><td>adding reagents a few drops at a time<\/td><td>one pipette per reagent<\/td><\/tr>\n<tr><td><strong>Spatula and distilled water<\/strong><\/td><td>dissolving a small portion of each solid<\/td><td>tap water contains chloride ions<\/td><\/tr>\n<tr><td><strong>Limewater in a test tube<\/strong><\/td><td>confirming carbon dioxide<\/td><td>fresh limewater turns cloudy quickly<\/td><\/tr>\n<tr><td><strong>Teat pipette and watch glass<\/strong><\/td><td>transferring and viewing small samples<\/td><td>view precipitates against a dark background<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Hazards and precautions:<\/strong> Bromine water and chlorine water: toxic and corrosive, use small volumes in a fume cupboard or a well-ventilated room, wear gloves.<\/p><p>Acidified potassium dichromate(VI): toxic and a suspected carcinogen, wear gloves, use small volumes and pour residues into the labelled waste bottle.<\/p><p>Sodium hydroxide solution: corrosive, eye protection throughout. Ethanol: flammable, so the hydrolysis tube is warmed in a water bath, never over a flame.<\/p><p>Silver nitrate: stains skin and clothing. Concentrated hydrochloric acid: corrosive, use a few drops only. Fehling&#8217;s solution is alkaline and contains copper: gloves and eye protection.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>Method: Step by Step<\/h2>\n<\/div>\n<p>The method below follows the sample set on this page: three colourless liquids A, B and C and three white solids X, Y and Z. Each test uses a fresh portion of the unknown, and every &#8220;why&#8221; is a mark-scheme point.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Step<\/th><th>What you do<\/th><th>Why<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>1<\/strong><\/td><td>Put about 1 cm\u00b3 of each liquid A, B and C into three separate test tubes and add bromine water dropwise with shaking.<\/td><td>An alkene decolourises bromine water (orange to colourless) by electrophilic addition across the C=C bond; the other liquids leave it orange.<\/td><\/tr>\n<tr><td><strong>2<\/strong><\/td><td>To fresh portions add acidified potassium dichromate(VI) solution and warm the tubes in a water bath at about 60 \u00b0C for a few minutes.<\/td><td>Primary and secondary alcohols and aldehydes are oxidised only on warming; the orange Cr\u2082O\u2087\u00b2\u207b(aq) is reduced to green Cr\u00b3\u207a(aq). Without warming a negative result means nothing.<\/td><\/tr>\n<tr><td><strong>3<\/strong><\/td><td>To fresh portions add Fehling&#8217;s solution and warm in the water bath.<\/td><td>Only an aldehyde reduces the blue Cu\u00b2\u207a complex to a brick-red precipitate of Cu\u2082O; a liquid that is oxidised by dichromate(VI) but leaves Fehling&#8217;s solution blue is an alcohol, not an aldehyde.<\/td><\/tr>\n<tr><td><strong>4<\/strong><\/td><td>To fresh portions add sodium hydroxide solution and a little ethanol, warm in the water bath, then cool.<\/td><td>The C\u2013X bond in a ${w.halo} is covalent, so there are no free halide ions until hydrolysis: R\u2013X + OH\u207b \u2192 R\u2013OH + X\u207b. Ethanol dissolves the organic liquid; the water bath avoids a flame near it.<\/td><\/tr>\n<tr><td><strong>5<\/strong><\/td><td>Acidify the cooled mixture with dilute nitric acid, then add silver nitrate solution. Test any precipitate with dilute, then concentrated, ammonia solution.<\/td><td>The acid neutralises the excess hydroxide ions, which would otherwise give a brown precipitate of Ag\u2082O with Ag\u207a. Nitric acid is used because HCl would add Cl\u207b. White, cream or yellow AgX identifies the halogen; ammonia confirms it.<\/td><\/tr>\n<tr><td><strong>6<\/strong><\/td><td>Dissolve a spatula tip of each solid X, Y and Z in about 2 cm\u00b3 of distilled water in separate tubes.<\/td><td>The ion tests need free ions in solution; a dry solid cannot react with AgNO\u2083(aq) or BaCl\u2082(aq). Distilled water adds no chloride ions.<\/td><\/tr>\n<tr><td><strong>7<\/strong><\/td><td>To one portion of each solution add dilute nitric acid, then silver nitrate solution; test any precipitate with dilute and concentrated ammonia solution.<\/td><td>The acid removes carbonate ions, which would give a white precipitate of Ag\u2082CO\u2083. AgCl white (dissolves in dilute NH\u2083), AgBr cream (dissolves only in concentrated NH\u2083), AgI yellow (insoluble in both).<\/td><\/tr>\n<tr><td><strong>8<\/strong><\/td><td>To a second portion add dilute hydrochloric acid, then barium chloride solution.<\/td><td>The acid removes carbonate ions, which would give white BaCO\u2083; a white precipitate that forms after acidifying is BaSO\u2084. Never acidify with sulfuric acid, which adds sulfate ions.<\/td><\/tr>\n<tr><td><strong>9<\/strong><\/td><td>Add dilute acid to a little of the solid in a boiling tube, fit the bung at once and lead the gas into limewater.<\/td><td>Effervescence suggests a carbonate; limewater turning cloudy (a white precipitate of CaCO\u2083) identifies the gas as carbon dioxide and completes the test.<\/td><\/tr>\n<tr><td><strong>10<\/strong><\/td><td>To a third portion add chlorine water, then shake with a little cyclohexane and let the layers settle.<\/td><td>Chlorine oxidises bromide ions to bromine (orange) and iodide ions to iodine (brown); the halogen dissolves in the upper cyclohexane layer, orange for bromine and purple for iodine, which makes the colour unmistakable.<\/td><\/tr>\n<tr><td><strong>11<\/strong><\/td><td>Identify the cation with the scheme described on the cation card below (sodium hydroxide and ammonia solutions, or the solubility of the hydroxide and sulfate), and test a portion for ammonium ions.<\/td><td>The anion tests give only half the identity; the cation must be shown separately, and an ammonium salt looks exactly like a metal salt.<\/td><\/tr>\n<tr><td><strong>12<\/strong><\/td><td>Record every observation in a results table at the time, then write the inference beside it.<\/td><td>Examiners credit &#8220;orange to colourless&#8221;, &#8220;cream precipitate&#8221;, &#8220;effervescence, limewater turns cloudy&#8221;; they do not credit &#8220;positive&#8221;, &#8220;reacted&#8221; or &#8220;clear&#8221;.<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;Warm with acidified potassium dichromate(VI) in a water bath: orange to green.&#8221; &#8220;Add dilute nitric acid, then silver nitrate solution: cream precipitate, insoluble in dilute ammonia, soluble in concentrated ammonia.&#8221; Reagent, condition, observation.<\/p>\n<\/div>\n<\/article>\n<!-- 3D card: analysis (27 Sep 2026) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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width:100%; height:100%; margin:0; border-radius:0; border:0; padding:16px 20px; display:flex; flex-direction:column; overflow:auto;}\n.ols-ana-001:fullscreen .ana-head{display:none;}\n.ols-ana-001:fullscreen .ana-stage{flex:1 1 auto; height:auto; min-height:320px;}\n.ols-ana-001:-webkit-full-screen{max-width:none; width:100%; height:100%; margin:0; border-radius:0; border:0; padding:16px 20px; display:flex; flex-direction:column; overflow:auto;}\n.ols-ana-001:-webkit-full-screen .ana-head{display:none;}\n.ols-ana-001:-webkit-full-screen .ana-stage{flex:1 1 auto; height:auto; min-height:320px;}\n\n.ols-cc-ana-001{\n  font-family:'Poppins',system-ui,-apple-system,'Segoe UI',Roboto,Helvetica,Arial,sans-serif;\n  margin:10px auto 0; text-align:center; font-size:11px; font-style:italic; color:#aab0c0;\n}\n.ols-cc-ana-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-ana-001 a:hover{text-decoration:underline;}\n<\/style>\n\n<div class=\"ana-head\">\n  <h2 class=\"ana-title\">Test Bench: Identify the Unknowns<\/h2>\n  <p class=\"ana-sub\">Watch three colourless liquids and three white solids meet their test-tube reagents, then follow every observation to an inference and an identity.