Question

when evaporation takes place on a uppper surface of a liquid temperature of the remaining liquid drops. explain

Answer 1

Evaporation happens only at the surface of a liquid and occurs at any temperature as long as the substance is a liquid at that temperature (between the melting point and the boiling point of that liquid). Evaporation is dependent on individual particles gaining enough energy to escape the surface of the liquid and become gas particles. However, we are aware that liquids evaporate faster at a higher temperature.

But where does that energy come from?

The energy comes from the liquid, hence, the temperature drops due to the change in energy.