{"id":12479,"date":"2026-10-03T08:32:26","date_gmt":"2026-10-03T07:32:26","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/predicting-solubility\/"},"modified":"2026-10-04T23:04:46","modified_gmt":"2026-10-04T22:04:46","slug":"predicting-solubility","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/predicting-solubility\/","title":{"rendered":"Predicting Solubility"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-kinetics-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      --soft-green: #f0f7f1;\n      --gold: #c9973a;\n      --grey-text: #667085;\n      --body-text: #1f2937;\n      --border: rgba(28, 36, 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0 8px;\n      font-size: 20px;\n      line-height: 1.3;\n      color: var(--navy);\n    }\n.ols-faq-item p {\n      margin: 0;\n      font-size: 16px;\n      line-height: 1.65;\n      color: var(--body-text);\n    }\n.ols-faq-card,\n      .ols-quicksnap-card,\n      .ols-attribution-card {\n        padding: 24px 18px;\n        border-radius: 22px;\n      }\n  <\/style>\n\n    <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 12 Entropy and Energetics<\/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-12-entropy-and-energetics\/\">Topic 12 Entropy and Energetics Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/entropy-and-the-direction-of-change\/\">Entropy and the Direction of Change<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/calculating-entropy-changes\/\">Calculating Entropy Changes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/feasibility-gibbs-energy-and-temperature\/\">Feasibility and Temperature<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/thermodynamic-and-kinetic-stability\/\">Thermodynamic and Kinetic Stability<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-and-born-haber-cycles\/\">Lattice Energy and Born\u2013Haber Cycles<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-trends-and-covalent-character\/\">Lattice Energy Trends and Covalent Character<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/enthalpy-of-solution-and-hydration\/\">Enthalpy of Solution and Hydration<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/predicting-solubility\/\">Predicting Solubility<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Other Sections<\/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-6-energetics\/\">Topic 6 Energetics (AS)<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-11-kinetics\/\">Topic 11 Kinetics<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/\">Topic 8B Groups 1 and 2<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/\">Core Practicals<\/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      .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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true;\r\n  }\r\n\r\n  if (!highlightActive()) {\r\n    document.addEventListener('DOMContentLoaded', highlightActive);\r\n  }\r\n})();\r\n<\/script>\r\n\n\n    <main class=\"ols-main\">\n\n      <!-- BREADCRUMBS - updated with full path and correct links -->\n      <nav class=\"ols-breadcrumbs\" aria-label=\"Breadcrumb\">\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/\">Revision Notes<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/\">A Level Chemistry<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\">Edexcel International<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/\">Topic 12 Entropy and Energetics<\/a> \/\n<span>Predicting Solubility<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Predicting Solubility<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to using enthalpy and entropy changes of solution to predict whether an ionic compound dissolves, and to explain why solubility runs one way for the Group 2 hydroxides and the other way for the Group 2 sulfates.<\/p>\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Exam board: Edexcel International<\/div>\n<div class=\"ols-badge\">Unit 4: WCH14\/01<\/div>\n<div class=\"ols-badge\">Topic 12: Entropy and Energetics<\/div>\n<\/div>\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 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\">\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      <\/header>\n\n      <article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>Dissolving as a Feasibility Question<\/h2>\n<\/div>\n<p>Whether a salt dissolves is a question of <strong>feasibility<\/strong>, and feasibility is decided by the total entropy change.<\/p>\n<p style=\"text-align:center\"><strong>\u0394S_total = \u0394S_system + \u0394S_surroundings<\/strong><\/p>\n<p>Both terms can be worked out for dissolving, and the enthalpy of solution from the previous page is only half of the story.<\/p>\n<p><strong>\u0394S_system<\/strong> is usually positive.