Carbon dioxide test. Image 2 of 2. Test tube of limewater after a positive test for the presence of carbon dioxide gas (CO2). The CO2 has been 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). As the gas was carbon dioxide, a chemical reaction occurred, producing calcium carbonate (CaCO3) as a white solid (precipitate). This has turned the clear limewater cloudy. The equation for the reaction is: CO2 + Ca(OH)2 ----> CaCO3 + H2O For the start of this test, see image A500/279.