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Crystallisation forms solid crystals from a solution by removing water, and slow cooling gives large, pure crystals. Free notes and 11 questions answered.
Crystallising Salt Solutions and Making Pure Samples is part of Reactions of acids in GCSE AQA Higher Chemistry.
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Crystallisation forms solid crystals from a solution by removing water, and slow cooling gives large, pure crystals. Free notes and 11 questions answered.
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Use this lesson to stay inside Reactions of acids while connecting the detail back to the wider Topic 4: Chemical changes topic.
Read the core explanations from Crystallising Salt Solutions and Making Pure Samples before testing yourself from memory.
You've made salt solutions in acid-base reactions, but how do you get the actual solid salt out? This isn't just academic - pharmaceutical companies need pure salts for medicines, and food manufacturers need pure sodium chloride for seasoning.
The challenge is removing all the water while keeping the salt pure. Too fast and you get impure crystals. Too slow and you waste time. There's a technique to getting it right.
Picture a beaker of salt solution. The salt particles are surrounded by water molecules, keeping them dissolved. As water evaporates, the solution becomes more concentrated. Eventually, there's not enough water to keep all the salt dissolved - that's when crystals start forming.
The key is controlling the speed. Slow crystallisation gives large, pure crystals. Fast crystallisation gives small, impure crystals that trap water and other substances.
Worked example: Preparing Pure Copper Sulfate Crystals
Make pure, dry crystals of copper sulphate from copper oxide and sulfuric acid.
Answer: Pure, dry copper sulphate crystals ready for use
Notice we used excess copper oxide, not excess acid. This ensures no unreacted acid remains in the final product. The excess solid is easily removed by filtration, but excess acid would contaminate the crystals.
The method depends on what reactants you're given. Here's how to choose your approach based on the information provided:
From metal + acid: Use excess metal, filter off unreacted metal, then crystallise. Example: zinc + hydrochloric acid → zinc chloride.
From metal oxide + acid: Use excess oxide, filter off unreacted oxide, then crystallise. Example: copper oxide + sulfuric acid → copper sulphate.
From alkali + acid: Use titration to find exact volumes, then mix those exact volumes and crystallise. Example: sodium hydroxide + nitric acid → sodium nitrate.
Exam questions often ask you to calculate the mass of salt produced. This requires stoichiometry - using the balanced equation to work out mass relationships.
Worked example: Calculating Mass of Salt Produced
Calculate the mass of copper sulphate produced when 4.0g of copper oxide reacts with excess sulfuric acid. CuO + H₂SO₄ → CuSO₄ + H₂O
Answer: 8.0 g of copper sulphate produced
Notice how the mass doubled - this is because copper sulphate has a much higher relative formula mass than copper oxide. The extra mass comes from the sulphate group.
Exam questions on salt preparation follow predictable patterns. Here's how to maximise your marks:
Common trap: Questions asking for sodium chloride preparation. You cannot use the standard method because sodium reacts violently with acid. Instead, use titration with sodium hydroxide and hydrochloric acid.
Master this method and you can make any soluble salt pure and dry. The key principles - excess insoluble reactant, filtration, gentle heating, slow cooling - apply whether you're making copper sulphate or zinc chloride.
Exam precision note: In Crystallising Salt Solutions and Making Pure Samples - Soluble salts - Reactions of acids, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
Crystallisation works because the amount of salt that remains dissolved depends on temperature and water content. After filtration, the filtrate contains the soluble salt and water. Gentle evaporation removes some water and makes the solution more concentrated. As the solution cools, it may become saturated and then crystallisation begins. Students should link each step to a purpose: excess insoluble base ensures all acid is neutralised, filtration removes the unreacted solid, evaporation concentrates the solution, and cooling allows crystals to form.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Crystallisation is the process of forming solid crystals from a solution by removing water through evaporation or cooling. As water is removed, the solution becomes more concentrated until there is not enough water to keep all the salt dissolved.
Sodium chloride is made by titration, because sodium reacts violently with acid and the excess-solid method cannot be used. Titration finds the exact volumes of sodium hydroxide and hydrochloric acid that react, then those volumes are mixed and the solution is crystallised.
Large, pure crystals form when crystallisation is slow, so the solution should be heated gently and then left to cool slowly at room temperature. Fast crystallisation gives small, impure crystals that trap water and other substances.
In making a pure salt, excess insoluble base ensures all the acid is neutralised, filtration removes the unreacted solid, evaporation concentrates the solution, and cooling allows crystals to form. The crystals are then washed with distilled water and dried between filter papers.
The mass of salt produced is calculated using moles and the mole ratio. For 4.0g of copper oxide with excess sulfuric acid: moles of CuO = 4.0 ÷ 79.5 = 0.0503 mol, the ratio is 1:1, so the mass of CuSO₄ = 0.0503 × 159.5 = 8.0 g.
Salt crystals are patted dry between filter papers rather than heated, because heating would make the crystals lose water. They are first washed with distilled water to remove surface impurities.
Excess solid is used instead of excess acid because the excess solid is easily removed by filtration, but excess acid would contaminate the crystals. Using excess copper oxide, for example, ensures no unreacted acid remains in the final product.
A salt is made from an alkali and an acid by using titration to find the exact volumes that react, then mixing those exact volumes and crystallising. For example, sodium hydroxide and nitric acid make sodium nitrate.
Crystals form as a salt solution cools because the amount of salt that stays dissolved depends on temperature and water content. As the solution cools, solubility decreases and the solution may become saturated, so the dissolved salt particles arrange themselves in a regular pattern.
The salt solution is heated gently to prevent the salt decomposing. Gentle evaporation removes some water and makes the solution more concentrated, and heating stops at about half the original volume, before crystals form.
The relative formula mass of copper sulphate, CuSO₄, is 159.5. The calculation is 63.5 + 32 + (4 × 16) = 159.5, which is about twice the Mr of copper oxide (79.5).