{"id":8097,"date":"2026-09-15T08:35:04","date_gmt":"2026-09-15T07:35:04","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-4-acids\/acids-and-alkalis\/"},"modified":"2026-09-22T16:37:07","modified_gmt":"2026-09-22T15:37:07","slug":"acids-and-alkalis","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-4-acids\/acids-and-alkalis\/","title":{"rendered":"Acids and Alkalis"},"content":{"rendered":"\n<!--\n===============================================================================\nONLINE LEARNING SYSTEM COPYRIGHT NOTICE\n\u00a9 Online Learning System. 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<p>OCR A Level Chemistry A<\/p>\n  <\/div>\n  <div class=\"ols-topic-group\">\n    <h4>2.1.4 Acids<\/h4>\n    <ul class=\"ols-topic-list\">\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-4-acids\/\">Section overview<\/a><\/li>\n<li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-4-acids\/acids-and-alkalis\/\">Acids and Alkalis<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-4-acids\/typical-reactions-of-acids\/\">Typical Reactions of Acids<\/a><\/li>\n<li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-4-acids\/standard-solutions-and-titration-technique\/\">Standard Solutions and Titration Technique<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-4-acids\/titration-calculations\/\">Titration Calculations<\/a><\/li>\n    <\/ul>\n  <\/div>\n  <div class=\"ols-topic-group\">\n    <h4>Other OCR Sections<\/h4>\n    <ul class=\"ols-topic-list\">\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-1-atomic-structure-and-isotopes\/\">2.1.1 Atomic Structure and Isotopes<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-2-compounds-formulae-and-equations\/\">2.1.2 Compounds, Formulae and Equations<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/\">2.1.3 Amount of Substance<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-1-electron-structure\/\">2.2.1 Electron Structure<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/\">2.2.2 Bonding and Structure<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-1-1-periodicity\/\">3.1.1 Periodicity<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/4-1-1-basic-concepts-of-organic-chemistry\/\">4.1.1 Basic Concepts of Organic Chemistry<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/4-1-2-alkanes\/\">4.1.2 Alkanes<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/4-1-3-alkenes\/\">4.1.3 Alkenes<\/a><\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\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    sidebar.querySelectorAll('.active, .active-main, .parent-active').forEach(function(item) {\r\n      item.classList.remove('active', 'active-main', 'parent-active');\r\n    });\r\n\r\n    sidebar.querySelectorAll('a[data-ols-disabled-parent=\"1\"], a[aria-current]').forEach(function(a) {\r\n      a.removeAttribute('aria-current');\r\n      a.removeAttribute('tabindex');\r\n      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{\r\n    document.addEventListener('DOMContentLoaded', highlightActive);\r\n  }\r\n})();\r\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\/2-1-4-acids\/\">2.1.4 Acids<\/a> \/\n<span>Acids and Alkalis<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Acids and Alkalis<\/h1>\n        <p class=\"ols-page-intro\">A concise OCR A Level Chemistry A revision guide to the formulae of the common acids and alkalis, acids as sources of H\u207a ions and alkalis as sources of OH\u207b ions in water, and the difference between strong and weak acids in terms of dissociation (2.1.4(a), (b)).<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Paper 1 and 2<\/div>\n<div class=\"ols-badge\">2.1.4 Acids<\/div>\n<div class=\"ols-badge\">H432\/01 and H432\/02<\/div>\n<\/div>\n\n        <div class=\"ols-author\">\n          <img decoding=\"async\" class=\"ols-author-avatar-img\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\" alt=\"Dr. Mohammed Al-Fatah\">\n          <div class=\"ols-author-content\">\n            <h2 class=\"ols-author-title\">Written by:<br><span>Dr. Mohammed Al-Fatah<\/span><\/h2>\n            <p class=\"ols-author-description\">Chemistry specialist revision notes for A Level Chemistry.<\/p>\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\"><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\"><\/path><\/svg>\n              View LinkedIn Profile\n            <\/a>\n          <\/div>\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: pH, Dilution and Neutralisation<\/h2>\n<p>Before you start, check what you already know about acids, alkalis and the pH scale.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"763\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>The Common Acids and Alkalis<\/h2>\n<\/div>\n<p>OCR expects the formulae of four acids and three alkalis to be known without prompting.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Acid<\/th><th>Formula<\/th><th>Alkali<\/th><th>Formula<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Hydrochloric acid<\/strong><\/td><td>HCl<\/td><td>Sodium hydroxide<\/td><td>NaOH<\/td><\/tr>\n<tr><td><strong>Sulfuric acid<\/strong><\/td><td>H\u2082SO\u2084<\/td><td>Potassium hydroxide<\/td><td>KOH<\/td><\/tr>\n<tr><td><strong>Nitric acid<\/strong><\/td><td>HNO\u2083<\/td><td>Ammonia<\/td><td>NH\u2083<\/td><\/tr>\n<tr><td><strong>Ethanoic acid<\/strong><\/td><td>CH\u2083COOH<\/td><td><\/td><td><\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Ethanoic acid is written CH\u2083COOH rather than C\u2082H\u2084O\u2082 to show that only <strong>one<\/strong> of its four hydrogens is acidic: the one on the carboxyl group.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Sulfuric acid is diprotic: one mole gives two moles of H\u207a. The other three acids are monoprotic.<\/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>Quick Check: How Much Alkali Is Needed<\/h2>\n<p>Decide which of these four acids needs twice as much alkali per mole.