The law of conservation of matter or principle of matter conservation states that the mass of an object or collection of objects never changes over time, no matter how the constituent parts rearrange themselves.
The mass can neither be created nor destroyed.
The law requires that during any nuclear reaction, radioactive decay or chemical reaction in an isolated system, the total mass of the reactants or starting materials must be equal to the mass of the products.
The concept of mass conservation is widely used in many fields such as chemistry, mechanics, and fluid dynamics. In chemistry the law of conservation of matter may be explained in the following way (see the picture of combustion of methane). The masses of a methane and oxygen together must be equal to the masses of carbon dioxide and water. In other words, during a chemical reaction, everything you start with, you must end up with, but it might look different.