Law of Constant Composition
Summary: A given compound always contains the same elements combined in the same fixed proportion by mass, regardless of its source or how it was made. Tags: igcse chemistry keyword Created: 2026-08-06
The law of constant composition (also called the law of definite proportions) states that a pure compound always contains the same elements in the same fixed proportion by mass, no matter where the compound comes from or how it was prepared. For example, pure water is always 11.1% hydrogen and 88.9% oxygen by mass — whether it comes from a tap, a lake, or is made in a laboratory, the proportion never changes.
This is because compounds form by elements combining in fixed ratios of atoms. Water is always H₂O: two hydrogen atoms for every one oxygen atom. The IGCSE exam point is that each compound has a unique fixed composition, and this idea underlies chemical formulae, calculations of percentage composition by mass, and distinguishing pure compounds from mixtures (which can vary in composition). It also explains why a compound cannot be broken into arbitrary ratios of its elements.
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025), Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| The law applies to mixtures as well as compounds | Mixtures can have any proportion of components; only pure compounds have a fixed composition by mass |
| A compound’s composition can vary with its source | The law states composition is identical whatever the source or method of preparation |
| Proportions are by number of particles only | The law concerns fixed proportion by mass, which follows from the fixed ratio of atoms in the formula |