Breaking the bond evaporation

A molecule in a mass of liquid water is pulled by the molecules around it, but it is pulled equally strongly from every direction. If it is at the surface of a body of liquid, however, it is pulled from the sides and from below, but not from above, so it is not quite so securely held. If it can acquire a little more energy, the molecule will move faster and faster until the hydrogen bonds linking it to its neighbors break and the molecule is free to enter the air.

It then enters the layer of air immediately above the surface. This boundary layer contains water molecules that left the liquid earlier. There is a limit to the number of water molecules the layer can hold, and if the arrival of more takes the total above the limit—the water vapor in the layer is then said to be saturated—some molecules will return to the water. Although, strictly speaking, it is the water vapor that is saturated, we usually think of saturation in terms of the air holding the water vapor and talk of the air being saturated.

If a water molecule enters air that is cooler than the water surface it escaped from, it loses energy and as soon as it approaches another water molecule a hydrogen bond forms between them. That is why the molecule returns to the body of water. If the air above the water is dry, however, the water molecule can remain. Air is dry either because it contains very few water molecules, in which case they rarely approach each other closely enough to form groups linked by hydrogen bonds, or because it is warm, in which case the molecules are moving too rapidly for hydrogen bonds to form. When the air above a water surface is dry, water molecules will pass through the layer immediately above the water surface, enter the air beyond that layer, and disperse. The amount of water vapor in the air will increase and the amount of liquid water will decrease. This is evaporation.

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