<\/p>\n<ul>\n<li>An ordered lattice becomes a set of mobile ions spread through the solution, which is a large increase in disorder.<\/li>\n<li>But water molecules that hydrate an ion are held in place around it.<\/li>\n<li>For a <strong>small, highly charged ion<\/strong> such as Mg\u00b2\u207a or Al\u00b3\u207a, so many water molecules are ordered so tightly that \u0394S_system can be negative overall.<\/li>\n<\/ul>\n<p><strong>\u0394S_surroundings<\/strong> = \u2212\u0394solH\/T.<\/p>\n<ul>\n<li>An exothermic \u0394solH heats the surroundings and gives a positive term.<\/li>\n<li>An endothermic \u0394solH gives a negative term.<\/li>\n<li>Because T is in the denominator, an endothermic salt becomes more soluble as the temperature rises.<\/li>\n<\/ul>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> A salt dissolves if \u0394S_total is positive: \u0394S_system (usually positive, the ions spread out) + \u0394S_surroundings (= \u2212\u0394solH\/T). A slightly endothermic salt dissolves because the entropy gain of the system outweighs the small negative surroundings term.<\/p>\n<\/div>\n<p>So there are two ways to predict solubility.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Method<\/th><th>What to do<\/th><th>Prediction<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>From \u0394solH alone<\/strong><\/td><td>look at the sign and size of \u0394solH<\/td><td>large endothermic value: insoluble; small value of either sign: soluble; exothermic value: soluble<\/td><\/tr>\n<tr><td><strong>From \u0394S_total<\/strong><\/td><td>put in both terms at the temperature asked<\/td><td>positive result: the salt dissolves<\/td><\/tr>\n<\/tbody>\n<\/table>\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\/entropy-t12-16-solpredict.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\/entropy-t12-16-solpredict.jpg\" alt=\"Flow chart for predicting whether a salt dissolves: a quick estimate from the sign and size of the enthalpy of solution on the left, and on the right the full method of working out \u0394S of the surroundings from \u2212\u0394solH \u00f7 T, adding \u0394S of the system and checking whether \u0394S total is positive\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-16-solpredict.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Predicting whether a salt dissolves: a quick estimate from the sign and size of \u0394solH, and the full method that adds \u0394S_system to \u0394S_surroundings and checks the sign of \u0394S_total.<\/p><\/div>\n<\/div>\n<p>In a series of related salts the lattice energy and the hydration enthalpies change at different rates down the group.<\/p>\n<p>The trend in \u0394solH, and so in \u0394S_surroundings, is what drives the trend in solubility.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> \u0394S_surroundings = \u2212\u0394solH \u00f7 T with \u0394solH in J mol\u207b\u00b9 and T in kelvin. Feasible when \u0394S_total > 0. Say which term dominates and why.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>Group 2 Hydroxides<\/h2>\n<\/div>\n<p>The solubility of the Group 2 hydroxides <strong>increases<\/strong> down the group. Magnesium hydroxide is sparingly soluble, and barium hydroxide is soluble enough to make a strongly alkaline solution.<\/p>\n<p>Both quantities in the solution cycle fall down the group, because the cation gets larger and its charge density falls.<\/p>\n<ul>\n<li>The lattice energy becomes less exothermic, so less energy is needed to break the lattice.<\/li>\n<li>The hydration enthalpy of the cation becomes less exothermic, so less energy is released on hydrating it.<\/li>\n<li>The trend in \u0394solH depends on which falls faster.<\/li>\n<\/ul>\n<p>The deciding factor is the <strong>small hydroxide ion<\/strong>.<\/p>\n<ol>\n<li>With a small anion, the size of the cation makes up a large part of the distance between the ion centres.<\/li>\n<li>So the lattice energy depends strongly on the cation and falls steeply from Mg(OH)\u2082 to Ba(OH)\u2082.<\/li>\n<li>The hydration enthalpy of the cation falls too, but more gently.<\/li>\n<li>The endothermic term shrinks faster than the exothermic term, so \u0394solH becomes <strong>less endothermic<\/strong> down the group.<\/li>\n<li>\u0394S_surroundings becomes less negative, \u0394S_total becomes more positive and the hydroxides become more soluble.<\/li>\n<\/ol>\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\/entropy-t12-11-solubility.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\/entropy-t12-11-solubility.jpg\" alt=\"Why solubility trends in Group 2 run opposite ways for hydroxides and sulfates: lattice energy and hydration enthalpy both fall down the group, but\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-11-solubility.