{"id":10487,"date":"2026-09-26T11:30:44","date_gmt":"2026-09-26T10:30:44","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/halide-ions-reducing-agents-and-tests\/"},"modified":"2026-10-04T23:02:54","modified_gmt":"2026-10-04T22:02:54","slug":"halide-ions-reducing-agents-and-tests","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/halide-ions-reducing-agents-and-tests\/","title":{"rendered":"Halide Ions: Reducing Agents and Tests"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-redox-groups-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: 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border-radius: 18px;\n      padding: 18px 20px;\n      box-shadow: var(--inner-shadow);\n    }\n.ols-faq-item h3 {\n      margin: 0 0 8px;\n      font-size: 20px;\n      line-height: 1.3;\n      color: var(--navy);\n    }\n.ols-faq-item p {\n      margin: 0;\n      font-size: 16px;\n      line-height: 1.65;\n      color: var(--body-text);\n    }\n.ols-faq-card,\n      .ols-quicksnap-card,\n      .ols-attribution-card {\n        padding: 24px 18px;\n        border-radius: 22px;\n      }\n  <\/style>\n\n    <aside class=\"ols-sidebar\">\n  <div class=\"ols-sidebar-header\">\n    <h3>Revision Notes<\/h3>\n    <p>A Level Chemistry<\/p>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Topic 8 Redox &amp; Groups 1, 2 and 7<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/\">Topic 8 Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/\">Topic 8A Redox Chemistry<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/oxidation-numbers\/\">Oxidation Numbers<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/electron-transfer-and-half-equations\/\">Electron Transfer and Half-Equations<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/disproportionation\/\">Disproportionation<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/redox-classification\/\">Redox Classification<\/a>\n          <\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/\">Topic 8B Groups 1 and 2<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/trends-down-groups-1-and-2\/\">Trends Down Groups 1 and 2<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/reactions-of-the-group-1-and-2-elements\/\">Reactions of the Group 1 and 2 Elements<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/oxides-hydroxides-and-carbonates\/\">Oxides, Hydroxides and Carbonates<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/solubility-trends-and-the-sulfate-test\/\">Solubility Trends and the Sulfate Test<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/thermal-stability-and-flame-colours\/\">Thermal Stability and Flame Colours<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/tests-for-ions\/\">Tests for Ions<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/titrations-and-measurement-uncertainty\/\">Titrations and Measurement Uncertainty<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/cp-3-concentration-of-hcl-by-titration\/\">Core Practical 3<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/cp-4-preparation-of-a-standard-solution\/\">Core Practical 4<\/a>\n          <\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/\">Topic 8C Group 7<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/trends-in-the-halogens\/\">Trends in the Halogens<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/halogens-as-oxidising-agents\/\">Halogens as Oxidising Agents<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/disproportionation-reactions-of-chlorine\/\">Disproportionation Reactions of Chlorine<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/halide-ions-reducing-agents-and-tests\/\">Halide Ions: Reducing Agents and Tests<\/a>\n          <\/li>\n        <\/ul>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Other Topics<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/\">Topic 6 Energetics<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/\">Topic 7 Intermolecular Forces<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/\">Topic 2 Atomic Structure &amp; The Periodic Table<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/\">Core Practicals<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\n\n<script>\r\n(function() {\r\n  function normalisePath(path) {\r\n    return String(path || '')\r\n      .split('?')[0]\r\n      .split('#')[0]\r\n      .replace(\/\\\/+$\/, '')\r\n      .toLowerCase();\r\n  }\r\n\r\n  function highlightActive() {\r\n    var sidebar = document.querySelector('.ols-sidebar');\r\n    if (!sidebar) return false;\r\n\r\n    var currentPath = normalisePath(window.location.pathname);\r\n    var links = sidebar.querySelectorAll('a[href]');\r\n    var matched = null;\r\n    var matchedLength = 0;\r\n\r\n    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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/\">Topic 8 Redox Chemistry and Groups 1, 2 and 7<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/\">Topic 8C Group 7<\/a> \/\n<span>Halide Ions: Reducing Agents and Tests<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Halide Ions: Reducing Agents and Tests<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to the halide ions as reducing agents, shown by the solid halides with concentrated sulfuric acid, the silver nitrate test followed by ammonia, and the hydrogen halides with ammonia and with water.<\/p>\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Exam board: Edexcel International<\/div>\n<div class=\"ols-badge\">Unit 2: WCH12\/01<\/div>\n<div class=\"ols-badge\">Topic 8C Group 7<\/div>\n<\/div>\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: Dr. Mohammed Al-Fatah\n      <\/h2>\n\n      <p class=\"ols-author-description\">\n        Chemistry specialist revision notes for A Level Chemistry.\n      <\/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      <article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>Halide Ions as Reducing Agents<\/h2>\n<\/div>\n<p>A halide ion acts as a <strong>reducing agent<\/strong> by giving up an electron to become the halogen.<\/p>\n<p>The <strong>reducing power increases down the group<\/strong>: iodide ions are strong reducing agents, bromide ions moderate, chloride ions weak and fluoride ions barely reducing at all.<\/p>\n<p>The larger the ion, the further the outer electrons are from the nucleus, the more they are shielded, and the more easily one is lost.<\/p>\n<p>This is the mirror image of the trend in the halogens as oxidising agents. Chlorine is the best at taking electrons; iodide is the best at giving them.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Oxidising power of the halogens falls down the group; reducing power of the halide ions rises down the group. Both follow atomic radius and shielding.<\/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>Solid Halides with Concentrated Sulfuric Acid<\/h2>\n<\/div>\n<p>Concentrated sulfuric acid is both an acid and an oxidising agent, so what it does to a solid halide depends on how good a reducing agent the halide ion is. Every halide first gives the <strong>hydrogen halide<\/strong> in an acid\u2013base reaction:<\/p>\n<p>NaCl + H\u2082SO\u2084 \u2192 NaHSO\u2084 + HCl &nbsp;&nbsp; (steamy fumes of HCl)<\/p>\n<p><strong>Chloride<\/strong> ions are too weak a reducing agent to reduce the sulfuric acid, so the reaction stops there. <strong>Bromide<\/strong> ions reduce the sulfuric acid to sulfur dioxide, and the hydrogen bromide formed is oxidised to bromine:<\/p>\n<p>NaBr + H\u2082SO\u2084 \u2192 NaHSO\u2084 + HBr &nbsp;&nbsp; then &nbsp;&nbsp; 2HBr + H\u2082SO\u2084 \u2192 Br\u2082 + SO\u2082 + 2H\u2082O<\/p>\n<p><strong>Iodide<\/strong> ions are strong enough to reduce sulfuric acid all the way from +6 to +4, 0 and \u22122:<\/p>\n<p>2HI + H\u2082SO\u2084 \u2192 I\u2082 + SO\u2082 + 2H\u2082O &nbsp;&nbsp; 6HI + H\u2082SO\u2084 \u2192 3I\u2082 + S + 4H\u2082O &nbsp;&nbsp; 8HI + H\u2082SO\u2084 \u2192 4I\u2082 + H\u2082S + 4H\u2082O<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Solid halide<\/th><th>Products<\/th><th>Observations<\/th><th>Lowest oxidation number of sulfur<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>NaCl<\/strong><\/td><td>HCl only<\/td><td>steamy white fumes<\/td><td>+6 (not reduced)<\/td><\/tr>\n<tr><td><strong>NaBr<\/strong><\/td><td>HBr, Br\u2082, SO\u2082<\/td><td>steamy fumes, orange-brown vapour, sharp choking gas<\/td><td>+4<\/td><\/tr>\n<tr><td><strong>NaI<\/strong><\/td><td>HI, I\u2082, SO\u2082, S, H\u2082S<\/td><td>steamy fumes, purple vapour and black solid, yellow solid, bad-egg smell<\/td><td>\u22122<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-figure-card ols-zoom-pop\">\n<div class=\"ols-figure-image\">\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/redox-groups-halide-ions-reducing-agents-tests.png\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/redox-groups-halide-ions-reducing-agents-tests.png\" alt=\"Silver nitrate test for halide ions and the reactions of solid sodium halides with concentrated sulfuric acid\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/redox-groups-halide-ions-reducing-agents-tests.png\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>The products on the right show the reducing power rising from chloride to iodide, and the precipitates on the left show the test that distinguishes them.