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Law of Constant Proportions

The law of constant proportions states that a pure chemical compound always contains the same elements combined in the same fixed ratio by mass.

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Student-friendly explanation

A compound has a definite composition. Water from any pure source contains hydrogen and oxygen in the same mass ratio, 1:8. The source or method of preparation does not change this fixed proportion.

How to write this in exams

  1. 1

    Start with the exact idea

    The law of constant proportions states that a pure chemical compound always contains the same elements combined in the same fixed ratio by mass.

  2. 2

    Then show how to use it

    Write the masses of elements. Reduce each sample to simplest ratio. Compare the ratios. If they match, the law is followed for the same pure compound.

  3. 3

    Add one concrete example

    In pure water, 2 g of hydrogen combines with 16 g of oxygen. The ratio by mass is 2:16, or 1:8.

  4. 4

    Avoid this incomplete answer

    Saying 6:16 is different from 3:8 is wrong because 6:16 simplifies to 3:8.

Definition

The law of constant proportions states that a pure chemical compound always contains the same elements combined in the same fixed ratio by mass.

Example

In pure water, 2 g of hydrogen combines with 16 g of oxygen. The ratio by mass is 2:16, or 1:8.

Rule to remember

For a pure compound, mass of element A : mass of element B is constant for every pure sample.

Memory hook

Pure compound, fixed mass ratio.

Examples and method

Worked example

Sample 1 has 3 g carbon and 8 g oxygen. Sample 2 has 6 g carbon and 16 g oxygen. Both give the ratio 3:8, so they can represent the same pure compound.

Method to apply

Write the masses of elements. Reduce each sample to simplest ratio. Compare the ratios. If they match, the law is followed for the same pure compound.

Diagram support

A two-column table comparing two pure samples can show that their simplified mass ratios are identical.

How CBSE asks it

Asked through sample comparison, fixed ratio calculation, or a reason why a compound has definite composition.

Avoid common mistakes

Common confusion

Students sometimes compare the number of atoms directly with the mass ratio. Atomic mass must be considered while finding mass proportion.

Common wrong answer

Saying 6:16 is different from 3:8 is wrong because 6:16 simplifies to 3:8.

Exam tip

For mass-ratio questions, reduce the given masses to the simplest ratio and check whether the ratio is constant.

Quick check

Why does pure water always have hydrogen and oxygen in the mass ratio 1:8?

Pure water always has the formula H2O, so its elements combine in a fixed composition. When atomic masses are considered, hydrogen and oxygen are present in the mass ratio 1:8.

Answer writing and exam use

1-mark answer

The law of constant proportions states that a pure chemical compound always contains the same elements combined in the same fixed ratio by mass.

2-mark answer

The law of constant proportions states that a pure chemical compound always contains the same elements combined in the same fixed ratio by mass. For a pure compound, mass of element A : mass of element B is constant for every pure sample. In pure water, 2 g of hydrogen combines with 16 g of oxygen. The ratio by mass is 2:16, or 1:8.

3-mark answer

A compound has a definite composition. Water from any pure source contains hydrogen and oxygen in the same mass ratio, 1:8. The source or method of preparation does not change this fixed proportion. For a pure compound, mass of element A : mass of element B is constant for every pure sample. Sample 1 has 3 g carbon and 8 g oxygen. Sample 2 has 6 g carbon and 16 g oxygen. Both give the ratio 3:8, so they can represent the same pure compound. Asked through sample comparison, fixed ratio calculation, or a reason why a compound has definite composition. Saying 6:16 is different from 3:8 is wrong because 6:16 simplifies to 3:8.
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