In everyday life, we constantly encounter three states of matter - liquid, gaseous and solid. We have a fairly clear idea of what solids and gases are. A gas is a collection of molecules that move randomly in all directions. All molecules of a solid body maintain their mutual arrangement. They only oscillate slightly.
Features of a liquid substance
And what are liquid substances? Their main feature is that, occupying an intermediate position between crystals and gases, they combine certain properties of these two states. For example, for liquids, as well as for solid (crystalline) bodies, the presence of volume is characteristic. However, at the same time, liquid substances, like gases, take the shape of the vessel in which they are located. Many of us believe that they do not have their own form. However, it is not. The natural form of any liquid -ball. Gravity usually prevents it from assuming this shape, so the liquid either assumes the shape of a vessel or spreads thinly over the surface.
In terms of its properties, the liquid state of a substance is especially complex, due to its intermediate position. It began to be studied since the time of Archimedes (2200 years ago). However, the analysis of how the molecules of a liquid substance behave is still one of the most difficult areas of applied science. There is still no generally accepted and completely complete theory of liquids. However, we can say something about their behavior quite definitely.
Behavior of molecules in a liquid
A fluid is something that can flow. The short-range order is observed in the arrangement of its particles. This means that the location of the neighbors closest to it, with respect to any particle, is ordered. However, as it moves away from others, its position in relation to them becomes less and less ordered, and then the order disappears altogether. Liquid substances are made up of molecules that move much more freely than in solids (and even more freely in gases). For a certain time, each of them rushes first in one direction, then in the other, without moving away from its neighbors. However, a liquid molecule breaks out of the environment from time to time. She gets to a new place by moving to another place. Here again, for a certain time, she makes wobble-like movements.
Y. I. Frenkel's contribution to the study of liquids
I. I. Frenkel, a Soviet scientist, has made great contributions to the development of a number ofproblems on such a topic as liquid substances. Chemistry advanced greatly thanks to his discoveries. He believed that thermal motion in liquids has the following character. For a certain time, each molecule oscillates around the equilibrium position. However, it changes its place from time to time, moving abruptly to a new position, which is separated from the previous one by a distance that is approximately the size of this molecule itself. In other words, inside the liquid, the molecules move, but slowly. Some of the time they stay near certain places. Consequently, their movement is something like a mixture of movements in the gas and in the solid body. Fluctuations in one place after a while are replaced by a free transition from place to place.
Pressure in liquid
Some properties of liquid matter are known to us due to constant interaction with them. So, from the experience of everyday life, we know that it acts on the surface of solid bodies that come into contact with it, with certain forces. They are called fluid pressure forces.
For example, when opening a water faucet with a finger and turning on the water, we feel how it presses on the finger. And a swimmer who has dived to great depths does not accidentally experience pain in his ears. It is explained by the fact that pressure forces act on the eardrum. Water is a liquid substance, so it has all its properties. In order to measure the temperature of water at a depth of the sea, very durablethermometers so they can't be crushed by fluid pressure.
This pressure is due to compression, that is, a change in the volume of the liquid. It has elasticity in relation to this change. The forces of pressure are the forces of elasticity. Therefore, if a fluid acts on bodies in contact with it, then it is compressed. Since the density of a substance increases during compression, we can assume that liquids have elasticity in relation to a change in density.
Continuing to consider the properties of a liquid substance, we turn to evaporation. Near its surface, as well as directly in the surface layer, forces act that ensure the very existence of this layer. They do not allow the molecules in it to leave the volume of the liquid. However, due to thermal motion, some of them develop rather high velocities, with the help of which it becomes possible to overcome these forces and leave the liquid. We call this phenomenon evaporation. It can be observed at any air temperature, however, with its increase, the intensity of evaporation increases.
If the molecules that have left the liquid are removed from the space near its surface, then all of it eventually evaporates. If the molecules that left it are not removed, they form steam. Vapor molecules that have fallen into a region near the surface of the liquid are drawn into it by the forces of attraction. This process is called condensation.
Hence,if the molecules are not removed, the rate of evaporation decreases over time. If the vapor density increases further, a situation is reached in which the number of molecules leaving the liquid in a certain time will be equal to the number of molecules returning to it in the same time. This creates a state of dynamic equilibrium. The vapor in it is called saturated. Its pressure and density increase with increasing temperature. The higher it is, the greater the number of liquid molecules has sufficient energy for evaporation and the greater the density of the vapor must be in order for condensation to equal evaporation.
When, in the process of heating liquid substances, a temperature is reached at which saturated vapors have the same pressure as the external environment, an equilibrium is established between saturated vapor and liquid. If the liquid imparts an additional amount of heat, the corresponding mass of liquid is immediately converted into vapor. This process is called boiling.
Boiling is the intense evaporation of a liquid. It occurs not only from the surface, but concerns its entire volume. Vapor bubbles appear inside the liquid. In order to go into vapor from a liquid, molecules need to acquire energy. It is needed to overcome the attractive forces that keep them in the liquid.
The boiling point is the one at whichthere is an equality of two pressures - external and saturated vapors. It increases as pressure increases and decreases as pressure decreases. Due to the fact that the pressure in the liquid changes with the height of the column, boiling in it occurs at different levels at different temperatures. Only saturated steam, which is above the surface of the liquid during the boiling process, has a certain temperature. It is determined only by external pressure. This is what we mean when we talk about the boiling point. It differs for different liquids, which is widely used in engineering, in particular, when distilling petroleum products.
Latent heat of vaporization is the amount of heat required to turn an isothermally defined amount of liquid into steam if the external pressure is the same as the saturated vapor pressure.
Properties of liquid films
We all know how to get foam by dissolving soap in water. This is nothing but a lot of bubbles, which are limited by the thinnest film consisting of liquid. However, a separate film can also be obtained from the foaming liquid. Its properties are very interesting. These films can be very thin: their thickness in the thinnest parts does not exceed a hundred-thousandth of a millimeter. However, they are sometimes very stable, despite this. The soap film can be subjected to deformation and stretching, a jet of water can pass through it without destroying it. How to explain such stability? In order for a film to appear, it is necessary to add substances that dissolve in it to a pure liquid. But not any, but such,which significantly lower surface tension.
Liquid films in nature and technology
In technology and nature, we mainly meet not with individual films, but with foam, which is their combination. It can often be observed in streams, where small streams fall into calm water. The ability of water to foam in this case is associated with the presence of organic matter in it, which is secreted by the roots of plants. This is an example of how natural liquid substances foam. But what about the technology? During construction, for example, special materials are used that have a cellular structure resembling foam. They are light, cheap, strong enough, poorly conduct sound and heat. To obtain them, foaming agents are added to special solutions.
So, we have learned what substances are liquid, found out that the liquid is an intermediate state of matter between gaseous and solid. Therefore, it has properties characteristic of both. Liquid crystals, which are widely used today in technology and industry (for example, liquid crystal displays) are a prime example of this state of matter. They combine the properties of solids and liquids. It is difficult to imagine what liquid substances science will invent in the future. However, it is clear that there is great potential in this state of matter that can be used for the benefit of humanity.
Special interest in the consideration of physical and chemical processes occurringin a liquid state, due to the fact that the person himself consists of 90% of water, which is the most common liquid on Earth. It is in it that all vital processes take place both in the plant and in the animal world. Therefore, it is important for all of us to study the liquid state of matter.