Understanding Saturation Pressure Effects in Water Vapour

The question asks about the consequences when the saturation pressure of water vapour increases. We need to evaluate the provided statements based on thermodynamic principles.

Saturation Pressure and Temperature Relationship

For any pure substance, including water, the saturation pressure and saturation temperature are intrinsically linked. As the saturation pressure increases, the saturation temperature required for the liquid-vapour phase change also increases. Therefore, the statement "Saturation temperature decreases" is incorrect.

Saturation Pressure and Enthalpy of Evaporation

The enthalpy of evaporation (also known as latent heat of vaporization) is the energy required to convert a substance from liquid to vapour at constant temperature and pressure. Thermodynamic principles, particularly the Clausius-Clapeyron relation, show that along the saturation curve, as temperature and pressure increase, the enthalpy of evaporation typically decreases.

This means that less energy is required to vaporize a unit mass of water at higher saturation pressures (and corresponding higher saturation temperatures).

Evaluating Option B

Based on the established thermodynamic relationship:

• When the saturation pressure of water vapour increases, the corresponding saturation temperature also increases.
• Simultaneously, the enthalpy of evaporation decreases.

Therefore, the statement "Enthalpy of evaporation decreases" is the correct consequence.

Conclusion

The correct statement when saturation pressure of water vapour increases is that the enthalpy of evaporation decreases.