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 as the substance cools from 150°C to 0°C?
Show the answer and explanation
Correct answer
B. The substance changes from gas to liquid at 120°C, then remains liquid until 10°C, then changes to solid.
Principle or equation
A pure substance has characteristic phase transition temperatures at a given pressure. During cooling, the gas condenses to liquid at the condensation point, and the liquid freezes to solid at the freezing point.
Why this answer is correct
At 150°C, the substance is above its condensation point (120°C), so it is a gas. As it cools to 120°C, it condenses to liquid. Between 120°C and 10°C, it remains liquid. At 10°C, it freezes to solid. Therefore, the correct description is that it changes from gas to liquid at 120°C and remains liquid until 10°C, then changes to solid.
Example
For water at 1 atm, cooling steam from 150°C to -20°C: gas condenses at 100°C, liquid freezes at 0°C. Similarly, the given substance has a condensation point at 120°C and freezing point at 10°C.
This published item includes a stored explanation and passed the platform’s publication workflow. It is independent preparation material, not a claim of an official or recalled examination question.