A sample of a pure substance is heated from -30°C to 180°C at 1 atm. The temperature remains constant at 40°C for a period, then rises, then remains constant at 140°C. Which of the following correctly describes the changes of state that occur?
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Correct answer
A. The substance melts at 40°C and boils at 140°C.
Principle or equation
During heating at constant pressure, the temperature remains constant during a phase change. The first plateau is the melting point (solid to liquid), and the second is the boiling point (liquid to gas).
Why this answer is correct
The substance is initially solid. At 40°C, the temperature stops rising, indicating a phase change from solid to liquid (melting). After that, temperature rises until 140°C, where it stops again, indicating the liquid to gas transition (boiling).
Example
For water, heating from -10°C to 120°C shows plateaus at 0°C (melting) and 100°C (boiling).
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