Tap water is placed in an open pan and allowed to evaporate naturally. After some time, how does the temperature of the remaining water typically change?

Introduction / Context:
Evaporation is a common everyday process in which a liquid slowly changes into vapour at the surface, even at temperatures below its boiling point. People often notice that water or sweat on the skin feels cool as it evaporates. This question asks what happens to the temperature of tap water left in an open pan when evaporation is allowed to occur over some time under ordinary conditions.

Given Data / Assumptions:

• Tap water is placed in a shallow, open pan exposed to air.
• The water is allowed to evaporate at room temperature without external heating.
• The process is slow and occurs over a moderate time interval.
• Ambient air conditions are reasonably constant.

Concept / Approach:
Evaporation is a cooling process. The most energetic molecules near the surface of the liquid escape into the vapour phase, taking away more kinetic energy on average than the molecules that remain. As a result, the average kinetic energy of the remaining molecules decreases, which means the temperature of the liquid falls. However, heat is also exchanged with the surrounding air and container, so the temperature does not keep falling indefinitely. Under normal room conditions, the water temperature drops slightly below ambient, producing mild cooling, but it does not decrease rapidly to very low values.

Step-by-Step Solution:
Step 1: Recognise that during evaporation, high energy molecules leave the liquid surface and become vapour. Step 2: Understand that the remaining liquid molecules have lower average kinetic energy after these energetic molecules depart. Step 3: Lower average kinetic energy corresponds to a lower temperature of the remaining liquid. Step 4: At the same time, the liquid can absorb heat from the surrounding air and the pan, partially offsetting the cooling. Step 5: In typical room conditions, the overall effect is a modest drop in water temperature below the starting value. Step 6: Therefore, the best description is that the temperature decreases slightly, not that it increases or drops extremely rapidly.

Verification / Alternative check:
Practical experience supports this explanation. If you wet your hand and hold it in moving air, it feels cool but does not freeze. Similarly, water in an evaporating dish used in laboratories feels slightly cooler than the surrounding air while evaporation is active. If the water is left long enough, it eventually comes back close to room temperature as the rate of evaporation decreases, but during active evaporation the net effect is mild cooling.

Why Other Options Are Wrong:
It increases to a very high value: There is no external heat source to raise the temperature significantly; evaporation removes energy rather than adding it.

It increases slightly: This contradicts the fundamental idea that evaporation is a cooling process where higher energy molecules leave the liquid.

It decreases extremely rapidly to freezing: Under normal room conditions, evaporation is not fast enough, and heat exchange with the surroundings prevents such a rapid temperature drop.

Common Pitfalls:
Some learners confuse boiling with evaporation and think any phase change to vapour must involve heating, leading them to expect a temperature increase. Others know evaporation causes cooling but overestimate the effect, imagining that water could quickly become very cold or freeze purely due to evaporation in ordinary conditions. Remember that evaporation cools the liquid by a modest amount, and the extent of cooling depends on humidity, air flow and heat supply from the environment.

Final Answer:
When tap water evaporates in an open pan, its temperature usually decreases slightly below the starting value.