Question

For a pure substance that expands on freezing, which of the following statement(s) is/are CORRECT?

• A. Temperature of the liquid phase can be lower than the temperature at the triple point
• B. Critical pressure is equal to the pressure at the triple point
• C. Highest pressure at which the vapour phase can exist is the pressure at the triple point
• D. Highest temperature at which solid-liquid phase change can happen is the temperature at the triple point

Hint:

The triple point is a unique state where all three phases of a substance coexist in equilibrium. However, a substance can exist in the liquid phase below the triple point temperature if the pressure conditions are right.

Answer 1

The Correct Option is A, D

Let's analyze each statement:
- Option (A): This statement is correct. For a pure substance that expands upon freezing, the temperature of the liquid phase can indeed be lower than the temperature at the triple point. This behavior is observed in certain substances, like water, where the liquid phase can exist below the triple point under certain conditions, especially when pressure is reduced.
- Option (B): This statement is incorrect. The critical pressure is not necessarily equal to the pressure at the triple point. The critical point occurs at a higher temperature and pressure than the triple point, where the substance can no longer exist in distinct phases (solid, liquid, or gas).
- Option (C): This statement is incorrect. The highest pressure at which the vapour phase can exist is not the pressure at the triple point. The pressure at the triple point is where all three phases of the substance coexist, but the vapour phase can exist at pressures higher than the triple point, depending on the temperature.
- Option (D): This statement is correct. The highest temperature at which the solid-liquid phase change can occur is the temperature at the triple point. At this point, the solid and liquid phases are in equilibrium, and any temperature above this would lead to a phase transition from solid to liquid without going through the triple point.
Thus, the correct answers are (A) and (D).