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.
Practice This ConceptMain explanation
Teacher 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.
Example
In pure water, 2 g of hydrogen combines with 16 g of oxygen. The ratio by mass is 2:16, or 1:8.
Simple analogy
Pure compound, fixed mass ratio.
Common confusion
Students sometimes compare the number of atoms directly with the mass ratio. Atomic mass must be considered while finding mass proportion.
Exam tip
For mass-ratio questions, reduce the given masses to the simplest ratio and check whether the ratio is constant.
Answer writing and exam use
1-mark use
Write the exact meaning of law of constant proportions in one clean line.
2-mark use
Define law of constant proportions and add one example or condition.
3-mark use
Explain law of constant proportions, show the method or example, and mention the common mistake.
Practice this concept with focused MCQs
Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.
Help improve this page
Found something confusing, incorrect, or missing?