Why does pure water always contain hydrogen and oxygen in a fixed ratio, regardless of its source?

Pure water always contains hydrogen and oxygen in the fixed ratio of 1:8 by mass (or 2:1 by number of atoms) because water is a compound, not a mixture.

• Elements in a compound are chemically bonded in a definite and fixed proportion this is the Law of Definite Proportions (Law of Constant Composition), stated by Joseph Proust.
• Every molecule of water is H₂O two hydrogen atoms covalently bonded to one oxygen atom regardless of whether water comes from a river, glacier, or laboratory.
• This fixed ratio distinguishes compounds from mixtures.

Water is a compound, so its elements always combine in a fixed ratio by the Law of Definite Proportions.