{"id":8137,"date":"2026-09-16T17:24:20","date_gmt":"2026-09-16T16:24:20","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/anomalous-properties-of-water\/"},"modified":"2026-10-18T17:38:12","modified_gmt":"2026-10-18T16:38:12","slug":"anomalous-properties-of-water","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/anomalous-properties-of-water\/","title":{"rendered":"Anomalous Properties of Water"},"content":{"rendered":"<script src=\"https:\/\/cdnjs.cloudflare.com\/ajax\/libs\/three.js\/r128\/three.min.js\"><\/script>\n\n<section class=\"ols-revision-page ols-imf-page\">\n  <style>\n    .ols-revision-page {\n      --navy: #1C244B;\n      --blue: #2563eb;\n      --soft-blue: #eef4ff;\n      --soft-red: #fff7f7;\n      --soft-purple: #f7f0ff;\n      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<aside class=\"ols-sidebar\">\n  <div class=\"ols-sidebar-header\">\n    <h3>Revision Notes<\/h3>\n    <p>A Level Chemistry<\/p>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Topic 7 Intermolecular Forces<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/\">Topic 7 Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/london-forces\/\">London Forces<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/permanent-dipole-dipole-forces\/\">Permanent Dipole-Dipole Forces<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/hydrogen-bonding\/\">Hydrogen Bonding<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/anomalous-properties-of-water\/\">Anomalous Properties of Water<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/boiling-temperature-trends\/\">Boiling Temperature Trends<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/choosing-solvents\/\">Choosing Solvents<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Other Topics<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-3-bonding-structure\/\">Bonding &amp; Structure<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/\">Alkenes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/\">Energetics<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8A-redox-chemistry\/\">Redox Chemistry<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\n\n<script>\r\n(function() {\r\n  function normalisePath(path) {\r\n    return String(path || '')\r\n      .split('?')[0]\r\n      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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/\">Revision Notes<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/\">A Level Chemistry<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\">Edexcel International<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/\">Topic 7 Intermolecular Forces<\/a> \/\n<span>Anomalous Properties of Water<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Anomalous Properties of Water<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to the anomalous properties of water: its high melting temperature and boiling temperature, the open structure and low density of ice, surface tension and the Group 4 to 7 hydride graph, all explained by hydrogen bonding, for Edexcel International A Level Chemistry.<\/p>\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Exam board: Edexcel International<\/div>\n<div class=\"ols-badge\">Unit 2: WCH12\/01<\/div>\n<div class=\"ols-badge\">Topic 7: Intermolecular Forces<\/div>\n<\/div>\n      <\/header>\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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text-align:center; font-size:11px; font-style:italic; color:#aab0c0;\n}\n.ols-cc-icel-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-icel-001 a:hover{text-decoration:underline;}\n.ols-icel-001 .icel-seg button{display:inline-flex; align-items:center;}\n.ols-icel-001 .icel-seg button .icel-n{display:inline-flex; align-items:center; justify-content:center; width:20px; height:20px; margin-right:9px; border-radius:50%; font-size:11px; font-weight:600; background:rgba(37,99,235,0.10); color:#2563eb; flex:0 0 auto;}\n.ols-icel-001 .icel-seg button[aria-pressed=\"true\"] .icel-n{background:#ffffff; color:#2563eb;}\n<\/style>\n\n<div class=\"icel-head\">\n  <h2 class=\"icel-title\">The Open Hexagonal Lattice of Ice<\/h2>\n  <p class=\"icel-sub\">Choose a step to watch liquid water freeze into ice, see the hexagonal channels its hydrogen bonds hold open, then melt it and watch the molecules move closer.