Question 1 (2 marks)
Method
Why add an excess insoluble metal oxide during salt preparation?
Show model answer / marking points
To ensure all acid reacts (1); leftover oxide can be removed by filtration (1).
Required Practical 08: Making Salts | AQA GCSE Science | The Science Department
About this practical and its aim
Aim
Prepare a pure, dry sample of a soluble salt from an insoluble oxide or carbonate and a dilute acid.
Introduction
In this practical, you get a chance to really put theory into practice. You will learn about the combinations of acids and either metal carbonates or metal oxides that can be used to make a specific soluble salt. Focus on the key steps of the process and ideas like why we potassium salts are made from potassium hydroxide rather than potassium metal.
The science behind the practical
Acid reacts with a suitable insoluble base to produce a salt and water (and carbon dioxide if a carbonate). Excess solid ensures no acid remains.
Key things to remember
Warm dilute acid where directed; add excess insoluble solid; filter; evaporate some solvent; crystallise and dry the crystals.
Equipment and apparatus
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Dilute acid
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insoluble metal oxide or carbonate
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beaker
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stirring rod
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filter funnel and paper
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conical flask
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evaporating basin
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water bath or heater
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balance
Method: How to carry out the experiment
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Add 25cm3 of sulfuric acid into a 100cm3 beaker
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Stir in 5g of copper oxide powder ensuring that it is still visible after stirring (in excess)
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Set up a filter paper and funnel inside a conical flask
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Pour the mixture through the funnel and collect the filtrate
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Pour the filtrate into an evaporating basin and heat gently. When the solution boils, remove the flame then return it when the boiling stops.
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Heat until crystals first appear around the edges then turn off the heat. It is critical that you leave plenty of solution in the basin and do not try to boil it dry.
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Leave the crystals to grow (cool and crystalise). You may remove some, when cool enough, using a spatula and pat them between two paper towels then place into an evaporating dish for next lesson.
Variables and fair testing
| Type of variable | What is measured or controlled? |
|---|---|
| Independent variable | Identity or amount of reactants (for an investigation) |
| Dependent variable | Mass, appearance or yield of dry salt crystals |
| Control variables | Acid concentration/volume; heating; drying method; reagent purity |
Safety and managing risks
Follow your teacher's instructions and your school's risk assessment. This page is not a substitute for a formal risk assessment.
| Hazard | Risk | Precaution |
|---|---|---|
Dilute acid and reagents |
Skin and eye irritation |
Use approved concentrations, eye protection and appropriate handling. |
Hot evaporating solution |
Burns or spitting |
Heat gently and never evaporate to dryness. |
Results, calculations and analysis
Recording, presenting and interpreting results
Record colour and appearance of reagents, filtrate and dry crystals; compare actual yield if measured.
Calculations
Possible extension: percentage yield = actual yield ÷ theoretical yield × 100.
Conclusions, accuracy and evaluation
Drawing a conclusion
Filtration removes excess unreacted insoluble solid; cooling a concentrated solution produces crystals.
Evaluating the investigation
Avoid boiling dry; wash/dry crystals appropriately; repeat using controlled heating.
More to learn
Separating an insoluble solid
Adding excess insoluble oxide ensures the acid is used up; filter out the excess before concentrating the filtrate.
Common exam mistake
Crystallisation involves partial evaporation followed by cooling; do not boil the solution dry.
GCSE examination practice
Answer each question in full before opening the model answer. Check your work against the marking points and look for scientific key words.
Question 2 (2 marks)
Method
Why is the solution not heated until completely dry?
Show model answer / marking points
Hot crystals/solution may spit (1), and controlled cooling allows crystals to form (1).
Question 3 (1 marks)
Knowledge
Name the process used to remove excess unreacted oxide.
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Filtration (1).
Question 4 (2 marks)
Evaluation
Suggest one improvement to make the investigation more reliable and explain why it helps.
Show model answer / marking points
Repeat measurements and calculate a mean (1), reducing the influence of random variation/anomalies (1). Alternative valid improvement specific to the experiment with explanation can gain both marks.
This page was updated on: 15th June 2026