Neutralisation and Salt Preparation

This lesson covers neutralisation reactions and the preparation of soluble salts, as required by the AQA GCSE Chemistry specification (4.4.2). You need to understand the process of neutralisation, know how to prepare pure, dry salt crystals from an acid and an insoluble base, and write balanced equations for neutralisation reactions. This is a core practical topic that connects theory with hands-on laboratory skills.

---

What Is Neutralisation?

Neutralisation is the reaction between an acid and a base (or alkali) to produce a salt and water. During neutralisation, the pH of the solution moves towards 7 (neutral).

General Equations

Reaction type	General word equation
Acid + alkali	salt + water
Acid + metal oxide	salt + water
Acid + metal hydroxide	salt + water
Acid + metal carbonate	salt + water + carbon dioxide

The Ionic Equation

For any neutralisation reaction between an acid and an alkali, the essential process is:

H+(aq) + OH-(aq) → H2O(l)

This shows that the hydrogen ions from the acid combine with the hydroxide ions from the alkali to form water. The remaining ions form the salt.

Exam Tip: The ionic equation H+(aq) + OH-(aq) → H2O(l) is the same for ALL acid-alkali neutralisation reactions. Learn it — you may be asked to write it or explain what it shows. It demonstrates that neutralisation is fundamentally the combination of H+ and OH- ions to form water.

---

Preparing Soluble Salts

There are two main methods for preparing soluble salts in the laboratory:

Method 1: Using an Insoluble Base or Insoluble Carbonate

This method is used when one of the reactants is insoluble in water (e.g. a metal oxide, an insoluble metal hydroxide, or a metal carbonate).

graph TD
    A["1. Warm the acid<br/>in a beaker"] --> B["2. Add excess insoluble<br/>base/carbonate"]
    B --> C["3. Stir until no more<br/>dissolves (excess visible)"]
    C --> D["4. Filter to remove<br/>excess solid"]
    D --> E["5. Pour filtrate into<br/>evaporating basin"]
    E --> F["6. Heat gently to<br/>evaporate some water"]
    F --> G["7. Leave to crystallise<br/>at room temperature"]
    G --> H["8. Pat dry with<br/>filter paper"]

    style A fill:#e74c3c,color:#fff
    style D fill:#3498db,color:#fff
    style G fill:#27ae60,color:#fff
    style H fill:#f39c12,color:#fff

Detailed Steps

1. Warm a measured volume of dilute acid (e.g. sulfuric acid) gently in a beaker using a Bunsen burner or water bath.
2. Add a spatula of the insoluble base (e.g. copper oxide) to the warm acid. Stir with a glass rod.
3. Continue adding the base until it is in excess — you will see some undissolved solid at the bottom of the beaker. This ensures all the acid has reacted.
4. Filter the mixture through filter paper in a funnel to remove the excess unreacted solid. The filtrate (liquid that passes through) contains the dissolved salt.
5. Pour the filtrate into an evaporating basin.
6. Heat the evaporating basin gently over a water bath (not a Bunsen burner, to avoid spitting) until about half the water has evaporated.
7. Leave the remaining solution to cool and crystallise slowly at room temperature. Slow crystallisation produces larger, more regular crystals.
8. Pat dry the crystals with filter paper and leave to dry completely.

Example: Making Copper Sulfate Crystals

Copper oxide + sulfuric acid → copper sulfate + water

• Copper oxide (black powder) is the insoluble base.
• Sulfuric acid is the acid.
• The solution turns blue as copper sulfate forms.
• Excess copper oxide is removed by filtration.
• Blue copper sulfate crystals form on evaporation and cooling.

Exam Tip: The reason for adding excess base is to ensure all the acid has reacted. If acid remained in the solution, the crystals would be contaminated. The excess solid is easily removed by filtration. This is a very commonly examined concept.

---

Method 2: Using an Alkali (Titration Method)

This method is used when both the acid and the base are soluble (i.e. the base is an alkali such as sodium hydroxide or potassium hydroxide).

Steps

1. Use a burette to add acid to a known volume of alkali in a conical flask, with an indicator (e.g. phenolphthalein or methyl orange).
2. Add the acid dropwise until the indicator shows neutralisation has occurred (the end point).
3. Record the volume of acid added.
4. Repeat the experiment without the indicator, using the same volumes, to obtain a pure salt solution (the indicator would contaminate the salt).
5. Evaporate and crystallise the salt solution as described above.

Why Not Use Universal Indicator?

Universal indicator is a mixture of several dyes, which would contaminate the salt and change its colour. Single indicators (phenolphthalein or methyl orange) are preferred because they give a sharp colour change at the end point, but even these must be omitted from the final preparation.

---

Soluble and Insoluble Salts

Not all salts are soluble. You need to know the solubility rules:

Salt type	Solubility rule	Exceptions
Sodium, potassium, and ammonium salts	Always soluble	None
Nitrates	Always soluble	None
Chlorides	Usually soluble	Silver chloride and lead 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, potassium, and calcium hydroxides are soluble

---

Making Insoluble Salts (Precipitation)

Insoluble salts are made by mixing two solutions that each contain one of the ions needed for the salt. When mixed, the insoluble salt forms as a precipitate (a solid that forms in solution).

General method:

1. Mix solutions of two soluble salts that together contain the ions of the insoluble salt.
2. The insoluble salt forms as a precipitate.
3. Filter the mixture to collect the precipitate.
4. Wash the precipitate with distilled water to remove any soluble impurities.
5. Dry the precipitate in a warm oven or desiccator.

Example: Making Barium Sulfate

Barium chloride solution + sodium sulfate solution → barium sulfate (precipitate) + sodium chloride solution

• Barium sulfate is insoluble and forms as a white precipitate.
• It is collected by filtration, washed, and dried.

Exam Tip: For precipitation reactions, you must be able to predict whether a precipitate will form by using the solubility rules. If you mix two solutions and one of the possible products is insoluble according to the rules, a precipitate will form. This is commonly tested in 3-4 mark questions.

---

Applications of Neutralisation

Neutralisation reactions have many practical applications: