{"id":4741,"date":"2026-06-02T07:46:47","date_gmt":"2026-06-02T06:46:47","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/equations-and-reaction-types\/writing-chemical-equations\/"},"modified":"2026-10-25T12:07:25","modified_gmt":"2026-10-25T12:07:25","slug":"writing-chemical-equations","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/equations-and-reaction-types\/writing-chemical-equations\/","title":{"rendered":"Writing Chemical Equations"},"content":{"rendered":"\n<!--\n===============================================================================\nONLINE LEARNING SYSTEM COPYRIGHT NOTICE\n\u00a9 Online Learning System. 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class=\"ols-sidebar-header\">\r\n    <h3>Revision Notes<\/h3>\r\n    <p>A Level Chemistry<\/p>\r\n  <\/div>\r\n\r\n  <div class=\"ols-topic-group\">\r\n    <h4>Topic 5 Formulae, Equations and Amounts of Substance<\/h4>\r\n\r\n    <ul class=\"ols-topic-list\">\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/atoms-elements-and-molecules\/\">Atoms, Elements and Molecules<\/a>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/equations-and-reaction-types\/\">Equations and Reaction Types<\/a>\r\n        <ul class=\"ols-subtopic-list\">\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/equations-and-reaction-types\/writing-chemical-equations\/\">Writing Chemical Equations<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/equations-and-reaction-types\/typical-reactions-of-acids\/\">Typical Reactions of Acids<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/equations-and-reaction-types\/ionic-and-full-equations\/\">Ionic and Full Equations<\/a>\r\n          <\/li>\r\n        <\/ul>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/mole-calculations\/\">Mole Calculations<\/a>\r\n        <ul class=\"ols-subtopic-list\">\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/mole-calculations\/comparing-masses-of-substances\/\">Comparing Masses of Substances<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/mole-calculations\/calculations-involving-moles\/\">Calculations involving Moles<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/mole-calculations\/calculations-using-reacting-masses\/\">Calculations using Reacting Masses<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/mole-calculations\/the-yield-of-a-reaction\/\">The Yield of a Reaction<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/mole-calculations\/atom-economy\/\">Atom Economy<\/a>\r\n          <\/li>\r\n        <\/ul>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/empirical-and-molecular-formulae\/\">Empirical and Molecular Formulae<\/a>\r\n        <ul class=\"ols-subtopic-list\">\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/empirical-and-molecular-formulae\/empirical-formulae\/\">Empirical Formulae<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/empirical-and-molecular-formulae\/molecular-formulae\/\">Molecular Formulae<\/a>\r\n          <\/li>\r\n        <\/ul>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/calculations-with-solutions-and-gases\/\">Calculations with Solutions and Gases<\/a>\r\n        <ul class=\"ols-subtopic-list\">\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/calculations-with-solutions-and-gases\/molar-volume-calculations\/\">Molar Volume Calculations<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/calculations-with-solutions-and-gases\/concentrations-of-solutions\/\">Concentrations of Solutions<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/calculations-with-solutions-and-gases\/titration-calculations\/\">Titration Calculations<\/a>\r\n          <\/li>\r\n        <\/ul>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/measurement-uncertainty-and-errors\/\">Measurement Uncertainty and Errors<\/a>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/risks-hazards-and-practical-precautions\/\">Risks, Hazards and Practical Precautions<\/a>\r\n      <\/li>\r\n\r\n    <\/ul>\r\n  <\/div>\r\n\r\n  <div class=\"ols-topic-group\">\r\n    <h4>Next Topics<\/h4>\r\n\r\n    <ul class=\"ols-topic-list\">\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-6-organic-chemistry-i\/\">Topic 6 Organic Chemistry I<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-7-modern-analytical-techniques-i\/\">Topic 7 Modern Analytical Techniques I<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-4-inorganic-chemistry-and-the-periodic-table\/\">Topic 4 Inorganic Chemistry &amp; The Periodic