Core practical: preparing hydrated copper sulfate crystals Revision Notes | GCSE Edexcel Higher Chemistry

1. Why Making Pure Salts Matters

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.

2. The Crystallisation Process

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.

3. Making Pure Salt Samples - The Method

Worked example: Preparing Pure Copper Sulfate Crystals

Make pure, dry crystals of copper sulphate from copper oxide and sulfuric acid.

1. Add excess copper oxide to dilute sulfuric acid until no more dissolves — Excess ensures all acid is neutralised. Black powder stops dissolving when reaction is complete.
2. Filter the mixture to remove excess copper oxide — Filtration removes insoluble impurities, leaving clear blue solution.
3. Heat gently until solution is about half the original volume — Gentle heating prevents salt decomposing. Stop before crystals form.
4. Leave solution to cool slowly at room temperature — Slow cooling allows large, pure crystals to form as solubility decreases.
5. Filter crystals and wash with distilled water, then dry between filter papers — Washing removes surface impurities. Pat dry - don't heat or crystals lose water.

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.

4. Different Salts, Different Methods

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.

5. Calculating Salt Yields

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

1. Moles of CuO = $\frac{4.0}{79.5} = 0.0503$ mol — Mr of CuO = 63.5 + 16 = 79.5. Use n = m/Mr
2. From equation: 1 mol CuO produces 1 mol CuSO₄ — Coefficients are all 1, so mole ratio is 1:1
3. Moles of CuSO₄ = 0.0503 mol — Same number of moles due to 1:1 ratio
4. Mass = $0.0503 \times 159.5 = 8.0$ g — Mr of CuSO₄ = 63.5 + 32 + (4×16) = 159.5

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.

6. Exam Technique and Common Mistakes

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.