Answer to Question #99653 in Molecular Physics | Thermodynamics for Dheeraj

Question

A beaker contains some water at room temperature. A student places a mercury-in-glass thermometer in the water with the bulb of the thermometer just beneath the surface of the water.
The student uses an electric fan to blow air across the open top of the beaker. She notices that the reading on the thermometer begins to decrease.
Question : Explain, in terms of water molecules, why the temperature of the water at the surface begins to decrease.

Answer 1

Over any liquid, its saturated vapors are present. When the surface is blown off, the steam above the water is simply blown off by the fan and the steam becomes unsaturated. And it turns out that the molecules from the water go into the environment trying to compensate for the fact that the vapor above the surface is unsaturated. At the same time, flying away water molecules take energy. As a result of this, the total energy of the system decreases, which leads to a decrease in temperature.