Table<\/a>\r\n      <\/li>\r\n    <\/ul>\r\n  <\/div>\r\n<\/aside>\r\n\r\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 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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/\">Edexcel<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/\">Topic 5 Formulae, Equations and Amounts of Substance<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/equations-and-reaction-types\/\">Equations and Reaction Types<\/a> \/\n        <span>Writing Chemical Equations<\/span>\n      <\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Writing Chemical Equations<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to writing and balancing chemical equations, including the difference between small subscript numbers in formulae and large coefficient numbers placed in front of formulae.<\/p>\n\n        <div class=\"ols-badges\">\n          <div class=\"ols-badge\">Unit: Paper 1<\/div>\n          <div class=\"ols-badge\">Topic 5: Formulae, Equations and Amounts of Substance<\/div>\n          <div class=\"ols-badge\">9CH0\/01<\/div>\n        <\/div>\n      <\/header>\n<!-- 3D card: balanced-equation-atoms (23 Oct 2026) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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align-items:center; text-align:center;\r\n  font-size:14px; font-weight:600; line-height:1; color:#1C244B; white-space:nowrap;\r\n  background:rgba(255,255,255,0.95); padding:6px 11px; border-radius:999px;\r\n  border:1.5px solid rgba(28,36,75,0.22); transition:border-color .3s, color .3s;\r\n}\r\n.ols-bea-001 .bea-tr b{text-align:left; font-weight:600;}\r\n.ols-bea-001 .bea-tr i{font-style:normal; font-weight:400; color:#aab0c0;}\r\n.ols-bea-001 .bea-tr span{display:inline-block; will-change:transform;}\r\n.ols-bea-001 .bea-tr.is-ok{border-color:rgba(47,158,87,0.85); color:#1f7a3a;}\r\n.ols-bea-001 .bea-tr.is-bad{border-color:rgba(212,58,50,0.75); color:#b02a24;}\r\n.ols-bea-001 .bea-key i.k-miss{background:rgba(205,45,40,0.08); border:1.5px dashed #cd2d28;}\r\n.ols-bea-001 .bea-key i.k-ghost{background:transparent; border:1.5px dashed #8b92a8;}\r\n@media (max-width:760px){\r\n  .ols-bea-001 .bea-tr{font-size:12px; padding:5px 9px; grid-template-columns:18px 26px 8px 26px;}\r\n  .ols-bea-001 .bea-th{font-size:9.5px; padding:0 9px; grid-template-columns:18px 26px 8px 26px;}\r\n}\r\n\/* on a phone the tally is one row of three chips under the caption *\/\r\n@media (max-width:480px){\r\n  .ols-bea-001 .bea-tally{flex-direction:row; gap:4px;}\r\n  .ols-bea-001 .bea-th{display:none;}\r\n  .ols-bea-001 .bea-tr{font-size:11px; padding:4px 7px; grid-template-columns:13px 15px 8px 15px;}\r\n}\r\n@media (prefers-reduced-motion:reduce){\r\n  .ols-bea-001 .bea-tr{transition:none;}\r\n}\r\n<\/style>\r\n\r\n<div class=\"bea-head\">\r\n  <h2 class=\"bea-title\">Balancing Equations: Atoms Are Conserved<\/h2>\r\n  <p class=\"bea-sub\">Choose a step to follow every atom as methane burns, count the atoms on each side of the arrow, balance the equation, then check that mass is conserved.<\/p>\r\n<\/div>\r\n\r\n<div class=\"bea-stage\" id=\"beaStage\">\r\n  <canvas class=\"bea-canvas\" id=\"beaCanvas\"><\/canvas>\r\n  <div class=\"bea-overlay\" id=\"beaOverlay\"><\/div>\r\n  <div class=\"bea-caption\" id=\"beaCaption\"><\/div>\r\n  <div class=\"bea-hint\" id=\"beaHint\">Drag to rotate<\/div>\r\n  <div class=\"bea-tally\" id=\"beaTally\" aria-label=\"Atom count\">\r\n    <div class=\"bea-th\"><span><\/span><span>left<\/span><span>right<\/span><\/div>\r\n    <div class=\"bea-tr\" id=\"beaTrC\"><b>C<\/b><span class=\"bea-tl\"><\/span><i>|<\/i><span class=\"bea-tr2\"><\/span><\/div>\r\n    <div class=\"bea-tr\" id=\"beaTrH\"><b>H<\/b><span class=\"bea-tl\"><\/span><i>|<\/i><span class=\"bea-tr2\"><\/span><\/div>\r\n    <div class=\"bea-tr\" id=\"beaTrO\"><b>O<\/b><span class=\"bea-tl\"><\/span><i>|<\/i><span class=\"bea-tr2\"><\/span><\/div>\r\n  <\/div>\r\n  <div class=\"bea-badges\" id=\"beaBadges\"><\/div>\r\n<\/div>\r\n\r\n<div class=\"bea-controls\">\r\n  <div class=\"bea-row\">\r\n    <span class=\"bea-rowlab\">Step<\/span>\r\n    <div class=\"bea-seg\" role=\"group\" aria-label=\"Step\">\r\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\"><span class=\"bea-n\">1<\/span>Reactants<\/button>\r\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\"><span class=\"bea-n\">2<\/span>Products<\/button>\r\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\"><span