Calcium oxide is produced when limestone is heated in a kiln. Describe how calcium oxide can be converted into calcium hydroxide, and explain one use of calcium hydroxide in agriculture.

WJEC GCSE Chemistry (Wales) — 1.6 Limestone · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Calcium oxide reacts with water:

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

The solid CaO (quicklime) is added to water, forming the soluble calcium hydroxide (slaked lime or limewater). This reaction is exothermic and produces a clear solution.

In agriculture, calcium hydroxide is applied to acidic soils. It neutralises the excess hydrogen ions, raising the pH towards neutral or slightly alkaline. This pH shift improves nutrient availability and promotes better crop growth.

Examiner tips

  • Show the balanced equation and mention the exothermic nature; examiners reward the reaction step. Mention the product as slaked lime or limewater. State the agricultural use as soil neutralisation. Explain the benefit as improved crop growth due to optimal pH.

Common mistakes

  • Writing CaO + H₂O → CaO·H₂O instead of Ca(OH)₂. Forgetting to mention the product as slaked lime. Using ‘fertiliser’ instead of ‘soil neutralisation’. Not linking the pH change to crop growth.

Mark scheme (4 marks)

  1. Calcium oxide is reacted with water (to produce calcium hydroxide)
  2. The reaction produces calcium hydroxide (slaked lime / limewater)
  3. Calcium hydroxide is used to neutralise acidic soils
  4. This improves crop growth (because most crops grow best in neutral or slightly alkaline conditions)

Key terms in this question

calcium oxide · calcium hydroxide

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