<\/p>\n<\/div>\n\n<div class=\"icel-stage\" id=\"icelStage\">\n  <canvas class=\"icel-canvas\" id=\"icelCanvas\"><\/canvas>\n  <div class=\"icel-overlay\" id=\"icelOverlay\"><\/div>\n  <div class=\"icel-caption\" id=\"icelCaption\"><\/div>\n  <div class=\"icel-hint\" id=\"icelHint\">Drag to rotate<\/div>\n  <div class=\"icel-badges\" id=\"icelBadges\"><\/div>\n<\/div>\n\n<div class=\"icel-controls\">\n  <div class=\"icel-row\">\n    <span class=\"icel-rowlab\">Step<\/span>\n    <div class=\"icel-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\"><span class=\"icel-n\">1<\/span>Liquid water<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\"><span class=\"icel-n\">2<\/span>Freezing: four bonds each<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\"><span class=\"icel-n\">3<\/span>Hexagonal channels<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\"><span class=\"icel-n\">4<\/span>Melting: density rises<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"icel-row\">\n    <span class=\"icel-rowlab\">View<\/span>\n    <button type=\"button\" class=\"icel-pill\" id=\"icelResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"icel-info\">\n  <h3 id=\"icelInfoTitle\"><\/h3>\n  <p id=\"icelInfoText\"><\/p>\n  <div class=\"icel-facts\" id=\"icelFacts\"><\/div>\n  <div class=\"icel-key\">\n    <span><i class=\"k-ball\" style=\"background:#ce2824\"><\/i>Oxygen<\/span>\n    <span><i class=\"k-ball\" style=\"background:#f2f2f6;border-color:rgba(120,126,140,0.85)\"><\/i>Hydrogen<\/span>\n    <span><i class=\"\" style=\"background:#ffffff;border-color:rgba(37,99,235,0.9)\"><\/i>Hydrogen bond (dashed)<\/span>\n    <span><i class=\"\" style=\"background:#ffffff;border-color:rgba(214,150,24,0.85)\"><\/i>Six-membered ring<\/span>\n    <span><i class=\"\" style=\"background:rgba(120,170,230,0.3);border-color:rgba(70,125,190,0.45)\"><\/i>Open channel<\/span>\n  <\/div>\n<\/div>\n\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/ice-lattice.js?v=20261018z\"><\/script>\n<\/section>\n\n<p class=\"ols-cc-icel-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\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: Solids, Density and Boiling<\/h2>\n<p>Before you start, check three GCSE ideas that water is about to disobey.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"774\"><\/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>Why Water Is Anomalous<\/h2>\n<\/div>\n<p>Water has a much higher melting temperature and boiling temperature, a solid that is less dense than its liquid, and a higher surface tension than any molecule of its size should have.<\/p><p>Every one of these <strong>anomalous properties<\/strong> is caused by <strong>hydrogen bonding<\/strong>.<\/p><p>A molecule with only 10 electrons would be expected to boil below -80 \u00b0C if London forces were the only force present; water boils at 100 \u00b0C.<\/p>\n<p>Each water molecule can form up to four hydrogen bonds, more than any other small molecule, so a very large amount of energy is needed to separate the molecules.<\/p><div class=\"ols-key-box\"><p><strong>Key idea:<\/strong> This is the single explanation behind all the anomalies on this page.<\/p><\/div><p>A hydrogen bond needs about 20 kJ mol\u207b\u00b9 to break, against about 460 kJ mol\u207b\u00b9 for the covalent O-H bond. That is why boiling water separates the molecules and leaves every covalent bond intact.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Two O-H bonds and two lone pairs per molecule mean water forms the maximum number of hydrogen bonds, and every anomalous property follows from that.<\/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: Count the O-H Bonds<\/h2>\n<p>Apply the counting argument to a molecule that is not on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-534\" class=\"h5p-iframe\" data-content-id=\"534\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Anomalous Water Drag: Methanol Has Only One O-H Group\"><\/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>High Melting temperature and Boiling temperature<\/h2>\n<\/div>\n<p>The hydrides of Group 6 show the effect most clearly. Going down the group from H\u2082S to H\u2082Te the boiling temperature rises steadily because the number of electrons rises and the London forces strengthen.<\/p><p>A larger electron cloud distorts more easily, so it produces larger momentary dipoles.<\/p><p>Extending that trend upwards predicts a boiling temperature for water of about -80 \u00b0C. The real value is 100 \u00b0C.<\/p><p>This is because hydrogen bonds between water molecules are far stronger than the London forces and permanent dipole-permanent dipole interactions that hold the other hydrides together.