{"id":8094,"date":"2026-09-15T08:34:57","date_gmt":"2026-09-15T07:34:57","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/hydrated-salts-and-water-of-crystallisation\/"},"modified":"2026-09-22T09:27:20","modified_gmt":"2026-09-22T08:27:20","slug":"hydrated-salts-and-water-of-crystallisation","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/hydrated-salts-and-water-of-crystallisation\/","title":{"rendered":"Hydrated Salts and Water of Crystallisation"},"content":{"rendered":"\n<!--\n===============================================================================\nONLINE LEARNING SYSTEM COPYRIGHT NOTICE\n\u00a9 Online Learning System. 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background: #2563eb !important;\n      color: #ffffff !important;\n    }\n\n    body.ols-lightbox-lock {\n      overflow: hidden !important;\n    }\n\n    @media (max-width: 760px) {\n      .ols-zoom-card {\n        border-radius: 22px !important;\n        overflow: hidden !important;\n      }\n\n      .ols-zoom-card-image {\n        min-height: auto !important;\n        padding: 12px !important;\n        overflow: hidden !important;\n        border-top-left-radius: 22px !important;\n        border-top-right-radius: 22px !important;\n      }\n\n      .ols-zoom-card img.ols-zoomable-img,\n      .ols-zoom-card img.ols-zoomable-img:hover {\n        transform: none !important;\n        box-shadow: none !important;\n      }\n\n      .ols-zoom-card-caption {\n        border-bottom-left-radius: 22px !important;\n        border-bottom-right-radius: 22px !important;\n      }\n\n      .ols-image-lightbox {\n        padding: 14px !important;\n      }\n\n      .ols-image-lightbox-img {\n        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<p>OCR A Level Chemistry A<\/p>\n  <\/div>\n  <div class=\"ols-topic-group\">\n    <h4>2.1.3 Amount of Substance<\/h4>\n    <ul class=\"ols-topic-list\">\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/\">Section overview<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/atoms-elements-and-molecules\/\">Atoms, Elements and Molecules<\/a><\/li>\n      <li>\n        <a 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        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/comparing-masses-of-substances\/\">Comparing Masses of Substances<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/calculations-involving-moles\/\">Calculations involving Moles<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/calculations-using-reacting-masses\/\">Calculations using Reacting Masses<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/the-yield-of-a-reaction\/\">The Yield of a Reaction<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/atom-economy\/\">Atom Economy<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a 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        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/empirical-and-molecular-formulae\/empirical-formulae\/\">Empirical Formulae<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/empirical-and-molecular-formulae\/molecular-formulae\/\">Molecular Formulae<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a 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        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/calculations-with-solutions-and-gases\/molar-volume-calculations\/\">Molar Volume Calculations<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/calculations-with-solutions-and-gases\/concentrations-of-solutions\/\">Concentrations of Solutions<\/a><\/li>\n<li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/hydrated-salts-and-water-of-crystallisation\/\">Hydrated Salts and Water of Crystallisation<\/a><\/li>\n<li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/measuring-mass-volume-and-gas-volume\/\">Measuring Mass, Volume and Gas Volume<\/a><\/li>\n        <\/ul>\n      <\/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\/2-1-4-acids\/\">2.1.4 Acids<\/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 = 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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\/2-1-3-amount-of-substance\/\">2.1.3 Amount of Substance<\/a> \/\n<span>Hydrated Salts and Water of Crystallisation<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Hydrated Salts and Water of Crystallisation<\/h1>\n        <p class=\"ols-page-intro\">A concise OCR A Level Chemistry A revision guide to anhydrous and hydrated salts, water of crystallisation, and finding the formula of a hydrated salt from percentage composition, mass data and heating to constant mass (2.1.3(d)).<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Paper 1 and 2<\/div>\n<div class=\"ols-badge\">2.1.3 Amount of Substance<\/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: Mass Lost When Crystals Are Heated<\/h2>\n<p>Before you start, check what the mass lost on heating crystals tells you.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"760\"><\/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>Anhydrous, Hydrated and Water of Crystallisation<\/h2>\n<\/div>\n<p>Many ionic solids crystallise with water molecules locked into the lattice in a fixed ratio. This is <strong>water of crystallisation<\/strong>. A salt containing it is <strong>hydrated<\/strong>; the same salt with the water driven off is <strong>anhydrous<\/strong>.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Term<\/th><th>Meaning<\/th><th>Example<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Hydrated salt<\/strong><\/td><td>Crystals that contain water of crystallisation in a fixed ratio<\/td><td>CuSO\u2084\u00b75H\u2082O, blue<\/td><\/tr>\n<tr><td><strong>Anhydrous salt<\/strong><\/td><td>The salt with no water of crystallisation<\/td><td>CuSO\u2084, white<\/td><\/tr>\n<tr><td><strong>Water of crystallisation<\/strong><\/td><td>The water molecules bound in the crystal lattice, shown after a dot in the formula<\/td><td>The 5H\u2082O in CuSO\u2084\u00b75H\u2082O<\/td><\/tr>\n<tr><td><strong>Degree of hydration<\/strong><\/td><td>The number of water molecules per formula unit, x in MgSO\u2084\u00b7xH\u2082O<\/td><td>x = 7 for Epsom salt<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>The dot in CuSO\u2084\u00b75H\u2082O does not mean multiply: it separates the salt from its water. The relative formula mass includes the water: 159.6 + 5 \u00d7 18.0 = 249.6.