<\/p>\n<\/div>\n\n<div class=\"ana-stage\" id=\"anaStage\" tabindex=\"0\" aria-label=\"Film of the practical. Space plays or pauses, the left and right arrow keys skip 10 seconds.\">\n  <canvas class=\"ana-canvas\" id=\"anaCanvas\" aria-hidden=\"true\"><\/canvas>\n  <div class=\"ana-overlay\" id=\"anaOverlay\"><\/div>\n  <div class=\"ana-subt\" id=\"anaSubt\"><\/div>\n  <div class=\"ana-hint\" id=\"anaHint\" hidden>Paused: drag to look around<\/div>\n  <button type=\"button\" class=\"ana-poster\" id=\"anaPoster\" aria-label=\"Play the film\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M7 4.5v15l12.5-7.5z\"\/><\/svg><span>Replay<\/span><\/button>\n<\/div>\n\n<div class=\"ana-player\" id=\"anaPlayer\" role=\"group\" aria-label=\"Film controls\">\n  <button type=\"button\" class=\"ana-pb\" id=\"anaPlay\" aria-label=\"Play\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M7 4.5v15l12.5-7.5z\"\/><\/svg><\/button>\n  <button type=\"button\" class=\"ana-pb\" id=\"anaBack\" aria-label=\"Back 10 seconds\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M12 4V1.5L7.5 5 12 8.5V6a7 7 0 1 1-7 7H3a9 9 0 1 0 9-9z\"\/><text x=\"12.2\" y=\"16.2\" text-anchor=\"middle\">10<\/text><\/svg><\/button>\n  <button type=\"button\" class=\"ana-pb\" id=\"anaFwd\" aria-label=\"Forward 10 seconds\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M12 4V1.5L16.5 5 12 8.5V6a7 7 0 1 0 7 7h2a9 9 0 1 1-9-9z\"\/><text x=\"11.8\" y=\"16.2\" text-anchor=\"middle\">10<\/text><\/svg><\/button>\n  <button type=\"button\" class=\"ana-pb\" id=\"anaRestart\" aria-label=\"Restart\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M5.5 5h2.2v14H5.5zM9.2 12l9.3 6.8V5.2z\"\/><\/svg><\/button>\n  <div class=\"ana-prog\" id=\"anaProg\" role=\"slider\" tabindex=\"0\" aria-label=\"Seek\" aria-valuemin=\"0\" aria-valuemax=\"209\" aria-valuenow=\"0\" aria-valuetext=\"0:00\">\n    <div class=\"ana-track\"><div class=\"ana-fill\" id=\"anaFill\"><\/div><\/div>\n    <div class=\"ana-thumb\" id=\"anaThumb\"><\/div>\n  <\/div>\n  <span class=\"ana-time\" id=\"anaTime\">0:00 \/ 3:29<\/span>\n  <button type=\"button\" class=\"ana-pb ana-speed\" id=\"anaSpeed\" aria-label=\"Playback speed 1\u00d7\">1\u00d7<\/button>\n  <button type=\"button\" class=\"ana-pb\" id=\"anaFs\" aria-label=\"Full screen\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M4 9V4h5v2H6v3zm11-5h5v5h-2V6h-3zM4 15h2v3h3v2H4zm14 3v-3h2v5h-5v-2z\"\/><\/svg><\/button>\n<\/div>\n<div class=\"ana-chips\" id=\"anaChips\" role=\"group\" aria-label=\"Chapters\"><\/div>\n<\/section>\n\n<p class=\"ols-cc-ana-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/analysis.js?v=20260928c\"><\/script>\n\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Organic Unknowns A, B and C<\/h2>\n<\/div>\n<p>Four tests separate the organic liquids: bromine water for a C=C bond, warm acidified dichromate(VI) for anything oxidisable, warm Fehling&#8217;s solution (Fehling&#8217;s solution or Tollens&#8217; reagent are accepted) for an aldehyde, and hydrolysis followed by silver nitrate for a halogenoalkane.<\/p><p>The table gives the sample observations and the inference each one allows. Notice that the inference column uses the negative results as much as the positive ones.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Test<\/th><th>A<\/th><th>B<\/th><th>C<\/th><th>Inference<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Bromine water, shaken<\/strong><\/td><td>stays orange<\/td><td>orange to colourless (decolourised)<\/td><td>stays orange<\/td><td>B contains a C=C bond, so B is an alkene; the product is a 1,2-dibromoalkane (accepted). A and C are saturated.<\/td><\/tr>\n<tr><td><strong>Acidified K\u2082Cr\u2082O\u2087(aq), warmed in a water bath<\/strong><\/td><td>orange to green<\/td><td>stays orange<\/td><td>stays orange<\/td><td>A is oxidised: orange Cr\u2082O\u2087\u00b2\u207b(aq) is reduced to green Cr\u00b3\u207a(aq). A is a primary or secondary alcohol or an aldehyde; B and C are not oxidised (not a tertiary alcohol test on its own).<\/td><\/tr>\n<tr><td><strong>Fehling&#8217;s solution, warmed in a water bath<\/strong><\/td><td>remains blue, no precipitate<\/td><td>remains blue, no precipitate<\/td><td>remains blue, no precipitate<\/td><td>No aldehyde is present. Combined with the dichromate(VI) result, A is a primary or secondary alcohol, not an aldehyde.<\/td><\/tr>\n<tr><td><strong>Warm with NaOH(aq) and ethanol, cool, acidify with dilute HNO\u2083, add AgNO\u2083(aq), then NH\u2083(aq)<\/strong><\/td><td>no precipitate<\/td><td>no precipitate<\/td><td>cream precipitate, insoluble in dilute NH\u2083(aq), dissolves in concentrated NH\u2083(aq)<\/td><td>C releases Br\u207b ions on hydrolysis, so C is a bromoalkane. The ammonia result rules out AgCl (dissolves in dilute) and AgI (insoluble in both).<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Three details carry marks. The dichromate(VI) and Fehling&#8217;s tubes are <strong>warmed<\/strong> in a water bath at about 60 \u00b0C; at room temperature both stay unchanged and the negative result is worthless.<\/p><p>Bromine water is <strong>decolourised<\/strong>, from orange to colourless, never &#8220;goes clear&#8221;.<\/p><p>The dichromate(VI) result alone does not identify A: dichromate(VI) would also oxidise an aldehyde, which is exactly why the negative Fehling&#8217;s result must be quoted in the inference.<\/p>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/fix-54.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/fix-54.jpg\" alt=\"Three test tubes of bromine water with unknowns A, B and C; only B is colourless\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Bromine water added to A, B and C: only B decolourises it, from orange to colourless, which is the evidence for a C=C bond.<\/p><\/div>\n<\/div>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/Acidified-dichromate-test-setup.webp\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/Acidified-dichromate-test-setup.webp\" alt=\"Water bath at 60 \u00b0C with three tubes of acidified potassium dichromate(VI); tube A is green, B and C remain orange\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>After warming in a water bath, only tube A has changed from orange to green: Cr\u2082O\u2087\u00b2\u207b(aq) reduced to Cr\u00b3\u207a(aq) as A is oxidised.<\/p><\/div>\n<\/div>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-ana-fehlings.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-ana-fehlings.jpg\" alt=\"Fehling&#039;s solution warmed with A, B and C: all three remain blue\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Fehling&#8217;s solution warmed with A, B and C: all three remain blue with no precipitate, so none of them is an aldehyde.