A pure substance is heated from -80°C to 250°C at 1 atm. Its temperature remains constant at -10°C for a period, then rises, then remains constant at 130°C. Which of the following correctly describes the changes of state that occur?
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Correct answer
A. Melting at -10°C and boiling at 130°C
Principle or equation
At constant pressure, a pure substance changes state at fixed temperatures: solid to liquid (melting) and liquid to gas (boiling).
Why this answer is correct
When heating a solid, the temperature stays constant during melting and boiling. The first plateau at -10°C corresponds to melting (solid to liquid), and the second plateau at 130°C corresponds to boiling (liquid to gas).
Example
For water at 1 atm, heating from -20°C to 120°C shows plateaus at 0°C (melting) and 100°C (boiling).
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