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Hydride<\/th><th>Electrons<\/th><th>boiling temperature \/ \u00b0C<\/th><th>Strongest intermolecular force<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>H\u2082O<\/strong><\/td><td>10<\/td><td>100<\/td><td>hydrogen bonds<\/td><\/tr>\n<tr><td><strong>H\u2082S<\/strong><\/td><td>18<\/td><td>-60<\/td><td>permanent dipole-permanent dipole interactions<\/td><\/tr>\n<tr><td><strong>H\u2082Se<\/strong><\/td><td>36<\/td><td>-41<\/td><td>permanent dipole-permanent dipole interactions (London forces growing)<\/td><\/tr>\n<tr><td><strong>H\u2082Te<\/strong><\/td><td>54<\/td><td>-2<\/td><td>London forces dominate<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\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-49.jpg\" alt=\"Line graph of the boiling temperatures of the Group 4 to 7 hydrides against period, with water, ammonia and HF ringed as hydrogen-bonding anomalies\"><\/div>\n<div class=\"ols-figure-caption\"><p>NH\u2083, H\u2082O and HF sit far above the trend of their groups because of hydrogen bonding; CH\u2084 does not, because carbon is not electronegative enough for C-H to hydrogen bond.<\/p><\/div>\n<\/div>\n<p>The same graph shows that methane is not anomalous. Carbon is not electronegative enough for a C-H hydrogen to form a hydrogen bond, so the Group 4 hydrides follow a smooth London forces trend.<\/p><p>Ammonia and hydrogen fluoride are anomalous but less so than water, because each forms on average only one hydrogen bond per molecule.<\/p><p>Hydrogen fluoride has three lone pairs but only one hydrogen to donate. Ammonia has three hydrogens but only one lone pair to accept. In each case the smaller number sets the count.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> Water has a much higher boiling temperature than H\u2082S because water molecules form hydrogen bonds, which are much stronger than the permanent dipole-permanent dipole interactions and London forces between H\u2082S molecules, so much more energy is needed to separate water molecules.<\/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: Explain a Hydride That Breaks Its Trend<\/h2>\n<p>Write a short explanation, then compare it with the mark points and the model answer.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-775\" class=\"h5p-iframe\" data-content-id=\"775\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Anomalous Water Explain: A Hydride That Boils Far Above Its Trend\"><\/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>The Density of Ice<\/h2>\n<\/div>\n<p>Ice floats on water because it is <strong>less dense<\/strong>: 0.92 g cm\u207b\u00b3 against 1.00 g cm\u207b\u00b3 for water at 4 \u00b0C.<\/p><p>Almost every other substance is denser as a solid than as a liquid, because the particles pack more closely when they stop moving. Water is different because of the geometry of its hydrogen bonds.<\/p>\n<p>In ice each water molecule is held by four hydrogen bonds to four neighbours arranged tetrahedrally around it, and the hydrogen bonds are fixed in place.<\/p>\n<p>This tetrahedral arrangement builds an <strong>open hexagonal lattice<\/strong> with large empty spaces, like a honeycomb. The molecules are held further apart than they would be if they could pack freely.<\/p>\n<p>When ice melts, some of the hydrogen bonds break and the molecules can move into the spaces, so the liquid is denser than the solid.<\/p>\n<p>Water reaches its maximum density at 4 \u00b0C. Between 0 \u00b0C and 4 \u00b0C open, ice-like clusters left over from the lattice are still collapsing, which lets the molecules pack closer and makes the density rise.<\/p>\n<p>Above 4 \u00b0C the molecules move faster and thermal expansion takes over, so the density falls again.<\/p>\n\n<p>This has consequences beyond the exam: ice on a pond insulates the water below it, so aquatic life survives winter, and water expanding as it freezes shatters rocks and bursts pipes.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> Ice is less dense than water because hydrogen bonds hold the water molecules in a fixed, open tetrahedral lattice with large spaces; when ice melts some hydrogen bonds break and the molecules move closer together.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: How Much Does Ice Expand?<\/h2>\n<p>Use the two densities on this card to work out the change in volume.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-536\" class=\"h5p-iframe\" data-content-id=\"536\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Anomalous Water Fill In: How Much Does Water Expand When It Freezes?\"><\/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>Surface Tension<\/h2>\n<\/div>\n<p>Water has a high <strong>surface tension<\/strong> because the molecules at the surface are pulled inwards and sideways by hydrogen bonds to their neighbours, with no molecules above to balance the pull.<\/p><p>The surface therefore behaves like a stretched skin that resists being broken or increased in area.