<\/p><\/div>\n<\/div>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/wave-14.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/wave-14.jpg\" alt=\"Sequence of hydrolysis, acidification and silver nitrate on A, B and C with a cream precipitate in tube C\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Hydrolysis, acidification with dilute nitric acid and then silver nitrate: only C gives a cream precipitate of silver bromide.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Final identities: A is a primary or secondary alcohol (never &#8220;an oxidisable alcohol&#8221;), B is an alkene, C is a bromoalkane. Each identity must quote the test that supports it and the negative test that rules out the alternative.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Three New Liquids<\/h2>\n<p>Propanal, propanone and but-1-ene meet the same three reagents. Predict every observation and decide which liquid is which.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1033\" class=\"h5p-iframe\" data-content-id=\"1033\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Analysing Unknowns Drag: Three New Liquids\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>Testing for an Aldehyde and the Rest of the Organic Menu<\/h2>\n<\/div>\n<p>The sample set gives no positive Fehling&#8217;s result, so you must still know what one looks like.<\/p>\n<p>On warming in a water bath an aldehyde reduces the deep blue copper(II) complex in Fehling&#8217;s solution to a <strong>brick-red precipitate<\/strong> of copper(I) oxide, Cu\u2082O.<\/p>\n<p>Cu\u00b2\u207a is reduced to Cu\u207a while the aldehyde is oxidised to a carboxylic acid: RCHO + 2Cu\u00b2\u207a + 4OH\u207b \u2192 RCOOH + Cu\u2082O + 2H\u2082O.<\/p>\n<p>A ketone has no hydrogen on its carbonyl carbon, so it cannot be oxidised under these conditions and the solution stays blue.<\/p>\n<p><strong>Tollens&#8217; reagent<\/strong> (ammoniacal silver nitrate) is the accepted alternative: warmed with an aldehyde it gives a silver mirror on the inside of the tube as Ag\u207a is reduced to Ag.<\/p>\n<p>The full organic test menu is longer than the four tests used on A, B and C, and any of it can appear in a question.<\/p>\n<p>A <strong>carbonyl<\/strong> group (aldehyde or ketone) gives an orange precipitate with 2,4-dinitrophenylhydrazine (2,4-DNPH); Fehling&#8217;s or Tollens&#8217; then decides between the two.<\/p>\n<p>A <strong>carboxylic acid<\/strong> effervesces with sodium carbonate or sodium hydrogencarbonate solution, and the gas turns limewater cloudy; alcohols and phenols do not.<\/p>\n<p>A <strong>tertiary alcohol<\/strong> is the one that leaves warm acidified dichromate(VI) orange but is still an alcohol, so it is identified by elimination once the other tests are negative.<\/p>\n<p>The oxidation chemistry behind the dichromate(VI) test is the subject of <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp5-distillation-of-a-product-oxidation-of-ethanol\/\">Required Practical 5 (oxidation of an alcohol)<\/a>.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Functional group<\/th><th>Reagent and conditions<\/th><th>Positive observation<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Alkene, C=C<\/strong><\/td><td>bromine water, shake at room temperature<\/td><td>orange to colourless<\/td><\/tr>\n<tr><td><strong>Primary or secondary alcohol, or aldehyde<\/strong><\/td><td>acidified K\u2082Cr\u2082O\u2087(aq), warm in a water bath<\/td><td>orange to green<\/td><\/tr>\n<tr><td><strong>Aldehyde<\/strong><\/td><td>Fehling&#8217;s solution, warm in a water bath<\/td><td>brick-red precipitate of Cu\u2082O<\/td><\/tr>\n<tr><td><strong>Aldehyde (alternative)<\/strong><\/td><td>Tollens&#8217; reagent, warm gently in a water bath<\/td><td>silver mirror<\/td><\/tr>\n<tr><td><strong>Aldehyde or ketone<\/strong><\/td><td>2,4-DNPH solution<\/td><td>orange precipitate<\/td><\/tr>\n<tr><td><strong>Carboxylic acid<\/strong><\/td><td>Na\u2082CO\u2083(aq) or NaHCO\u2083(aq)<\/td><td>effervescence; gas turns limewater cloudy<\/td><\/tr>\n<tr><td><strong>Halogenoalkane<\/strong><\/td><td>warm with NaOH(aq) and ethanol, cool, dilute HNO\u2083, AgNO\u2083(aq), then NH\u2083(aq)<\/td><td>white, cream or yellow precipitate; solubility in NH\u2083(aq) confirms<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/Positive-Fehlings-test-in-lab-setting.webp\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/Positive-Fehlings-test-in-lab-setting.webp\" alt=\"Test tube showing a brick-red precipitate of copper(I) oxide below a pale blue solution\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>A positive Fehling&#8217;s result for reference: brick-red copper(I) oxide settles under the pale blue liquid after warming with an aldehyde. Not part of the sample data.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Dichromate(VI) says &#8220;oxidisable&#8221;; Fehling&#8217;s or Tollens&#8217; says &#8220;aldehyde&#8221;. You need both results to name a primary alcohol with confidence.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Aldehyde or Not?<\/h2>\n<p>Two questions on using a positive and a negative result together for liquids that are not on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1034\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">6<\/div>\n<h2>Testing C for the Halogen<\/h2>\n<\/div>\n<p>The halogenoalkane test has five steps and every one of them is examined. First, warm a few drops of C with sodium hydroxide solution and a little ethanol in a water bath.<\/p>\n<p>The <strong>C\u2013Br bond is covalent<\/strong>, so there are no bromide ions in the liquid until it has been hydrolysed: CH\u2083CH\u2082CH\u2082Br + OH\u207b \u2192 CH\u2083CH\u2082CH\u2082OH + Br\u207b (1-bromopropane is used as the example).<\/p>\n<p>Ethanol is the co-solvent that lets the organic liquid mix with the aqueous alkali, and it is flammable, so the tube is warmed in a water bath and never over a flame.<\/p>\n<p>Second, cool the tube and <strong>acidify with dilute nitric acid<\/strong>. The alkali was added in excess, and Ag\u207a reacts with OH\u207b to give a brown precipitate of silver oxide, Ag\u2082O, which would hide the halide result: 2Ag\u207a(aq) + 2OH\u207b(aq) \u2192 Ag\u2082O(s) + H\u2082O(l).<\/p><p>Nitric acid is chosen because hydrochloric acid would add chloride ions and give a white precipitate of AgCl whatever the unknown, and sulfuric acid would add sulfate ions.<\/p><p>Third, add silver nitrate solution: Ag\u207a(aq) + Br\u207b(aq) \u2192 AgBr(s), a <strong>cream precipitate<\/strong>.<\/p><p>Fourth and fifth, add dilute ammonia solution (the cream precipitate stays) and then concentrated ammonia solution (it dissolves): AgBr(s) + 2NH\u2083(aq) \u2192 [Ag(NH\u2083)\u2082]\u207a(aq) + Br\u207b(aq).<\/p><div class=\"ols-key-box\"><p><strong>Exam focus:<\/strong> Without the ammonia step an examiner can only accept &#8220;AgBr or AgI&#8221;, because cream and pale yellow are hard to tell apart in a small tube.<\/p><\/div>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-ana-halotest.