The change of water from a gas to a liquid?

The process of changing liquid water into a gas is called evaporation. Energy is added to the liquid water - generally in the form of heat. When enough energy is added, it allows the most energetic molecules at the surface of the liquid to break free of the liquid and enter the gas phase. The more energy that is added, the more water molecules will have enough energy to move into the gas phase. If enough energy is added, all the molecules will leave the liquid and from then on adding energy will just make the gas warmer.

Another way to change liquid water into gas is to drop the pressure above the liquid. In this case, it takes less energy for the molecules in the liquid to get away from the liquid so some of them will already be energetic enough to get away.

Any mass of liquid water will have a range of energies for the molecules, some will be low energy, others high energy with each collision between the molecules, they either gain or loose energy. If they gain enough energy, they can become energetic enough to escape the liquid as a gas. The higher the pressure, the more energy is required to become a gas. If the liquid water is in contact with some gas - air for example. The liquid will evaporate until the gas is saturated with water vapor. At that point the rate of water molecules dropping out of the gas back into the water (because they don't have enough energy to stay in the gas) is equal to the rate of water molecules escaping from the liquid to become gas. This point is also known as "equilibrium".