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2.6.2Inorganic Chemistry

Acids, Alkalis and Neutralisation — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 28 of 52

📚 Edexcel International GCSE Science Double Award 4SD0 - Chemistry Hub / Topical Revision
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Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
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Part 1 of 5Acids and Alkalis in Water

2.6.2 Acids, Alkalis and Neutralisation

The behaviour of acids and alkalis in water depends on the ions they produce.

Acids
  • Acids produce hydrogen ions, H⁺, when they dissolve in water, and these hydrogen ions make the solution acidic.
Alkalis
  • Alkalis produce hydroxide ions, OH⁻, when they dissolve in water, and these hydroxide ions make the solution alkaline.
pH scale
  • The pH scale shows how acidic or alkaline a solution is, and it is a measure of the hydrogen ion concentration.
Neutral water
  • Pure water contains equal numbers of hydrogen and hydroxide ions, so it has a pH of 7.

Part 2 of 5Neutralisation

Neutralisation takes place when an acid reacts with an alkali, so the solution becomes less acidic or alkaline.

1
An acid and an alkali react together, and the hydrogen ions from the acid combine with the hydroxide ions from the alkali.
2
The hydrogen and hydroxide ions combine to form water, which is neutral, so the solution becomes neutral.
3
The other ions remain in solution and form a salt, which can be recovered by evaporation and crystallisation.
4
The general equation is: acid + alkali → salt + water.

Part 3 of 5Uses of Neutralisation

Uses of Neutralisation
Soil treatment
  • Farmers add lime or limestone to acidic soil to raise its pH, because many crops grow poorly below pH 7.
Indigestion
  • Antacids contain bases that neutralise excess stomach acid, which relieves discomfort.
Industrial waste
  • Acidic waste can be neutralised with a base before it is released, to protect the environment.
Salt making
  • Neutralisation is the basis of preparing soluble salts from an acid and an insoluble base or an alkali.
AcidAlkaliSalt formed
Hydrochloric acidSodium hydroxideSodium chloride
Sulfuric acidPotassium hydroxidePotassium sulfate
Nitric acidCalcium hydroxideCalcium nitrate

Part 4 of 5Worked Example

Question Write the balanced equation and the ionic equation for the reaction of sulfuric acid with potassium hydroxide solution.

  1. Write the word equationsulfuric acid + potassium hydroxide → potassium sulfate + water.
  2. Balance the symbol equationH₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l).
  3. Remove the spectator ions K⁺ and SO₄²⁻, leaving only the ions that react.

Answer Ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l)

Part 5 of 5Exam Checklist & Tips

2.6.2 – Key Points for the Exam

  • Neutralisation A reaction between an acid and a base or alkali that forms a salt and water.
  • Hydrogen ion An H⁺ ion, which causes a solution to be acidic.
  • Hydroxide ion An OH⁻ ion, which causes a solution to be alkaline.
  • Salt An ionic compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion.

Common Mistakes

  • Stating that a metal reacting with an acid is a neutralisation;
    • no water is formed, so it does not fit the definition.
  • Writing the ionic equation with the spectator ions included.
  • Saying that neutral solutions have a pH of 0; neutral solutions have a pH of 7.
  • Forgetting that a salt is formed alongside the water in every neutralisation reaction.

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