Humber Admissions Practice Test

What is the Law of Constant Composition?

1. All elements are created equal

2. A given compound contains elements in a fixed proportion

3. Only metals can form compounds

4. Compounds can have varying compositions

The correct answer is: A given compound contains elements in a fixed proportion

The Law of Constant Composition states that a given compound always contains its constituent elements in a definite and fixed ratio by mass, regardless of the source of the compound or how it was produced. This fundamental concept in chemistry illustrates that the composition of a pure chemical compound is constant; for example, water (H₂O) always consists of two hydrogen atoms and one oxygen atom, no matter where it is found. Understanding this law is essential for distinguishing compounds from mixtures and recognizing how chemical reactions work. In contrast, the other options do not accurately reflect this law. The idea that all elements are created equal does not relate to composition, nor does it describe the fixed ratios integral to pure compounds. Similarly, the notion that only metals can form compounds is misleading, as nonmetals also play significant roles in compound formation. Lastly, the statement that compounds can have varying compositions contradicts the very essence of the Law of Constant Composition, which asserts that a specific compound's composition remains unchanged.