class=\"bea-n\">3<\/span>Balance H<\/button>\r\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\"><span class=\"bea-n\">4<\/span>Balance O<\/button>\r\n      <button type=\"button\" data-step=\"5\" aria-pressed=\"false\"><span class=\"bea-n\">5<\/span>States and mass<\/button>\r\n    <\/div>\r\n  <\/div>\r\n\r\n  <div class=\"bea-row\">\r\n    <span class=\"bea-rowlab\">View<\/span>\r\n    <button type=\"button\" class=\"bea-pill\" id=\"beaResetV\">Reset view<\/button>\r\n  <\/div>\r\n<\/div>\r\n\r\n<div class=\"bea-info\">\r\n  <h3 id=\"beaInfoTitle\"><\/h3>\r\n  <p id=\"beaInfoText\"><\/p>\r\n  <div class=\"bea-facts\" id=\"beaFacts\"><\/div>\r\n  <div class=\"bea-key\">\r\n    <span><i class=\"k-ball\" style=\"background:#464a54\"><\/i>Carbon<\/span>\r\n    <span><i class=\"k-ball\" style=\"background:#ececf0\"><\/i>Hydrogen<\/span>\r\n    <span><i class=\"k-ball\" style=\"background:#cd2d28\"><\/i>Oxygen<\/span>\r\n    <span><i class=\"k-ball k-miss\"><\/i>Missing atom<\/span>\r\n    <span><i class=\"k-ball k-ghost\"><\/i>Reactants as written (used up)<\/span>\r\n  <\/div>\r\n<\/div>\r\n\r\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/balanced-equation-atoms.js?v=20261025p\"><\/script>\r\n<\/section>\r\n\r\n<p class=\"ols-cc-bea-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n\n\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: Conservation of Mass<\/h2>\n<p>Before you start, check what you remember from GCSE about conservation of mass and diatomic elements.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"630\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">1<\/div><h2>What a Chemical Equation Shows<\/h2><\/div>\n        <p>A <strong>chemical equation<\/strong> represents a chemical reaction using formulae and symbols. The substances that react are written on the left-hand side, and the substances formed are written on the right-hand side.<\/p>\n        <p>The arrow means <strong>reacts to form<\/strong>. For example, when hydrogen reacts with oxygen, water is made:<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Example:<\/strong> 2H<sub>2<\/sub> + O<sub>2<\/sub> \u2192 2H<sub>2<\/sub>O<\/p><\/div>\n        <p>When writing equations, the aim is to show the correct substances and the correct number of each type of atom on both sides.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Key idea:<\/strong> atoms are rearranged during a chemical reaction. They are not created or destroyed.<\/p><\/div>\n      <\/article>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">2<\/div><h2>What Small Subscript Numbers Mean<\/h2><\/div>\n        <p>The small subscript number inside a formula tells you how many atoms of the element immediately before it are present in one formula unit or molecule.<\/p>\n        <p>For example, in <strong>H<sub>2<\/sub>SO<sub>4<\/sub><\/strong>, the 2 applies only to hydrogen and the 4 applies only to oxygen. There is no small number after sulfur, so there is one sulfur atom.<\/p>\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Formula<\/th>\n                <th>Atom count<\/th>\n                <th>Meaning<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Formula\">H<sub>2<\/sub>SO<sub>4<\/sub><\/td>\n                <td data-label=\"Atom count\">2 H, 1 S, 4 O<\/td>\n                <td data-label=\"Meaning\">The 2 applies to H only, and the 4 applies to O only.<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Formula\">CH<sub>4<\/sub><\/td>\n                <td data-label=\"Atom count\">1 C, 4 H<\/td>\n                <td data-label=\"Meaning\">The 4 applies to hydrogen only.<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Formula\">Ca(OH)<sub>2<\/sub><\/td>\n                <td data-label=\"Atom count\">1 Ca, 2 O, 2 H<\/td>\n                <td data-label=\"Meaning\">The 2 outside the brackets applies to everything inside the brackets.<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n        <div class=\"ols-zoom-card\">\n          <div class=\"ols-zoom-card-image\">\n            <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t1-img-01-atomcount.jpg\" alt=\"Eight tiles from Na to H\u2082SO\u2084 with plain coloured counters showing how subscripts and numbers in front change the atom count\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n          <\/div>\n          <div class=\"ols-zoom-card-caption\">\n            <p>Use the position of each number carefully. A small number inside a formula applies only to the atom or bracketed group immediately before it.