In the world around us, a huge variety of different physical phenomena and processes are constantly and continuously happening. One of the important can be considered the evaporation process. 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 vapor state. It is characteristic only for substances of liquid consistency. However, something similar is observed in solids, only this phenomenon is called sublimation. This can be seen with careful observation of the bodies. For example, a bar of soap dries up over time and begins to crack, this is due to the fact that water droplets in its composition evaporate and pass into a gaseous state of H 2 O.
Definition in physics
Evaporation is an endothermic process in which the heat of 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 a break occurs between the connected molecules;
- the heat required during the work of expansion of the molecules in the process of converting liquid substances into steam or gas.
How does this happen?
The transition of a substance from a liquid to a gaseous state can occur in two ways:
- Evaporation is a process in which molecules evaporate 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 liquid matter into 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, then it occurs as follows:
- Molecules in liquid substances are in constant chaotic motion, they all have completely different speeds. Meanwhile, particles are attracted to each other due to attractive forces. Each time they collide with each other, their speeds change. At some point, some develop a very high speed, which allows them to overcome the forces of attraction.
- These elements, which appeared on the surface of the liquid, have such kinetic energy that they are able to overcome intermolecular bonds and leave the liquid.
- It is these very fast molecules that fly off 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 is becoming smaller. This explains the cooling of the liquid. Remember how in childhood we were taught to blow on hot liquid so that it would soon cool down. It turns out that we accelerated the process of evaporation of water, and the decline in temperature occurred much faster.
What factors depends on?
There are many conditions necessary for the occurrence of this process. It occurs 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 the leaves of plants. It can be concluded that evaporation is a very significant and indispensable process for the environment and all living things.
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 whose heat of vaporization is lower will convert faster. Compare two processes: the evaporation of alcohol and ordinary water. In the first case, the conversion to the gaseous state occurs faster, because the specific heat of vaporization and condensation in alcohol is 837 kJ / kg, and in water almost three times as much - 2260 kJ / kg.
- The speed also depends on the initial temperature of the liquid: the larger it is, the faster steam is formed. As an example, let's take a glass of water when there is boiling water inside the vessel, then the evaporation occurs at a much greater 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 in a large-diameter plate, hot soup cools faster than in a small saucer.
- The rate of propagation of substances in the air largely determines the rate of evaporation, that is, the faster diffusion occurs, the faster vaporization occurs. For example, with 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 will talk about this in more detail below.
What is the role of air humidity?
Due to the fact that the evaporation process takes place everywhere continuously and continuously, particles of water are always present in the air. In molecular form, they look like a group of H 2 O elements. Liquids can evaporate depending on the volume indicator of water vapor in the atmosphere, this coefficient is called air humidity. It is of two types:
- Relative humidity is the ratio of the amount of water vapor in the air to the density of saturated steam at the same temperature as a percentage. For example, an indicator of 100% indicates that the atmosphere is completely saturated with H 2 O molecules.
- Absolute, however, 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 evaporation process and air humidity can be determined as follows. The lower the relative humidity, the faster the evaporation from the surface of the earth and other objects.
Evaporation of various substances
For various 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 a moonshine still only when it reaches the boiling point, it is approximately 78 degrees. However, the actual evaporation temperature of alcohol will be slightly higher, because in the original product (for example, Braga) it represents compounds with various aromatic oils and water.
Condensation and Sublimation
The following phenomenon can be observed every time water boils in the kettle. Please note that when boiling, water changes from a liquid to a gaseous state. This happens in this way: a hot stream of water vapor flies out of the kettle with a high speed through its spout. In this case, the resulting vapor is not visible directly at the exit of the nozzle, but at a small distance from it. This process is called condensation, that is, 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 gaseous, 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, assuming a liquid state. However, in some cases, at negative temperatures, ice transforms into a vaporous form, bypassing the liquid phase.
The effect of evaporation on the human body
Thanks to evaporation, thermoregulation occurs in our body. This process occurs through a self-cooling system. On a hot, sultry day, a person who does certain physical labor gets very hot. This means that it increases internal energy. And as you know, at temperatures above 42 °, the protein in the blood of a person begins to curl up, 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 extremely acceptable, active sweating begins through the pores on the skin. And then evaporation takes place from the surface of the skin, which absorbs excess body energy. In other words, evaporation is a process that helps cool the body to a normal state.