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-511\" class=\"h5p-iframe\" data-content-id=\"511\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Acids and Alkalis MCQ: How Much Alkali Each Acid Needs\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>What Makes an Acid an Acid<\/h2>\n<\/div>\n<p>An <strong>acid<\/strong> releases <strong>hydrogen ions, H\u207a<\/strong>, when it dissolves in water. Hydrogen chloride gas is a covalent molecule, but in water it dissociates: <strong>HCl(aq) \u2192 H\u207a(aq) + Cl\u207b(aq)<\/strong>. The H\u207a ions are what give acids their reactions with metals, carbonates and bases.<\/p>\n<p>An <strong>alkali<\/strong> is a soluble base that releases <strong>hydroxide ions, OH\u207b<\/strong>, in water. Sodium hydroxide is ionic and simply separates: <strong>NaOH(aq) \u2192 Na\u207a(aq) + OH\u207b(aq)<\/strong>. Ammonia is different: it reacts with water to make hydroxide ions, <strong>NH\u2083(aq) + H\u2082O(l) \u21cc NH\u2084\u207a(aq) + OH\u207b(aq)<\/strong>, which is why ammonia solution is alkaline even though the formula contains no OH.<\/p>\n<p>Neutralisation is the reaction of these two ions: <strong>H\u207a(aq) + OH\u207b(aq) \u2192 H\u2082O(l)<\/strong>, the ionic equation for every acid-alkali reaction.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> Acids release H\u207a ions in aqueous solution; alkalis release OH\u207b ions in aqueous solution; neutralisation forms water from H\u207a and OH\u207b.<\/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>Quick Check: A Base with No OH Group<\/h2>\n<p>Work out where the hydroxide ions in this solution come from.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-513\" class=\"h5p-iframe\" data-content-id=\"513\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Acids and Alkalis MCQ: Why a Solution of Methylamine Is Alkaline\"><\/iframe><\/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>Strong and Weak Acids<\/h2>\n<\/div>\n<p>The difference is not concentration but the <strong>extent of dissociation<\/strong>.<\/p>\n<ul>\n<li>A <strong>strong acid<\/strong> dissociates <strong>completely<\/strong> in water: every HCl molecule becomes H\u207a and Cl\u207b. Hydrochloric, sulfuric and nitric acids are strong.<\/li>\n<li>A <strong>weak acid<\/strong> dissociates only <strong>partially<\/strong>: in ethanoic acid a small fraction of the molecules give up H\u207a and most stay as CH\u2083COOH. The equilibrium <strong>CH\u2083COOH(aq) \u21cc CH\u2083COO\u207b(aq) + H\u207a(aq)<\/strong> lies well to the left.<\/li>\n<\/ul>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-figure-card\">\n<div class=\"ols-figure-image\"><img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/fix-56.jpg\" alt=\"Particle diagrams of a strong acid fully ionised and a weak acid only slightly ionised at the same concentration\"><\/div>\n<div class=\"ols-figure-caption\"><p>Particle diagrams of a strong acid, HCl, fully ionised into H\u207a and Cl\u207b, and a weak acid, CH\u2083COOH, in which only a few molecules have dissociated into CH\u2083COO\u207b and H\u207a<\/p><\/div>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Same concentration, different dissociation: a strong acid is fully ionised, a weak acid only slightly.<\/p><\/div>\n<\/div>\n<p>Because a weak acid has far fewer H\u207a ions at the same concentration, it has a higher pH, conducts electricity less well and reacts more slowly with magnesium or a carbonate. It still needs the <strong>same volume<\/strong> of alkali in a titration, because the equilibrium shifts right as H\u207a is removed until every molecule has reacted.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Strong means fully dissociated; concentrated means a lot of acid per dm\u00b3. A dilute strong acid and a concentrated weak acid are both possible.<\/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>Quick Check: Two Acids, One Concentration<\/h2>\n<p>Use the two pH values to complete the comparison.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-510\" class=\"h5p-iframe\" data-content-id=\"510\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Strong and Weak Acids Drag: Two Acids at the Same Concentration\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Strong, Weak, Concentrated or Dilute<\/h2>\n<p>In each round, pick the one statement that is accurate.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-512\" class=\"h5p-iframe\" data-content-id=\"512\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Strong and Weak Acids Summary: Strength Is Not Concentration\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Common Exam Mistakes<\/h2>\n<\/div>\n<ul>\n<li>Writing that a weak acid is a dilute acid. Strength is about dissociation, concentration about moles per dm\u00b3.<\/li>\n<li>Saying ammonia contains OH\u207b: it makes OH\u207b by reacting with water.<\/li>\n<li>Treating all four hydrogens in CH\u2083COOH as acidic; only the carboxyl hydrogen dissociates.<\/li>\n<li>Using a single arrow for the dissociation of a weak acid; it is an equilibrium.<\/li>\n<\/ul>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> A strong acid is fully dissociated in aqueous solution; a weak acid is only partially dissociated, so an equilibrium exists between the acid molecules and their ions.<\/p>\n<\/div>\n<\/article>\n\n<section class=\"ols-related-card\">\n        <h2>Related Topics<\/h2>\n        <p>Continue through this section by linking this page to equations, formulae and mole calculation revision.<\/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\/2-1-2-compounds-formulae-and-equations\/equations-and-reaction-types\/\">Equations and Reaction Types<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-2-compounds-formulae-and-equations\/equations-and-reaction-types\/writing-chemical-equations\/\">Writing Chemical Equations<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/\">Mole Calculations<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/empirical-and-molecular-formulae\/\">Empirical and Molecular Formulae<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/calculations-with-solutions-and-gases\/\">Calculations with Solutions and Gases<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-1-atomic-structure-and-isotopes\/\">Atomic Structure &amp; The Periodic Table<\/a>\n        <\/div>\n      <\/section>\n\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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