A substance is heated from -20°C to 150°C at 1 atm. Its temperature remains constant at 50°C for a period, then rises, then remains constant at 120°C. Which of the following correctly describes the changes of state that occur?
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Correct answer
A. The substance melts at 50°C and boils at 120°C.
Principle or equation
During heating, a pure substance's temperature remains constant at its melting point (solid to liquid) and at its boiling point (liquid to gas).
Why this answer is correct
The first plateau at 50°C corresponds to melting (solid to liquid), and the second plateau at 120°C corresponds to boiling (liquid to gas).
Example
For water, melting at 0°C and boiling at 100°C show plateaus on a heating curve.
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