Neutralisation Reactions and Salts

This lesson covers neutralisation reactions and the formation of salts as required by the AQA GCSE Combined Science Trilogy specification (8464). You must be able to write word and balanced symbol equations, predict the products of neutralisation reactions, and understand the ionic equation for neutralisation.

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What Is Neutralisation?

Neutralisation is the reaction between an acid and a base to form a salt and water. During neutralisation, the pH moves towards 7.

General Equation

$\text{acid} + \text{base} \rightarrow \text{salt} + \text{water}$acid+base→salt+water

The Ionic Equation for Neutralisation

Every acid–base neutralisation involves the same essential ionic change:

$\text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l)$H+(aq)+OH−(aq)→H2​O(l)

The H⁺ ions from the acid combine with the OH⁻ ions from the base to form water.

Exam Tip: If you are asked to write an ionic equation for any neutralisation reaction, the answer is always: H⁺(aq) + OH⁻(aq) → H₂O(l). This equation applies regardless of which specific acid and base are used.

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Acid + Metal Oxide → Salt + Water

When an acid reacts with a metal oxide (an insoluble base), the products are a salt and water.

Acid + Metal Oxide	Word Equation	Symbol Equation
HCl + CuO	Hydrochloric acid + copper oxide → copper chloride + water	2HCl + CuO → CuCl₂ + H₂O
H₂SO₄ + MgO	Sulfuric acid + magnesium oxide → magnesium sulfate + water	H₂SO₄ + MgO → MgSO₄ + H₂O
HNO₃ + ZnO	Nitric acid + zinc oxide → zinc nitrate + water	2HNO₃ + ZnO → Zn(NO₃)₂ + H₂O

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Acid + Metal Hydroxide → Salt + Water

When an acid reacts with a metal hydroxide (an alkali), the products are also a salt and water.

Acid + Metal Hydroxide	Word Equation	Symbol Equation
HCl + NaOH	Hydrochloric acid + sodium hydroxide → sodium chloride + water	HCl + NaOH → NaCl + H₂O
H₂SO₄ + 2KOH	Sulfuric acid + potassium hydroxide → potassium sulfate + water	H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O
HNO₃ + NaOH	Nitric acid + sodium hydroxide → sodium nitrate + water	HNO₃ + NaOH → NaNO₃ + H₂O

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Acid + Metal → Salt + Hydrogen

When an acid reacts with a reactive metal (one above hydrogen in the reactivity series), the products are a salt and hydrogen gas.

$\text{acid} + \text{metal} \rightarrow \text{salt} + \text{hydrogen}$acid+metal→salt+hydrogen

Acid + Metal	Symbol Equation
2HCl + Mg	MgCl₂ + H₂
H₂SO₄ + Zn	ZnSO₄ + H₂
2HCl + Fe	FeCl₂ + H₂

Test for hydrogen: Hold a burning splint near the gas — it gives a squeaky pop.

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Acid + Metal Carbonate → Salt + Water + Carbon Dioxide

When an acid reacts with a metal carbonate, the products are a salt, water and carbon dioxide gas.

$\text{acid} + \text{metal carbonate} \rightarrow \text{salt} + \text{water} + \text{carbon dioxide}$acid+metal carbonate→salt+water+carbon dioxide

Acid + Carbonate	Symbol Equation
2HCl + CaCO₃	CaCl₂ + H₂O + CO₂
H₂SO₄ + Na₂CO₃	Na₂SO₄ + H₂O + CO₂
2HNO₃ + MgCO₃	Mg(NO₃)₂ + H₂O + CO₂

Test for carbon dioxide: Bubble the gas through limewater — it turns milky (cloudy).

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Predicting Salts

flowchart TD
    A["Which acid is used?"] --> B["HCl → forms a<br/>CHLORIDE salt"]
    A --> C["H₂SO₄ → forms a<br/>SULFATE salt"]
    A --> D["HNO₃ → forms a<br/>NITRATE salt"]

    B --> E["Name the metal<br/>from the base/metal"]
    C --> E
    D --> E

    E --> F["Salt = metal name<br/>+ acid ending<br/>(e.g. magnesium chloride)"]

    style B fill:#1565c0,color:#fff
    style C fill:#2e7d32,color:#fff
    style D fill:#e65100,color:#fff

Examples

Acid	Base / Metal / Carbonate	Salt Produced
HCl	NaOH	Sodium chloride
H₂SO₄	CuO	Copper sulfate
HNO₃	KOH	Potassium nitrate
HCl	CaCO₃	Calcium chloride
H₂SO₄	Mg	Magnesium sulfate

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Practical Applications of Neutralisation

Application	Acid/Base Involved
Treating indigestion	Antacid tablets (base, e.g. Mg(OH)₂) neutralise excess stomach acid (HCl)
Treating acidic soil	Farmers add lime (CaO or Ca(OH)₂) to neutralise acidic soil
Treating acidic lakes	Powdered limestone (CaCO₃) is added to counteract acid rain effects
Bee stings	Bee venom is acidic — can be neutralised with bicarbonate (base)
Wasp stings	Wasp venom is alkaline — can be neutralised with vinegar (acid)

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Common Mistakes

Mistake	Correction
Writing "acid + metal → salt + water"	The products with a metal are salt + hydrogen (not water)
Forgetting CO₂ from carbonates	Acid + carbonate → salt + water + carbon dioxide
Thinking neutralisation only involves alkalis	Neutralisation occurs with any base, not just soluble ones (alkalis)
Naming salts incorrectly	Remember: HCl → chloride, H₂SO₄ → sulfate, HNO₃ → nitrate

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Worked Example

Question: Write the word and balanced symbol equation for the reaction of sulfuric acid with calcium carbonate.

Answer:

• Word equation: Sulfuric acid + calcium carbonate → calcium sulfate + water + carbon dioxide
• Symbol equation: H₂SO₄ + CaCO₃ → CaSO₄ + H₂O + CO₂

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Summary