{"id":10479,"date":"2026-09-26T11:30:32","date_gmt":"2026-09-26T10:30:32","guid":{"rendered":"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\/"},"modified":"2026-10-04T23:02:41","modified_gmt":"2026-10-04T22:02:41","slug":"oxides-hydroxides-and-carbonates","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\/8b-groups-1-and-2\/oxides-hydroxides-and-carbonates\/","title":{"rendered":"Oxides, Hydroxides and Carbonates"},"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\/8b-groups-1-and-2\/\">Topic 8B Groups 1 and 2<\/a> \/\n<span>Oxides, Hydroxides and Carbonates<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Oxides, Hydroxides and Carbonates<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to the oxides and hydroxides of the s-block metals with water and dilute acid, the pH of the solutions rising down Group 2, the carbonates with acid, and the uses of calcium hydroxide, magnesium hydroxide and calcium carbonate as bases.<\/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 8B Groups 1 and 2<\/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>Oxides with Water<\/h2>\n<\/div>\n<p>The s-block oxides are <strong>basic oxides<\/strong>. Those that dissolve react with water to form the metal hydroxide, and the solution is alkaline because hydroxide ions are released.<\/p>\n<p>Na\u2082O + H\u2082O \u2192 2NaOH &nbsp;&nbsp; CaO + H\u2082O \u2192 Ca(OH)\u2082 &nbsp;&nbsp; BaO + H\u2082O \u2192 Ba(OH)\u2082<\/p>\n<p>Calcium oxide (quicklime) reacts vigorously, giving out enough heat to make the water steam, and the product, calcium hydroxide, is slaked lime. Magnesium oxide reacts only slightly, because magnesium hydroxide is barely soluble, so the solution is only weakly alkaline.<\/p>\n<p>The <strong>pH of the solution rises down Group 2<\/strong>. Magnesium hydroxide solution is about pH 9 to 10, calcium hydroxide about 12, and barium hydroxide about 13. The trend follows the solubility of the hydroxides: the more hydroxide dissolves, the higher the concentration of OH\u207b(aq) and the higher the pH.<\/p>\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-oxides-hydroxides-carbonates.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-oxides-hydroxides-carbonates.png\" alt=\"Group 2 oxides, hydroxides and carbonates with water and acid, pH of solutions and uses\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/redox-groups-oxides-hydroxides-carbonates.png\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>The pH strip shows why the trend exists: more hydroxide dissolves down the group, so the solution holds more OH\u207b ions.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Link pH to solubility, not to &#8220;reactivity&#8221;. Barium hydroxide gives the most alkaline solution because it is the most soluble hydroxide, so its solution has the highest concentration of hydroxide ions.<\/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: Oxides and pH<\/h2>\n<p>Predict and explain the pH of solutions made from oxides and hydroxides not named above.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-870\" class=\"h5p-iframe\" data-content-id=\"870\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Redox and Groups MCQ: Oxides and pH\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>Oxides and Hydroxides with Acids<\/h2>\n<\/div>\n<p>As bases, the oxides and hydroxides neutralise acids to give a <strong>salt and water<\/strong>. The salt takes its name from the acid: chlorides from hydrochloric acid, sulfates from sulfuric acid, nitrates from nitric acid.<\/p>\n<p>MgO + 2HCl \u2192 MgCl\u2082 + H\u2082O &nbsp;&nbsp; CaO + H\u2082SO\u2084 \u2192 CaSO\u2084 + H\u2082O &nbsp;&nbsp; 2KOH + H\u2082SO\u2084 \u2192 K\u2082SO\u2084 + 2H\u2082O<\/p>\n<p>Ca(OH)\u2082 + 2HNO\u2083 \u2192 Ca(NO\u2083)\u2082 + 2H\u2082O &nbsp;&nbsp; ionic: OH\u207b + H\u207a \u2192 H\u2082O &nbsp;&nbsp; and for an oxide: O\u00b2\u207b + 2H\u207a \u2192 H\u2082O<\/p>\n<p>These are acid\u2013base reactions, not redox reactions: no oxidation number changes. The magnesium stays at +2 and the hydrogen at +1 throughout.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Metal oxide or hydroxide + acid \u2192 salt + water. The only difference between the two is that the oxide needs two H\u207a ions for each oxide ion.<\/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>Carbonates<\/h2>\n<\/div>\n<p>The Group 1 carbonates are soluble in water and give alkaline solutions; the Group 2 carbonates are insoluble, so they do not react with water at all. Both react with acids to give a salt, water and <strong>carbon dioxide<\/strong>, which is the fizzing seen when an acid is added to any carbonate:<\/p>\n<p>CaCO\u2083 + 2HCl \u2192 CaCl\u2082 + H\u2082O + CO\u2082 &nbsp;&nbsp; MgCO\u2083 + H\u2082SO\u2084 \u2192 MgSO\u2084 + H\u2082O + CO\u2082 &nbsp;&nbsp; ionic: CO\u2083\u00b2\u207b + 2H\u207a \u2192 H\u2082O + CO\u2082<\/p>\n<p>The gas turns limewater cloudy, which is the test for a carbonate ion. The thermal decomposition of the carbonates is treated on the <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> page.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Remember:<\/strong> Carbonate + acid gives three products, not two. Forgetting the carbon dioxide is the most common balancing error in this section.<\/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-carbonacid.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-carbonacid.jpg\" alt=\"Comparison of Group 1 and Group 2 carbonates with water, the carbonate and acid equations, and the limewater test.\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/t8x-carbonacid.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Group 1 carbonates dissolve to give alkaline solutions while Group 2 carbonates are insoluble, but every carbonate fizzes with acid to give a salt, water and carbon dioxide.