Carbon dioxide test. Image 1 of 2. Test tube of limewater to be used to test for the presence of carbon dioxide gas (CO2). The gas to be tested is passed through the limewater using the thin tube running down from top centre. The limewater is a solution of calcium hydroxide (Ca[OH]2) in water (H2O). If the gas is carbon dioxide, a chemical reaction will occur, producing calcium carbonate (CaCO3) as a white solid (precipitate). This will cause the clear limewater to turn cloudy. The equation for the reaction is: CO2 + Ca(OH)2 ----> CaCO3 + H2O For the result of this test, see image A500/280.