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-ana-halotest.jpg\" alt=\"Five steps for testing a C\u2013Br compound: NaOH and ethanol, water bath, HNO\u2083, AgNO\u2083, ammonia\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The five steps for testing an organic liquid for a halogen: hydrolyse with NaOH(aq) and ethanol in a water bath, cool, acidify with dilute nitric acid, add silver nitrate, confirm with dilute then concentrated ammonia.<\/p><\/div>\n<\/div>\n<p>The same silver nitrate chemistry is used to compare the rates of hydrolysis of chloro-, bromo- and iodoalkanes in <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rates-of-hydrolysis-of-halogenoalkanes\/\">Practical Skills: Rates of Hydrolysis (hydrolysis of halogenoalkanes)<\/a>, where the time taken for the precipitate to appear is the measurement.<\/p>\n<p>Here the precipitate is only the evidence for which halogen is present.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam trap:<\/strong> Adding silver nitrate straight to the organic liquid gives no precipitate and proves nothing: the halogen is covalently bonded. Hydrolyse, acidify with nitric acid, then test.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">7<\/div>\n<h2>Inorganic Unknowns X, Y and Z<\/h2>\n<\/div>\n<p>The three white solids are tested for their anions in solution, so the first step is to <strong>dissolve<\/strong> a spatula tip of each in a little distilled water.<\/p><p>A dry solid cannot react with silver nitrate or barium chloride solution, and tap water would add chloride ions.<\/p><p>On this board the cation is identified separately, by the scheme in the cation card below; in this sample set X is a calcium salt, Y a sodium salt and Z a potassium salt.<\/p><p>The table gives the anion evidence with the inference beside each observation.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Test<\/th><th>X<\/th><th>Y<\/th><th>Z<\/th><th>Inference<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Dissolve in distilled water<\/strong><\/td><td>colourless solution<\/td><td>colourless solution<\/td><td>colourless solution<\/td><td>All three are soluble salts; the ion tests can now be done on portions of each solution.<\/td><\/tr>\n<tr><td><strong>Dilute HNO\u2083, then AgNO\u2083(aq), then NH\u2083(aq)<\/strong><\/td><td>cream precipitate; does not dissolve in dilute NH\u2083(aq), dissolves in concentrated NH\u2083(aq)<\/td><td>no precipitate<\/td><td>effervescence with the acid, then no precipitate<\/td><td>X contains Br\u207b (AgBr). Y and Z contain no halide. Z reacts with the acid itself.<\/td><\/tr>\n<tr><td><strong>Dilute HCl, then BaCl\u2082(aq)<\/strong><\/td><td>no precipitate<\/td><td>white precipitate<\/td><td>effervescence with the acid, then no precipitate<\/td><td>Y contains SO\u2084\u00b2\u207b (BaSO\u2084). The acid first removes any carbonate, which would give white BaCO\u2083.<\/td><\/tr>\n<tr><td><strong>Dilute acid on the solid, gas into limewater<\/strong><\/td><td>no gas<\/td><td>no gas<\/td><td>effervescence; limewater turns cloudy<\/td><td>Z contains CO\u2083\u00b2\u207b: the gas is CO\u2082, shown by the white precipitate of CaCO\u2083 in the limewater.<\/td><\/tr>\n<tr><td><strong>Chlorine water, then shake with cyclohexane<\/strong><\/td><td>orange solution; orange upper (cyclohexane) layer<\/td><td>no change<\/td><td>no colour change (slight fizzing from the acidic chlorine water)<\/td><td>X contains Br\u207b: chlorine oxidises bromide ions to bromine. Consistent with the cream AgBr.<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Identities: X is calcium bromide, CaBr\u2082; Y is sodium sulfate, Na\u2082SO\u2084; Z is potassium carbonate, K\u2082CO\u2083.<\/p><p>Each anion has one primary test and one confirmation: silver nitrate then ammonia for the halide, acid then barium chloride for the sulfate, acid then limewater for the carbonate.<\/p><p>The chlorine water result for X is a second, independent line of evidence for bromide, which is what makes the identification secure.<\/p>\n<p>The acid before each precipitation test is not decoration. Carbonate ions give a white precipitate with silver ions (Ag\u2082CO\u2083) and with barium ions (BaCO\u2083), so a carbonate would give a false positive in both tests.<\/p><p>Dilute acid converts the carbonate to carbon dioxide and removes the problem.<\/p><p>The acid must add nothing that is being tested for: <strong>nitric acid<\/strong> before silver nitrate (hydrochloric acid would add Cl\u207b), <strong>hydrochloric or nitric acid<\/strong> before barium chloride (sulfuric acid would add SO\u2084\u00b2\u207b).<\/p>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/fix-4.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/fix-4.jpg\" alt=\"Table of silver chloride, bromide and iodide colours with their solubility in dilute and concentrated ammonia\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The silver halide test: acidify with dilute nitric acid, add silver nitrate, then use dilute and concentrated ammonia to tell white AgCl, cream AgBr and yellow AgI apart.<\/p><\/div>\n<\/div>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/Sulfate-ion-test-infographic-and-lab-setup.webp\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/Sulfate-ion-test-infographic-and-lab-setup.webp\" alt=\"Three tubes showing white barium sulfate, no precipitate and a carbonate effervescing with the acid\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The sulfate test: acidify with dilute hydrochloric acid, then add barium chloride solution. A white precipitate of BaSO\u2084 shows sulfate; a carbonate fizzes with the acid instead.<\/p><\/div>\n<\/div>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-ana-carbonate.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-ana-carbonate.jpg\" alt=\"Carbonate test: acid added to the solid in a boiling tube, gas bubbled into limewater\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The carbonate test done properly: acid added to the solid in a boiling tube, bung fitted at once, and the gas bubbled through limewater, which turns cloudy.<\/p><\/div>\n<\/div>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-ana-inorgguide.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-ana-inorgguide.jpg\" alt=\"Anion tests on X, Y and Z: silver nitrate, barium chloride, limewater and chlorine water with the cyclohexane layer on top\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The anion tests on X, Y and Z side by side, with the orange bromine in the upper cyclohexane layer of the chlorine water tube for X.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;Add dilute hydrochloric acid, then barium chloride solution: a white precipitate shows sulfate ions. The acid removes carbonate ions, which would also give a white precipitate.&#8221; Reagent, observation, inference and the reason for the acid.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: A New Solid, W<\/h2>\n<p>Follow the evidence trail for a solid that is not on this page: a yellow silver precipitate, a purple cyclohexane layer and a white precipitate with dilute sulfuric acid.