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Give the oxidation number of sulfur for each product: +6 in H\u2082SO\u2084, +4 in SO\u2082, 0 in S, \u22122 in H\u2082S. The further it falls, the stronger the reducing agent that caused it.<\/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: Halides and Concentrated Sulfuric Acid<\/h2>\n<p>Predict the products and observations for a halide not used above, and give the oxidation number changes of sulfur.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"895\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card purple\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>The Silver Nitrate Test<\/h2>\n<\/div>\n<p>To identify a halide ion in solution, first add dilute <strong>nitric acid<\/strong>, then <strong>silver nitrate solution<\/strong>. The halide ions precipitate as silver halides of different colours:<\/p>\n<p>Ag\u207a(aq) + Cl\u207b(aq) \u2192 AgCl(s) white &nbsp;&nbsp; Ag\u207a(aq) + Br\u207b(aq) \u2192 AgBr(s) cream &nbsp;&nbsp; Ag\u207a(aq) + I\u207b(aq) \u2192 AgI(s) yellow<\/p>\n<p>The nitric acid removes carbonate and hydroxide ions, which would otherwise give precipitates of silver carbonate or silver oxide and confuse the result. Hydrochloric acid cannot be used because it contains chloride ions. Fluoride ions give no precipitate, because silver fluoride is soluble.<\/p>\n<p>The colours are close, so the result is confirmed with <strong>ammonia solution<\/strong>. Silver chloride dissolves in dilute ammonia, silver bromide dissolves only in concentrated ammonia, and silver iodide does not dissolve in either.<\/p>\n<p>The silver ions are removed from the solid as the complex ion [Ag(NH\u2083)\u2082]\u207a, and the less soluble the halide, the harder this is.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Halide<\/th><th>Precipitate<\/th><th>Dilute NH\u2083(aq)<\/th><th>Concentrated NH\u2083(aq)<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Chloride<\/strong><\/td><td>white AgCl<\/td><td>dissolves<\/td><td>dissolves<\/td><\/tr>\n<tr><td><strong>Bromide<\/strong><\/td><td>cream AgBr<\/td><td>insoluble<\/td><td>dissolves<\/td><\/tr>\n<tr><td><strong>Iodide<\/strong><\/td><td>yellow AgI<\/td><td>insoluble<\/td><td>insoluble<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;Acidify with dilute nitric acid, add silver nitrate solution: a cream precipitate that is insoluble in dilute ammonia but dissolves in concentrated ammonia shows bromide ions.&#8221;<\/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: Reading a Silver Nitrate Result<\/h2>\n<p>Identify the halide from precipitate and ammonia results for samples not described above, and explain a false result.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-896\" class=\"h5p-iframe\" data-content-id=\"896\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Redox and Groups Summary: Reading a Silver Nitrate Result\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>The Hydrogen Halides<\/h2>\n<\/div>\n<p>The hydrogen halides are colourless gases that fume in moist air.<\/p>\n<p>They dissolve in water to give <strong>acids<\/strong>, because the H\u2013X bond breaks heterolytically and releases hydrogen ions: HCl(g) + H\u2082O(l) \u2192 H\u2083O\u207a(aq) + Cl\u207b(aq), often written HCl \u2192 H\u207a + Cl\u207b.<\/p>\n<p>Hydrochloric, hydrobromic and hydriodic acids are strong acids. Hydrofluoric acid is weak, because the H\u2013F bond is so strong and the fluoride ion is strongly hydrogen bonded to water.<\/p>\n<p>With <strong>ammonia gas<\/strong> the hydrogen halides form white smoke of the solid ammonium halide, an acid\u2013base reaction in which a hydrogen ion moves from the acid to the lone pair on ammonia:<\/p>\n<p>HCl(g) + NH\u2083(g) \u2192 NH\u2084Cl(s) &nbsp;&nbsp; HBr(g) + NH\u2083(g) \u2192 NH\u2084Br(s)<\/p>\n<p>The white smoke forming where the two gases meet is the classic test for either gas.