 There are many indications that a chemical change is occurring. In the decomposition reaction that we are going to observe, we have some blue-green copper-2 carbonate that we are going to heat. The copper-2 carbonate decomposes upon heating to produce copper-2 oxide, which is black in color. The copper-2 carbonate also gives off carbon dioxide. The carbon dioxide is a colorless gas which is difficult to observe, but if we bubble the carbon dioxide through a solution of lime water, the carbon dioxide reacts with the lime water, which is calcium hydroxide, to produce insoluble calcium carbonate. This appears as a white solid in the test tube.