Evaporation of a liquid produces cooling of the liquid because the

Step 1: Analyze the question We are asked why evaporation causes cooling of the remaining liquid.

Step 2: The concept Molecules in a liquid move at different speeds. The fastest, most energetic ones can overcome the attractive forces holding them in the liquid and escape from the surface as vapour. This is evaporation.

Step 3: Reasoning

• When the fastest molecules leave, the molecules that remain are the slower ones.
• So the average kinetic energy of the molecules left in the liquid falls.
• Temperature is a measure of the average kinetic energy of the molecules, so the liquid's temperature drops. This is the cooling effect.

Checking the options

• A. Attraction between vapour and liquid molecules being great would tend to hold molecules back. It is not the reason for cooling.
• B. Average kinetic energy of the liquid is reduced. Correct, as explained above.
• C. Density change is not the cause of cooling.
• D. Rising vapour pressure above the liquid actually slows further evaporation. It does not explain cooling.
• E. Volume decrease is a result of evaporation, not the reason for cooling.

Tip: Remember the chain "fastest molecules escape → average KE of the rest drops → temperature drops." This is why sweating cools your body and why a wet cloth feels cold. Any NECO, WASSCE or JAMB question on evaporative cooling is usually pointing to the reduction in average kinetic energy.