Chemical change happens when a substance combines with another substance to form a new substance. It produces chemical reactions and generally is not reversible except by further chemical reactions. Heat is sometimes produced in some reactions. When chemical reactions occur, the atoms are rearranged and an energy change happens along with it as new products are generations. At a molecular level, chemical changes involves the making or breaking of bonds between atoms. Some examples of chemical change are iron rusting (iron oxide forms) and bread rising (yeast converts carbohydrates into carbon dioxide gas).