answer:

Scenario 1: Melting point of two solid samples

SOLID A

Appearance: white crystalline powder

Observation while melting: Solid A melts completely within a short period of time.

SOLID B

Appearance: white crystalline powder

Observation while melting: Solid B has portions that seem to be not melting.

Note: The appearance of both substance as stated on the given scenario is that, both substance are white crystalline powder.

Question:

Which solid sample is a pure substance?

Answer: Solid A