In the world around us, a huge variety of different physical phenomena and processes are constantly and continuously happening. One of the most important is the process of evaporation. There are several prerequisites for this phenomenon. In this article, we will analyze each of them in more detail.
What is evaporation?
This is the process of converting substances into a gaseous or vaporous state. It is typical only for substances of liquid consistency. However, something similar is observed in solids, only this phenomenon is called sublimation. This can be seen by careful observation of the bodies. For example, a bar of soap dries out over time and begins to crack, this is due to the fact that the water droplets in its composition evaporate and go into a gaseous state H2O.
Definition in physics
Evaporation is an endothermic process in which the source of absorbed energy is the heat of phase transition. It includes two components:
- a certain amount of heat needed to overcome the molecular forces of attraction when there is a break between the connected molecules;
- heat required for the work of expanding molecules in the process of converting liquid substances into vapor or gas.
How is this happening?
The transition of a substance from a liquid state to a gaseous state can occur in two ways:
- Evaporation is a process in which molecules escape from the surface of a liquid substance.
- Boiling is the process of vaporization from a liquid by bringing the temperature to the specific heat of boiling of a substance.
Despite the fact that both of these phenomena transform a liquid substance into a gas, there are significant differences between them. Boiling is an active process that occurs 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, while 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, then it happens as follows:
- Molecules in liquid substances are in constant chaotic motion, they all have completely different speeds. Meanwhile, the particles are attracted to each other due to the forces of attraction. Every time they collide with each other, their speeds change. At some point, some develop very high speed, allowing them to overcome the forces of gravity.
- These elements, which appeared on the surface of the liquid, have such kinetic energy that they are able to overcomeintermolecular bonds and leave the liquid.
- It is these very fastest molecules that fly out from the surface of a liquid substance, and this process takes place constantly and continuously.
- Once in the air, they turn into steam - this is called vaporization.
- As a consequence of this, the average kinetic energy of the remaining particles becomes smaller and smaller. This explains the cooling of the liquid. Remember how in childhood we were taught to blow on a hot liquid so that it cools down faster. It turns out that we accelerated the process of water evaporation, and the temperature dropped much faster.
What factors does it depend on?
There are many conditions necessary for this process to occur. It comes from everywhere where particles of water are present: these are lakes, seas, rivers, all wet objects, the covers of the bodies of animals and people, as well as plant leaves. 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:
- The evaporation rate directly 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 converted faster. Let's compare two processes: evaporation of alcohol and ordinary water. In the first case, the conversion to a gaseous state occurs faster, because the specific heat of vaporization and condensation for alcohol is 837 kJ / kg, and for water almost three timesmore - 2260 kJ/kg.
- The speed also depends on the initial temperature of the liquid: the higher it is, the faster the steam is formed. As an example, let's take a glass of water, when there is boiling water inside the vessel, then vaporization occurs at a much higher rate than when the water temperature is lower.
- Another factor that determines the speed of this process is the surface area of the liquid. Remember that hot soup cools faster in a large diameter bowl than in a small saucer.
- The rate of distribution of substances in the air largely determines the rate of evaporation, that is, the faster diffusion occurs, the faster vaporization occurs. For example, in strong winds, water droplets evaporate faster from the surface of lakes, rivers and reservoirs.
- The air temperature in the room also plays an important role. We'll talk more about this below.
What is the role of air humidity?
Due to the fact that the process of evaporation occurs from everywhere continuously and constantly, there are always particles of water in the air. In molecular form, they look like a group of elements H2O. Liquids can evaporate depending on the volume of water vapor in the atmosphere, this coefficient is called air humidity. It comes in two types:
- 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, a score of 100% indicatesthat the atmosphere is completely saturated with molecules of H2O.
- The absolute characterizes the density of water vapor in the air, denoted by the letter f and shows how much water molecules are contained in 1m3 air.
The connection between the evaporation process and air humidity can be determined as follows. The lower the relative humidity of the air, the faster the evaporation from the surface of the earth and other objects will occur.
Evaporation of various substances
In different substances, this process proceeds differently. For example, alcohol evaporates faster than many liquids due to its low specific heat of vaporization. Often such liquid substances are called volatile, because water vapor literally evaporates from them at almost any temperature.
Alcohol can also evaporate even at room temperature. In the process of preparing wine or vodka, alcohol is driven through the moonshine, only reaching the boiling point, which is approximately equal to 78 degrees. However, the actual evaporation temperature of alcohol will be slightly higher, because in the original product (for example, mash) it is a combination with various aromatic oils and water.
Condensation and sublimation
The following phenomenon can be observed every time the water boils in the kettle. Note that when water boils, it changes from a liquid state to a gaseous state. It happens in this way: a hot jet of water vapor withflies out of the kettle through its spout at high speed. In this case, the formed steam is not visible directly at the exit from the spout, but at a short distance from it. This process is called condensation, i.e. water vapor thickens to such an extent that it becomes visible to our eyes.
The evaporation of a solid is called sublimation. At the same time, they pass from the state of aggregation 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°C, it begins to melt, taking on a liquid state. However, in some cases, at negative temperatures, ice passes into a vapor form, bypassing the liquid phase.
Effect of evaporation on the human body
Thanks to evaporation, thermoregulation occurs in our body. This process takes place through a self-cooling system. On a hot sultry day, a person who is engaged in certain physical labor becomes very hot. This means that it increases the internal energy. And as you know, at a temperature above 42 ° protein in human blood begins to fold, if this process is not stopped in time, it will lead to death.
The self-cooling system is designed just in such a way as to regulate the temperature for normal life. When the temperature becomes the maximum allowable, active sweating begins through the pores on the skin. And then from the surface of the skin occursevaporation, which absorbs excess body energy. In other words, evaporation is a process that contributes to the cooling of the body to a normal state.
