A liquid is in equilibrium with its vapour at its boiling point. On the average, the molecules in the two phases have equal:
1)potential energy
2)total energy
3)kinetic energy
4)intermolecular forces.

Options

A. potential energy
B. total energy
C. kinetic energy
D. intermolecular forces.

Explanation

To solve the question, we need to analyze the conditions under which a liquid is in equilibrium with its vapor at the boiling point.

### Step-by-Step Solution:

1. **Understanding Equilibrium**: At the boiling point, a liquid is in equilibrium with its vapor. This means that the rate of evaporation of the liquid equals the rate of condensation of the vapor.

2. **Temperature at Boiling Point**: At the boiling point, both the liquid and vapor phases are at the same temperature. This is a key point because temperature is directly related to the kinetic energy of the molecules.

3. **Kinetic Energy Relation**: According to the kinetic theory of gases, the average kinetic energy of gas molecules is given by the equation:
\[ KE = \frac{3}{2} k_B T \]
where \( k_B \) is the Boltzmann constant and \( T \) is the temperature in Kelvin. This shows that kinetic energy is directly proportional to temperature.

4. **Equal Kinetic Energy**: Since both phases (liquid and vapor) are at the same temperature at the boiling point, the average kinetic energy of the molecules in the liquid phase will be equal to the average kinetic energy of the molecules in the vapor phase.

5. **Conclusion**: Therefore, the correct answer is that on average, the molecules in the two phases (liquid and vapor) have equal **kinetic energy**.

### Final Answer:
3) kinetic energy