All chemical reactions have a conservation of mass. The law of conservation of mass states that atoms cannot be created nor destroyed in a chemical reaction. This discovery, made by Antoine Lavoisier in1789, is the basic foundation for modern chemistry.
Conservation of mass is why equations depicting chemical reactions must be balanced. When this is done, the numbers of each type of atoms are the same on both sides of the equation. In a closed system, the mass of all products and reactants is the same at any point during the reaction. Conservation of mass is possible because elements found in the earth are very stable.