Matter is neither created nor destroyed in a chemical reaction ?
It is also called the Law of conservation of mass. It was proposed by Antoine Lavoisier.
According to this law, in a closed system, mass can neither be created nor be destroyed. In other words it means that in a closed system there is no change in the total mass of the initial matter to the mass of the final matter. Thus, no energy or mass is lost or gained by the system. Energy can change form but the mass shall remain unchanged. So when light or physical energy is transformed into particles, these particles will contribute exactly the same mass as the light or physical energy had contributed to the system.
In chemistry, when a reaction takes place in a closed system, then the mass of the reactants will be equal to the products formed. Hence, no mass is lost during the process.