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1035\" class=\"h5p-iframe\" data-content-id=\"1035\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Analysing Unknowns Quick Choice: A New Solid, W\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">8<\/div>\n<h2>Identifying the Cation: Hydroxide and Sulfate Solubility Trends<\/h2>\n<\/div>\n<p>On this board the metal ions you are asked to identify are the Group 2 metals magnesium, calcium, strontium and barium, and the tool is <strong>solubility<\/strong>, not flame colour.<\/p><p>Add sodium hydroxide solution to a solution of the unknown: the <strong>hydroxides become more soluble<\/strong> down the group, so magnesium gives a thick white precipitate of Mg(OH)\u2082, calcium a white precipitate, strontium a slight white precipitate and barium usually none.<\/p><p>Then add dilute sulfuric acid to a fresh portion: the <strong>sulfates become less soluble<\/strong> down the group, so magnesium gives no precipitate, calcium a slight white precipitate, and strontium and barium a thick white precipitate of SrSO\u2084 or BaSO\u2084.<\/p>\n<p>Neither test on its own is enough. Magnesium is identified by a hydroxide precipitate with no sulfate precipitate; barium by a sulfate precipitate with no hydroxide precipitate; calcium and strontium sit in between and need the pattern of both.<\/p><div class=\"ols-key-box\"><p><strong>Common mistake:<\/strong> Students lose marks by writing &#8220;no reaction&#8221; when a soluble product forms in solution (the reaction happens, the product is simply soluble), and by describing Mg(OH)\u2082 as &#8220;cloudy&#8221; rather than as a white precipitate.<\/p><\/div>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Metal ion<\/th><th>NaOH(aq) added<\/th><th>Dilute H\u2082SO\u2084 added<\/th><th>Inference<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Mg\u00b2\u207a<\/strong><\/td><td>white precipitate, Mg(OH)\u2082<\/td><td>no precipitate (MgSO\u2084 soluble)<\/td><td>magnesium<\/td><\/tr>\n<tr><td><strong>Ca\u00b2\u207a<\/strong><\/td><td>white precipitate, Ca(OH)\u2082<\/td><td>slight white precipitate, CaSO\u2084<\/td><td>calcium<\/td><\/tr>\n<tr><td><strong>Sr\u00b2\u207a<\/strong><\/td><td>slight white precipitate, Sr(OH)\u2082<\/td><td>white precipitate, SrSO\u2084<\/td><td>strontium<\/td><\/tr>\n<tr><td><strong>Ba\u00b2\u207a<\/strong><\/td><td>no precipitate (Ba(OH)\u2082 soluble)<\/td><td>white precipitate, BaSO\u2084<\/td><td>barium<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Ionic equations with state symbols are expected: Mg\u00b2\u207a(aq) + 2OH\u207b(aq) \u2192 Mg(OH)\u2082(s) and Ba\u00b2\u207a(aq) + SO\u2084\u00b2\u207b(aq) \u2192 BaSO\u2084(s).<\/p><p>The barium sulfate test is the same chemistry as the sulfate test on the anion side of this page, which is why an unknown containing barium must be tested for sulfate with a reagent that adds barium ions, not the other way round.<\/p>\n<h3>The ammonium ion<\/h3>\n<p>Ammonium salts look like any other white solid, so the <strong>ammonium test<\/strong> is part of every scheme.<\/p><p>Warm a little of the solid (or its solution) with sodium hydroxide solution and hold a piece of <strong>damp red litmus paper<\/strong> at the mouth of the tube without letting it touch the liquid.<\/p><p>Ammonia gas is given off and the paper turns blue: NH\u2084\u207a(aq) + OH\u207b(aq) \u2192 NH\u2083(g) + H\u2082O(l).<\/p><p>Students lose marks by writing &#8220;litmus turns blue&#8221; without saying the paper was damp and red, and by forgetting that the alkali itself would turn the paper blue if it splashed on it.<\/p>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-ana-ammonium.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-ana-ammonium.jpg\" alt=\"Ammonium ion test: warm with NaOH(aq), damp red litmus turns blue\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The ammonium ion test: warm the solid with sodium hydroxide solution and test the gas with damp red litmus paper, which turns blue.<\/p><\/div>\n<\/div>\n<h3>The hydroxide ion<\/h3>\n<p>Hydroxide ions are in the anion list for this practical, and they are identified by their <strong>alkalinity<\/strong>.<\/p><p>Touch a drop of the solution on to red litmus paper with a glass rod, or add a few drops of universal indicator: hydroxide ions turn red litmus paper blue and universal indicator blue or purple.<\/p><p>A solution of a carbonate is also alkaline, so check that the solution does not effervesce with dilute acid before concluding that hydroxide is the anion.<\/p><p>A confirmatory test uses the ammonium test in reverse: warm a portion of the solution with a little solid ammonium chloride and hold damp red litmus paper at the mouth of the tube.<\/p><p>Ammonia is given off and the paper turns blue, NH\u2084\u207a(aq) + OH\u207b(aq) \u2192 NH\u2083(g) + H\u2082O(l).<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Hydroxides: solubility increases down the group (magnesium precipitates, barium does not). Sulfates: solubility decreases down the group (barium precipitates, magnesium does not).<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">9<\/div>\n<h2>Chlorine Water: a Stronger Oxidising Agent<\/h2>\n<\/div>\n<p>Chlorine water confirms bromide and iodide ions because <strong>chlorine is a stronger oxidising agent<\/strong> than bromine or iodine.<\/p><p>Reactivity falls down Group 7: the atoms get larger, the outer shell is further from the nucleus and more shielded, so the attraction for an extra electron weakens.<\/p><p>Chlorine therefore takes electrons from bromide and iodide ions, oxidising them to the element: 2Br\u207b(aq) + Cl\u2082(aq) \u2192 Br\u2082(aq) + 2Cl\u207b(aq) and 2I\u207b(aq) + Cl\u2082(aq) \u2192 I\u2082(aq) + 2Cl\u207b(aq).<\/p><div class=\"ols-key-box\"><p><strong>Exam tip:<\/strong> Say &#8220;chlorine oxidises bromide ions to bromine&#8221;, never &#8220;chlorine displaces bromide&#8221;: what is displaced is the element bromine, and the mechanism is a transfer of electrons.<\/p><\/div>\n<p>The colours in water are orange for bromine and brown for iodine, and dilute solutions of the two can look alike.<\/p><p>Shaking with a few drops of <strong>cyclohexane<\/strong> (or hexane) settles it: the halogen dissolves in the non-polar solvent, which is less dense than water and forms the <strong>upper layer<\/strong>, orange for bromine and purple (violet) for iodine, above a colourless aqueous layer.<\/p><p>Chloride ions give no change, because chlorine cannot oxidise chloride. With X the orange upper layer agrees with the cream AgBr precipitate: two independent tests, one inference.<\/p>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/Chlorine-water-displacement-reactions.webp\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/Chlorine-water-displacement-reactions.webp\" alt=\"Three tubes after adding chlorine water: chloride colourless, bromide orange, iodide brown\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Chlorine water added to chloride, bromide and iodide solutions: no change, orange bromine, brown iodine (purple when shaken with cyclohexane).<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Oxidising power falls down ${w.g7}. Chlorine oxidises Br\u207b and I\u207b; bromine oxidises I\u207b only; iodine oxidises neither. The cyclohexane layer sits on top and shows the halogen colour clearly.