Concept:The Law of Constant Proportions states that a pure chemical compound always contains its constituent elements in a fixed ratio by mass, regardless of its source or method of preparation.
Explanation:This law, also called the Law of Definite Proportions, was proposed by Joseph Proust in 1794.
For example, water (
H2​O) is always composed of hydrogen and oxygen in a mass ratio of approximately
1:8, whether from a river, ocean, or laboratory.
The law applies only to pure compounds, not to mixtures, where composition can vary.
Among the given options:
• Option A correctly states: "A chemical compound always contains the same elements in the same proportion by mass." This matches the law exactly.
• Option B is false because elements combine in fixed, not random, ratios.
• Option C describes the Law of Conservation of Mass (Lavoisier), not constant proportions.
• Option D is false; atoms rearrange in chemical reactions but are not divided into smaller particles (that occurs only in nuclear reactions).
Thus, option A is the best explanation.
Answer:Option A