Making Soluble Salts
Revise Making Soluble Salts for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. The required practical makes a pure, dry salt by reacting excess insoluble base with acid, then filtering, evaporating and crystallising.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
This required practical prepares a pure, dry sample of a soluble salt from an insoluble base and an acid.
Warm the acid gently, then add the insoluble base a little at a time while stirring, until no more dissolves and some solid remains. Adding excess ensures all the acid has reacted, so none remains to contaminate the product.
Filter to remove the unreacted excess base. Then heat the filtrate gently in an evaporating basin until crystals begin to form, and leave the solution to cool so crystals grow. Finally filter and dry the crystals, ideally between filter papers or in a warm oven.
Revision notes
Reacting the acid
Warm the acid gently to speed up the reaction, then add the insoluble base a little at a time while stirring.
Continue until no more base dissolves and some solid remains, showing the base is in excess. This guarantees all the acid has reacted.
Removing the excess
Filter the mixture to remove the unreacted base.
The filtrate is a solution of the pure salt. If the excess were not removed, the final crystals would be contaminated with unreacted base.
Obtaining crystals
Heat the filtrate gently in an evaporating basin until crystals begin to form at the edges.
Stop heating and leave to cool so crystals grow slowly. Filter the crystals and dry them between filter papers or in a warm oven. Slow cooling gives larger, purer crystals.
Key points
- Warm the acid before adding the base.
- Add the base until it is in excess.
- Excess base ensures all the acid has reacted.
- Filter to remove the unreacted base.
- Evaporate until crystals begin to form.
- Leave to cool so crystals grow slowly.
Worked examples
Example 1
Explain why the insoluble base is added in excess. [2 marks]
Model answer
Adding excess ensures that all of the acid has reacted (1 mark), so none remains to contaminate the salt produced (1 mark).
Example 2
Explain why the mixture is filtered before evaporation. [2 marks]
Model answer
Filtering removes the unreacted excess base (1 mark), which would otherwise contaminate the final crystals (1 mark).
Example 3
Explain why heating is stopped when crystals first appear. [2 marks]
Model answer
Heating to dryness could decompose the salt or produce very small crystals (1 mark), so the solution is left to cool slowly instead, allowing larger and purer crystals to form (1 mark)).
Common mistakes
Not adding the base in excess.
Unreacted acid would remain and contaminate the product.
Forgetting to filter.
The excess base must be removed before evaporation.
Evaporating to dryness.
Stop when crystals first appear and let the solution cool.
Drying crystals with a Bunsen flame.
Dry them between filter papers or in a warm oven.
Exam tips
- Describe the method as an ordered sequence of steps.
- Explain why excess base is used and why filtering matters.
- Stop heating when crystals first appear.
- Mention slow cooling for larger, purer crystals.
Key terms
- Soluble salt
- A salt that dissolves in water.
- Excess
- More than enough of a reactant to use up the other completely.
- Filtrate
- The liquid passing through the filter paper.
- Crystallisation
- Forming crystals by evaporating a solution.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.