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Chlorine Water and Iodide Ions<\/h2>\n<p>Complete the ionic equation for chlorine water and potassium iodide solution, give the colours in water and in cyclohexane, and name what is oxidised.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1036\" class=\"h5p-iframe\" data-content-id=\"1036\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Analysing Unknowns Fill In: Chlorine Water and Iodide Ions\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">10<\/div>\n<h2>Ionic Equations and Reagent Roles<\/h2>\n<\/div>\n<p>Every precipitation and gas equation in this practical is expected as an <strong>ionic equation with state symbols<\/strong>; an equation without them loses the mark. The full set for the sample unknowns is below, together with the equations for the confirmatory and organic steps.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Reaction<\/th><th>Equation<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Carbonate with acid<\/strong><\/td><td>CO\u2083\u00b2\u207b(aq) + 2H\u207a(aq) \u2192 CO\u2082(g) + H\u2082O(l)<\/td><\/tr>\n<tr><td><strong>Carbon dioxide with limewater<\/strong><\/td><td>Ca(OH)\u2082(aq) + CO\u2082(g) \u2192 CaCO\u2083(s) + H\u2082O(l)<\/td><\/tr>\n<tr><td><strong>Silver bromide precipitate<\/strong><\/td><td>Ag\u207a(aq) + Br\u207b(aq) \u2192 AgBr(s)<\/td><\/tr>\n<tr><td><strong>Silver bromide in concentrated ammonia<\/strong><\/td><td>AgBr(s) + 2NH\u2083(aq) \u2192 [Ag(NH\u2083)\u2082]\u207a(aq) + Br\u207b(aq)<\/td><\/tr>\n<tr><td><strong>Barium sulfate precipitate<\/strong><\/td><td>Ba\u00b2\u207a(aq) + SO\u2084\u00b2\u207b(aq) \u2192 BaSO\u2084(s)<\/td><\/tr>\n<tr><td><strong>Barium carbonate (the false positive the acid prevents)<\/strong><\/td><td>Ba\u00b2\u207a(aq) + CO\u2083\u00b2\u207b(aq) \u2192 BaCO\u2083(s)<\/td><\/tr>\n<tr><td><strong>Silver oxide (what forms if excess NaOH is not neutralised)<\/strong><\/td><td>2Ag\u207a(aq) + 2OH\u207b(aq) \u2192 Ag\u2082O(s) + H\u2082O(l)<\/td><\/tr>\n<tr><td><strong>Bromide oxidised by chlorine<\/strong><\/td><td>2Br\u207b(aq) + Cl\u2082(aq) \u2192 Br\u2082(aq) + 2Cl\u207b(aq)<\/td><\/tr>\n<tr><td><strong>Hydrolysis of the bromoalkane<\/strong><\/td><td>CH\u2083CH\u2082CH\u2082Br + OH\u207b \u2192 CH\u2083CH\u2082CH\u2082OH + Br\u207b<\/td><\/tr>\n<tr><td><strong>Dichromate(VI) reduced (half-equation)<\/strong><\/td><td>Cr\u2082O\u2087\u00b2\u207b(aq) + 14H\u207a(aq) + 6e\u207b \u2192 2Cr\u00b3\u207a(aq) + 7H\u2082O(l)<\/td><\/tr>\n<tr><td><strong>Bromine adding to an alkene<\/strong><\/td><td>C\u2082H\u2084(g) + Br\u2082(aq) \u2192 C\u2082H\u2084Br\u2082(l)<\/td><\/tr>\n<tr><td><strong>Ammonium ion with hydroxide<\/strong><\/td><td>NH\u2084\u207a(aq) + OH\u207b(aq) \u2192 NH\u2083(g) + H\u2082O(l)<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Three reagent roles are asked about again and again. <strong>Dilute nitric acid before silver nitrate<\/strong> has two jobs.<\/p><p>In the inorganic test it removes carbonate ions, which would give a white precipitate of Ag\u2082CO\u2083 and mask the halide. In the organic test it neutralises the excess sodium hydroxide from the hydrolysis, which would otherwise give brown Ag\u2082O.<\/p><p><strong>Why nitric acid<\/strong> and not another acid: hydrochloric acid adds chloride ions (false white AgCl) and sulfuric acid adds sulfate ions (false white BaSO\u2084 in the sulfate test).<\/p><p><strong>Dilute hydrochloric acid before barium chloride<\/strong> removes carbonate ions, which would give white BaCO\u2083; sulfuric acid can never be used here.<\/p>\n<p>The ammonia steps are also reagent roles. Dilute ammonia dissolves AgCl by forming the soluble complex [Ag(NH\u2083)\u2082]\u207a; AgBr is less soluble and needs concentrated ammonia; AgI is too insoluble to dissolve in either.<\/p><p>So the ammonia result is not a second test for the same thing, it is what separates the three halides once a precipitate has been seen.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam trap:<\/strong> Two different reasons for the same acid: nitric acid removes carbonate ions in the inorganic test and neutralises excess hydroxide ions in the organic test. Give the reason that fits the question.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Diagnosing Test Results<\/h2>\n<p>Explain four results from halogenoalkane tests on compounds that are not on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1074\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">11<\/div>\n<h2>Exam-Style Data Practice<\/h2>\n<\/div>\n<p>Two kinds of question follow this practical: an unknown-solution matrix, in which four labelled solutions are mixed pairwise and identified from the pattern, and a composition-and-spectra question that leads from percentage composition to a structure. Both are answered by the evidence-trail habit.<\/p>\n<h3>Unknown solution matrix<\/h3>\n<p>Four solutions are sodium carbonate, barium chloride, dilute hydrochloric acid and dilute sulfuric acid. The trap is that barium chloride gives a <strong>white precipitate with sodium carbonate<\/strong> as well as with sulfuric acid: Ba\u00b2\u207a(aq) + CO\u2083\u00b2\u207b(aq) \u2192 BaCO\u2083(s). A student who forgets this misassigns C and D.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Letter<\/th><th>Identity<\/th><th>Evidence from the pairwise mixing<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>A<\/strong><\/td><td>sulfuric acid, H\u2082SO\u2084<\/td><td>white precipitate with C (BaSO\u2084) and effervescence with D; no other change<\/td><\/tr>\n<tr><td><strong>B<\/strong><\/td><td>hydrochloric acid, HCl<\/td><td>effervescence with D only; no precipitate with anything<\/td><\/tr>\n<tr><td><strong>C<\/strong><\/td><td>barium chloride, BaCl\u2082<\/td><td>white precipitate with A (BaSO\u2084, insoluble in acid) and with D (BaCO\u2083, which dissolves with effervescence when A or B is added)<\/td><\/tr>\n<tr><td><strong>D<\/strong><\/td><td>sodium carbonate, Na\u2082CO\u2083<\/td><td>effervescence with A and B; white precipitate with C<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>The distinguishing observations are that A gives both a precipitate and a gas, B gives only a gas, C gives two precipitates and D gives two lots of gas and one precipitate. Adding acid to the C + D precipitate dissolves it with effervescence, which tells BaCO\u2083 from BaSO\u2084.<\/p>\n<h3>Composition and spectra<\/h3>\n<p>An organic compound contains 38.7% carbon, 9.7% hydrogen and 51.6% oxygen by mass (the three values must add to 100%). Its mass spectrum has a molecular ion peak at m\/z 62 and a base peak at m\/z 31; its infrared spectrum has a broad absorption at about 3400 cm\u207b\u00b9 and no absorption near 1700 cm\u207b\u00b9.