<\/p>\n          <\/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\/10\/t1-mv-02-displayed.svg\" 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\/10\/t1-mv-02-displayed.svg\" alt=\"Displayed formulae of nitrogen, N\u2082 (N\u2261N), water, H\u2082O, sulfuric acid, H\u2082SO\u2084 (two S=O, two S\u2013O\u2013H)\">\n<\/a>\n<\/div>\n<\/div>\n<\/article>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">3<\/div><h2>What Big Numbers in Front Mean<\/h2><\/div>\n        <p>A large number placed in front of a formula is called a <strong>coefficient<\/strong>. It multiplies the whole formula.<\/p>\n        <p>For example, <strong>5CH<sub>4<\/sub><\/strong> means five molecules of CH<sub>4<\/sub>. Each CH<sub>4<\/sub> contains one carbon atom and four hydrogen atoms, so 5CH<sub>4<\/sub> contains <strong>5 carbon atoms<\/strong> and <strong>20 hydrogen atoms<\/strong>.<\/p>\n        <div class=\"ols-rule-list\">\n          <div class=\"ols-rule-item\">\n            <h3>2H<sub>2<\/sub>O<\/h3>\n            <p>Two water molecules. Total atoms: 4 H and 2 O.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>3CO<sub>2<\/sub><\/h3>\n            <p>Three carbon dioxide molecules. Total atoms: 3 C and 6 O.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>4Al<sub>2<\/sub>O<sub>3<\/sub><\/h3>\n            <p>Four formula units of aluminium oxide. Total atoms: 8 Al and 12 O.<\/p>\n          <\/div>\n        <\/div>\n        <div class=\"ols-key-box\"><p><strong>Important rule:<\/strong> you may change coefficients when balancing an equation, but you must not alter the small subscript numbers inside a correct formula.<\/p><\/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: Counting Atoms in Formulae<\/h2>\n<p>Count the atoms in each formula on paper, then type the number.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-631\" class=\"h5p-iframe\" data-content-id=\"631\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Chemical Formulae Flashcards: Counting Atoms with Subscripts, Brackets and Coefficients\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card orange\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">4<\/div><h2>Why Equations Must Be Balanced<\/h2><\/div>\n        <p>A balanced equation has the same number of each type of atom on both sides. This follows the conservation of mass: atoms are not lost, gained or changed into different atoms during ordinary chemical reactions.<\/p>\n        <p>Balancing an equation means adjusting the coefficients in front of formulae until the atom counts match on the left-hand side and right-hand side.<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Balanced equation:<\/strong> an equation in which the number of atoms of each element is the same on both sides of the reaction arrow.<\/p><\/div>\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-8.jpg\" alt=\"Step-by-step balancing of Zn + 2HCl to ZnCl\u2082 + H\u2082 with an atom-count check table\"><\/div>\n<div class=\"ols-figure-caption\"><p>Balancing chemical equations made easy using coefficients and atom counts<\/p><\/div>\n<\/div>\n          <div class=\"ols-zoom-card-caption\">\n            <p>Balancing is a counting process. The formulae stay fixed, while the numbers in front are adjusted to make the atom counts equal.<\/p>\n          <\/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>Quick Check: Which Change Is Allowed?<\/h2>\n<p>Choose the one correct balanced equation.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-632\" class=\"h5p-iframe\" data-content-id=\"632\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Balancing Equations MCQ: Which Change Is Allowed?\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">5<\/div><h2>A Systematic Method for Balancing<\/h2><\/div>\n        <p>Balance equations methodically. Work across the equation and check one element at a time. If an element appears in several places, leave it until later because changing one coefficient may affect more than one atom count.<\/p>\n        <ol>\n          <li>Write the correct formulae for the reactants and products.<\/li>\n          <li>Count the atoms of each element on both sides.<\/li>\n          <li>Add coefficients in front of formulae to balance one element at a time.<\/li>\n          <li>Keep bracketed groups together if the group is unchanged on both sides.<\/li>\n          <li>Check every atom count again at the end.