IGCSE Edexcel Chemistry Practice Test

Limewater is used to test for carbon dioxide. When carbon dioxide dissolves in the calcium hydroxide solution, it reacts to form calcium carbonate, a solid that does not dissolve and makes the solution go milky. The key reaction is CO2 + Ca(OH)2 → CaCO3(s) + H2O. This cloudiness is a distinct sign of carbon dioxide because the other gases—ammonia, oxygen, and hydrogen—do not react with limewater to form a precipitate, so the solution stays clear.