# What is evaporation? Condensation?

Dec 26, 2016

Evaporation and condensation are phase changes between a liquid and a gas.

#### Explanation:

Evaporation is the movement of molecules to from the liquid phase to the gaseous state at less than the boiling point.

Condensation is the reverse: molecules in the gaseous state returning to the liquid phase.

The average kinetic energy of the molecules of the molecules in a liquid is less than the energy needed to break the intermolecular attraction that holds them together.

However the average is not true of all the molecules. Some of the molecules have more than the average amount of kinetic energy.

These molecules have enough energy to break free from the other liquid molecules and become gas.

Given enough time and an average energy less than ${100}^{\circ}$C, all the molecules in liquid water will become gas, and evaporate.

Condensation is the return of molecules from the gaseous state to the surface of the liquid.

For example, an individual water molecule will remain in suspension in the atmosphere. When enough water molecules join together, the molecules will form a "rain drop" or snow crystal and fall to the earth.

Also when the temperature drops below the "dew point", the kinetic energy of the water molecules is not enough to remain in suspension. When they hit a cold surface, they condense or join together and become a liquid. This condensation or "dew" is often seen on cold surfaces such as car windows.