{"id":8074,"date":"2026-09-15T08:24:16","date_gmt":"2026-09-15T07:24:16","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/writing-chemical-equations\/"},"modified":"2026-09-25T09:31:30","modified_gmt":"2026-09-25T08:31:30","slug":"writing-chemical-equations","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\">\n    <h3>Revision Notes<\/h3>\n    <p>Cambridge International AS Level Chemistry<\/p>\n  <\/div>\n  <div class=\"ols-topic-group\">\n    <h4>Topic 2 Atoms, Molecules and Stoichiometry<\/h4>\n    <ul class=\"ols-topic-list\">\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/\">Topic overview<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/atoms-elements-and-molecules\/\">2.1 \/ 2.2 Atoms, Elements and Molecules<\/a><\/li>\n<li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/ions-and-ionic-formulae\/\">2.3 Ions and Ionic Formulae<\/a><\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/\">2.3 Equations and Reaction Types<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/writing-chemical-equations\/\">Writing Chemical Equations<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/typical-reactions-of-acids\/\">Typical Reactions of Acids<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/ionic-and-full-equations\/\">Ionic and Full Equations<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/empirical-and-molecular-formulae\/\">2.3 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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/empirical-and-molecular-formulae\/empirical-formulae\/\">Empirical Formulae<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/empirical-and-molecular-formulae\/molecular-formulae\/\">Molecular Formulae<\/a><\/li>\n<li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/hydrated-salts-and-water-of-crystallisation\/\">2.3 Hydrated Salts and Water of Crystallisation<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/\">2.4 Mole Calculations<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/calculations-involving-moles\/\">Calculations involving Moles<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/calculations-with-solutions-and-gases\/\">2.4 Solutions and Gases<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/calculations-with-solutions-and-gases\/titration-calculations\/\">Titration Calculations<\/a><\/li>\n        <\/ul>\n      <\/li>\n    <\/ul>\n  <\/div>\n  <div class=\"ols-topic-group\">\n    <h4>Other Topics<\/h4>\n    <ul class=\"ols-topic-list\">\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-1-atomic-structure\/\">Topic 1 Atomic Structure<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/\">Topic 3 Chemical Bonding<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-4-states-of-matter\/\">Topic 4 States of Matter<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-9-the-periodic-table-chemical-periodicity\/\">Topic 9 Chemical Periodicity<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-13-an-introduction-to-as-level-organic-chemistry\/\">Topic 13 Introduction to Organic Chemistry<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-14-hydrocarbons\/\">Topic 14 Hydrocarbons<\/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\"], 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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\">AS Level<\/div>\n<div class=\"ols-badge\">Topic 2: Atoms, Molecules and Stoichiometry<\/div>\n<div class=\"ols-badge\">9701 Papers 1 and 2<\/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: Conservation of Mass<\/h2>\n<p>Before you start, check the two GCSE ideas that balancing equations is built on.<\/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\/09\/wave-32.jpg\" alt=\"Atom-counting examples with the 3N2 and 5H2O pictures clearly separated and sulfur spelled with an f.\" 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      <\/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>Use subscripts, brackets and the number in front to count atoms in formulae you have not seen on this page.<\/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>All four equations have matching atom counts, but only one is an acceptable 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\/05\/3-Balancing-chemical-equations-guide.webp\" alt=\"Step-by-step guide for balancing chemical equations\" 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>Count atoms on both sides and 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>Balance five new equations by typing the number that goes in front of each formula.<\/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 your formulae and numbers.<\/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\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      <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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/\">Displacement Reactions<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/ionic-and-full-equations\/\">Ionic Equations<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/\">Mole Calculations<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-1-atomic-structure\/\">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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