What is wrong with this experiment? "when it was heated, the sample of carbon decomposed?"?

The statement is wrong because carbon does not decompose in the presence of heat. Since carbon contains a large potential energy translated to its calorific value, it is a combustible matter. Thus, in the presence of heat, it gets combusted, or in simple words, it will be burned. As a result, it will yield products in the form of carbon dioxide and water.