The Law of Definite Proportions
Imagine you are making lemonade. You decide the perfect glass has 2 spoons of sugar, the juice of 1 lemon, and 200 ml of water. If you make it exactly this way every time, the taste is identical. But if you double the water or halve the sugar, you get a different drink — not the same lemonade anymore.
A chemical compound works the same way. Water is not just "hydrogen and oxygen" — it is hydrogen and oxygen in a very specific, unchangeable mass ratio. If you change that ratio, you no longer have water. You might get hydrogen peroxide, or just a mixture of gases, but not the same compound.
The law applies to compounds (pure substances made of different elements bonded together), not to mixtures. In a mixture, you can vary the proportions freely — think of a handful of sand and salt.
The Precise Statement
A given chemical compound always contains its constituent elements in a fixed ratio by mass, regardless of its source or method of preparation.
This means: every molecule of water, whether it comes from rain, a river, your tap, or is synthesized in a lab, has exactly the same mass ratio of hydrogen to oxygen.
The Numbers That Never Change
Take water (H2O). In every single water molecule:
- 2 hydrogen atoms (atomic mass ≈ 1 u each) → total mass = 2 u
- 1 oxygen atom (atomic mass ≈ 16 u) → total mass = 16 u
The mass ratio of hydrogen to oxygen is always 2:16, which simplifies to 1:8.
mass of oxygenmass of hydrogen=162=81
If you decompose 18 grams of water, you will always get exactly 2 grams of hydrogen and 16 grams of oxygen. Not 2.5 g and 15.5 g — always 2 g and 16 g.
Why This Matters …