Testing for carbonate. Test tube containing a compound added to acid (left) producing a gas which is bubbled through limewater (calcium hydroxide solution), turning it milky. Carbon dioxide (CO2) passed into limewater produces a milky suspension of calcium carbonate. Therefore the gas produced here is carbon dioxide. This shows that the compound added to the acid is a metal carbonate as a metal carbonate added to acid produces a metal salt, water and carbon dioxide.