Chemistry · Some Basic Concepts Of Chemistry · NEET
The Law of Definite Proportions says one pure compound always has the same elements combined in the same ratio by mass, no matter where it comes from or how it is made. For example, pure water is always 1 part hydrogen to 8 parts oxygen by mass. Memory hook: "Same compound = same recipe by weight, every single time."
Water from any source (natural or lab-made) always has hydrogen and oxygen in the same fixed mass ratio of 1:8 — the core idea of the Law of Definite Proportions.
Your doubts, answered
What exactly does the Law of Definite Proportions say?
It says a given pure compound always contains the same elements joined together in the same fixed ratio by mass (by weight). It does not matter if the sample is natural or made in a lab. Example: water (H2O) is always 11.1% hydrogen and 88.9% oxygen by mass, so H:O = 1:8 by mass, always.
Who gave the Law of Definite Proportions and how?
The French chemist Joseph Proust gave it. He took two samples of cupric carbonate, one natural and one made synthetically in the lab. He found that both samples had exactly the same proportion of elements by mass. So he concluded that source does not matter. It is also called the Law of Constant Proportions or Law of Definite Composition.
What is the difference between definite proportions and multiple proportions?
Definite Proportions is about ONE compound: its element ratio by mass is always fixed. Multiple Proportions is about TWO compounds made from the SAME two elements (like H2O and H2O2). It compares how the mass of one element changes for a fixed mass of the other, and says those masses are in a simple whole-number ratio. In short: definite = one compound stays fixed; multiple = compares two different compounds.
Does the way I make a compound change its composition?
No. This is the whole point of the law. Whether CO2 comes from burning carbon, from breathing, or from a factory, it is always about 27.3% carbon and 72.7% oxygen by mass. The mass ratio C:O stays 3:8. The source and the method do not change the fixed recipe of a pure compound.
Why does this law matter for NEET?
NEET often asks you to match a law with its correct name or with what Dalton's atomic theory could or could not explain. You must know that Dalton's theory explains conservation of mass, definite (constant) proportions and multiple proportions, but it could NOT explain the law of gaseous volumes. This exact set-up was asked in NEET 2025.
⚠️ The NEET trap ✗ Thinking the Law of Constant Proportions is the one Dalton's atomic theory could NOT explain. ✓ Dalton's atomic theory CAN explain the law of constant (definite) proportions, conservation of mass and multiple proportions. It could NOT explain Gay-Lussac's law of gaseous volumes (that needed Avogadro's idea). 🧠 Dalton handles the MASS laws; he fails at the VOLUME (gas) law.
Real NEET questions
NEET 2025
Dalton's atomic theory could not explain which of the following?
A · Law of multiple proportions
B · Law of gaseous volumes ✓
C · Law of conservation of mass
D · Law of constant proportions
Solution: Dalton's atomic theory successfully explained the law of conservation of mass, the law of constant (definite) proportions and the law of multiple proportions, because these are all about the masses of atoms combining. It could NOT explain Gay-Lussac's law of combining gaseous volumes. That law was later explained by Avogadro's hypothesis, which says equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. So the correct answer is (B) Law of gaseous volumes.
Solved Some Basic Concepts Of Chemistry NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Is the Law of Definite Proportions the same as the Law of Constant Proportions?
Yes. Law of Definite Proportions, Law of Constant Proportions and Law of Definite Composition are three names for the exact same law given by Joseph Proust.
Give one simple numerical example of this law.
In pure water, hydrogen and oxygen are always in the mass ratio 1:8. So 9 g of water always has 1 g of hydrogen and 8 g of oxygen, whatever the source of the water.
Which scientist proposed this law?
Joseph Proust, a French chemist, proposed it around 1799 after studying two samples of cupric carbonate.
Does this law work for mixtures too?
No. It works only for pure compounds. Mixtures (like air or salt water) can have any proportion of parts, so their composition is not fixed.
