The Law of Conservation of Mass is fundamental to understanding how chemical reactions work. This states that mass is neither destroyed nor created in a chemical reaction . In a simple way, the total mass of the products after the reaction is equal to the reactants before a chemical reaction, where no matter is lost or gained (the reaction takes place in a closed system). For example, if you burn a piece of wood in a closed container, the mass of the wood and oxygen before the reaction will be the same as the mass of the ash, gases, and other products after the reaction.