Conservation of mass

The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. Thus, the amount of matter cannot change. Sir Antoine Lavoisier promoted this idea. 
This law says that when a chemical reaction rearranges atoms into a new product, the mass of the reactants (chemicals before the chemical reaction) is the same as the mass of the products (the new chemicals made). More simply, whatever you do, you will still have the same amount of stuff. 
The law is accurate for all chemical reactions. However, certain nuclear reactions (fusion and fission) can convert a small part of the mass into energy. But if this energy is turned back into mass, you will have the same amount of mass.
History.
The principle of conservation of mass was first outlined by Mikhail Lomonosov (1711–1765) in 1748. He proved it by experiments—though this is sometimes challenged. Antoine Lavoisier (1743–1794) had expressed these ideas in 1774. Others whose ideas pre-dated the work of Lavoisier include Joseph Black (1728–1799), Henry Cavendish (1731–1810), and Jean Rey (1583–1645).
Other conservation laws.
The law of conservation of mass is one of the conservation laws of physics. Each of these laws says that something is never created or destroyed.