IGCSE Edexcel Chemistry Practice Test 2026 - Free Chemistry Practice Questions and Study Guide

Carbon dioxide reacts with limewater (calcium hydroxide) to form calcium carbonate, which is insoluble and forms a milky white precipitate. When CO2 is bubbled through limewater, the solution turns cloudy because CaCO3 precipitates. This cloudiness is the clear sign that CO2 is present. For contrast, the other observations point to different gases: a squeaky pop comes from hydrogen, a glowing splint relights in the presence of oxygen, and damp red litmus blue indicates a basic gas—not carbon dioxide, which is acidic in water. So turning limewater cloudy is the definitive test for carbon dioxide.