A sample of a pure substance is heated from -120°C to 350°C at 1 atm. The temperature remains constant at -50°C for a period, then rises, then remains constant at 200°C. Which of the following statements correctly describes the changes of state that occur?
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Correct answer
A. The substance melts at -50°C and boils at 200°C.
Principle or equation
During heating at constant pressure, a pure substance shows plateaus in the temperature-time curve at its melting point and boiling point. The first plateau corresponds to melting (solid to liquid), the second to boiling (liquid to gas).
Why this answer is correct
The sample starts as a solid (below -50°C). At -50°C, the temperature stays constant while the solid melts to liquid. After melting, the temperature rises until 200°C, where it stays constant while the liquid boils to gas. Thus, -50°C is the melting point and 200°C is the boiling point.
Example
For water, heating from -20°C to 120°C shows plateaus at 0°C (melting) and 100°C (boiling).
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