Law of constant proportions
- Olympiad · NSEJS
The law of constant proportions says that a pure chemical compound always contains the same elements joined in the same fixed ratio by mass.
It is also called the law of definite proportions. It does not matter where the compound comes from or how it was made. Pure water from a river, a lab or a rain cloud always has hydrogen and oxygen in the mass ratio 1 : 8.
The reason lies in atoms. A molecule of water is always H₂O: two hydrogen atoms and one oxygen atom. A hydrogen atom has a mass of about 1 unit and an oxygen atom about 16. So the ratio is 2 : 16, which is 1 : 8.
A small example
How much hydrogen and oxygen are there in 27 g of water?
- The ratio is 1 : 8, so there are 1 + 8 = 9 parts in all.
- Hydrogen: 1 part out of 9, so 27 × 1/9 = 3 g.
- Oxygen: 8 parts out of 9, so 27 × 8/9 = 24 g.
The same idea works for carbon dioxide, CO₂. Carbon is 12 and two oxygens are 32. So carbon and oxygen are in the ratio 12 : 32, which is 3 : 8. This fixed ratio is what lets a balanced chemical equation predict masses.
A mixture is different. Salt water can have a little salt or a lot. Only a compound keeps one fixed ratio.
Where it fits
This goes beyond the Class 10 board syllabus. HBCSE's Junior Science syllabus lists it under atoms and molecules. That syllabus is for NSEJS, which the Indian Association of Physics Teachers (IAPT) conducts.
Sources
Facts last checked against these sources on 30 September 2026.
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