The chemical change you described, where sugar (sucrose) breaks down into carbon and water vapor when heated, is called "thermal decomposition" or "pyrolysis." During this process, heat is applied to the sugar, causing it to undergo a chemical reaction that breaks it down into simpler substances, carbon (in the form of charred residue) and water vapor (in the form of steam). This is a type of chemical change because the chemical composition of the sugar molecules is altered, leading to the formation of different substances.