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GCSE Chemistry · key term
An aqueous solution of calcium hydroxide used to test for carbon dioxide.
Used by AQA, Edexcel and OCR
Specification 4.8.2.3, 4.8.3.3 · Paper 2
Revise: Test for Carbon DioxideCarbon dioxide is colourless, so its appearance alone cannot identify it. Instead, use a qualitative test: shake the gas with limewater, or bubble the gas through it.
Specification 3.12 · Paper 1, Paper 2
Revise: Tests for GasesBubble the gas through the limewater so that the gas and solution mix.
A solution of calcium hydroxide that turns milky when carbon dioxide is passed through it.
A clear solution that turns milky in the presence of carbon dioxide.
Specification C4.2a · Paper 2 (Foundation), Paper 4 (Higher)
A solution of calcium hydroxide used to test for carbon dioxide.
Revise: Gas IdentificationLimewater is a clear calcium hydroxide solution. Carbon dioxide reacts with it to form tiny particles of solid calcium carbonate. This precipitate makes the liquid look milky.
Paper 1 (Foundation), Paper 2 (Foundation), Paper 3 (Higher), Paper 4 (Higher)
Revise: Gas testsPass the gas through limewater, or transfer some gas into a test tube of limewater and mix them. If the gas is carbon dioxide, the limewater turns milky or cloudy white.
State both the reagent and the positive result: use limewater; it turns milky or cloudy.
Do not identify carbon dioxide from bubbling or fizzing alone. The gas must give the positive limewater result.
Do not identify a carbonate from bubbling alone: state that the gas produced turns limewater milky or cloudy white.
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