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Remember:<\/strong> Hydrogen halide + water is an acid forming; hydrogen halide + ammonia is a salt forming. Neither is a redox reaction: the halogen stays at \u22121 throughout.<\/p>\n<\/div>\n<div class=\"ols-figure-card ols-zoom-pop\">\n<div class=\"ols-figure-image\">\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/t8x-hxreact.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/t8x-hxreact.jpg\" alt=\"Poster of hydrogen halides forming acids in water, HF weak, and forming white ammonium halide smoke with ammonia.\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/t8x-hxreact.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>The hydrogen halides dissolve in water to give acids, all strong except hydrofluoric acid, and react with ammonia to give white smoke of an ammonium halide without any redox change.<\/p><\/div>\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: Reactions of the Hydrogen Halides<\/h2>\n<p>Write equations for the hydrogen halides with water and with ammonia, and decide which reactions on this page are redox.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"897\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Explain why iodide ions are stronger reducing agents than chloride ions<\/h3><p>The iodide ion is larger, its outer electrons are further from the nucleus and more shielded, so an electron is lost more easily.<\/p>\n<h3>Give the products when sodium bromide reacts with concentrated sulfuric acid<\/h3><p>HBr, then Br\u2082 and SO\u2082 with water; sulfur is reduced from +6 to +4.<\/p>\n<h3>Describe how to distinguish chloride, bromide and iodide ions<\/h3><p>Acidified silver nitrate gives white, cream or yellow precipitates; dilute then concentrated ammonia dissolves AgCl, only AgBr, and neither, respectively.<\/p>\n<h3>Do not say<\/h3><p>&#8220;Sodium chloride gives chlorine with concentrated sulfuric acid&#8221;; &#8220;acidify with hydrochloric acid&#8221;; &#8220;silver iodide dissolves in concentrated ammonia&#8221;.<\/p>\n<\/article>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the halide ion reactions and the silver nitrate test.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why are iodide ions better reducing agents than chloride ions?<\/h3>\n<p>The iodide ion is larger, so its outer electrons are further from the nucleus and more shielded, and one is lost more easily.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What does sodium chloride give with concentrated sulfuric acid?<\/h3>\n<p>Only hydrogen chloride, seen as steamy fumes, because chloride ions cannot reduce sulfuric acid.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is a bad-egg smell seen with sodium iodide?<\/h3>\n<p>Iodide ions reduce sulfuric acid all the way to hydrogen sulfide, in which sulfur is \u22122.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why acidify with nitric acid before adding silver nitrate?<\/h3>\n<p>To remove carbonate and hydroxide ions, which would also give precipitates with silver ions, without adding chloride ions.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why does silver chloride dissolve in ammonia but silver iodide does not?<\/h3>\n<p>Ammonia forms the complex ion [Ag(NH\u2083)\u2082]\u207a with silver ions. Silver chloride is soluble enough to supply them, silver iodide is too insoluble.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Topic 8C Group 7 Pages<\/h2>\n<p>Use these pages to connect the ideas across Group 7 and the rest of Topic 8.<\/p>\n<div class=\"ols-related-grid\">\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/trends-in-the-halogens\/\">Trends in the Halogens<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/halogens-as-oxidising-agents\/\">Halogens as Oxidising Agents<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/disproportionation-reactions-of-chlorine\/\">Disproportionation Reactions of Chlorine<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/\">Topic 8C Group 7 Overview<\/a>\n<\/div>\n<\/section>\n<section class=\"ols-attribution-card\">\n        <p><strong>Copyright notice:<\/strong> This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. 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