Required Practical: Making Salts — AQA GCSE Chemistry

This required practical prepares a pure, dry sample of a soluble salt from an acid and an insoluble base.

The reaction

An acid reacts with an insoluble base (a metal oxide or carbonate) to make a soluble salt and water: acid + base → salt + water

For example: sulfuric acid + copper oxide → copper sulfate + water.

Method

1. Warm the dilute acid (do not boil) in a beaker.
2. Add the insoluble base (e.g. copper oxide) a little at a time, stirring, until no more reacts — the base is in excess (some unreacted solid remains and the colour stops changing).
3. Filter the mixture to remove the excess solid base — the filtrate is the salt solution.
4. Pour the solution into an evaporating basin and heat gently to evaporate some water, until the crystallisation point (crystals start to form at the edge).
5. Leave the solution to cool and crystallise slowly, then dry the crystals (e.g. by patting with filter paper).

Why each step matters

• Excess base ensures all the acid reacts (so no acid is left in the salt).
• Filtering removes the unreacted solid.
• Slow crystallisation gives good, pure crystals.

Exam tips

• Use excess base so all the acid is used up, then filter off the excess.
• Crystallise to get the pure, dry salt (don't evaporate to dryness rapidly).
• Know which salt forms from which acid (sulfate, chloride, nitrate).
• Explain the purpose of each step.