Making Soluble Salts

This lesson covers how to prepare soluble salts from acids and insoluble bases (or insoluble carbonates), as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the practical method, why an excess of the insoluble substance is used, and the key steps of filtration, evaporation and crystallisation.

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Why Make Salts?

Salts are essential chemical compounds with a wide range of uses:

• Sodium chloride (NaCl) — flavouring and preserving food
• Copper sulfate (CuSO₄) — fungicide, electroplating
• Ammonium nitrate (NH₄NO₃) — fertiliser
• Calcium sulfate (CaSO₄) — plaster casts

In the laboratory, you can make a pure, dry sample of a soluble salt by reacting an acid with an insoluble base or an insoluble carbonate.

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Choosing the Right Reactants

To make a particular salt, you need to choose the correct acid and base:

Salt Required	Acid	Base (Insoluble)
Copper sulfate	Sulfuric acid (H₂SO₄)	Copper oxide (CuO) or copper carbonate (CuCO₃)
Zinc chloride	Hydrochloric acid (HCl)	Zinc oxide (ZnO) or zinc carbonate (ZnCO₃)
Magnesium sulfate	Sulfuric acid (H₂SO₄)	Magnesium oxide (MgO) or magnesium carbonate (MgCO₃)
Nickel nitrate	Nitric acid (HNO₃)	Nickel oxide (NiO) or nickel carbonate (NiCO₃)

Exam Tip: You must use an insoluble base or carbonate for this method. If the base were soluble (like NaOH), you would not know when the reaction is complete because the excess would dissolve.

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The Method — Step by Step

1. Warm the Acid

Gently warm a measured volume of dilute acid in a beaker using a Bunsen burner or water bath. Warming speeds up the reaction.

2. Add Excess Insoluble Base or Carbonate

Add the insoluble base (e.g. copper oxide) in small portions to the warm acid, stirring after each addition. Continue adding until the base is in excess — you will know it is in excess when some solid remains undissolved at the bottom of the beaker after stirring.

3. Why Use an Excess?

Using an excess of the insoluble reactant ensures that all the acid has reacted. If any acid remained, the salt would be impure (contaminated with acid). The excess solid is removed in the next step.

4. Filter

Pour the mixture through filter paper in a funnel into a conical flask or evaporating basin. The filter paper traps the excess insoluble solid (the residue), and the dissolved salt solution passes through as the filtrate.

5. Evaporate

Transfer the filtrate to an evaporating basin. Heat gently over a water bath (not a direct flame, to avoid spitting) to evaporate some of the water. Stop heating when crystallisation begins at the edge of the solution.

6. Crystallise

Leave the saturated solution to cool slowly. As it cools, crystals of the salt will form. Slow cooling produces larger, more regular crystals.

7. Dry

Pat the crystals dry between sheets of filter paper or leave them in a warm place (e.g. a drying oven at low temperature).

graph TD
    A["1. Warm the dilute acid"] --> B["2. Add insoluble base/carbonate<br/>in excess (solid remains)"]
    B --> C["3. Filter to remove<br/>excess solid"]
    C --> D["4. Evaporate filtrate<br/>gently on a water bath"]
    D --> E["5. Leave to crystallise<br/>(cool slowly)"]
    E --> F["6. Pat dry with<br/>filter paper"]

    style A fill:#2980b9,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#8e44ad,color:#fff
    style E fill:#c0392b,color:#fff
    style F fill:#2c3e50,color:#fff

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Example: Making Copper Sulfate Crystals

Word Equation

copper oxide + sulfuric acid → copper sulfate + water

Symbol Equation

CuO + H₂SO₄ → CuSO₄ + H₂O

Observations

Stage	Observation
Adding CuO to warm H₂SO₄	Black copper oxide powder dissolves; the solution turns blue
Excess reached	Black powder remains undissolved at the bottom
After filtering	A clear blue solution is collected (filtrate); black residue remains on the filter paper
Evaporation and crystallisation	Blue crystals of copper sulfate form

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Example: Making Zinc Chloride

Word Equation

zinc oxide + hydrochloric acid → zinc chloride + water

Symbol Equation

ZnO + 2HCl → ZnCl₂ + H₂O

Observations

• Zinc oxide is a white powder that dissolves in the acid.
• The solution remains colourless (zinc chloride is a white/colourless salt).
• Excess zinc oxide remains as white undissolved solid.

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Using a Carbonate Instead of an Oxide

You can also use an insoluble carbonate instead of an oxide. The reaction produces the same salt plus water plus carbon dioxide (observed as fizzing/effervescence).

Example — Copper Carbonate + Sulfuric Acid

copper carbonate + sulfuric acid → copper sulfate + water + carbon dioxide

CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂

When adding the carbonate, you will see fizzing (due to CO₂ being released). The fizzing stops when all the acid has reacted or when the carbonate is in excess.

Exam Tip: When the question says "describe how to make a pure, dry sample of a soluble salt," you must include: (1) using an excess of the insoluble reactant, (2) filtering to remove the excess, (3) evaporating the filtrate, (4) crystallising, and (5) drying the crystals. Missing any step will lose marks.

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Solubility Rules — Which Salts Are Soluble?

Salt Type	Soluble?	Exceptions
Sodium, potassium and ammonium salts	Always soluble	None
Nitrates	Always soluble	None
Chlorides	Usually soluble	Lead chloride and silver chloride are insoluble
Sulfates	Usually soluble	Barium sulfate, calcium sulfate and lead sulfate are insoluble
Carbonates	Usually insoluble	Sodium, potassium and ammonium carbonates are soluble
Hydroxides	Usually insoluble	Sodium and potassium hydroxides are soluble; calcium hydroxide is slightly soluble