Neutralisation Reactions

This lesson covers neutralisation reactions as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the general equations for neutralisation, predict products of reactions between acids and bases, metals or carbonates, and write word and balanced symbol equations.

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What Is Neutralisation?

Neutralisation is the reaction between an acid and a base. The acid and base cancel each other out, producing a salt and water.

General Equation

acid + base → salt + water

In terms of ions, every neutralisation reaction involves the same essential change:

H⁺(aq) + OH⁻(aq) → H₂O(l)

The hydrogen ions from the acid combine with the hydroxide ions from the base to form water. This is why the pH moves towards 7.

Exam Tip: The ionic equation H⁺ + OH⁻ → H₂O applies to every acid–base neutralisation. If you are asked to write an ionic equation for neutralisation, this is the answer.

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Acid + Base (Metal Oxide or Metal Hydroxide)

When an acid reacts with a metal oxide or a metal hydroxide, a salt and water are produced.

Acid + Metal Oxide → Salt + Water

Reaction	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 + Na₂O	Nitric acid + sodium oxide → sodium nitrate + water	2HNO₃ + Na₂O → 2NaNO₃ + H₂O

Acid + Metal Hydroxide → Salt + Water

Reaction	Word Equation	Symbol Equation
HCl + NaOH	Hydrochloric acid + sodium hydroxide → sodium chloride + water	HCl + NaOH → NaCl + H₂O
H₂SO₄ + Ca(OH)₂	Sulfuric acid + calcium hydroxide → calcium sulfate + water	H₂SO₄ + Ca(OH)₂ → CaSO₄ + 2H₂O

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Acid + Metal → Salt + Hydrogen

When an acid reacts with a reactive metal, a salt and hydrogen gas are produced.

acid + metal → salt + hydrogen

Reaction	Word Equation	Symbol Equation
HCl + Mg	Hydrochloric acid + magnesium → magnesium chloride + hydrogen	2HCl + Mg → MgCl₂ + H₂
H₂SO₄ + Zn	Sulfuric acid + zinc → zinc sulfate + hydrogen	H₂SO₄ + Zn → ZnSO₄ + H₂
HCl + Fe	Hydrochloric acid + iron → iron(II) chloride + hydrogen	2HCl + Fe → FeCl₂ + H₂

Testing for Hydrogen Gas

• Collect the gas in an inverted test tube.
• Hold a burning splint to the mouth of the tube.
• Hydrogen burns with a squeaky pop.

Exam Tip: Not all metals react with acids. Unreactive metals like copper, silver and gold do NOT react with dilute acids — they are below hydrogen in the reactivity series.

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Acid + Carbonate → Salt + Water + Carbon Dioxide

When an acid reacts with a carbonate (or hydrogen carbonate), three products are formed: a salt, water and carbon dioxide gas.

acid + carbonate → salt + water + carbon dioxide

Reaction	Word Equation	Symbol Equation
HCl + CaCO₃	Hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide	2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂
H₂SO₄ + Na₂CO₃	Sulfuric acid + sodium carbonate → sodium sulfate + water + carbon dioxide	H₂SO₄ + Na₂CO₃ → Na₂SO₄ + H₂O + CO₂
HCl + NaHCO₃	Hydrochloric acid + sodium hydrogen carbonate → sodium chloride + water + carbon dioxide	HCl + NaHCO₃ → NaCl + H₂O + CO₂

Testing for Carbon Dioxide

• Bubble the gas through limewater (calcium hydroxide solution).
• If CO₂ is present, the limewater turns milky/cloudy (due to formation of insoluble calcium carbonate).

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Naming Salts

The name of the salt produced depends on the acid used and the metal (or metal in the base/carbonate):

Acid	Salt Produced	Example
Hydrochloric acid (HCl)	Metal chloride	NaCl — sodium chloride
Sulfuric acid (H₂SO₄)	Metal sulfate	MgSO₄ — magnesium sulfate
Nitric acid (HNO₃)	Metal nitrate	Ca(NO₃)₂ — calcium nitrate

graph TD
    A["Which acid was used?"] --> B["HCl → Chloride salt"]
    A --> C["H₂SO₄ → Sulfate salt"]
    A --> D["HNO₃ → Nitrate salt"]
    B --> E["e.g. NaCl, MgCl₂, CuCl₂"]
    C --> F["e.g. Na₂SO₄, MgSO₄, CuSO₄"]
    D --> G["e.g. NaNO₃, Mg(NO₃)₂, Cu(NO₃)₂"]

    style A fill:#2c3e50,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#2980b9,color:#fff
    style D fill:#8e44ad,color:#fff
    style E fill:#1a1a2e,color:#fff
    style F fill:#1a1a2e,color:#fff
    style G fill:#1a1a2e,color:#fff

Exam Tip: To name a salt, take the metal from the base/carbonate/metal and the ending from the acid: HCl → chloride, H₂SO₄ → sulfate, HNO₃ → nitrate.

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Practical Uses of Neutralisation

Neutralisation has many everyday applications:

Application	Acid	Base/Alkali	Why
Treating indigestion	Excess stomach acid (HCl)	Antacid (e.g. calcium carbonate, magnesium hydroxide)	Neutralises the excess acid
Treating acidic soil	Acidic soil	Lime — calcium hydroxide Ca(OH)₂	Raises the pH of the soil
Treating acid rain damage in lakes	Acidic lake water	Powdered limestone (CaCO₃)	Raises the pH of the water
Making fertilisers	Nitric acid	Ammonia solution	Produces ammonium nitrate

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Balancing Equations — A Reminder

When writing balanced symbol equations, remember:

1. Write the correct formulae for reactants and products.
2. Count the atoms of each element on both sides.
3. Adjust coefficients (large numbers in front) to balance — never change the subscripts in a formula.
4. Check the equation is balanced.