We are all well aware of one serious life fact from childhood. In order to cool hot tea, it is necessary to pour it into a cold saucer and blow it over its surface for a long time. When you are six or seven years old, you don’t really think about the laws of physics, you just take them for granted or, in physical terms, you take them as an axiom. However, as we learn science over time, we discover interesting similarities between axioms and consistent proofs, smoothly translating our childhood assumptions into adult theorems. The same goes for hot tea. None of us could have imagined that this way of cooling it is directly related to the evaporation of liquid.
Physics of the process
In order to answer the question of what determines the rate of evaporation of a liquid, it is necessary to understand the very physics of the process. Evaporation is the process of phase transition of a substance from a liquid state of aggregation to a gaseous state. Any liquid substance can evaporate, including very viscous. In appearanceand you can’t 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 this process is called sublimation.
How it happens
Starting to figure out what the rate of evaporation of a liquid depends on, one should start from the fact that this is an endothermic process, that is, a process that takes place with the absorption of heat. The heat of phase transition (heat of evaporation) transfers energy to the molecules of a substance, increasing their speed and increasing the likelihood of their separation, while weakening the forces of molecular cohesion. Breaking away from the bulk of the substance, the fastest molecules break out of its boundaries, and the substance loses its mass. At the same time, the ejected liquid molecules instantly boil, carrying out the process of phase transition upon separation, and their exit is already in the gaseous state.
Application
Understanding the reasons on which the rate of evaporation of a liquid depends, it is possible to correctly regulate the technological processes occurring on their basis. For example, the operation of an air conditioner, in the heat exchanger-evaporator of which the refrigerant boils, taking heat from the cooled room, or the boiling of water in the pipes of an industrial boiler, the heat of which is transferred to the needs of heating and hot water supply. Understanding the conditions on which the rate of evaporation of a liquid depends provides an opportunity to design and manufacture modern and technological equipment of compact dimensions and with an increased coefficientheat transfer.
Temperature
Liquid state of aggregation is extremely unstable. With our earthly n. y. (the concept of "normal conditions", i.e. suitable for human life), it periodically tends to move into a solid or gaseous phase. How does this happen? What determines the rate of evaporation of a liquid?
The primary criterion is, of course, the temperature. The more we heat the liquid, the more energy we bring to the molecules of the substance, the more molecular bonds we break, the faster the phase transition process goes. Apotheosis is achieved with a steady nucleate boil. Water boils at 100°C at atmospheric pressure. The surface of a pot or, for example, a kettle, where it boils, is only at first glance perfectly smooth. 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 due to the small heat exchange surface, water instantly boils, forming an air bubble that rises to the surface, where it collapses. That is why such boiling is called bubbly. The rate of water evaporation is maximum.
Pressure
The second important parameter, on which the rate of evaporation of a liquid depends, is pressure. When the pressure drops below atmospheric, water begins to boil at lower temperatures. The work of the famous pressure cookers is based on this principle - special pans, from where the air was pumped out, and the water boiled already at 70-80 ºС. The increase in pressure, on the other hand,increases the boiling point. This useful property is used when supplying superheated water from a thermal power plant to the central heating and ITP, where, in order to maintain the potential of transferred heat, 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 surface of the liquid with a temperature higher than the temperature of the supplied air jet is another factor that determines the rate of evaporation of the liquid. Examples of this can be taken from everyday life. Blowing the surface of the lake with the wind, or the example with which we started the story: blowing hot tea poured into a saucer. It cools down 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. A saucer is wider than a mug, so more mass of water can potentially escape from its square.
The type of liquid itself also affects the rate of evaporation: some liquids evaporate faster, others, on the contrary, slower. The state of the surrounding air also has an important influence on the evaporation process. If the absolute moisture content is high (very humid air, such as near the sea), the evaporation process will be slower.
