Phase Equilibria

In equilibrium, the number of degrees of freedom for a system with two phases (liquid and vapor) is zero because the temperature and pressure are fixed.

Step 1: Define the degree of freedom.
In a two-component system, the degree of freedom is given by the formula: where is the number of components and is the number of phases. Since the system is two-component, . Thus, the formula becomes:

Step 2: Conclusion.
The degree of freedom for the system is , which corresponds to option (2).