 When you heat a metal carbonate, the metal carbonate decomposes to give carbon dioxide and the metal oxide. We will heat a sample of copper to carbonate. As we heat the copper carbonate, the copper carbonate decomposes to give a characteristic black copper oxide. The carbon dioxide that is released is bubbled through lime water. The lime water is a solution of calcium hydroxide. The carbonic acid and the calcium hydroxide react to produce calcium carbonate, which is insoluble. The lime water turns cloudy because the carbon dioxide as it bubbles through the water produces carbonic acid.