The law of conservation of mass states that the mass of the reactants and the mass of products in a chemical reaction are equal. This refers to matter not being destroyed or created.
When using a closed system, the total mass of all products and reactants remain the same throughout the entire process. This finding is thanks to Antoine Lavoisier in 1789 and it revolutionized science. This law is the basis for all modern chemistry because it tells scientists that chemical reactions are not capable of destroying or creating mass. This remains true today because scientists know that chemical reactions do not turn atoms into other elements.