{"id":11294,"date":"2026-09-27T14:52:24","date_gmt":"2026-09-27T13:52:24","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-4-pag-7-qualitative-analysis-of-ions-and-functional-groups\/"},"modified":"2026-10-03T08:38:12","modified_gmt":"2026-10-03T07:38:12","slug":"pag-4-pag-7-qualitative-analysis-of-ions-and-functional-groups","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-4-pag-7-qualitative-analysis-of-ions-and-functional-groups\/","title":{"rendered":"PAG 4 and PAG 7 Qualitative Analysis of Unknowns"},"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; 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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>OCR A A Level Chemistry<\/p>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Practical Activity Groups (PAGs)<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-1-molar-volume-of-a-gas\/\">PAG 1: Molar Volume of a Gas<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-3-enthalpy-determination\/\">PAG 3: Enthalpy Change via Hess's Law<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-2-acid-base-titration\/\">PAG 2: Concentration of HCl by Titration<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-2-preparing-a-standard-solution\/\">PAG 2: Preparation of a Standard Solution<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/rates-of-hydrolysis-of-haloalkanes\/\">Hydrolysis rates: Hydrolysis of Halogenoalkanes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-5-synthesis-of-an-organic-liquid\/\">PAG 5: Chlorination of 2-methylpropan-2-ol<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-5-oxidation-of-an-alcohol\/\">PAG 5: Oxidation of an Alcohol<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-4-pag-7-qualitative-analysis-of-ions-and-functional-groups\/\">PAG 4 and 7: Analysis of Inorganic and Organic Unknowns<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/measuring-rates-of-reaction\/\">Rates: Rates of Reaction<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-1-moles-determination-by-mass\/\">PAG 1: 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\/ocr-a\/\">OCR A Chemistry<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/\">Practical Activity Groups (PAGs)<\/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\/ocr-a\/\">OCR A<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/\">Practical Activity Groups (PAGs)<\/a> \/\n<span>PAG 4 and PAG 7 Qualitative Analysis of Unknowns<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>PAG 4 and PAG 7 Qualitative Analysis of Unknowns<\/h1>\n        <p class=\"ols-page-intro\">PAG 4 and PAG 7 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\">Practical endorsement<\/div>\n<div class=\"ols-badge\">PAG 4 and PAG 7<\/div>\n<div class=\"ols-badge\">H432<\/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 OCR A 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>PAG 4 and PAG 7 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 17 chemistry, the tests for alkenes, alcohols, aldehydes and haloalkanes, 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>Then comes 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 (practical skills are examined in every paper) 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 haloalkane.<\/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\/ocr-a\/practical-activity-groups\/pag-5-oxidation-of-an-alcohol\/\">PAG 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>Haloalkane<\/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 haloalkane 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\/ocr-a\/practical-activity-groups\/rates-of-hydrolysis-of-haloalkanes\/\">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>The Order of the Anion Tests and the Ammonium Ion<\/h2>\n<\/div>\n<p>On this board the anion tests must be carried out in a fixed <strong>order<\/strong>: carbonate first, then sulfate, then halide, and the reason for the order is a standard question.<\/p><p>Barium ions give a white precipitate with carbonate ions as well as with sulfate ions, and silver ions give precipitates with carbonate and sulfate ions as well as with halide ions.<\/p><p>If you tested for the halide first, a white precipitate of Ag\u2082CO\u2083 or Ag\u2082SO\u2084 would look like AgCl; if you tested for sulfate before carbonate, white BaCO\u2083 would look like BaSO\u2084.<\/p>\n<p>Testing for carbonate first removes the problem: if the solution fizzes with dilute nitric acid, the carbonate is used up as carbon dioxide, and the acidified solution can go on to the sulfate test.<\/p><p>Once barium nitrate has shown that no sulfate is present (or has removed it as BaSO\u2084, which is filtered off), silver nitrate can be added and any precipitate must be a silver halide.<\/p><div class=\"ols-key-box\"><p><strong>Common mistake:<\/strong> Students lose marks by giving the order without the reason, and by saying &#8220;carbonate interferes&#8221; without naming the false precipitate.<\/p><\/div>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Order<\/th><th>Test<\/th><th>Why here<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>1<\/strong><\/td><td>dilute HNO\u2083: look for effervescence, gas turns limewater cloudy<\/td><td>CO\u2083\u00b2\u207b would give false white precipitates with Ba\u00b2\u207a (BaCO\u2083) and Ag\u207a (Ag\u2082CO\u2083) in the later tests; the acid also removes it<\/td><\/tr>\n<tr><td><strong>2<\/strong><\/td><td>Ba(NO\u2083)\u2082(aq) or BaCl\u2082(aq): white precipitate<\/td><td>SO\u2084\u00b2\u207b would give a white precipitate of Ag\u2082SO\u2084 with silver ions; use Ba(NO\u2083)\u2082 if a halide test follows, because BaCl\u2082 adds Cl\u207b<\/td><\/tr>\n<tr><td><strong>3<\/strong><\/td><td>AgNO\u2083(aq), then dilute and concentrated NH\u2083(aq)<\/td><td>now any precipitate is a silver halide: white, cream or yellow, distinguished by ammonia<\/td><\/tr>\n<tr><td><strong>4<\/strong><\/td><td>NaOH(aq), warm, damp red litmus<\/td><td>the ammonium test is done on a separate portion of the solid<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Nitric acid and barium nitrate are preferred because they add only ions that take no part in any of the tests: NO\u2083\u207b forms no insoluble salt with barium or silver. Hydrochloric acid would add Cl\u207b (false AgCl) and sulfuric acid would add SO\u2084\u00b2\u207b (false BaSO\u2084).<\/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<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;Carbonate first, because carbonate ions would give a white precipitate with barium ions and with silver ions; sulfate second, because sulfate ions would give a white precipitate with silver ions; halide last.&#8221; Name the false precipitate each time.<\/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 17: 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 PAG 4 and PAG 7 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 Practical Activity Groups 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\/ocr-a\/practical-activity-groups\/pag-1-molar-volume-of-a-gas\/\">PAG 1: Molar Volume of a Gas<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-3-enthalpy-determination\/\">PAG 3: 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\/ocr-a\/practical-activity-groups\/pag-2-acid-base-titration\/\">PAG 2: Concentration of HCl by Titration<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-2-preparing-a-standard-solution\/\">PAG 2: Preparation of a Standard Solution<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/rates-of-hydrolysis-of-haloalkanes\/\">Hydrolysis rates: Hydrolysis of Halogenoalkanes<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-5-synthesis-of-an-organic-liquid\/\">PAG 5: Chlorination of 2-methylpropan-2-ol<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-5-oxidation-of-an-alcohol\/\">PAG 5: Oxidation of an Alcohol<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/\">Practical Activity Groups (PAGs) 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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