Evaporation and Kinetic Energy Loss

Steps:

• Evaporation happens at the liquid surface where molecules must overcome intermolecular forces to escape as vapor.
• Molecules with higher kinetic energy have greater speed and are more likely to break free from the liquid.
• When these faster, more energetic molecules escape, they remove energy from the system.
• The remaining molecules thus have a lower average kinetic energy, cooling the liquid.

Why C is correct:

• According to the kinetic molecular theory, evaporation selectively removes high-KE molecules, reducing the average KE of those left behind, as KE is proportional to temperature (KE_avg = 3/2 kT).

Why the others are wrong:

• A: Less energetic molecules do not escape; higher KE ones do, so average KE decreases, not from low-KE loss.
• B: Less energetic molecules do not escape, so average KE does not increase.
• D: More energetic molecules escape, but this decreases average KE, not increases it.

Final answer: C