Reactions of Acids » Required Practical: Making a Pure Dry Salt

The practical you need to know

This is the first of the required practicals in AQA GCSE Chemistry. You may be asked about it in the exam even if you never did it yourself, so you need to know the method well enough to describe it, explain each step and spot mistakes in someone else's method.

The spec asks you to make a pure, dry sample of a soluble salt from an insoluble oxide or carbonate. You use a Bunsen burner to heat the dilute acid, then a water bath or electric heater to evaporate the solution.

The usual version, and the one most exam questions use, is making copper(II) sulfate from copper(II) oxide and dilute sulfuric acid:

copper(II) oxide + sulfuric acid → copper(II) sulfate + water

CuO + H₂SO₄ → CuSO₄ + H₂O

It is a good practical to learn because the colours tell you what is happening. Copper(II) oxide is a black powder. Copper(II) sulfate solution is blue. The crystals you finish with are blue too.

Key terms:

• Water bath: a container of hot water that a dish sits in, so the dish is heated gently and evenly, never above about 100 °C.
• Evaporating basin: a shallow dish that a solution is heated in, so water can evaporate from its wide surface.
• Hazard: something that could cause harm, such as a hot object or a chemical that irritates the skin.
• Risk: the chance that a hazard will actually cause harm, which you lower by working safely.

Apparatus

Part of this practical is choosing the right equipment for each job. Here is what you need and why.

Method

Here is the method as numbered steps. Learn the order and the reason for each step, because 6-mark questions reward both.

1. Put on eye protection. Measure about 40 cm³ of dilute sulfuric acid with a measuring cylinder and pour it into a beaker.
2. Set the beaker on a gauze on a tripod and warm the acid gently with a Bunsen burner. Do not let it boil. Warm acid reacts with the oxide faster.
3. Turn off the Bunsen burner. Add copper(II) oxide a little at a time using a spatula, stirring with a glass rod after each addition. The black powder disappears and the solution turns blue.
4. Keep adding until some black powder stays in the beaker even after stirring. This shows the copper(II) oxide is in excess and all the acid has reacted.
5. Leave the beaker to cool a little, then filter the mixture into a conical flask. The excess black copper(II) oxide stays in the filter paper. The blue filtrate is copper(II) sulfate solution.
6. Pour the filtrate into an evaporating basin. Heat it on a water bath or electric heater to evaporate some of the water, until roughly half the solution is left or crystals start to form at the edge.
7. Take the basin off the heat and leave it in a cool place until crystals form slowly as the water evaporates.
8. Remove the crystals and pat them dry between two pieces of filter paper.

Why two different heaters?

This is one of the most common questions on this practical, and the spec names both heaters, so make sure you can explain it.

A water bath cannot get hotter than the boiling point of water, which is what makes it so gentle. An electric heater does a similar job with no naked flame.

Working safely

Exam questions often ask for a hazard and how to reduce the risk. Always pair the two.

• Dilute sulfuric acid can irritate the eyes and skin. Wear eye protection and wash any splashes off with plenty of water.
• Copper(II) oxide and copper(II) sulfate are harmful if swallowed and can irritate the eyes and skin. Wear eye protection, do not eat or drink in the lab, and wash your hands afterwards.
• Hot equipment and liquids can burn. Let the beaker cool before filtering and use tongs or wait before touching the evaporating basin.
• The Bunsen burner flame can set hair or clothing alight. Tie back long hair, stand up while heating and turn the burner off when you are not using it.
• Spitting during evaporation can throw hot solution out. This is another reason to use a water bath or electric heater and to stop before the basin is dry.

There are no variables to test

Most required practicals change one thing and measure another. This one is different. You are not testing anything, you are making a product. So you will not be asked for an independent or dependent variable here. Instead, questions focus on the method, the reasons for each step, the choice of apparatus and safety.

You might be asked how to get a better result. Larger crystals form when the solution cools and evaporates slowly, so leave it longer in a cool place rather than speeding it up.

Exam-style question

A student wrote this method to make copper(II) sulfate crystals.

1. Pour dilute sulfuric acid into a beaker.
2. Add one spatula of copper(II) oxide and stir.
3. Filter the mixture.
4. Heat the filtrate strongly with a Bunsen burner until all the water has gone.

Suggest three improvements to the method and give a reason for each. (6 marks)