Vaporization, which occurs only from the free surface of the liquid, bordering on the gaseous environment or vacuum, is called evaporation.

Consider the evaporation process from the point of view of molecular kinetic theory. As you know, the potential energy of the molecules of the liquid by increasing the distance between them should increase. Therefore, to leave the liquid, the molecule must do work by reducing its kinetic energy. Among the randomly moving molecules of the liquid in the surface layer there are always molecules, seeking to depart from the fluid. When such a molecule is beyond the surface layer, then there is a force, pulling the molecule back into the liquid (rice. 7.1). So out of the fluid only those molecules, have the kinetic energy more work, necessary to overcome impeding forces in the molecular layer thickness r_m, where r_m — the radius of molecular action.

 Left the liquid molecules together constitute a vapor above its surface. As liquid molecules are emitted with sufficiently large kinetic energy, and remain in it molecule with a smaller kinetic energy, the average value of energy E_post For molecules, remaining in the liquid, in the process of evaporation decreases, t. e. the liquid is cooled by evaporation. This explains the sensation of cold upon exiting the water after swimming, cooling hands, soaked in ether, and t. p.

Some vapor molecules in chaotic motion over the surface of the liquid fly back to liquid. This means, that along with the evaporation of a liquid always takes place and condensation of its vapor. Since the vapor molecules, flying in a fluid, partially transfer their kinetic energy to the molecules of the liquid, when this occurs increase E_post The molecules of the liquid, t. e. the increase in internal energy of the liquid (the heated fluid).

So, on the surface of the liquid at the same time evaporation and condensation. When the process of evaporation, liquid cooled, and when condensation predominates, the liquid is heated.

Since the strength of molecular interactions depend on the nature of molecules, the evaporation rate depends on type of liquid. This dependence is easy to detect on experience. If in the same open vessels to pour equal volumes of different liquids, then after a while you will see, that the rapidity of evaporation of these fluids are different. It turns out, the ether evaporates faster than alcohol, and the alcohol faster than water.

Experience shows, the rate of evaporation of liquid depends on the area of its free surface. The more this area, the faster evaporates the liquid. (Give examples, confirming this relationship, and explain it.)

Evaporation occurs at any temperature. Therefore, over the free liquid surface is always in the vapor of this liquid.

It is easy to notice, that with increasing temperature the rate of evaporation of liquid increases. For example, hot water evaporates faster than cold. The reason is, that with increasing temperature increases the average kinetic energy of the molecules of the liquid and becomes larger such molecules, which can overcome the counteraction of the surface layer of the liquid and fly beyond.

As noted above, along with the evaporation of a liquid always takes place and condensation of its vapor, which reduces the rate of evaporation. By increasing the density of the vapour molecules above the liquid surface vapor condensation increases and the evaporation is slowed down. Therefore, less than the density of vapor of liquid over its surface, the greater the rate of evaporation. Therefore pumping vapor from the surface accelerates the evaporation.

It, the cooling fluid in the process of evaporation is not always visible. The reason is, between the liquid and surrounding the heat exchange bodies exist, which compensates for the loss of her energy, slimming cooling liquid. However, for a high evaporation rate, the fluid temperature can drop significantly.