Let's analyze the relationship between saturation pressure and various properties of water vapour to determine the correct statement.

Understanding Saturation Pressure and Saturation Temperature

When we talk about the saturation pressure and saturation temperature of water, we are referring to the conditions under which water can exist in equilibrium between its liquid and vapor phases. These two properties are directly related. For a pure substance like water, there is a unique saturation temperature for every saturation pressure, and vice versa, along the vapor-liquid saturation curve.

• As the pressure increases along the saturation curve, the corresponding saturation temperature also increases.
• As the pressure decreases along the saturation curve, the corresponding saturation temperature also decreases.

This relationship holds true until the critical point is reached.

Analyzing Enthalpy of Evaporation

The enthalpy of evaporation, also known as the latent heat of vaporization, is the amount of energy required to transform a unit mass of water from its liquid state to its gaseous (vapor) state at a constant pressure and temperature. This energy is used to overcome the intermolecular forces holding the liquid molecules together.

Consider what happens as the saturation pressure (and thus saturation temperature) increases. As the temperature of the liquid water approaches the critical temperature, the distinction between the liquid phase and the vapor phase diminishes. The intermolecular forces in the liquid become weaker relative to the kinetic energy of the molecules.

At the critical point, the liquid and vapor phases become indistinguishable, and the enthalpy of evaporation becomes zero. This means no energy is required to change the phase at the critical point because the distinction between liquid and vapor vanishes.

Therefore, as the saturation pressure and temperature increase along the saturation curve towards the critical point, the energy required for evaporation decreases.

Examining Specific Volume Change of Phase

The specific volume change of phase refers to the difference between the specific volume of the saturated vapor ($\nu_g$) and the specific volume of the saturated liquid ($\nu_f$), i.e., $\Delta \nu = \nu_g - \nu_f$.

As pressure increases along the saturation curve:

• The specific volume of the saturated liquid ($\nu_f$) increases slightly.
• The specific volume of the saturated vapor ($\nu_g$) decreases significantly.

The decrease in $\nu_g$ is much more substantial than the slight increase in $\nu_f$. Consequently, the difference $\nu_g - \nu_f$ decreases as pressure increases and approaches zero at the critical point, where $\nu_g = \nu_f = \nu_{critical}$.

Evaluating the Given Statements

Let's evaluate each statement based on our understanding:

1. Saturation temperature decreases

   This statement is incorrect. As discussed, saturation temperature increases when saturation pressure increases.

2. Enthalpy of evaporation decreases

   This statement is correct. As saturation pressure and temperature increase towards the critical point, the enthalpy of evaporation decreases, becoming zero at the critical point.

3. Enthalpy of evaporation increases

   This statement is incorrect. The enthalpy of evaporation decreases as saturation pressure increases.

4. Specific volume change of phase increases

   This statement is incorrect. The specific volume change of phase ($\nu_g - \nu_f$) decreases as saturation pressure increases towards the critical point.

Based on the analysis, the correct statement is that the enthalpy of evaporation decreases when the saturation pressure of water vapour increases.

Revision Table: Effects of Increasing Saturation Pressure of Water Vapour

Additional Information on Saturation Properties and Phase Change

The behavior of water during phase change is often represented on property diagrams, such as the T-v diagram or P-v diagram. The saturation curve on these diagrams illustrates the relationship between pressure, temperature, and specific volume during the phase transition. The region under the dome formed by the saturation curve represents the two-phase liquid-vapor mixture region.

• The critical point is the peak of the saturation dome. Above the critical point, the liquid and vapor phases cannot coexist in equilibrium, and there is no distinct phase transition from liquid to vapor; the substance exists as a supercritical fluid.
• The enthalpy of evaporation is represented on a T-s diagram or h-s diagram as the horizontal distance between the saturated liquid line and the saturated vapor line at a constant temperature (and pressure). This distance shrinks as temperature/pressure increase towards the critical point.
• The specific volume change of phase is related to the latent heat by the Clausius-Clapeyron equation: $\left(\frac{dP}{dT}\right)_{sat} = \frac{h_{fg}}{T \cdot \nu_{fg}}$, where $h_{fg}$ is the enthalpy of evaporation and $\nu_{fg} = \nu_g - \nu_f$. This equation also highlights the interdependence of pressure, temperature, enthalpy of evaporation, and specific volume change during phase transition.