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Why the Group 2 hydroxides and sulfates run opposite ways: the lattice energy and the total hydration enthalpy both fall down the group, but with the small OH\u207b ion the lattice energy falls faster and with the large SO\u2084\u00b2\u207b ion the hydration enthalpy falls faster.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> Hydroxides: &#8220;down the group the lattice energy decreases faster than the hydration enthalpy because the OH\u207b ion is small, so \u0394solH becomes less endothermic and solubility increases&#8221;.<\/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>Check: Reasoning With \u0394solH and \u0394S<\/h2>\n<p>Use given enthalpy and entropy data for salts not on this page to decide whether each dissolves at a stated temperature, and identify which term dominates.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1120\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>Group 2 Sulfates<\/h2>\n<\/div>\n<p>The Group 2 sulfates run the other way: solubility <strong>decreases<\/strong> down the group.<\/p>\n<ul>\n<li>Magnesium sulfate is very soluble (Epsom salts).<\/li>\n<li>Calcium sulfate is sparingly soluble.<\/li>\n<li>Barium sulfate is so insoluble that it is the basis of the sulfate test: a white precipitate with acidified barium chloride.<\/li>\n<li>Barium sulfate can be swallowed safely as a barium meal even though barium ions are toxic.<\/li>\n<\/ul>\n<p>The difference is the <strong>large sulfate ion<\/strong>.<\/p>\n<ol>\n<li>SO\u2084\u00b2\u207b is so big that the distance between the ion centres is dominated by the anion.<\/li>\n<li>Changing the cation from Mg\u00b2\u207a to Ba\u00b2\u207a alters that distance by a small fraction, so the lattice energy falls only slightly down the group.<\/li>\n<li>The hydration enthalpy of the cation, however, depends only on the cation and falls steeply as before.<\/li>\n<li>Now the exothermic term shrinks faster than the endothermic term, so \u0394solH becomes <strong>more endothermic<\/strong> down the group.<\/li>\n<li>\u0394S_surroundings becomes more negative and the sulfates become less soluble.<\/li>\n<\/ol>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Trend<\/th><th>Anion<\/th><th>Change in lattice energy down the group<\/th><th>Change in cation hydration enthalpy<\/th><th>Result<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Hydroxides: solubility increases<\/strong><\/td><td>OH\u207b, small<\/td><td>falls steeply (cation size dominates the ion separation)<\/td><td>falls<\/td><td>\u0394solH less endothermic down the group<\/td><\/tr>\n<tr><td><strong>Sulfates: solubility decreases<\/strong><\/td><td>SO\u2084\u00b2\u207b, large<\/td><td>falls slightly (anion size dominates the ion separation)<\/td><td>falls steeply by comparison<\/td><td>\u0394solH more endothermic down the group<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>The same reasoning covers the carbonates, which like the sulfates become less soluble down the group because CO\u2083\u00b2\u207b is also large.<\/p>\n<p>The full set of trends, with the reactions and tests that go with them, is in <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/\">Topic 8B Groups 1 and 2<\/a>.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> Sulfates: &#8220;the SO\u2084\u00b2\u207b ion is large, so the lattice energy changes little down the group while the hydration enthalpy of the cation decreases markedly, so \u0394solH becomes more endothermic and solubility decreases&#8221;.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Say<\/h3>\n<ul>\n<li>&#8220;A salt dissolves when \u0394S_total is positive.&#8221;<\/li>\n<li>&#8220;\u0394S_surroundings = \u2212\u0394solH\/T, so an endothermic \u0394solH gives a negative surroundings term.&#8221;<\/li>\n<li>&#8220;Down the group both the lattice energy and the hydration enthalpy become less exothermic; the trend in \u0394solH depends on which changes more.&#8221;<\/li>\n<\/ul>\n<h3>Do not say<\/h3>\n<ul>\n<li>&#8220;The hydroxides dissolve because their lattice energy is small&#8221; (it is the rate of change down the group, not the size, that sets the trend).<\/li>\n<li>&#8220;\u0394S_system is always positive on dissolving&#8221; (small, highly charged ions order the water).<\/li>\n<li>&#8220;Barium sulfate is insoluble because it is unreactive.&#8221;<\/li>\n<\/ul>\n<h3>Watch for<\/h3>\n<ul>\n<li>A question that gives \u0394solH and \u0394S_system for a salt and asks whether it dissolves at 298 K: convert \u0394solH to joules, work out \u0394S_surroundings and add.<\/li>\n<li>A question that asks why an endothermic salt dissolves more at higher temperature: the surroundings term gets smaller.<\/li>\n<li>A pair of trends given in one question, hydroxides and sulfates, with the answer hinging on the size of the anion.<\/li>\n<\/ul>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Explaining a Solubility Trend<\/h2>\n<p>Explain the direction of a solubility trend for a series of salts not on this page from how the lattice energy and the hydration enthalpy change down the group.