AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Soluble salts: Required practical activity 8: preparation of a pure, dry sample of a soluble salt from an insoluble oxide or carbonate, using a Bunsen burner to heat dilute acid and a water bath or electric heater to evaporate the solution.
This practical prepares a soluble salt from an insoluble oxide or carbonate.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This practical prepares a soluble salt from an insoluble oxide or carbonate. You warm dilute acid using a Bunsen burner, add the insoluble solid in excess, and stir until no more dissolves. Filter to remove unreacted solid, keeping the filtrate. Then crystallise the filtrate by gentle evaporation using a water bath or electric heater, not a direct Bunsen flame, to avoid spitting and decomposition. Cool, filter off crystals, and dry them between filter papers or in a warm place. For example, copper(II) oxide with dilute sulfuric acid gives blue copper(II) sulfate crystals. The method separates the soluble salt from insoluble impurities and controls evaporation to obtain a pure, dry sample.
What earns the marks
- Warm the dilute acid using a Bunsen burner to increase the rate of reaction safely before adding the solid.
- Add the insoluble oxide or carbonate in excess, a little at a time, until no more dissolves, so all the acid is neutralised.
- Filter the mixture to remove unreacted solid, collecting the filtrate containing the soluble salt.
- Evaporate the filtrate slowly using a water bath or electric heater to the point of crystallisation, then cool to form crystals.
- Separate crystals by filtration and dry them between filter papers or in a warm place to obtain a pure, dry sample.
Common mistakes and how to put them right
Heating the solution directly with a Bunsen flame during evaporation.
Put it right: Use a water bath or electric heater for gentle evaporation.
Adding insufficient oxide or carbonate.
Put it right: Add the solid in excess until no more dissolves.
Evaporating to dryness before cooling.
Put it right: Evaporate to the point of crystallisation, then cool slowly.
Exam tips
- State clearly why excess solid is used: to ensure all the acid reacts, so the filtrate contains only the salt and water.
- Describe evaporation as gentle, using a water bath or electric heater, and explain that this prevents spitting or decomposition.
- When asked about purity, refer to removing unreacted solid by filtration and drying the crystals to remove water.
Try the lesson
The start of the Soluble salts: Required practical activity 8: preparation of a pure, dry sample of a soluble salt from an insoluble oxide or carbonate, using a Bunsen burner to heat dilute acid and a water bath or electric heater to evaporate the solution. lesson, made from checked teaching notes for this specification point.
Have a go first
During the preparation of a pure, dry sample of a soluble salt, a student uses a water bath to heat the salt solution. What is the purpose of using a water bath at this stage of the practical?
Listen
This practical prepares a soluble salt from an insoluble oxide or carbonate. You warm dilute acid using a Bunsen burner, add the insoluble solid in excess, and stir until no more dissolves. Filter to remove unreacted solid, keeping the filtrate. Then crystallise the filtrate by gentle evaporation using a water bath or electric heater, not a direct Bunsen flame, to avoid spitting and decomposition. Cool, filter off crystals, and dry them between filter papers or in a warm place. For example, copper(II) oxide with dilute sulfuric acid gives blue copper(II) sulfate crystals. The method separates the soluble salt from insoluble impurities and controls evaporation to obtain a pure, dry sample. A strong answer earns marks for these points. Warm the dilute acid using a Bunsen burner to increase the rate of reaction safely before adding the solid. Add the insoluble oxide or carbonate in excess, a little at a time, until no more dissolves, so all the acid is neutralised. Filter the mixture to remove unreacted solid, collecting the filtrate containing the soluble salt. Evaporate the filtrate slowly using a water bath or electric heater to the point of crystallisation, then cool to form crystals. A common slip: Heating the solution directly with a Bunsen flame during evaporation: this can cause spitting, loss of product, or decomposition. Instead, use a water bath or electric heater for gentle evaporation. Exam tip: State clearly why excess solid is used: to ensure all the acid reacts, so the filtrate contains only the salt and water.
See it
Put it in order
Put these steps in the order they happen.
- 1. Add insoluble oxide or carbonate in excess until no more dissolves
- 2. Filter mixture to remove unreacted solid
- 3. Keep the filtrate containing the soluble salt
- 4. Evaporate slowly using a water bath or electric heater to the point of crystallisation
- 5. Cool to form crystals
- 6. Separate crystals by filtration
- 7. Dry crystals between filter papers or in a warm place
- 8. Warm dilute acid using a Bunsen burner
Key words
Say what each word means before you turn the card.
excess
Try answering aloud or in your head before turning the card.
filtrate
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gentle evaporation
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point of crystallisation
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See every definition
- excess
- Added a little at a time until no more dissolves, so all the acid is neutralised.
- filtrate
- The liquid collected after filtering the mixture, containing the soluble salt.
- gentle evaporation
- Slow evaporation of the filtrate using a water bath or electric heater, not a direct Bunsen flame, to avoid spitting and decomposition.
- point of crystallisation
- The stage of evaporation reached before cooling, so the salt does not decompose or trap impurities.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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