A sample of a pure substance is heated from -50°C to 200°C at 1 atm. The temperature remains constant at 0°C for a period, then rises, then remains constant at 100°C. Which of the following correctly describes the changes of state that occur?
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Correct answer
A. The substance melts at 0°C and boils at 100°C.
Principle or equation
At constant pressure, a pure substance changes state at fixed temperatures: melting (solid to liquid) and boiling (liquid to gas). Constant temperature during heating indicates a phase change.
Why this answer is correct
The temperature plateau at 0°C indicates a phase change from solid to liquid (melting), and the plateau at 100°C indicates a change from liquid to gas (boiling).
Example
For water, the melting point is 0°C and the boiling point is 100°C at 1 atm.
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