Carbon dioxide test. Student using limewater (calcium hydroxide solution, Ca[OH]2) to test for the presence of carbon dioxide gas (CO2). The test tube at left contains a reaction mixture that has evolved a gas. This gas is passed through the limewater in the tube at right. A reaction between the gas and the calcium hydroxide has led to the previously clear limewater turning cloudy. This is interpreted as a precipitate of calcium carbonate (CaCO3) being produced, and the gas can then be identified as carbon dioxide. Water (H2O) is also formed. The equation for the reaction is: CO2 + Ca(OH)2 ----> CaCO3 + H2O.
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