Required Practical: Making Salts

This lesson covers the AQA Required Practical for preparing a pure, dry sample of a soluble salt from an insoluble oxide or carbonate, as specified in AQA GCSE Chemistry (Required Practical 1, specification 4.4.2). You must be able to describe the apparatus, method, safety precautions, and expected results. This practical is frequently examined and you should be confident with every step of the procedure.

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Aim

To prepare a pure, dry sample of a soluble salt by reacting an insoluble base (metal oxide or carbonate) with a dilute acid.

The most common example used in this practical is:

Copper oxide + sulfuric acid → copper sulfate + water

or

Zinc carbonate + hydrochloric acid → zinc chloride + water + carbon dioxide

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Apparatus and Materials

Item	Purpose
Beaker (250 cm3)	To hold the acid and carry out the reaction
Measuring cylinder (50 cm3)	To measure the volume of dilute acid
Dilute sulfuric acid (or dilute hydrochloric acid)	The acid reactant
Copper oxide (or zinc carbonate)	The insoluble base/carbonate reactant
Spatula	To add the solid to the acid
Glass stirring rod	To stir the mixture and aid dissolving
Bunsen burner, tripod, gauze, and heatproof mat	To warm the acid (gently)
Filter funnel and filter paper	To remove excess unreacted solid
Conical flask or beaker	To collect the filtrate
Evaporating basin	To evaporate water from the salt solution
Water bath (beaker of hot water)	To heat the evaporating basin gently
Crystallising dish or watch glass	Optional, for slow crystallisation

Exam Tip: You may be asked to draw or label the apparatus used in this practical. Make sure you can draw a filtration set-up (funnel with filter paper in a ring on a stand, with a beaker or flask underneath) and an evaporation set-up (evaporating basin on a water bath or tripod and gauze). Labelling every piece of apparatus gains marks.

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Method

Follow these steps carefully to prepare pure, dry copper sulfate crystals from copper oxide and dilute sulfuric acid:

Step 1: Warm the Acid

• Measure 25 cm3 of dilute sulfuric acid using a measuring cylinder and pour it into a beaker.
• Warm the acid gently using a Bunsen burner on a blue flame (or use a water bath). Do not boil the acid.
• Warming the acid increases the rate of reaction with the solid.

Step 2: Add the Insoluble Base

• Using a spatula, add one spatula measure of copper oxide powder to the warm acid.
• Stir the mixture with a glass stirring rod.
• The copper oxide (black powder) will react with the sulfuric acid and gradually dissolve, turning the solution blue (due to the formation of copper sulfate).

Step 3: Add Excess Solid

• Continue adding copper oxide, one spatula at a time, stirring after each addition.
• Keep adding until the copper oxide is in excess — you will see undissolved black powder remaining at the bottom of the beaker after stirring.
• This ensures that all the acid has been used up (neutralised). No unreacted acid remains in the solution.

graph TD
    A["Warm dilute<br/>sulfuric acid"] -->|"Add copper oxide<br/>+ stir"| B["Black powder<br/>dissolves, solution<br/>turns blue"]
    B -->|"Keep adding<br/>copper oxide"| C{"Does excess<br/>solid remain?"}
    C -->|"No"| B
    C -->|"Yes — all acid<br/>has reacted"| D["Filter the<br/>mixture"]
    D --> E["Blue filtrate<br/>= copper sulfate<br/>solution"]
    D --> F["Black residue<br/>= excess copper oxide<br/>(discarded)"]
    E --> G["Evaporate gently<br/>on water bath"]
    G --> H["Leave to crystallise<br/>at room temperature"]
    H --> I["Pat dry with<br/>filter paper"]

    style A fill:#e74c3c,color:#fff
    style D fill:#3498db,color:#fff
    style E fill:#2980b9,color:#fff
    style H fill:#27ae60,color:#fff
    style I fill:#f39c12,color:#fff

Step 4: Filter the Mixture

• Set up a filter funnel with filter paper folded into a cone, resting in the funnel on a ring and stand.
• Place a clean beaker or conical flask underneath to collect the filtrate.
• Pour the mixture through the filter paper.
• The excess copper oxide (black solid) is trapped by the filter paper as the residue.
• The blue copper sulfate solution passes through as the filtrate.

Step 5: Evaporate

• Pour the blue filtrate into an evaporating basin.
• Place the evaporating basin on a water bath (a beaker of hot water heated by a Bunsen burner).
• Heat gently until about half the water has evaporated. Do not boil dry — this produces tiny, poor-quality crystals.
• To check if enough water has evaporated, dip a cold glass rod into the solution. If crystals form on the rod, the solution is saturated and ready to crystallise.

Step 6: Crystallise

• Remove the evaporating basin from the heat and leave it to cool slowly at room temperature.
• As the solution cools, blue copper sulfate crystals will form.
• Slow cooling produces larger, more regular crystals. Rapid cooling or boiling dry produces small, irregular crystals.

Step 7: Dry the Crystals

• Carefully pour away any remaining liquid.
• Pat the crystals dry with filter paper.
• Leave to dry completely in a warm place or a desiccator.

Exam Tip: A common question asks "Why is excess copper oxide added?" The answer is: to ensure all the acid has reacted, so that the salt solution is not contaminated with unreacted acid. The excess solid is then removed by filtration. This reasoning is worth 2 marks in most mark schemes.

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Safety Precautions

Hazard	Precaution
Dilute sulfuric acid — irritant	Wear safety goggles; avoid skin contact; wash spills immediately
Copper oxide — harmful if inhaled	Avoid creating dust; add carefully with a spatula
Hot liquids — risk of scalding	Heat gently; do not boil; use a water bath rather than direct heating
Bunsen burner — fire risk	Tie back long hair; keep flammable materials away
Glassware — breakage risk	Handle carefully; report any breakage immediately

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Expected Results

Observation	Explanation
Black powder (copper oxide) dissolves in the acid	The neutralisation reaction is occurring
Solution turns blue	Copper sulfate (a blue salt) is forming in solution
Excess black powder remains undissolved	All the acid has been used up; copper oxide is in excess
After filtration, the filtrate is a clear blue solution	The pure copper sulfate solution has been separated from excess copper oxide
Blue crystals form on cooling	Copper sulfate is crystallising out of the saturated solution

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Variations of this Practical

The same method can be used with different combinations:

Acid	Insoluble base/carbonate	Salt produced	Colour of crystals
Dilute sulfuric acid	Copper oxide	Copper sulfate	Blue
Dilute hydrochloric acid	Copper oxide	Copper chloride	Green-blue
Dilute sulfuric acid	Zinc carbonate	Zinc sulfate	White/colourless
Dilute hydrochloric acid	Zinc carbonate	Zinc chloride	White/colourless
Dilute sulfuric acid	Magnesium oxide	Magnesium sulfate	White/colourless