From childhood, we all know one serious fact of life. In order to cool hot tea, it is necessary to pour it into a cold saucer and to blow continuously over its surface. When you are six or seven years old, you don’t really think about the laws of physics, you just take them as given or, physically speaking, take them for an axiom. However, comprehending science over time, we find interesting similarities between axioms and consistent evidence, smoothly translating our children's assumptions into adult theorems. The same goes for hot tea. None of us could have thought that such a method of cooling it is directly related to the evaporation of the liquid.
Process physics
In order to answer the question of what the rate of evaporation of a liquid depends, it is necessary to understand the physics of the process itself. Evaporation is the process of the phase transition of a substance from a liquid state of aggregation to a gaseous state. Any liquid substance can evaporate, including a very viscous one. In appearance, you cannot say that a certain jelly-like slurry can lose part of its mass due to evaporation, but under certain conditions this is exactly what happens. A solid can also evaporate, only such a process is called sublimation.
How is going
Having started to understand what the evaporation rate of a liquid depends on, it should be based on the fact that this is an endothermic process, that is, a process that takes place with the absorption of heat. The heat of the phase transition (heat of evaporation) transfers energy to the molecules of the substance, increasing their speed and increasing the likelihood of their detachment, while weakening the forces of molecular cohesion. Separating from the bulk of the substance, the fastest molecules break out beyond its borders, and the substance loses its mass. In this case, the released liquid molecules boil instantly, performing a phase transition process upon separation, and their output is already in a gaseous state.
Application
Understanding what causes the evaporation rate of a liquid to depend on, it is possible to correctly regulate the technological processes occurring on their basis. For example, the operation of an air conditioner, in which a refrigerant boils in a heat exchanger-evaporator, taking heat from a refrigerated room, or boiling water in the pipes of an industrial boiler, the heat of which is transferred to the needs of heating and hot water. Awareness of the conditions on which the rate of evaporation of a liquid depends, provides the opportunity to design and manufacture modern and technological equipment of compact dimensions and with an increased heat transfer coefficient.
Temperature
The liquid state of aggregation is extremely unstable. With our earthly n. at. (the concept of "normal conditions", that is, people suitable for life), it periodically tends to go into the solid or gaseous phase. How does this happen? What determines the rate of evaporation of a liquid?
The primary criterion is, of course, temperature. The stronger we heat the liquid, the more energy we bring to the molecules of the substance, the more molecular bonds we break, the faster the process of phase transition. Apotheosis is achieved with steady bubble boiling. Water boils at 100 º at atmospheric pressure. The surface of a pot or, for example, a teapot, where it boils, is ideally smooth only at first glance. With a multiple increase in the picture, we will see endless sharp peaks, like in the mountains. Heat is supplied pointwise to each of these peaks, and because of the small heat exchange surface, water instantly boils, forming a bubble of air that rises to the surface, where it collapses. That is why this boiling is called bubble. The rate of evaporation of water is maximum.
Pressure
The second important parameter on which the rate of evaporation of a liquid depends is pressure. With a decrease in pressure below atmospheric, water begins to boil at lower temperatures. The work of famous pressure cookers is based on this principle - special pots from where air was pumped out, and water boiled already at 70-80 º. An increase in pressure, on the contrary, increases the boiling point. This useful property is used when supplying superheated water from the CHPP to the central heating and heat treatment plants, where, in order to maintain the potential of the transferred heat, the water is heated to temperatures of 150-180 degrees, when it is necessary to exclude the possibility of boiling in pipes.
Other factors
Intensive blowing of the liquid surface with a temperature higher than the temperature of the supplied air stream is another factor on which the rate of liquid evaporation depends. Examples of this can be taken from everyday life. Blowing wind over the surface of the lake or the example with which we started the story: blowing hot tea poured in a saucer. It cools due to the fact that, breaking away from the bulk of the substance, the molecules take part of the energy with them, cooling it. Here you can also see the effect of surface area. The saucer is wider than the mug, so a larger amount of the mass of water can potentially leave its square.
The type of liquid itself also affects the evaporation rate: some liquids evaporate faster, others, on the contrary, slower. An important influence on the evaporation process is also exerted by the state of the surrounding air. At high absolute moisture content (very humid air, for example, near the sea), the evaporation process will go slower.