AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Soluble salts: Salt solutions can be crystallised to produce solid salts.
Crystallisation recovers a dissolved salt as a solid.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 3 of 5
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Crystallisation recovers a dissolved salt as a solid. A salt solution contains water plus dissolved ions, for example sodium chloride from hydrochloric acid neutralised by sodium hydroxide. Gently warming evaporates some water, raising the concentration until the solution becomes saturated. Cooling then lowers solubility for many salts, so crystals form; the solid is separated by filtration, washed with a little cold distilled water to remove soluble impurities, and dried between filter papers or in a warm oven. Strong heating must be avoided because it can drive off water of crystallisation or decompose the salt. The method works because solubility usually falls as temperature falls, so the dissolved salt leaves the solution as an ordered solid lattice.
What earns the marks
- A solution is warmed to evaporate some solvent and produce a saturated or nearly saturated solution.
- The concentrated solution is left to cool so that crystals of the salt form.
- Crystals are separated from the remaining solution by filtration.
- Crystals are washed with a small volume of cold distilled water to remove soluble impurities.
- Crystals are dried gently, for example between filter papers or in a warm oven, not by strong direct heating.
- The method relies on the solubility of the salt decreasing as the temperature falls.
Common mistakes and how to put them right
Evaporating the solution to complete dryness over a strong flame.
Put it right: This can decompose the salt or remove water of crystallisation, so the solution should be evaporated only until crystals begin to appear and then cooled.
Washing crystals with hot water.
Put it right: This redissolves much of the product, so cold distilled water should be used sparingly.
Describing filtration as the step that forms crystals.
Put it right: Filtration only separates crystals already formed, so crystallisation must be explained as the cooling and saturation step.
Exam tips
- Name the salt and the solution it comes from so the answer is specific rather than generic.
- Sequence the steps clearly: evaporate some water, cool, filter, wash, dry.
- Explain why each step is done, especially why gentle heating and cold washing are used.
Try the lesson
The start of the Soluble salts: Salt solutions can be crystallised to produce solid salts. lesson, made from checked teaching notes for this specification point.
Have a go first
A student has a solution of copper sulfate in water and wants to obtain solid copper sulfate crystals. Which process should the student use?
Listen
Crystallisation recovers a dissolved salt as a solid. A salt solution contains water plus dissolved ions, for example sodium chloride from hydrochloric acid neutralised by sodium hydroxide. Gently warming evaporates some water, raising the concentration until the solution becomes saturated. Cooling then lowers solubility for many salts, so crystals form; the solid is separated by filtration, washed with a little cold distilled water to remove soluble impurities, and dried between filter papers or in a warm oven. Strong heating must be avoided because it can drive off water of crystallisation or decompose the salt. The method works because solubility usually falls as temperature falls, so the dissolved salt leaves the solution as an ordered solid lattice. A strong answer earns marks for these points. A solution is warmed to evaporate some solvent and produce a saturated or nearly saturated solution. The concentrated solution is left to cool so that crystals of the salt form. Crystals are separated from the remaining solution by filtration. Crystals are washed with a small volume of cold distilled water to remove soluble impurities. Exam tip: Name the salt and the solution it comes from so the answer is specific rather than generic.
See it
Put it in order
Put these steps in the order they happen.
- 1. Crystals form
- 2. Dry gently between filter papers or in a warm oven
- 3. Separate crystals by filtration
- 4. Salt solution
- 5. Warm gently to evaporate some water
- 6. Wash with a little cold distilled water
- 7. Solution becomes saturated
- 8. Leave to cool
Key words
Say what each word means before you turn the card.
Crystallisation
Try answering aloud or in your head before turning the card.
Filtration
Try answering aloud or in your head before turning the card.
See every definition
- Crystallisation
- The process in which a salt solution is warmed to evaporate some water, raising the concentration until saturated, then cooled so that crystals of the salt form.
- Filtration
- The step that separates crystals already formed from the remaining solution.
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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