A sample of a pure substance is cooled from 200°C to -50°C at 1 atm. During cooling, it condenses from gas to liquid at 120°C and then freezes from liquid to solid at 10°C. Which of the following statements correctly describes the changes that occur?
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Correct answer
A. The substance is a gas at 150°C and a liquid at 50°C.
Principle or equation
At constant pressure, a pure substance changes state at characteristic temperatures: gas condenses to liquid at the boiling point, and liquid freezes to solid at the melting point. Above the boiling point it is gas, between boiling and melting points it is liquid, and below the melting point it is solid.
Why this answer is correct
The condensation temperature is 120°C, so above 120°C the substance is gas. The freezing point is 10°C, so between 10°C and 120°C it is liquid, and below 10°C it is solid. At 150°C (above 120°C) it is gas; at 50°C (between 10°C and 120°C) it is liquid. Thus option A is correct.
Example
For water at 1 atm, boiling point 100°C and freezing point 0°C: at 120°C it is gas, at 50°C it is liquid, at -10°C it is solid.
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