<\/p><p>This is why water forms near-spherical droplets, why a steel needle can be floated on water and why pond skaters walk on it.<\/p>\n<p>Liquids held only by London forces, such as hexane, have a much lower surface tension because far less energy is needed to create new surface. Adding a detergent to water lowers its surface tension by placing molecules at the surface that cannot hydrogen bond as strongly.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Surface tension is not named in the Edexcel International specification, but it appears in data-response questions; explain it with the same hydrogen-bonding argument.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: A Liquid With a Lower Surface Tension<\/h2>\n<p>Explain why a liquid that does form hydrogen bonds still has a much lower surface tension than water.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-537\" class=\"h5p-iframe\" data-content-id=\"537\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Anomalous Water MCQ: Why Ethanol Has a Lower Surface Tension\"><\/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>Other Consequences of Hydrogen Bonding in Water<\/h2>\n<\/div>\n<p>Water also has an unusually high specific heat capacity and a high enthalpy change of vaporisation.<\/p><p>This is because energy supplied to water is used to break hydrogen bonds before the molecules can move faster or escape as a gas. These properties make water an excellent coolant and moderate the climate of coastal regions.<\/p><p>Water is also an excellent solvent for ionic compounds and for molecules that can hydrogen bond, which the <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/choosing-solvents\/\">Choosing Solvents<\/a> page explains.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Property<\/th><th>Value for water<\/th><th>Comparison<\/th><th>Cause<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Boiling temperature<\/strong><\/td><td>100 \u00b0C<\/td><td>H\u2082S: -60 \u00b0C<\/td><td>four hydrogen bonds per molecule<\/td><\/tr>\n<tr><td><strong>Density of solid<\/strong><\/td><td>0.92 g cm\u207b\u00b3<\/td><td>liquid: 1.00 g cm\u207b\u00b3<\/td><td>open hydrogen-bonded lattice<\/td><\/tr>\n<tr><td><strong>Surface tension<\/strong><\/td><td>72 mN m\u207b\u00b9<\/td><td>hexane: 18 mN m\u207b\u00b9<\/td><td>surface molecules pulled inwards by hydrogen bonds<\/td><\/tr>\n<tr><td><strong>Specific heat capacity<\/strong><\/td><td>4.18 J g\u207b\u00b9 K\u207b\u00b9<\/td><td>ethanol: 2.44 J g\u207b\u00b9 K\u207b\u00b9<\/td><td>energy breaks hydrogen bonds before raising temperature<\/td><\/tr>\n<tr><td><strong>Enthalpy change of vaporisation<\/strong><\/td><td>41 kJ mol\u207b\u00b9<\/td><td>H\u2082S: 19 kJ mol\u207b\u00b9<\/td><td>hydrogen bonds must be broken before a molecule can escape<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Whatever the anomalous property, the answer begins &#8220;because water molecules form hydrogen bonds&#8221; and ends with what those bonds do.<\/p>\n<\/div>\n<div class=\"ols-figure-card ols-zoom-pop\">\n<div class=\"ols-figure-image\">\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/t17x-s3waterprops.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/t17x-s3waterprops.jpg\" alt=\"Five tiles comparing water with H\u2082S, hexane and ethanol for boiling, density, surface tension, heat capacity and vaporisation.\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/t17x-s3waterprops.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Hydrogen bonding explains why water has a high boiling temperature, a less dense solid, a high surface tension, a high specific heat capacity and a high enthalpy change of vaporisation.<\/p><\/div>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Pick the Accurate Statement<\/h2>\n<p>In each round, choose the one statement that is accurate.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-776\" class=\"h5p-iframe\" data-content-id=\"776\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Anomalous Water Summary: One Cause Behind Every Anomaly\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">6<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Explain why ice floats<\/h3><p>Hydrogen bonds hold molecules in an open tetrahedral lattice with spaces; on melting some bonds break and molecules move closer, so the liquid is denser.<\/p>\n<h3>Explain the boiling temperature of water compared with H\u2082S<\/h3><p>Hydrogen bonds between water molecules are stronger than the permanent dipole-permanent dipole interactions and London forces between H\u2082S molecules.<\/p>\n<h3>Why is CH\u2084 not anomalous?<\/h3><p>Carbon is not electronegative enough for C-H hydrogens to form hydrogen bonds.