Laws of chemical combination

Law of conservation of mass (Antoine Lavoisier): in any physical or chemical change, matter is neither created nor destroyed, so total mass stays the same.

Law of definite proportions (Joseph Proust): a given compound always contains the same elements combined in the same proportion by mass, whatever its source.

Law of multiple proportions (John Dalton): if the same two elements make two or more different compounds, then fixing the mass of one element and comparing the masses of the other element across those compounds gives a small whole-number ratio.

Example of multiple proportions: 2 g of hydrogen combines with 16 g of oxygen in water and with 32 g of oxygen in hydrogen peroxide, a 1 : 2 ratio.

Gay Lussac's law of gaseous volumes: when gases react or are produced, their volumes, measured at the same temperature and pressure, bear a simple whole-number ratio.

Gay Lussac's law is a statement about volumes; the law of definite proportions is a statement about masses.

Avogadro's law: at a fixed temperature and pressure, a given volume of any gas holds the same number of molecules, whatever the gas.

Avogadro separated the ideas of atoms and molecules, which explained results like 2 volumes of hydrogen plus 1 volume of oxygen giving 2 volumes of water vapour.