A sample of a pure substance is heated from -80°C to 120°C at 1 atm. The temperature remains constant at -20°C for a period, then rises, then remains constant at 80°C. Which of the following statements correctly describes the changes of state during this heating?
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Correct answer
A. The substance melts at -20°C and boils at 80°C.
Principle or equation
During heating at constant pressure, a pure substance shows constant temperature during phase changes. The first plateau corresponds to melting (solid to liquid) and the second to boiling (liquid to gas).
Why this answer is correct
The temperature plateaus indicate phase transitions. The lower plateau at -20°C is the melting point, and the higher plateau at 80°C is the boiling point. Thus, the substance melts at -20°C and boils at 80°C.
Example
For water, heating from -10°C to 120°C shows a plateau at 0°C (melting) and at 100°C (boiling).
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