How does the law of conservation of matter apply to chemical reactions?

The law of conservation of mass, often referred to as Lavoisier’s law, states that matter cannot be created or destroyed during chemical reactions. Instead, atoms and molecules simply recombine into new substances.1

This principle serves as the fundamental basis for balancing chemical equations. Because the total mass of the reactants must equal the total mass of the products, there must be the same number of atoms of each element on both sides of a chemical equation.1

To apply this law when balancing equations, chemists adjust the coefficients—the numbers placed in front of molecules—to ensure the count of each atom type is equal on both sides. It is important to note that one should never change the subscripts of a molecule, as doing so would alter the chemical identity of the substance itself. By systematically counting atoms and adjusting coefficients, scientists ensure that their equations accurately reflect the conservation of matter.1