Which of the following is good evidence that an unknown sample is a pure substance?

• A. It dissolves in water to form a cloudy mixture.
• B. It has a constant melting point.
• C. Its color changes when exposed to air.
• D. It can be separated into two components by filtration.

Answer: (B)

Verified step by step guidance

1. Understand the definition of a pure substance: a material that has a fixed composition and distinct properties, such as a constant melting point.
2. Recognize that a constant melting point is a key physical property of a pure substance because impurities typically cause melting point depression and broadening.
3. Analyze the other options: dissolving to form a cloudy mixture suggests a suspension or mixture, not purity; color change upon exposure to air indicates a chemical reaction or instability, not purity; separation by filtration implies the sample is a mixture, not a pure substance.
4. Conclude that the presence of a constant melting point is strong evidence that the sample is a pure substance because it reflects uniform composition and consistent physical behavior.
5. Therefore, the correct evidence for purity is that the sample has a constant melting point.