Heating a substance to constant mass is a quantitative chemistry technique where a single chemical species or group of reactants is heated with constant weighing until the mass does not change. Single chemical compounds may combust or decompose from constant heating causing their masses to change as gaseous products evolve.
Heating a group of reactants may impart these reactants with the energy they need to undergo an endothermic chemical reaction that could not proceed without additional energy input. The heating to constant mass principle is based on the law of conservation of mass, where the total mass of reactants in a chemical reaction must equal the total mass of the products.
This law allows the indirect inference of the mass of the evolved gas products through simple subtraction. If 10 ounces of reactant that are heated to constant mass yield seven ounces of product, then it is deducted that three ounces of product left the system as a gas or evaporated liquid. Because compounds react with one another in fixed ratios, this facilitates the determination of the kind of products. If a reactant of known quantity and quality is heated to constant mass, the ratio of the reactant mass to product mass can be used to empirically determine the type of product.