Flexi Says: Fire uses and releases energy as part of a chemical reaction in which fuel reacts with oxygen in presence of heat. Fire requires activation energy to start and it is an exothermic reaction that results in a net release of energy. Matter is conserved in all chemical reactions, including combustion. The same number and type of atoms remain after fuel is burned; the atoms are just rearranged to form different molecules. Thus, fire does not convert matter to heat. Heat is released because the potential energy of the product molecules (water and carbon dioxide) is lower than the potential energy of the reactant molecules (fuel and oxygen).