Required Practical: Making a Soluble Salt

This lesson covers AQA Required Practical 1 (Chemistry) — preparing a pure, dry sample of a soluble salt from an insoluble oxide or carbonate and a dilute acid, as specified in the AQA GCSE Combined Science Trilogy specification (8464).

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Aim

To prepare a pure, dry sample of a soluble salt by reacting an insoluble base (e.g. copper oxide) with a dilute acid (e.g. sulfuric acid).

Context

In this practical, you make copper sulfate crystals from copper oxide and dilute sulfuric acid:

$\text{CuO}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{CuSO}_4(aq) + \text{H}_2\text{O}(l)$CuO(s)+H2​SO4​(aq)→CuSO4​(aq)+H2​O(l)

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Apparatus

Item	Purpose
Dilute sulfuric acid (H₂SO₄)	The acid reactant
Copper oxide powder (CuO)	The insoluble base
Beaker (250 cm³)	To carry out the reaction
Bunsen burner, tripod, gauze, heatproof mat	To warm the acid
Glass stirring rod	To stir the mixture
Filter funnel and filter paper	To remove excess solid
Conical flask or beaker	To collect the filtrate
Evaporating basin	To evaporate water from the solution
Water bath	For gentle, controlled evaporation

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Method

flowchart TD
    A["1. Warm 25 cm³ dilute<br/>sulfuric acid in a beaker"] --> B["2. Add copper oxide<br/>powder, one spatula<br/>at a time, stirring"]
    B --> C["3. Continue adding until<br/>copper oxide is in EXCESS<br/>(black powder remains<br/>undissolved)"]
    C --> D["4. Filter the mixture<br/>to remove excess<br/>copper oxide"]
    D --> E["5. Pour the blue filtrate<br/>into an evaporating basin"]
    E --> F["6. Heat gently on a<br/>water bath until crystals<br/>start to form at the edges"]
    F --> G["7. Leave to cool and<br/>crystallise slowly"]
    G --> H["8. Pat dry between<br/>sheets of filter paper"]

    style A fill:#1565c0,color:#fff
    style D fill:#2e7d32,color:#fff
    style F fill:#e65100,color:#fff
    style H fill:#7b1fa2,color:#fff

Step-by-Step Detail

1. Warm the acid — Gently warm about 25 cm³ of dilute sulfuric acid in a beaker using a Bunsen burner. Warming increases the rate of reaction.

2. Add the base in excess — Add copper oxide powder one spatula at a time, stirring after each addition. The black copper oxide reacts and dissolves, turning the solution blue (copper sulfate).

3. Recognise excess — Keep adding copper oxide until some black powder remains undissolved at the bottom of the beaker. This means all the acid has reacted — there is no remaining acid to contaminate the salt.

4. Filter — Set up a filter funnel and filter paper in a conical flask. Pour the mixture through the filter. The excess copper oxide (residue) is trapped on the filter paper. The blue copper sulfate solution (filtrate) passes through.

5. Evaporate — Pour the filtrate into an evaporating basin. Heat gently on a water bath (NOT a direct flame) until crystals begin to appear at the edges of the liquid.

6. Crystallise — Remove from the heat and leave the solution to cool slowly. Slow cooling produces larger, more regular crystals.

7. Dry — Pat the crystals gently between sheets of filter paper to absorb remaining water.

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Why Use an Insoluble Base?

An insoluble base (such as CuO or MgO) is used because you can see when it is in excess — the unreacted solid settles at the bottom. With a soluble base (like NaOH), the excess dissolves and you cannot tell when all the acid has been used up.

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Why Use a Water Bath?

• Provides gentle, even heating.
• Prevents the solution from spitting or boiling dry.
• Avoids decomposition of the salt.
• Maximum temperature is limited to 100°C (the boiling point of water).

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Key Safety Points

Hazard	Precaution
Dilute sulfuric acid (irritant)	Wear safety goggles; avoid skin contact
Hot apparatus	Use tongs; be aware of hot beakers and evaporating basins
Copper oxide (harmful if inhaled)	Avoid creating dust; do not inhale

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Expected Results

• Colour change: The solution turns from colourless (acid) to blue (copper sulfate) as copper oxide dissolves.
• Excess indicator: Black copper oxide remains undissolved.
• Crystals: Blue copper sulfate crystals form on cooling.

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Common Mistakes

Mistake	Correction
Not adding enough base	If the base is not in excess, the salt will be contaminated with unreacted acid
Heating too strongly	Always use a water bath — direct flames can cause spitting or decompose the salt
Evaporating all the water	Stop heating when crystals appear at the edges; then leave to cool for slow crystallisation
Forgetting to filter	The excess solid must be removed by filtration before evaporation

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Adapting the Method for Other Salts

Salt Required	Acid	Insoluble Base
Copper sulfate	Sulfuric acid	Copper oxide
Magnesium chloride	Hydrochloric acid	Magnesium oxide
Zinc sulfate	Sulfuric acid	Zinc oxide
Nickel nitrate	Nitric acid	Nickel oxide

Exam Tip: The method always follows the same pattern: warm acid → add insoluble base in excess → filter → evaporate on water bath → cool and crystallise → dry.

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Worked Example

Question: Describe how you would prepare pure, dry crystals of magnesium sulfate from magnesium oxide and dilute sulfuric acid.

Answer:

1. Warm dilute sulfuric acid in a beaker.
2. Add magnesium oxide powder, one spatula at a time, stirring until excess magnesium oxide remains undissolved.
3. Filter the mixture to remove excess magnesium oxide.
4. Pour the filtrate into an evaporating basin and heat gently on a water bath until crystals start to form at the edges.
5. Leave to cool slowly, then pat the crystals dry between filter paper.

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Summary