The boiling point and melting point of a substance differ by 80 degrees Celsius. If the boiling point is 56 C, what is the melting point?

• A. 24 C
• B. 36 C
• C. -24 C
• D. -36 C

Answer: (C)

The temperatures where a substance changes phase are distinct: melting point is where a solid becomes a liquid, and boiling point is where a liquid becomes a gas. The difference between these two temperatures is given as 80°C. If the boiling point is 56°C, the melting point must be 56 − 80 = −24°C. So the substance melts at −24°C, meaning it stays solid until that temperature is reached and becomes liquid above it. The other values would not give an 80-degree gap (for example, 56 − 24 = 32, 56 − 36 = 20, or 56 − (−36) = 92).