According to the kinetic molecular theory, how does the average kinetic energy of particles change during a phase change at constant temperature?

• A. It fluctuates randomly due to energy transfer.
• B. It decreases because particles slow down during the phase change.
• C. It increases rapidly as the substance absorbs heat.
• D. It remains constant because temperature does not change during the phase change.

Answer: (D)

Verified step by step guidance

1. Recall that according to the kinetic molecular theory, the average kinetic energy of particles is directly proportional to the temperature of the substance. This relationship is given by the equation \(\text{KE}_{\text{avg}} = \frac{3}{2} k_B T\), where \(k_B\) is the Boltzmann constant and \(T\) is the absolute temperature.
2. Understand that during a phase change (such as melting or boiling), the temperature of the substance remains constant even though heat is being added or removed. This is because the energy goes into changing the phase rather than increasing the temperature.
3. Since the temperature remains constant during the phase change, the average kinetic energy of the particles also remains constant, because it depends only on temperature.
4. The energy added during a phase change is used to overcome intermolecular forces, changing the potential energy of the system, not the kinetic energy of the particles.
5. Therefore, the average kinetic energy does not increase or decrease during a phase change at constant temperature; it remains constant.