<\/li>\n        <\/ol>\n        <div class=\"ols-zoom-card\">\n          <div class=\"ols-zoom-card-image\">\n            <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t1-img-02-balancefraction.jpg\" alt=\"Five steps balancing the heating of lead(II) nitrate with \u00bdO\u2082, then doubling to 2Pb(NO\u2083)\u2082 \u2192 2PbO + 4NO\u2082 + O\u2082, with an atom tally\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n          <\/div>\n          <div class=\"ols-zoom-card-caption\">\n            <p>A systematic approach reduces errors, especially when several elements or bracketed groups are present.<\/p>\n          <\/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>Quick Check: Balanced or Not?<\/h2>\n<p>Click every equation that is already balanced.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-633\" class=\"h5p-iframe\" data-content-id=\"633\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Balancing Equations Mark the Words: Find the Balanced Equations\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">6<\/div><h2>Worked Example: Balancing an Equation<\/h2><\/div>\n        <p>Consider the reaction between magnesium and oxygen:<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Unbalanced:<\/strong> Mg + O<sub>2<\/sub> \u2192 MgO<\/p><\/div>\n        <p>There are two oxygen atoms on the left because oxygen exists as O<sub>2<\/sub>. There is only one oxygen atom in MgO on the right. Place a 2 in front of MgO:<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Step:<\/strong> Mg + O<sub>2<\/sub> \u2192 2MgO<\/p><\/div>\n        <p>This now gives two magnesium atoms on the right, so place a 2 in front of Mg:<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Balanced:<\/strong> 2Mg + O<sub>2<\/sub> \u2192 2MgO<\/p><\/div>\n        <div class=\"ols-key-box\"><p><strong>Check:<\/strong> left side = 2 Mg and 2 O. Right side = 2 Mg and 2 O. The equation is balanced.<\/p><\/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: Type the Coefficients<\/h2>\n<p>Type the smallest whole-number coefficients that balance each equation.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-286\" class=\"h5p-iframe\" data-content-id=\"286\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Balancing Equations Fill in the Blanks: Type the Coefficients\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card orange\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">7<\/div><h2>Using Bracketed Groups Efficiently<\/h2><\/div>\n        <p>If a group of atoms remains unchanged on both sides of an equation, it is often quicker to count the whole group rather than each individual atom inside it.<\/p>\n        <p>For example, if the sulfate group, SO<sub>4<\/sub><sup>2-<\/sup>, appears unchanged on both sides, count sulfate groups first. This saves time and reduces the chance of losing track of sulfur and oxygen atoms separately.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Exam focus:<\/strong> bracketed groups are useful when the same group appears unchanged on both sides. If the group changes, count the individual atoms instead.<\/p><\/div>\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-9.jpg\" alt=\"Step-by-step balancing of 2AgNO3 + CaCl\u2082 to Ca(NO\u2083)2 + 2AgCl, counting the nitrate group as a whole\"><\/div>\n<div class=\"ols-figure-caption\"><p>Balancing chemical equations guide showing atom counting and unchanged groups<\/p><\/div>\n<\/div>\n          <div class=\"ols-zoom-card-caption\">\n            <p>Keep unchanged polyatomic groups together while balancing, then complete a final atom-by-atom check.<\/p>\n          <\/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>Quick Check: From Word Equation to Balanced Equation<\/h2>\n<p>Write each balanced symbol equation on paper, then flip the card to check.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-287\" class=\"h5p-iframe\" data-content-id=\"287\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Balanced Symbol Equations: Word Equation Practice\"><\/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: Writing Equations Round-Up<\/h2>\n<p>Pick the one fully correct statement in each round to pull the whole page together.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1306\" class=\"h5p-iframe\" data-content-id=\"1306\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Writing Equations Summary: Formulae, Coefficients and Balancing\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">8<\/div><h2>Common Exam Points<\/h2><\/div>\n        <ul>\n          <li>Small subscript numbers are part of the formula and must not be changed when balancing.