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1121\" class=\"h5p-iframe\" data-content-id=\"1121\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Entropy and Energetics Explain: Explaining a Solubility Trend\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check how solubility is predicted and why the Group 2 trends run opposite ways.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why does barium sulfate not dissolve?<\/h3>\n<p>Because the hydration enthalpies of Ba\u00b2\u207a and SO\u2084\u00b2\u207b are far too small to pay for breaking the lattice. The barium ion is large, so its hydration enthalpy is weak, but the sulfate ion is also large, so the lattice term hardly changes down the group. The enthalpy of solution becomes markedly positive and the entropy gain cannot rescue it.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why do the Group 2 hydroxides get more soluble down the group but the sulfates get less soluble?<\/h3>\n<p>The two terms fall at different rates. With a small anion such as OH\u207b the lattice term falls quickly down the group as the cation grows, faster than the hydration enthalpy falls, so dissolving gets easier. With the large SO\u2084\u00b2\u207b ion the lattice term is already small and barely changes, while the hydration enthalpy of the cation still falls, so dissolving gets harder.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Do I need both the enthalpy and the entropy of solution to predict solubility?<\/h3>\n<p>Yes. Dissolving is feasible when the total entropy change is positive. The enthalpy of solution decides the entropy change of the surroundings (\u2212\u0394solH\/T) and the entropy of solution is the change in the system. A salt with a positive enthalpy of solution can still dissolve if its entropy of solution is large and positive.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why can the entropy change of solution be negative?<\/h3>\n<p>Because small, highly charged ions organise the water molecules around themselves into ordered hydration shells. That loss of freedom for the water can outweigh the gain from breaking up the lattice, so for ions such as Mg\u00b2\u207a the entropy of solution is negative even though the lattice has been dispersed.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is solubility not simply decided by the sign of the enthalpy of solution?<\/h3>\n<p>Because the entropy term also matters, and it becomes more important as the temperature rises. A salt with \u0394solH slightly positive dissolves if the entropy gain is large enough, and one with \u0394solH slightly negative may still be only sparingly soluble if the entropy change is negative. The prediction must use the total entropy change.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Topic 12 Entropy and Energetics Pages<\/h2>\n<p>Use these pages to connect the ideas across Topic 12 Entropy and Energetics and the rest of the course.<\/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-12-entropy-and-energetics\/entropy-and-the-direction-of-change\/\">Entropy and the Direction of Change<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/calculating-entropy-changes\/\">Calculating Entropy Changes<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/feasibility-gibbs-energy-and-temperature\/\">Feasibility and Temperature<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/thermodynamic-and-kinetic-stability\/\">Thermodynamic and Kinetic Stability<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-and-born-haber-cycles\/\">Lattice Energy and Born\u2013Haber Cycles<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-trends-and-covalent-character\/\">Lattice Energy Trends and Covalent Character<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/enthalpy-of-solution-and-hydration\/\">Enthalpy of Solution and Hydration<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/\">Topic 12 Entropy and Energetics Overview<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/\">Topic 8B Groups 1 and 2<\/a>\n<\/div>\n<\/section>\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. 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-12-entropy-and-energetics\/predicting-solubility\/#webpage\",\n      \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/predicting-solubility\/\",\n      \"name\": \"Predicting Solubility | Topic 12 Entropy and Energetics | Online Learning System\",\n      \"description\": \"Edexcel International A Level Chemistry revision notes on predicting solubility from enthalpy and entropy changes of solution, with the Group 2 hydroxide and sulfate trends explained.\",\n      \"isPartOf\": {\n        \"@id\": 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