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Step<\/th><th>Working<\/th><th>Result<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>1. Moles in 100 g<\/strong><\/td><td>C: 38.7 \u00f7 12.0 = 3.23; H: 9.7 \u00f7 1.0 = 9.7; O: 51.6 \u00f7 16.0 = 3.23<\/td><td>C : H : O = 3.23 : 9.7 : 3.23<\/td><\/tr>\n<tr><td><strong>2. Simplest ratio<\/strong><\/td><td>divide by 3.23: C 1.00, H 3.00, O 1.00<\/td><td>empirical formula CH\u2083O, relative mass 12.0 + 3.0 + 16.0 = 31.0<\/td><\/tr>\n<tr><td><strong>3. Molecular formula<\/strong><\/td><td>Mr from the molecular ion is 62; 62 \u00f7 31 = 2<\/td><td>C\u2082H\u2086O\u2082<\/td><\/tr>\n<tr><td><strong>4. Infrared<\/strong><\/td><td>broad absorption at about 3400 cm\u207b\u00b9: O\u2013H, broadened by hydrogen bonding; nothing near 1700 cm\u207b\u00b9: no C=O<\/td><td>an alcohol, not an acid or a carbonyl compound<\/td><\/tr>\n<tr><td><strong>5. Fragment<\/strong><\/td><td>m\/z 31 is CH\u2082OH\u207a (12 + 2 + 16 + 1), formed when the C\u2013C bond in HOCH\u2082\u2013CH\u2082OH breaks<\/td><td>a CH\u2082OH group is present<\/td><\/tr>\n<tr><td><strong>6. Structure<\/strong><\/td><td>two carbons, two OH groups, a CH\u2082OH fragment<\/td><td>ethane-1,2-diol, HOCH\u2082CH\u2082OH<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Two errors are common enough to name. Rounding the ratio to CH\u2082O gives C\u2082H\u2084O\u2082, which is ethanoic acid, a compound that would show a strong C=O absorption and no m\/z 31 fragment; the data rule it out.<\/p><p>CHO\u207a has m\/z 29, not 31: check every fragment by adding the atomic masses.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Composition gives the empirical formula; the molecular ion gives Mr and so the molecular formula; the infrared spectrum names the functional groups; the fragments place them. Quote each piece of evidence for its own conclusion.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Formula From Composition and Spectra<\/h2>\n<p>A different compound, Q: 52.2% carbon, 13.0% hydrogen, 34.8% oxygen, M\u207a at m\/z 46, broad absorption at 3300 cm\u207b\u00b9 and a base peak at m\/z 31. Work through the same steps.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1038\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">12<\/div>\n<h2>Errors, Uncertainty and Improvements<\/h2>\n<\/div>\n<p>A qualitative practical has no numerical result to compare with an accepted value, so the evaluation questions ask about <strong>false positives<\/strong>, <strong>false negatives<\/strong> and the steps that prevent them. The table lists the errors students actually make and what each does to the conclusion.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Source of error<\/th><th>Effect on the result<\/th><th>Improvement<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Dichromate(VI) or Fehling&#8217;s tube not warmed<\/strong><\/td><td>false negative: an alcohol or aldehyde appears unreactive<\/td><td>warm in a water bath at about 60 \u00b0C for several minutes before recording a negative<\/td><\/tr>\n<tr><td><strong>Solid tested directly with AgNO\u2083(aq) or BaCl\u2082(aq)<\/strong><\/td><td>false negative: no ions in solution, no precipitate<\/td><td>dissolve a small portion in distilled water first<\/td><\/tr>\n<tr><td><strong>Hydrochloric acid used before silver nitrate<\/strong><\/td><td>false positive: white AgCl in every tube<\/td><td>acidify with dilute nitric acid<\/td><\/tr>\n<tr><td><strong>Sulfuric acid used before barium chloride<\/strong><\/td><td>false positive: white BaSO\u2084 in every tube<\/td><td>acidify with dilute hydrochloric (or nitric) acid<\/td><\/tr>\n<tr><td><strong>No acid before silver nitrate on the hydrolysed liquid<\/strong><\/td><td>brown Ag\u2082O masks the halide colour<\/td><td>neutralise the excess NaOH with dilute nitric acid, testing with litmus<\/td><\/tr>\n<tr><td><strong>Carbonate not removed before the barium or silver test<\/strong><\/td><td>false positive: white BaCO\u2083 or Ag\u2082CO\u2083<\/td><td>acidify first and wait until effervescence stops<\/td><\/tr>\n<tr><td><strong>Effervescence recorded as &#8220;carbonate&#8221; without the gas test<\/strong><\/td><td>inference not supported<\/td><td>bubble the gas through limewater and record the cloudiness<\/td><\/tr>\n<tr><td><strong>Tap water used to dissolve the solids<\/strong><\/td><td>false positive for chloride<\/td><td>use distilled or deionised water<\/td><\/tr>\n<tr><td><strong>Same pipette used for two reagents<\/strong><\/td><td>cross-contamination, unexpected precipitates<\/td><td>one labelled pipette per reagent; rinse tubes between tests<\/td><\/tr>\n<tr><td><strong>Too much solid in the tube<\/strong><\/td><td>cloudy suspension hides or imitates a precipitate<\/td><td>use a spatula tip only; view against a dark background<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Where a quantity is measured, its percentage uncertainty still follows the usual rule.<\/p><p>If a method asks for &#8220;about 0.5 g&#8221; of solid and a 2 d.p. balance reads 0.50 g, the uncertainty is \u00b10.005 g, so the percentage uncertainty is (0.005 \u00f7 0.50) \u00d7 100 = <strong>1.0%<\/strong>.<\/p><p>Measuring 5 cm\u00b3 of water with a 10 cm\u00b3 measuring cylinder (\u00b10.5 cm\u00b3) gives (0.5 \u00f7 5) \u00d7 100 = <strong>10%<\/strong>.<\/p><p>Neither matters here, because a precipitate forms or does not form whatever the exact mass or volume, and that is the answer to &#8220;why is &#8216;approximately&#8217; acceptable in this method&#8221;: the result is qualitative, so the measurements need only be roughly right.<\/p><p>The composition data in the exam question are different: 38.7% quoted to 1 decimal place has an uncertainty of \u00b10.05%, about 0.1% of the value.<\/p><p>That is why the mole ratio comes out as 1.00 : 3.00 : 1.00 and not as something that needs rounding.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;Warming the tube in a water bath makes a negative result reliable.&#8221; &#8220;Dilute nitric acid is used because hydrochloric acid would add chloride ions and give a false positive.&#8221; State the error, its effect and the fix.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">13<\/div>\n<h2>Common Mistakes<\/h2>\n<\/div>\n<ul>\n<li>Writing &#8220;clear&#8221; for a solution with no colour. Bromine water is <strong>decolourised<\/strong>; a solution with no colour is <strong>colourless<\/strong>; &#8220;clear&#8221; only means not cloudy.<\/li>\n<li>Not warming the dichromate(VI) and Fehling&#8217;s tubes, then reporting &#8220;no reaction&#8221;. A negative result at room temperature proves nothing.<\/li>\n<li>Calling A &#8220;an oxidisable alcohol&#8221;. The inference is &#8220;a primary or secondary alcohol&#8221;; the negative Fehling&#8217;s result is what rules out an aldehyde.<\/li>\n<li>Adding silver nitrate straight to the organic liquid. The halogen is covalently bonded; hydrolyse first, then acidify with nitric acid, then test.<\/li>\n<li>Testing a solid with silver nitrate or barium chloride solution without dissolving it in distilled water first.<\/li>\n<li>Giving &#8220;fizzes with acid&#8221; as proof of a carbonate. The gas must turn limewater cloudy.<\/li>\n<li>Stopping at &#8220;cream precipitate&#8221;. Without the dilute and concentrated ammonia results, AgBr has not been distinguished from AgI.<\/li>\n<li>Writing &#8220;chlorine displaces bromide ions&#8221;. Chlorine oxidises bromide ions to bromine; the element bromine is what is displaced.<\/li>\n<li>Ionic equations without state symbols, and minus signs missing from Br\u207b and SO\u2084\u00b2\u207b.<\/li>\n<li>Rounding the mole ratio carelessly (CH\u2082O instead of CH\u2083O) and quoting a fragment whose mass does not add up (CHO\u207a is 29, CH\u2082OH\u207a is 31).