<\/li>\n          <li>A coefficient in front of a formula multiplies the whole formula.<\/li>\n          <li>If there is no small subscript number after a symbol, there is one atom of that element in the formula.<\/li>\n          <li>Brackets multiply everything inside them. For example, Ca(OH)<sub>2<\/sub> contains two oxygen atoms and two hydrogen atoms.<\/li>\n          <li>Only coefficients are changed during balancing.<\/li>\n          <li>Balance one element at a time, then check the whole equation again at the end.<\/li>\n          <li>Leave elements that appear in several formulae until later where possible.<\/li>\n          <li>Count unchanged groups, such as sulfate or nitrate, as whole groups when this is quicker.<\/li>\n          <li>A balanced equation must have the same number of each type of atom on both sides.<\/li>\n        <\/ul>\n      <\/article>\n\n      \n\n      <section class=\"ols-quicksnap-card\">\n        <h2>QuickSnap<\/h2>\n        <p>This text summary condenses the page into the essential exam ideas.<\/p>\n        <ul class=\"ols-quicksnap-list\">\n          <li><strong>Subscripts:<\/strong> small numbers inside a formula show how many atoms of the element immediately before them are present.<\/li>\n          <li><strong>Coefficients:<\/strong> big numbers in front of formulae multiply the whole formula.<\/li>\n          <li><strong>Do not alter formulae:<\/strong> never change a small subscript to balance an equation.<\/li>\n          <li><strong>Balance with coefficients:<\/strong> write numbers in front of formulae until the atoms match on both sides.<\/li>\n          <li><strong>Bracketed groups:<\/strong> a number outside brackets multiplies everything inside the brackets.<\/li>\n          <li><strong>Final check:<\/strong> recount every element at the end to make sure no earlier balance has been disrupted.<\/li>\n        <\/ul>\n      <\/section>\n\n      <div class=\"ols-image-lightbox\" id=\"olsImageLightboxWritingChemicalEquations001\" 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-writing-chemical-equations-page\");\n          if (!page) {\n            return;\n          }\n\n          var lightbox = page.querySelector(\"#olsImageLightboxWritingChemicalEquations001\");\n          if (!lightbox) {\n            return;\n          }\n\n          var lightboxImage = lightbox.querySelector(\".ols-image-lightbox-img\");\n          var closeButton = lightbox.querySelector(\".ols-image-lightbox-close\");\n          var clickableImages = page.querySelectorAll(\"img.ols-lightbox-target\");\n\n          function openLightbox(image) {\n            lightboxImage.src = image.currentSrc || image.src;\n            lightboxImage.alt = image.alt || \"Expanded revision image\";\n            lightbox.classList.add(\"is-open\");\n            lightbox.setAttribute(\"aria-hidden\", \"false\");\n\n            if (!document.body.dataset.olsPreviousOverflow) {\n              document.body.dataset.olsPreviousOverflow = document.body.style.overflow || \"default\";\n            }\n\n            document.body.style.overflow = \"hidden\";\n          }\n\n          function closeLightbox() {\n            lightbox.classList.remove(\"is-open\");\n            lightbox.setAttribute(\"aria-hidden\", \"true\");\n            lightboxImage.src = \"\";\n\n            if (document.body.dataset.olsPreviousOverflow) {\n              document.body.style.overflow = document.body.dataset.olsPreviousOverflow === \"default\" ? \"\" : document.body.dataset.olsPreviousOverflow;\n              delete document.body.dataset.olsPreviousOverflow;\n            }\n          }\n\n          clickableImages.forEach(function(image){\n            image.addEventListener(\"click\", function(event){\n              event.preventDefault();\n              event.stopPropagation();\n              openLightbox(image);\n            });\n          });\n\n          closeButton.addEventListener(\"click\", closeLightbox);\n\n          lightbox.addEventListener(\"click\", function(event){\n            if (event.target === lightbox) {\n              closeLightbox();\n            }\n          });\n\n          document.addEventListener(\"keydown\", function(event){\n            if (event.key === \"Escape\" && lightbox.classList.contains(\"is-open\")) {\n              closeLightbox();\n            }\n          });\n        })();\n      <\/script>\n\n<section class=\"ols-faq-card\">\n        <h2>FAQs<\/h2>\n        <p>These questions address the most common mistakes students make when writing and balancing equations.