A pure substance is cooled from 150°C to -40°C at 1 atm. During cooling, it condenses from gas to liquid at 80°C and then freezes from liquid to solid at -10°C. Which of the following statements correctly describes the changes of state?
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Correct answer
C. The substance is a solid at -20°C and a gas at 90°C.
Principle or equation
At a given pressure, a pure substance has characteristic boiling and melting points. Above the boiling point it is a gas, between boiling and melting points it is a liquid, and below the melting point it is a solid.
Why this answer is correct
The substance condenses at 80°C, so it is a gas above 80°C and a liquid between 80°C and -10°C. It freezes at -10°C, so it is a solid below -10°C. At 90°C it is a gas; at -20°C it is a solid. Therefore option C is correct.
Example
Water boils at 100°C and freezes at 0°C. At 120°C it is a gas, at 50°C it is a liquid, and at -10°C it is a solid.
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