What you'll learn
Making a pure, dry sample of a salt is one of the classic school chemistry practicals, and it brings together neutralisation, filtration and crystallisation in a single method. For AQA GCSE Chemistry you need to know how to prepare a soluble salt from an insoluble base and an acid, why an excess of base is used, and how to obtain pure, dry crystals at the end. This guide covers the reaction involved, the full method step by step, the reasoning behind each stage, and how to purify the salt by crystallisation. By the end you should be able to describe the practical precisely, explain each step, and identify the salt produced from a given acid and base.
Key terms and definitions
Salt — A compound formed when the hydrogen in an acid is replaced by a metal.
Base — A substance that neutralises an acid; an insoluble base is often a metal oxide, hydroxide or carbonate.
Neutralisation — The reaction between an acid and a base to form a salt and water.
Excess — More than enough of a reactant, used to make sure all of the other reactant is used up.
Filtration — Separating an insoluble solid from a liquid using filter paper.
Crystallisation — Forming crystals from a solution by evaporating some of the water.
Soluble — Able to dissolve in water.
Evaporation — Turning a liquid into a gas, used to remove water from a solution.
Core concepts
The reaction involved
A soluble salt can be made by reacting an acid with an insoluble base, such as a metal oxide, metal hydroxide or metal carbonate. The general reaction is:
acid + base → salt + water
For example, sulfuric acid reacting with copper oxide makes copper sulfate and water:
sulfuric acid + copper oxide → copper sulfate + water
The salt produced depends on the acid and the metal: hydrochloric acid makes chlorides, sulfuric acid makes sulfates, and nitric acid makes nitrates.
Why an insoluble base is used
Using an insoluble base is the key to this method. Because the base does not dissolve, you can simply keep adding it until no more reacts. Any leftover base stays as a solid and can be filtered off, leaving a pure salt solution. This is a neat way of making sure all the acid is neutralised without leaving any acid behind.
The method step by step
To make copper sulfate crystals from sulfuric acid and copper oxide:
- Measure a fixed volume of dilute sulfuric acid into a beaker and warm it gently (warming speeds up the reaction). Do not boil.
- Add copper oxide (the insoluble base) a little at a time, stirring, until it is in excess — that is, until no more dissolves and some solid copper oxide remains unreacted. This shows all the acid has been used up.
- Filter the mixture to remove the excess solid copper oxide. The filtrate is a pure solution of copper sulfate.
- Pour the filtrate into an evaporating basin and gently heat it to evaporate some of the water, until crystals just begin to form (the "point of crystallisation").
- Leave the solution to cool and crystallise slowly. As it cools, copper sulfate crystals form.
- Remove the crystals, and dry them by patting gently between sheets of filter paper or leaving them in a warm place.
Why an excess of base is added
Adding the base in excess makes sure that all the acid reacts. If you did not use excess, some acid might be left over, and the salt would be contaminated with acid. Because the excess base is insoluble, it can be filtered off easily, so it does not contaminate the salt. This is the whole reason an insoluble base is chosen.
Why the solution is not evaporated to dryness
When obtaining the crystals, you evaporate only some of the water and then let the rest cool slowly, rather than evaporating all the water quickly. Evaporating to dryness would drive off the water of crystallisation and could decompose the salt, giving a powder instead of proper crystals. Slow crystallisation produces larger, purer crystals.
Using a carbonate instead of an oxide
The same method works if you use a metal carbonate as the insoluble base instead of a metal oxide. The reaction then also produces carbon dioxide:
acid + metal carbonate → salt + water + carbon dioxide
For example, copper carbonate with sulfuric acid gives copper sulfate, water and carbon dioxide. You can tell the reaction is happening because it fizzes as carbon dioxide gas is given off, and you know it is complete when the fizzing stops and excess solid remains. The rest of the method — filtering off the excess, evaporating to the point of crystallisation, and cooling — is exactly the same.
Safety in the practical
This practical involves warming acid, so a few safety points matter. Wear eye protection because the acid is an irritant. Warm the acid gently and do not boil it, to avoid spitting. Take care when handling the hot evaporating basin, using tongs, and allow it to cool before touching it. Following these precautions keeps the practical safe while still producing good crystals.
Identifying the salt
The name of the salt tells you which acid and metal were used:
| Acid | Salt produced |
|---|---|
| Hydrochloric acid | Chloride |
| Sulfuric acid | Sulfate |
| Nitric acid | Nitrate |
So zinc oxide with hydrochloric acid gives zinc chloride, and magnesium oxide with nitric acid gives magnesium nitrate.
Worked examples
Example 1: Naming the salt
What salt is produced when copper oxide reacts with hydrochloric acid? Hydrochloric acid produces chlorides, and the metal is copper, so the salt is copper chloride.
Example 2: Explaining the excess
Explain why copper oxide is added until it is in excess. Adding copper oxide in excess makes sure that all the acid is neutralised, so no acid is left to contaminate the salt. Because copper oxide is insoluble, the unreacted excess can be filtered off easily.
Example 3: The purpose of filtration
State what is removed by filtration in this practical and why. Filtration removes the excess unreacted copper oxide, which is an insoluble solid. This leaves a pure copper sulfate solution (the filtrate) with no solid impurities.
Example 4: Getting good crystals
Explain why the solution is left to cool slowly rather than heated until dry. Heating until dry would remove the water of crystallisation and could decompose the salt, giving a powder. Cooling slowly allows larger, purer crystals to form as the salt comes out of solution.
Common mistakes and how to avoid them
A common error is not stating how you know the acid has been used up. The sign is that excess solid remains and no more dissolves — mention this, as it is a marking point.
Students often forget to filter before crystallising, or filter at the wrong stage. Filtration must come after adding excess base, to remove the leftover solid, and before evaporating.
Another mistake is saying to "evaporate all the water". You evaporate only some of the water to the point of crystallisation, then cool slowly. Evaporating to dryness spoils the crystals.
When naming salts, take care to match the acid to the correct ending: sulfuric → sulfate, hydrochloric → chloride, nitric → nitrate. Mixing these up loses easy marks.
Finally, remember to dry the crystals gently (between filter paper or in a warm place), not by strong heating, which could decompose them.
Exam technique for "Required practical: making salts"
Method questions expect the full sequence in order: warm the acid, add insoluble base until in excess, filter off the excess, evaporate to the point of crystallisation, then cool and dry the crystals. Missing or reordering a step loses marks.
Explanation questions almost always ask why excess base is used (to neutralise all the acid, then filter it off because it is insoluble) and why you crystallise slowly rather than evaporating to dryness. Learn both explanations.
Be ready to name the salt from a given acid and base, and to write a word equation. When describing how you know the reaction is complete, always state that excess solid remains undissolved. Use precise terms — filtrate, excess, crystallisation — throughout.
Quick revision summary
- A soluble salt is made from an acid and an insoluble base: acid + base → salt + water.
- Add the base in excess so all the acid reacts; you know it is complete when excess solid remains.
- Filter to remove the excess insoluble base, leaving a pure salt solution.
- Evaporate to the point of crystallisation, then cool slowly to form crystals — do not evaporate to dryness.
- Dry the crystals gently between filter paper or in a warm place.
- Salt names: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.