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Heating a hydrated salt drives off the water of crystallisation; the mass lost is the mass of water, and the mass left is the anhydrous salt.<\/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: Reading a Dot Formula<\/h2>\n<p>Four quick questions on what the water written after the dot adds to a formula.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-505\" class=\"h5p-iframe\" data-content-id=\"505\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrated Salts Quick Fire: Reading a Dot Formula\"><\/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>Finding x from Percentage Composition<\/h2>\n<\/div>\n<p>If a hydrated salt is 36.1% water by mass, the rest is the anhydrous salt. Treat it exactly like an empirical formula calculation with two &#8220;elements&#8221;: the anhydrous salt and water.<\/p>\n<p><strong>Example.<\/strong> Hydrated calcium chloride, CaCl\u2082\u00b7xH\u2082O, contains 49.3% water.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th><\/th><th>CaCl\u2082<\/th><th>H\u2082O<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Mass in 100 g<\/strong><\/td><td>50.7 g<\/td><td>49.3 g<\/td><\/tr>\n<tr><td><strong>M<\/strong><\/td><td>111.0<\/td><td>18.0<\/td><\/tr>\n<tr><td><strong>Moles<\/strong><\/td><td>0.457<\/td><td>2.74<\/td><\/tr>\n<tr><td><strong>Ratio<\/strong><\/td><td>1<\/td><td>6.0<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>So x = 6 and the salt is <strong>CaCl\u2082\u00b76H\u2082O<\/strong>.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> Divide the mass of anhydrous salt and the mass of water by their relative formula masses, then divide both by the smaller number of moles to find x.<\/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: Finding x from a Percentage<\/h2>\n<p>Find the degree of hydration of a salt from the percentage of water in its crystals.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-503\" class=\"h5p-iframe\" data-content-id=\"503\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrated Salts MCQ: Finding x from the Percentage of Water\"><\/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>Finding x from Heating to Constant Mass<\/h2>\n<\/div>\n<p>In the laboratory the water is driven off by gentle heating in a crucible until the mass no longer changes: <strong>heating to constant mass<\/strong>. The difference between the hydrated and anhydrous masses is the water lost.<\/p>\n<p><strong>Example.<\/strong> 2.50 g of hydrated copper(II) sulfate is heated to constant mass, leaving 1.60 g of anhydrous salt.<\/p>\n<ol>\n<li>Mass of water = 2.50 \u2212 1.60 = 0.90 g, so n(H\u2082O) = 0.90 \/ 18.0 = 0.0500 mol<\/li>\n<li>n(CuSO\u2084) = 1.60 \/ 159.6 = 0.0100 mol<\/li>\n<li>Ratio CuSO\u2084 : H\u2082O = 1 : 5, so the formula is <strong>CuSO\u2084\u00b75H\u2082O<\/strong><\/li>\n<\/ol>\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\/home-olsnet-revision_notes-ocr-module-2-images-heating-a-hydrated-salt-to-constant-mass.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\/home-olsnet-revision_notes-ocr-module-2-images-heating-a-hydrated-salt-to-constant-mass.jpg\" alt=\"Heating a hydrated salt to constant mass: a lidded crucible on a pipe-clay triangle over a Bunsen flame with tongs, and a results table in which the mass after the second heating equals the mass after the first\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Heat, cool in a desiccator, weigh, and repeat until two masses agree: only then has all the water gone.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Constant mass proves the reaction is complete. Two successive weighings that agree are the evidence.<\/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: Work Out the Degree of Hydration<\/h2>\n<p>Work each card out on paper, then flip it to check every step of your working.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-502\" class=\"h5p-iframe\" data-content-id=\"502\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Water of Crystallisation Flip Cards: Work Out the Degree of Hydration\"><\/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>Evaluating the Experiment<\/h2>\n<\/div>\n<p>Exam questions often ask why the value of x came out too high or too low.<\/p>\n<ul>\n<li><strong>Not heated to constant mass:<\/strong> some water remains, the &#8220;anhydrous&#8221; mass is too high, the water mass too low, so x is too low.<\/li>\n<li><strong>Heated too strongly:<\/strong> the salt itself decomposes (copper(II) sulfate gives black copper(II) oxide), too much mass is lost, so x is too high.<\/li>\n<li><strong>Crystals absorb water while cooling:<\/strong> the anhydrous mass rises again, so x is too low; cool in a desiccator.<\/li>\n<li><strong>Spitting:<\/strong> solid lost from the crucible counts as &#8220;water&#8221;, so x is too high; use a lid.<\/li>\n<\/ul>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> If the salt is not heated to constant mass, water remains in the solid, the mass of water is underestimated and the calculated value of x is too low.<\/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: Too High or Too Low?<\/h2>\n<p>In each round, pick the statement that gets the effect on x right.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-504\" class=\"h5p-iframe\" data-content-id=\"504\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrated Salts Summary: Why x Came Out Wrong\"><\/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>Common Exam Mistakes<\/h2>\n<\/div>\n<ul>\n<li>Treating the dot as a multiplication sign.<\/li>\n<li>Leaving the water out of the relative formula mass of the hydrated salt.<\/li>\n<li>Dividing the water moles by the salt moles the wrong way round: x is water per salt.<\/li>\n<li>Rounding a ratio of 4.9 to 4; it is 5.<\/li>\n<\/ul>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> x = moles of water \u00f7 moles of anhydrous salt, where the moles of water come from the mass lost on heating to constant mass.<\/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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