<\/p>\n        <div class=\"ols-faq-list\">\n          <div class=\"ols-faq-item\">\n            <h3>Can I change a small subscript number when balancing an equation?<\/h3>\n            <p>No. A small subscript number is part of the chemical formula. Changing it would change the substance. Balance equations by changing only the numbers in front of formulae.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>What does a number in front of a formula mean?<\/h3>\n            <p>A coefficient multiplies the whole formula. For example, 3H<sub>2<\/sub>O contains six hydrogen atoms and three oxygen atoms.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>Why does an equation need to be balanced?<\/h3>\n            <p>An equation must be balanced because atoms are conserved in a chemical reaction. The same number of each type of atom must appear on both sides of the reaction arrow.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>When should I count a group of atoms as a whole group?<\/h3>\n            <p>You can count a group such as sulfate, SO<sub>4<\/sub>, or nitrate, NO<sub>3<\/sub>, as a whole group if it appears unchanged on both sides of the equation.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>What should I do if an element appears in several formulae?<\/h3>\n            <p>Leave it until later where possible. Balance simpler elements or unchanged groups first, then return to the element that appears in several places.<\/p>\n          <\/div>\n        <\/div>\n      <\/section>\n\n      <!-- author card (moved to the end, 10 Oct 2026) -->\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\n\n      <section class=\"ols-related-card\">\n        <h2>Related Topics<\/h2>\n        <p>Use these pages next to strengthen the surrounding skills needed for equations and calculations.<\/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\/topic-5-formulae-equations-and-amounts-of-substance\/atoms-elements-and-molecules\/\">Atoms, Elements and Molecules<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/equations-and-reaction-types\/typical-reactions-of-acids\/\">Typical Reactions of Acids<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/equations-and-reaction-types\/\">Displacement Reactions<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/equations-and-reaction-types\/ionic-and-full-equations\/\">Ionic Equations<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/mole-calculations\/\">Mole Calculations<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-1-atomic-structure-and-the-periodic-table\/\">Atomic Structure &amp; The Periodic Table<\/a>\n        <\/div>\n      <\/section>\n\n<section class=\"ols-attribution-card ols-attr-std\" aria-label=\"Copyright notice\">\n<style id=\"ols-attribution-card-css\">.ols-attribution-card.ols-attr-std{background:linear-gradient(135deg,#ffffff,#f8fbff);border:1px solid rgba(28,36,75,0.14);border-radius:28px;box-shadow:0 18px 45px rgba(28,36,75,0.10);padding:34px;margin:24px 0;overflow:hidden;font-family:Poppins,Arial,sans-serif;box-sizing:border-box}.ols-attribution-card.ols-attr-std *{box-sizing:border-box}.ols-attribution-card.ols-attr-std p{margin:0;font-size:14px;line-height:1.65;font-weight:300;color:#667085}.ols-attribution-card.ols-attr-std strong{font-weight:700;color:#1C244B}@media (max-width:760px){.ols-attribution-card.ols-attr-std{padding:24px 18px;border-radius:22px}}<\/style>\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. It may not be copied, reproduced, redistributed or adapted without written permission.<\/p>\n<\/section>\n\n      <script type=\"application\/ld+json\">\n      {\n        \"@context\": \"https:\/\/schema.org\",\n        \"@graph\": [\n          {\n            \"@type\": \"WebPage\",\n            \"@id\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/equations-and-reaction-types\/writing-chemical-equations\/#webpage\",\n            \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/equations-and-reaction-types\/writing-chemical-equations\/\",\n            \"name\": \"Writing Chemical Equations - Edexcel A Level Chemistry\",\n            \"description\": \"Revision notes explaining how to write and balance chemical equations, including subscripts, coefficients, atom counting, bracketed 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