AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to describe how to make pure, dry samples of named…
Full specification wording
Soluble salts: Students should be able to describe how to make pure, dry samples of named soluble salts from information provided.
To make a named soluble salt, choose a dilute acid and an insoluble reactant (base, carbonate, or metal) that supply the required ions.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 4 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
To make a named soluble salt, choose a dilute acid and an insoluble reactant (base, carbonate, or metal) that supply the required ions. For copper(II) sulfate, warm dilute sulfuric acid and add insoluble copper(II) oxide until it is in excess, ensuring all the acid reacts. Filter off the unreacted solid oxide to leave a solution containing only the salt and water. Evaporate some water to reach saturation, cool to crystallise, filter, wash with a little cold distilled water and dry gently. The excess solid ensures no unreacted acid remains, and filtration removes the excess solid, giving a pure salt. The same pattern works for other named salts, such as magnesium sulfate from magnesium oxide and sulfuric acid.
What earns the marks
- Select reactants that provide the required cation and anion, using an insoluble oxide, hydroxide, carbonate or metal with a dilute acid.
- Warm the dilute acid and add the insoluble solid until it is in excess, so the acid is fully neutralised.
- Filter the mixture to remove unreacted excess solid and obtain a clear salt solution.
- Evaporate some water, cool to crystallise, then filter, wash with cold distilled water and dry gently.
- Name the salt correctly and link its formula to the ions supplied by the chosen reactants.
- Explain that excess solid guarantees the acid is used up, improving purity of the product.
Common mistakes and how to put them right
Adding too little solid so acid remains.
Put it right: The salt would be contaminated with acid, so the solid must be added until no more dissolves.
Choosing two soluble reactants (like NaOH and HCl) and adding an excess of one.
Put it right: The excess soluble reactant cannot be filtered off and will contaminate the target salt. An insoluble reactant must be used, or a titration performed.
Forgetting to filter before crystallising.
Put it right: Unreacted solid would contaminate the crystals, so filtration must remove it first.
Exam tips
- State the named salt and give the two reactants with their formulae.
- Use the words excess, filter, crystallise, wash and dry in a logical order.
- Explain the purpose of each key step rather than listing steps only.
Try the lesson
The start of the Soluble salts: Students should be able to describe how to make pure, dry samples of named soluble salts from information provided. lesson, made from checked teaching notes for this specification point.
Have a go first
A student wants to make pure, dry copper sulfate crystals from copper oxide and dilute sulfuric acid. Which step should be carried out first?
Listen
To make a named soluble salt, choose a dilute acid and an insoluble reactant (base, carbonate, or metal) that supply the required ions. For copper(II) sulfate, warm dilute sulfuric acid and add insoluble copper(II) oxide until it is in excess, ensuring all the acid reacts. Filter off the unreacted solid oxide to leave a solution containing only the salt and water. Evaporate some water to reach saturation, cool to crystallise, filter, wash with a little cold distilled water and dry gently. The excess solid ensures no unreacted acid remains, and filtration removes the excess solid, giving a pure salt. The same pattern works for other named salts, such as magnesium sulfate from magnesium oxide and sulfuric acid. A strong answer earns marks for these points. Select reactants that provide the required cation and anion, using an insoluble oxide, hydroxide, carbonate or metal with a dilute acid. Warm the dilute acid and add the insoluble solid until it is in excess, so the acid is fully neutralised. Filter the mixture to remove unreacted excess solid and obtain a clear salt solution. Evaporate some water, cool to crystallise, then filter, wash with cold distilled water and dry gently. Exam tip: State the named salt and give the two reactants with their formulae.
See it
Put it in order
Put these steps in the order they happen.
- 1. Filter, wash with cold distilled water and dry gently
- 2. Cool to crystallise
- 3. Add insoluble solid until it is in excess
- 4. Evaporate some water to reach saturation
- 5. Choose dilute acid and insoluble reactant supplying required ions
- 6. Warm the dilute acid
- 7. Filter off unreacted excess solid to leave salt solution
Key words
Say what each word means before you turn the card.
excess
Try answering aloud or in your head before turning the card.
crystallise
Try answering aloud or in your head before turning the card.
purity
Try answering aloud or in your head before turning the card.
See every definition
- excess
- Adding the insoluble solid until it is in excess means the acid is fully neutralised and no unreacted acid remains.
- crystallise
- After evaporating some water, cooling allows the salt to crystallise before it is filtered, washed and dried gently.
- purity
- The excess solid guarantees the acid is used up and filtration removes the excess solid, giving a pure salt.
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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