<\/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: Salt, Water and Gas<\/h2>\n<p>Complete and balance equations for oxides, hydroxides and carbonates with acids, using compounds that are not on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-871\" class=\"h5p-iframe\" data-content-id=\"871\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Redox and Groups Fill In: Salt, Water and Gas\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card purple\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Uses as Bases<\/h2>\n<\/div>\n<p>Because they are cheap, mild bases, several Group 2 compounds are used to neutralise acids in everyday settings.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Compound<\/th><th>Use<\/th><th>Equation<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Calcium hydroxide, Ca(OH)\u2082 (slaked lime)<\/strong><\/td><td>spread on fields to neutralise acidic soil so crops grow better<\/td><td>Ca(OH)\u2082 + 2H\u207a \u2192 Ca\u00b2\u207a + 2H\u2082O<\/td><\/tr>\n<tr><td><strong>Magnesium hydroxide, Mg(OH)\u2082 (milk of magnesia)<\/strong><\/td><td>antacid that neutralises excess stomach acid<\/td><td>Mg(OH)\u2082 + 2HCl \u2192 MgCl\u2082 + 2H\u2082O<\/td><\/tr>\n<tr><td><strong>Calcium carbonate, CaCO\u2083<\/strong><\/td><td>antacid tablets; also neutralises acidic soil and lakes<\/td><td>CaCO\u2083 + 2HCl \u2192 CaCl\u2082 + H\u2082O + CO\u2082<\/td><\/tr>\n<tr><td><strong>Calcium oxide, CaO (quicklime)<\/strong><\/td><td>neutralises acidic soil; removes sulfur dioxide from flue gases<\/td><td>CaO + SO\u2082 \u2192 CaSO\u2083<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Magnesium hydroxide is chosen as an antacid rather than sodium hydroxide because it is only sparingly soluble, so it neutralises acid without making the stomach alkaline. Calcium carbonate gives off carbon dioxide as it works, which is why some tablets cause belching.<\/p>\n\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> When asked why magnesium hydroxide is safe to swallow, answer with its low solubility and its weak alkalinity, not simply &#8220;it is a base&#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: Choosing a Base<\/h2>\n<p>Decide which compound suits each use and justify the choice from solubility and the products formed.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-872\" class=\"h5p-iframe\" data-content-id=\"872\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Redox and Groups Summary: Choosing a Base\"><\/iframe><\/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>Write the equation for a Group 2 oxide with water<\/h3><p>MO + H\u2082O \u2192 M(OH)\u2082, and state that the solution is alkaline because of the hydroxide ions.<\/p>\n<h3>Explain the trend in pH of the hydroxide solutions<\/h3><p>Solubility increases down the group, so the concentration of hydroxide ions increases and the pH rises.<\/p>\n<h3>Give a use of calcium hydroxide with an equation<\/h3><p>Neutralising acidic soil: Ca(OH)\u2082 + 2H\u207a \u2192 Ca\u00b2\u207a + 2H\u2082O.<\/p>\n<h3>Do not say<\/h3><p>&#8220;Barium hydroxide is a stronger base&#8221; (it is more soluble); &#8220;carbonate + acid \u2192 salt + water&#8221; without the carbon dioxide.<\/p>\n<\/article>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the oxides, hydroxides and carbonates.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why is a solution of calcium oxide alkaline?<\/h3>\n<p>Calcium oxide reacts with water to form calcium hydroxide, which releases hydroxide ions into the solution.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why does the pH of the hydroxide solutions rise down Group 2?<\/h3>\n<p>The hydroxides become more soluble down the group, so more hydroxide ions are in solution and the pH is higher.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Is a metal oxide reacting with an acid a redox reaction?<\/h3>\n<p>No. It is an acid\u2013base reaction: the oxide ion takes two hydrogen ions to form water and no oxidation number changes.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is magnesium hydroxide a safe antacid?<\/h3>\n<p>It is only sparingly soluble and weakly alkaline, so it neutralises excess acid without making the stomach alkaline.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What happens when a carbonate reacts with an acid?<\/h3>\n<p>A salt, water and carbon dioxide form; the fizzing is the carbon dioxide, which turns limewater cloudy.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Topic 8B Groups 1 and 2 Pages<\/h2>\n<p>Use these pages to connect the ideas across groups 1 and 2 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\/8b-groups-1-and-2\/trends-down-groups-1-and-2\/\">Trends Down Groups 1 and 2<\/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\/8b-groups-1-and-2\/reactions-of-the-group-1-and-2-elements\/\">Reactions of the Group 1 and 2 Elements<\/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\/8b-groups-1-and-2\/solubility-trends-and-the-sulfate-test\/\">Solubility Trends and the Sulfate Test<\/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\/8b-groups-1-and-2\/thermal-stability-and-flame-colours\/\">Thermal Stability and Flame Colours<\/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\/8b-groups-1-and-2\/tests-for-ions\/\">Tests for Ions<\/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\/8b-groups-1-and-2\/titrations-and-measurement-uncertainty\/\">Titrations and Measurement Uncertainty<\/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\/8b-groups-1-and-2\/\">Topic 8B Groups 1 and 2 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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