An unknown substance undergoes a chemical change that gives off heat. Which statement is true? The types of molecules of the substance before and after the chemical change are different, and the change was endothermic. The types of molecules of the substance before and after the chemical change are the same, and the change was exothermic. The types of molecules of the substance before and after the chemical change are the same, and the change was endothermic. The types of molecules of the substance before and after the chemical change are different, and the change was exothermic.