<\/p>\n<h3>Do not say<\/h3><p>&#8220;Ice has stronger hydrogen bonds than water&#8221; (the bonds are the same strength; ice has more of them held rigidly); &#8220;covalent bonds break when ice melts&#8221;.<\/p>\n<\/article>\n\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the explanations for water that Edexcel International questions ask for.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why is the boiling temperature of water so high?<\/h3>\n<p>Each water molecule can form up to four hydrogen bonds, which are far stronger than the permanent dipole-permanent dipole interactions and London forces holding together hydrides such as H\u2082S. Extending the Group 6 trend would predict about -80 \u00b0C; water boils at 100 \u00b0C.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why does ice float on water?<\/h3>\n<p>In ice the hydrogen bonds hold each molecule to four neighbours in a fixed tetrahedral arrangement, building an open hexagonal lattice with large spaces. When ice melts some hydrogen bonds break and the molecules move closer together, so the liquid is denser.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is methane not anomalous?<\/h3>\n<p>Carbon is not electronegative enough for the hydrogen atoms in CH\u2084 to carry a large \u03b4+ charge, so methane cannot hydrogen bond. The Group 4 hydrides follow a smooth trend set by their number of electrons.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why does water have a high surface tension?<\/h3>\n<p>Molecules at the surface have no neighbours above them, so the hydrogen bonds pull them inwards and sideways. Making new surface therefore costs energy, which is why water forms beads and supports a floating needle.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Are the hydrogen bonds in ice stronger than in water?<\/h3>\n<p>No, they are the same strength. Ice simply has more of them and holds them in fixed positions, which is what produces the open structure.<\/p>\n<\/div>\n<\/div>\n<\/section>\n\n      <!-- author card (moved to the end, 10 Oct 2026) -->\n      <div class=\"ols-author\">\n\n    <img decoding=\"async\"\n      class=\"ols-author-avatar-img\"\n      src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\"\n      alt=\"Dr. Mohammed Al-Fatah\"\n    >\n\n    <div class=\"ols-author-content\">\n\n      <h2 class=\"ols-author-title\">\n        Written by: Dr. Mohammed Al-Fatah\n      <\/h2>\n\n      <p class=\"ols-author-description\">\n        Chemistry specialist revision notes for A Level Chemistry.\n      <\/p>\n\n      <a\n        class=\"ols-linkedin-pill\"\n        href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\"\n        target=\"_blank\"\n        rel=\"noopener noreferrer\"\n      >\n        <svg class=\"ols-linkedin-icon\" viewBox=\"0 0 24 24\" fill=\"currentColor\" aria-hidden=\"true\">\n          <path d=\"M4.98 3.5C4.98 4.88 3.86 6 2.48 6S0 4.88 0 3.5 1.12 1 2.48 1s2.5 1.12 2.5 2.5zM.5 8h4V24h-4V8zm7 0h3.8v2.2h.1c.5-.9 1.8-2.2 3.9-2.2 4.2 0 5 2.8 5 6.4V24h-4v-7.6c0-1.8 0-4.2-2.6-4.2s-3 2-3 4v7.8h-4V8z\"\/>\n        <\/svg>\n        View LinkedIn Profile\n      <\/a>\n\n    <\/div>\n\n  <\/div>\n\n<section class=\"ols-related-card\">\n<h2>Related Topic 7 Pages<\/h2>\n<p>Use these pages to connect the three intermolecular forces with the physical properties they explain.<\/p>\n<div class=\"ols-related-grid\">\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/london-forces\/\">London Forces<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/permanent-dipole-dipole-forces\/\">Permanent Dipole-Dipole Forces<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/hydrogen-bonding\/\">Hydrogen Bonding<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/boiling-temperature-trends\/\">Boiling Temperature Trends<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/choosing-solvents\/\">Choosing Solvents<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-3-bonding-structure\/polar-and-non-polar-molecules\/\">Polar &amp; Non-Polar Molecules<\/a>\n<\/div>\n<\/section>\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    <\/main>\n  \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-international\/topic-7-intermolecular-forces\/anomalous-properties-of-water\/#webpage\",\n      \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/anomalous-properties-of-water\/\",\n      \"name\": \"Anomalous Properties of Water | Topic 7 Intermolecular Forces | Online Learning System\",\n      \"description\": \"Anomalous properties of water for Edexcel International A Level Chemistry: high boiling temperature, density of ice, surface tension and the hydride boiling temperature graph explained by hydrogen bonding.\",\n      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