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:

For water-like substances (expand on freezing): - Solid-liquid equilibrium line in \(P-T\) diagram slopes negatively. - Triple point marks the highest temperature of solid-liquid coexistence.

Answer 1

The Correct Option is A, D

Step 1: Understanding substance that expands on freezing.
Water is the common example. When it freezes, volume increases. This implies the slope of solid-liquid line in \(P\)-\(T\) diagram is negative (Clapeyron equation).

Step 2: Check each statement.
(A) True. At pressures below triple point, liquid can exist at temperatures lower than triple point (due to negative slope). (B) False. Critical pressure is the pressure at critical point, not at triple point. (C) False. Vapour can exist even above triple point pressure until critical point. (D) True. At triple point, the maximum temperature for solid-liquid coexistence is defined.

Step 3: Correct statements.
Thus, correct = (A) and (D). Final Answer:
\[ \boxed{(A) \text{ and } (D)} \]