Which of the following can be considered a pure substance with constant composition?

• A. A homogeneous alloy with constant composition
• B. A mixture of sugar and water
• C. A heterogeneous rock
• D. A beverage with varying sugar content

Answer: (A)

The key idea is that a pure substance has a fixed composition that is uniform throughout. When a material is a homogeneous alloy with a constant composition, every part of it contains the same definite ratio of elements, so the substance behaves as a single, uniform material with consistent properties everywhere. That makes it fit the idea of a pure substance with constant composition.

In contrast, a mixture of sugar and water can have different amounts of sugar in different samples, so its composition isn’t fixed throughout. A heterogeneous rock contains different minerals in different places, so its composition varies by location. A beverage with varying sugar content isn’t uniform either, so it doesn’t have a single fixed composition.