Analyzing Water Phase States

This question requires understanding the specific conditions of water at its critical point and triple point. Let's break down what happens at these points and evaluate each statement.

Defining the Critical Point

The critical point represents the highest temperature and pressure at which a substance can exist as a liquid. Beyond this point, the liquid and gas phases merge into a single phase called a supercritical fluid. At the critical point ($T_c$, $P_c$), the properties of the saturated liquid and saturated vapor become identical. This means there's no longer a distinct boundary or difference between the liquid and vapor states.

Defining the Triple Point

The triple point is a unique combination of temperature and pressure where all three phases of a substance – solid, liquid, and gas (or vapor) – can coexist in thermodynamic equilibrium. For water, this occurs at a very specific temperature (0.01°C or 273.16 K) and a low pressure (0.6117 kPa). At this point, phase transitions between solid, liquid, and vapor are constantly occurring simultaneously.

Evaluating Each Statement

• Statement 1: "At the critical point, all the three phases of Water coexist in equilibrium." This is incorrect. Only the liquid and vapor phases merge into one at the critical point. The solid phase does not coexist here.
• Statement 2: "At the critical point, saturated liquid and saturated vapour phases are identical." This statement is correct. It accurately describes the critical point, where the distinction between liquid and vapor disappears, making their properties identical.
• Statement 3: "At the triple point, all the three phases of Water coexist in non-equilibrium." This is incorrect. The defining characteristic of the triple point is the coexistence of all three phases in stable equilibrium.
• Statement 4: "At the triple point, saturated liquid and saturated vapour phases are identical." This is incorrect. While liquid and vapor coexist at the triple point, they remain distinct phases with different properties. They only become identical at the critical point.

Summary of Water Phase Behavior

The critical point is where liquid and vapor phases become indistinguishable, while the triple point is where solid, liquid, and vapor phases coexist in equilibrium. Therefore, the statement accurately describing these conditions is the second one.