At critical point, the enthalpy of vaporization is zero. This is because the critical point is the highest temperature and pressure at which a substance can exist in equilibrium between its liquid and vapor phases. At this point, the liquid and vapor phases become indistinguishable, and the distinction between liquid and gas disappears.

Enthalpy of Vaporization:
The enthalpy of vaporization is the amount of heat energy required to convert a given quantity of a substance from its liquid phase to its gaseous phase at a constant temperature and pressure. It is a measure of the strength of the attractive forces between the molecules in the liquid phase.

Explanation:
At the critical point, the temperature and pressure are at their maximum values for the given substance. This means that the attractive forces between the molecules in the liquid phase have been weakened to the point where they are no longer able to hold the molecules together in a distinct liquid phase. As a result, the substance exists as a homogeneous mixture of liquid and vapor, with no clear boundary between the two phases.

Since there is no clear boundary between the liquid and vapor phases at the critical point, there is no heat energy required to overcome the attractive forces and convert the substance from one phase to another. Therefore, the enthalpy of vaporization is zero at the critical point.

Significance:
The enthalpy of vaporization is an important property of substances, as it determines their boiling points and the energy required for various industrial processes such as distillation and evaporation. At the critical point, the absence of enthalpy of vaporization means that substances can exist in a unique state where they possess the properties of both liquid and gas.

Conclusion:
In conclusion, at the critical point, the enthalpy of vaporization is zero because the liquid and vapor phases are indistinguishable and the attractive forces between the molecules in the liquid phase have been weakened to the point where they no longer exist. This unique state at the critical point has important implications for the behavior and properties of substances at high temperatures and pressures.