Carbon dioxide is a colourless gas with no smell. The test for carbon dioxide uses limewater, which is a clear solution of calcium hydroxide in water.
Bubble the gas through limewater, or shake the gas with limewater in a test tube. If the gas is carbon dioxide, the limewater turns milky, which is also called cloudy.
The carbon dioxide reacts with the calcium hydroxide to make calcium carbonate. Calcium carbonate does not dissolve in water, so tiny solid particles spread through the liquid and make it look milky: Ca(OH)2 + CO2 → CaCO3 + H2O.
If the limewater stays clear, the gas is not carbon dioxide. Remember that the limewater starts clear and colourless, not milky.
