Conservation of mass
From Academic Kids
The law of conservation of mass/matter states that the mass of a system of substances is constant, regardless of the processes acting inside the system. An equivalent statement is that matter changes form, but cannot be created or destroyed. This implies that for any chemical process in a closed system, the mass of the reactants must equal the mass of the products.
The law of conservation of mass fails for nuclear processes, where the equivalence of matter and energy, and hence conservation of energy, applies. The law also fails for relativistic situations. However, it is to a very high accuracy applicable in chemical reactions, since in every case the mass-energy of the reactants is huge in comparison to the energy absorbed or released when they react. By way of example, a gram of gasoline releases 48 kJ of energy when burned. However, the mass-energy of a gram of gasoline (or anything else) is 90 TJ, or about 2 billion times as much — and this does not include the mass-energy of the oxygen used in combustion.
The law was first clearly and unambiguously formulated by Antoine Lavoisier, who is often referred to as the father of modern chemistry. However, other scientists such as Mikhail Lomonosov had previously expressed similar ideas.