Carbon dioxide bubbling through limewater. Carbon dioxide (CO2) is produced in the test tube on the left. It is a product of the reaction between calcium carbonate (CaCO3) and an acid. The carbon dioxide follows the thin pipe and bubbles through limewater in the test tube on the right. Limewater (water in which calcium hydroxide is dissolved) is used as an indicator of carbon dioxide. It reacts with carbon dioxide to form calcium carbonate, turning the limewater cloudy.