Making SaltsOxford AQA IGCSE Chemistry: Revision notes

Soluble salts can be made by reacting an acid with one of three types of reactant:

• A metal, e.g. acid + magnesium → salt + hydrogen
• An insoluble base (metal oxide or metal hydroxide), e.g. acid + copper oxide → salt + water
• An alkali, e.g. acid + sodium hydroxide → salt + water

The method used depends on whether the base is soluble or not.

When reacting an acid with a metal or an insoluble base, excess solid is used so all the acid reacts, leaving no acid behind to contaminate the salt.

1. Add excess metal (or insoluble base) to the acid, a little at a time, until no more reacts (effervescence/dissolving stops)
2. Filter off the unreacted excess solid
3. The filtrate is a solution of the pure salt in water
4. Crystallise the filtrate to obtain the solid salt

An alkali is soluble, so you cannot simply add excess and filter — there is no solid to filter off. Instead, an indicator is used to show exactly when the reaction is complete (neutralisation).

1. Add a few drops of indicator to a measured volume of alkali
2. Add acid from a burette, a little at a time, swirling, until the indicator shows the solution is neutral
3. Repeat without the indicator using the same volumes of acid and alkali, so the resulting salt solution is pure
4. Crystallise the solution to obtain the solid salt

Once a pure salt solution has been obtained (by either method), the solid salt is recovered by crystallisation:

1. Gently heat/evaporate the solution to remove some of the water and produce a saturated solution
2. Leave the saturated solution to cool, so that crystals of the salt form
3. Filter off the crystals and dry them (e.g. between filter paper)