Purity from melting point range

Steps:

• Pure substances melt at a single, sharp temperature with no range.
• Impure substances show a melting range due to mixed components.
• Substance 1 melts exactly at 117°C, indicating purity.
• Substances 2, 3, and 4 melt over ranges (e.g., 36–40°C for 2, 101–105°C for 4, and similar for 3), indicating impurity.

Why B is correct:

• Impure substances melt over a temperature range, as defined by the depression and broadening of the melting point in mixtures (colligative property).

Why the others are wrong:

• A: Substances 2 and 3 could also be pure if their data showed sharp melting, but ranges indicate otherwise.
• C: Water melts sharply at 0°C, not over 101–105°C.
• D: Room temperature is ~20–25°C; Substance 2 melts at 36–40°C, so it's liquid at room temperature.

Final answer: B