Evaporation is ... The process of phase transition of a substance from a liquid state to a vapor

In the world around us, a lot of different physical phenomena and processes are constantly and continuously happening. One of the important is the process of evaporation. There are several mandatory conditions for this phenomenon. In this article, we will discuss each of them in more detail.

What is evaporation?

This is the process of converting substances into a gaseous or vaporous state. It is characteristic only of substances of liquid consistency. However, something similar is observed in solid bodies, only this phenomenon is called sublimation. This can be seen with careful observation of the bodies. For example, a piece of soap dries up over time and starts to crack, this is because the droplets of water in its composition evaporate and go to the gaseous state of H ₂ O.

Definition in physics

Evaporation is an endothermic process, in which the heat of the phase transition serves as a source of absorbed energy. It includes two components:

• A certain amount of heat necessary to overcome the molecular forces of attraction, when there is a gap between the connected molecules;
• The heat required for the work of the expansion of molecules in the process of the transformation of liquid substances into vapor or gas.

How does this happen?

The transition of a substance from a liquid state to a gaseous state can occur in two ways:

1. Evaporation is a process in which molecules evaporate from the surface of a liquid substance.
2. Boiling is the process of vaporization from a liquid by adjusting the temperature to the specific heat of the substance boiling.

Despite the fact that both of these phenomena convert a liquid substance into a gas, there are significant differences between them. Boiling is an active process that takes place only at a certain temperature, while evaporation occurs under any conditions. Another difference is that boiling is characteristic of the entire thickness of the liquid, and the second phenomenon occurs only on the surface of liquid substances.

Molecular-kinetic theory of evaporation

If we consider this process at the molecular level, it occurs as follows:

1. Molecules in liquid substances are in constant chaotic motion, they all have absolutely different velocities. Meanwhile, the particles are attracted to each other by the forces of attraction. Every time they collide with each other, their speed changes. At some point, some develop a very high speed, allowing you to overcome the forces of attraction.
2. These elements, which appeared on the surface of the liquid, have such a kinetic energy that they are able to overcome intermolecular bonds and leave the liquid.
3. It is these very fast molecules that escape from the surface of the liquid substance, and this process takes place continuously and continuously.
4. Once in the air, they turn into steam - it's called steaming.
5. As a consequence, the average kinetic energy of the remaining particles becomes ever smaller. This explains the cooling of the liquid. Remember how in childhood we were taught to blow on a hot liquid, so that it would soon cool off. It turns out that we accelerated the process of evaporation of water, and the temperature drop occurred much faster.

On what factors depends?

There are many conditions necessary for the emergence of this process. It occurs from everywhere, where there are particles of water: it is lakes, seas, rivers, all wet objects, covers of bodies of animals and people, as well as leaves of plants. It can be concluded that evaporation is a very significant and indispensable process for the surrounding world and all living beings.

Here are the factors that influence this phenomenon:

1. The rate of evaporation depends on the composition of the liquid itself. It is known that each of them has its own characteristics. For example, those substances, in which the heat of vaporization is lower, will be transformed faster. Let's compare two processes: evaporation of alcohol and ordinary water. In the first case, the conversion into a gaseous state occurs faster, because the specific heat of vaporization and condensation in alcohol is 837 kJ / kg, and in water it is almost three times more - 2260 kJ / kg.
2. The speed also depends on the initial temperature of the liquid: the higher it is, the faster steam is produced. As an example, take a glass of water, when there is boiling water inside the vessel, the vaporization occurs at a much higher rate than when the water temperature is lower.
3. Another factor determining the rate of flow of this process is the surface area of the liquid. Remember that in a plate of large diameter hot soup cools faster than in a small saucer.
4. The speed of distribution of substances in the air environment largely determines the rate of evaporation, i.e., the faster the diffusion occurs, the faster the vaporization takes place. For example, in strong winds, water droplets evaporate faster from the surface of lakes, rivers and reservoirs.
5. The air temperature in the room also plays an important role. More on this, we'll talk just below.

What is the role of air humidity?

Due to the fact that the process of evaporation occurs from everywhere continuously and continuously, there are always particles of water in the air. In molecular form, they look like a group of elements H ₂ O. Liquids can evaporate depending on the volume of water vapor in the atmosphere, this coefficient is called air humidity. It can be of two types:

1. Relative humidity is the ratio of the amount of water vapor in the air to the density of saturated vapor at the same temperature as a percentage. For example, the 100% indicator indicates that the atmosphere is completely saturated with H ₂ O molecules.
2. The absolute characterizes the density of water vapor in the air, is denoted by the letter f and shows how much water molecules are contained in 1 m ^{3 of} air.

The relationship between the process of evaporation and air humidity can be determined as follows. The lower the relative humidity of air, the faster evaporation from the surface of the earth and other objects will occur.

Evaporation of various substances

For different substances this process proceeds in different ways. For example, the evaporation of alcohol occurs faster than in many liquids due to its low specific heat of vaporization. Often, such liquid substances are called volatile, because water vapor literally volatilize from them practically at any temperatures.

Alcohol can also evaporate even at room temperature. In the process of cooking wine or vodka, alcohol is expelled through the brewing apparatus, only reaching the boiling point, it is approximately equal to 78 degrees. However, the actual temperature of evaporation of alcohol will be slightly greater, because in the initial product (for example, brag) it is compounds with various aromatic oils and water.

Condensation and sublimation

The next phenomenon can be observed every time the water in the kettle boils. Note that when boiling water passes from the liquid state to the gaseous state. This happens in this way: a hot jet of water vapor flies from the kettle through its spout with great speed. At the same time, the formed steam is visible not at the exit from the spout, but at a small distance from it. This process is called condensation, i.e., water vapor condenses to such an extent that it becomes visible to our eyes.

Evaporation of a solid is called sublimation. In this case, they pass from the aggregate state to the gaseous state, bypassing the liquid stage. The most famous case of sublimation is associated with ice crystals. In its original form, ice is a solid, at temperatures above 0 ° it begins to melt, assuming a liquid state. However, in some cases, at negative temperatures, ice passes into a vaporous form, bypassing the liquid phase.

The effect of evaporation on the human body

Thanks to evaporation in our body, thermoregulation takes place. This process takes place through a self-cooling system. On a hot, sultry day, a person who engages in certain physical labor becomes very hot. This means that it increases the internal energy. And as you know, at a temperature above 42 °, the protein in the person's blood begins to curdle, if you do not stop this process in time, it will lead to death.

The self-cooling system is arranged just in such a way as to regulate the temperature for normal vital activity. When the temperature becomes extremely permissible, active sweating begins through the pores on the skin. And then, from the surface of the skin, evaporation takes place, which absorbs excess body energy. In other words, evaporation is a process that helps to cool the body to a normal state.