what is law of conservation of mass

The law of conservation of mass, also known as the principle of mass conservation, states that for any system closed to all transfers of matter and energy, the mass of the system must remain constant over time, as the systems mass cannot be created or destroyed, although it may be rearranged in space, or the entities associated with it may be changed in form. This law is widely used in many fields such as chemistry, mechanics, and fluid dynamics. The concept of mass conservation was primarily demonstrated in the 17th century and finally confirmed by Antoine Lavoisier in the late 18th century. The law of conservation of mass can only be formulated in classical mechanics, in which the energy scales associated with an isolated system are much smaller than the mass of a typical object in the system. The law has to be modified to comply with the laws of quantum mechanics and special relativity under the principle of mass-energy equivalence, which states that energy and mass form one conserved quantity. The law of conservation of mass is important in understanding chemical reactions, as the mass of the chemical components before the reaction is equal to the mass of the components after the reaction.