Cambridge O Level Biology · Syllabus 5090 · Human Gas Exchange
Limewater Test
What is Limewater Test?
A laboratory test for carbon dioxide. Limewater is a solution of calcium hydroxide, and it is clear and colourless. When carbon dioxide is bubbled through it, insoluble calcium carbonate is formed as a fine suspension, and the limewater turns milky or cloudy. The more concentrated the carbon dioxide in the gas being bubbled through, the more quickly the cloudiness appears. In gas-exchange work the test is used comparatively: air breathed out is bubbled through one sample of limewater and atmospheric air through an identical sample, and the sample exposed to expired air becomes cloudy first. The result shows that expired air contains a higher concentration of carbon dioxide; it does not measure that concentration.
This definition is part of the Human Gas Exchange chapter in Cambridge O Level Biology.