Making Soluble Salts

This lesson covers the preparation of soluble salts for Edexcel GCSE Chemistry (1CH0). You need to understand how to name salts, describe the method for preparing a pure, dry sample of a soluble salt, and know the core practical for preparing copper sulfate crystals. This is a frequently examined topic.

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Naming Salts

A salt is a compound formed when the hydrogen in an acid is replaced by a metal (or ammonium). The name of the salt has two parts:

1. The first part comes from the metal (or base) used.
2. The second part comes from the acid used.

Acid Used	Produces Salts Called	Example
Hydrochloric acid (HCl)	Chlorides	Sodium chloride (NaCl), Copper chloride (CuCl₂)
Sulfuric acid (H₂SO₄)	Sulfates	Copper sulfate (CuSO₄), Zinc sulfate (ZnSO₄)
Nitric acid (HNO₃)	Nitrates	Calcium nitrate (Ca(NO₃)₂), Magnesium nitrate (Mg(NO₃)₂)

Examples of Naming

• Magnesium + hydrochloric acid → magnesium chloride + hydrogen
• Zinc + sulfuric acid → zinc sulfate + hydrogen
• Calcium carbonate + nitric acid → calcium nitrate + water + carbon dioxide

Exam Tip: A common question gives you a salt and asks which acid and metal/base you would use. Work backwards: copper sulfate → "sulfate" means sulfuric acid was used, "copper" means a copper compound was the base (e.g. copper oxide or copper carbonate).

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Method: Making a Soluble Salt from an Insoluble Base

The standard method for preparing a soluble salt involves reacting an excess insoluble base (or insoluble carbonate) with a warm dilute acid, then filtering and crystallising.

Why Use an Excess of the Base?

By adding an excess of the insoluble base, you ensure that all the acid has reacted. This means the final solution contains only the dissolved salt (and water), with no leftover acid. The unreacted excess base can be removed by filtration because it is insoluble.

Step-by-Step Method

graph TD
    A["1. Warm the dilute acid<br/>in a beaker"] --> B["2. Add the insoluble base<br/>(e.g. CuO) in small portions<br/>while stirring"]
    B --> C["3. Continue adding until<br/>the base is in EXCESS<br/>(some remains undissolved)"]
    C --> D["4. FILTER the mixture<br/>to remove excess<br/>unreacted base"]
    D --> E["5. Collect the FILTRATE<br/>(the salt solution)"]
    E --> F["6. Pour filtrate into an<br/>evaporating basin"]
    F --> G["7. Heat gently to evaporate<br/>some water, then leave<br/>to CRYSTALLISE slowly"]
    G --> H["8. Filter the crystals,<br/>rinse with cold distilled<br/>water, pat dry"]

    style A fill:#3498db,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#27ae60,color:#fff
    style G fill:#9b59b6,color:#fff
    style H fill:#e74c3c,color:#fff

Detailed Steps Explained

1. Warm the acid — Gently warm about 25 cm³ of dilute sulfuric acid in a beaker on a tripod and gauze with a Bunsen burner. Warming speeds up the reaction.
2. Add the base — Add copper oxide powder a spatula-tip at a time, stirring with a glass rod after each addition.
3. Identify the excess — You know the base is in excess when it stops dissolving and you can see unreacted black solid at the bottom of the beaker after stirring.
4. Filter — Set up filter paper in a funnel over a conical flask. Pour the mixture through. The excess copper oxide stays on the filter paper (the residue); the blue copper sulfate solution passes through (the filtrate).
5. Evaporate and crystallise — Pour the filtrate into an evaporating basin. Heat gently until about half the water has evaporated (crystals start forming at the edge). Remove from heat and leave to cool slowly — slow cooling produces larger, more regular crystals.
6. Dry the crystals — Filter the crystals, rinse with a small amount of cold distilled water, and dry by pressing between sheets of filter paper or leaving in a warm oven.

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Core Practical: Preparation of Copper Sulfate Crystals

This is a required practical for Edexcel GCSE Chemistry.

Reaction

Word equation: copper oxide + sulfuric acid → copper sulfate + water

Balanced symbol equation: CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

Observations During the Reaction

• The black copper oxide powder is added to the colourless sulfuric acid.
• As the reaction proceeds, the solution turns blue (due to dissolved Cu²⁺ ions).
• The black powder gradually disappears as it reacts.
• When excess copper oxide is added, some black powder remains undissolved at the bottom.

After Filtering and Crystallising

• The filtrate is a clear blue solution of copper sulfate.
• After slow evaporation, blue crystals of hydrated copper sulfate (CuSO₄· 5H₂O) form.

Key Points for Exam Answers

• Explain why the base is added in excess: to ensure all the acid has reacted.
• Explain how you know the base is in excess: the solid stops dissolving and remains visible after stirring.
• Explain why you filter: to remove the excess unreacted solid.
• Explain why you evaporate slowly: to allow crystals to form; rapid boiling gives tiny crystals and may spit dangerously.

Exam Tip: This practical is very frequently examined. You must be able to describe the full method, explain each step, and state the observations at each stage. A 6-mark extended writing question could ask you to describe the entire preparation of copper sulfate from copper oxide and sulfuric acid.

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Alternative Methods of Making Salts

Using a Metal Carbonate

You can also make a salt by reacting an acid with an insoluble metal carbonate. For example:

Word equation: copper carbonate + sulfuric acid → copper sulfate + water + carbon dioxide

Balanced symbol equation: CuCO₃(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l) + CO₂(g)

Observation: Fizzing/effervescence occurs (CO₂ gas released). The green copper carbonate dissolves and the solution turns blue.

The rest of the method (add excess, filter, evaporate, crystallise) is identical.

Using a Metal

Some salts can be made by reacting a metal directly with an acid:

Zinc + sulfuric acid → zinc sulfate + hydrogen

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

However, this is not always practical (some metals react too violently, and copper does not react with dilute acids).

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Solubility Rules and Choosing the Right Method

To make a particular salt, you first need to know whether the salt is soluble or insoluble in water. For Edexcel GCSE, the key solubility rules are:

Ion	Soluble or Insoluble?	Exceptions
Sodium, potassium and ammonium salts	Always soluble	None
Nitrates	Always soluble	None
Chlorides	Usually soluble	Lead chloride (PbCl₂) and silver chloride (AgCl) are insoluble
Sulfates	Usually soluble	Lead sulfate (PbSO₄), barium sulfate (BaSO₄), and calcium sulfate (CaSO₄) are insoluble
Carbonates	Usually insoluble	Sodium carbonate, potassium carbonate, and ammonium carbonate are soluble
Hydroxides	Usually insoluble	Sodium hydroxide and potassium hydroxide are soluble; calcium hydroxide is slightly soluble

Choosing the Correct Method

• Soluble salt from an insoluble base or carbonate: Use the excess solid method described above (add excess, filter, evaporate, crystallise).
• Soluble salt from a soluble base (alkali): Use a titration method — add the acid and alkali in exact proportions (using an indicator to find the endpoint), then evaporate to crystallise. You cannot use the excess method because both the acid and alkali are soluble, so you would not be able to separate excess reactant by filtration.

Exam Tip: If asked why you cannot use the excess method with a soluble base, explain that both the base and the salt are soluble, so you cannot filter out the excess base. Instead, a titration is needed to add exact quantities.

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Summary