<\/li>\n<\/ul>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Technique Check<\/h2>\n<p>Six sets of statements about the technique of this practical. Pick the accurate one in each set.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1039\" class=\"h5p-iframe\" data-content-id=\"1039\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Analysing Unknowns Summary: Technique Check\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">14<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Say<\/h3><p>&#8220;Warm with acidified potassium dichromate(VI) in a water bath: orange to green, so the liquid is oxidised.&#8221;<\/p><p>&#8220;Fehling&#8217;s solution remains blue on warming, so it is not an aldehyde; A is a primary or secondary alcohol.&#8221;<\/p><p>&#8220;Dissolve the solid in distilled water, add dilute nitric acid, then silver nitrate solution: cream precipitate, insoluble in dilute ammonia, soluble in concentrated ammonia, so bromide ions.&#8221;<\/p><p>&#8220;Effervescence; the gas turns limewater cloudy, so carbonate ions.&#8221;<\/p><p>&#8220;Chlorine oxidises bromide ions to bromine, which gives an orange upper layer in cyclohexane.&#8221;<\/p>\n<h3>Do not say<\/h3><p>&#8220;Goes clear.&#8221; &#8220;Positive result.&#8221; &#8220;Oxidisable alcohol.&#8221; &#8220;Chlorine displaces bromide.&#8221; &#8220;Add acid&#8221; without saying which acid and why. &#8220;Fizzes, so carbonate&#8221; without the limewater step. Equations without state symbols. &#8220;No reaction&#8221; when a soluble product has formed.<\/p>\n<h3>Watch for<\/h3><p>The order of the tests and the reason for it (acid before silver nitrate or barium chloride; carbonate removed first).<\/p>\n<p>Which acid: nitric before silver nitrate, hydrochloric or nitric before barium chloride, never sulfuric. Water bath, not a naked flame, for every warming step.<\/p>\n<p>Percentages in a composition question must add to 100%, and every fragment mass must add up.<\/p>\n<\/article>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Quick answers to the questions students ask most about Required Practicals 4 and 6 and the tests for unknown organic liquids and inorganic solids.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why must the acidified dichromate(VI) and Fehling&#8217;s tubes be warmed?<\/h3>\n<p>Both oxidations are slow at room temperature.<\/p><p>Warming in a water bath at about 60 \u00b0C for a few minutes lets the reaction happen, so a tube that stays orange or blue after warming is a genuine negative result.<\/p><p>Without warming a negative result proves nothing, and the water bath keeps any naked flame away from flammable organic liquids.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is nitric acid, and not hydrochloric acid, added before silver nitrate?<\/h3>\n<p>Hydrochloric acid would add chloride ions and give a white precipitate of silver chloride in every tube, a false positive. Nitric acid adds only nitrate ions, which form no insoluble silver salt.<\/p><p>The acid removes carbonate ions in the inorganic test and neutralises excess sodium hydroxide after hydrolysing the organic liquid, which would otherwise give brown silver oxide.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I tell silver bromide from silver iodide?<\/h3>\n<p>Cream and pale yellow are easy to confuse in a small tube, so use ammonia: silver chloride dissolves in dilute ammonia, silver bromide dissolves only in concentrated ammonia, and silver iodide dissolves in neither. Quote the ammonia result as part of the inference.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Does fizzing with acid prove that a solid is a carbonate?<\/h3>\n<p>No. Effervescence suggests a carbonate (or hydrogencarbonate) but the gas must be identified: bubble it through limewater, which turns cloudy as a white precipitate of calcium carbonate forms. The observation and its confirmation are separate mark points.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What is the difference between &#8220;clear&#8221; and &#8220;colourless&#8221;?<\/h3>\n<p>Colourless means the solution has no colour; clear means it is not cloudy. Bromine water that has reacted with an alkene is decolourised, orange to colourless. A blue Fehling&#8217;s solution is clear but not colourless. Examiners penalise &#8220;goes clear&#8221; because it does not describe a colour change.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why shake the chlorine water mixture with cyclohexane?<\/h3>\n<p>Bromine and iodine dissolve much better in the non-polar solvent than in water, and cyclohexane floats as the upper layer, so the colour is concentrated and easy to read: orange for bromine, purple for iodine. Dilute aqueous solutions of the two can look alike.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Required Practicals Pages<\/h2>\n<p>Use these pages to connect the practical techniques with each other and with the rest of the course.<\/p>\n<div class=\"ols-related-grid\">\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/molar-volume-of-a-gas\/\">Gas volumes: Molar Volume of a Gas<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp2-measurement-of-an-enthalpy-change\/\">RP2: Enthalpy Change via Hess&#8217;s Law<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp1-acid-base-titration\/\">RP1: Concentration of HCl by Titration<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp1-making-a-volumetric-solution\/\">RP1: Preparation of a Standard Solution<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rates-of-hydrolysis-of-halogenoalkanes\/\">Hydrolysis rates: Hydrolysis of Halogenoalkanes<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/preparing-an-organic-liquid-chlorination-of-2-methylpropan-2-ol\/\">Organic prep: Chlorination of 2-methylpropan-2-ol<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp5-distillation-of-a-product-oxidation-of-ethanol\/\">RP5: Oxidation of an Alcohol<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/\">Required Practicals Overview<\/a>\n<\/div>\n<\/section>\n<section class=\"ols-attribution-card\">\n        <p><strong>Copyright and author footprint:<\/strong> This OLS revision page was written for Online Learning System by <strong>Dr. Mohammed Al-Fatah<\/strong>. It is designed for A Level Chemistry revision and should not be copied or redistributed without permission.<\/p>\n      <\/section>\n\n      <div class=\"ols-image-lightbox\" id=\"olsImageLightboxNatureCovalentBonding9ch0\" aria-hidden=\"true\" role=\"dialog\" aria-modal=\"true\" aria-label=\"Expanded revision image\">\n        <div class=\"ols-image-lightbox-inner\">\n          <button class=\"ols-image-lightbox-close\" type=\"button\" aria-label=\"Close enlarged image\">\u00d7<\/button>\n          <img decoding=\"async\" class=\"ols-image-lightbox-img\" src=\"\" alt=\"\">\n        <\/div>\n      <\/div>\n\n      <script>\n        (function(){\n          var page = document.querySelector(\".ols-nature-of-covalent-bonding-9ch0-page\");\n          if (!page) { return; }\n          var lightbox = page.querySelector(\"#olsImageLightboxNatureCovalentBonding9ch0\");\n          if (!lightbox) { return; }\n          var lightboxImage = lightbox.querySelector(\".ols-image-lightbox-img\");\n    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