Question

A pure substance is a substance of constant composition throughout its mass and follows

• A. One component, one phase
• B. One component, one or more phases
• C. More than one component, one phase
• D. More than one component, one or more phases

Hint:

A pure substance is one component (e.g., water, oxygen) and can exist in multiple phases while maintaining constant composition.

Answer 1

The Correct Option is B

Step 1: Define a pure substance.
A pure substance is a material with a uniform and constant chemical composition throughout its mass. This means it consists of only one component (a single chemical species), such as water, oxygen, or nitrogen. Step 2: Analyze the phase aspect.
A pure substance can exist in one or more phases (solid, liquid, gas) while maintaining its chemical composition. For example:
Water (H₂O) as a pure substance can exist as ice (solid), liquid water, or steam (gas), or in multiple phases (e.g., liquid and vapor during boiling), but it remains one component.
Step 3: Evaluate the options.
(1) One component, one phase: This is too restrictive. A pure substance can exist in multiple phases (e.g., water at its triple point). Incorrect.
(2) One component, one or more phases: This matches the definition. A pure substance has one component and can exist in one or more phases. Correct.
(3) More than one component, one phase: This describes a mixture (e.g., air, which is a mixture of gases in one phase), not a pure substance. Incorrect.
(4) More than one component, one or more phases: This also describes a mixture (e.g., oil and water, two components, two phases), not a pure substance. Incorrect.
Step 4: Select the correct answer.
A pure substance has one component and can exist in one or more phases, matching option (2).