Law of Definite Proportions
The principle that a given chemical compound always contains its constituent elements in the same fixed mass ratio.
Unit
Dimensionless
In depth
Proposed by Proust, this law distinguishes compounds from mixtures and implies that compounds have discrete formulae. A handful of non-stoichiometric solids, called berthollides, deviate slightly because of lattice defects, but the law holds for essentially all molecular compounds.
Examples in the real world
Water from any source — rain, a glacier or a laboratory synthesis — is always 11.19% hydrogen and 88.81% oxygen by mass.