Liquid substances and their properties. Liquid state of matter

In everyday life, we are constantly confronted with three states of matter - liquid, gaseous and solid. About what solid bodies and gases are, we have a fairly clear idea. Gas is a collection of molecules that move randomly in all directions. All molecules of a solid body retain their mutual arrangement. They make only minor fluctuations.

Features of 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 inherent. However, at the same time, liquid substances, like gases, take the form 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 fluid is a sphere. Gravity usually prevents it from taking this form, so the liquid either takes the form of a vessel, or spreads over the surface in a thin layer.

In terms of its properties, the liquid state of the substance is particularly difficult, which is due to its intermediate position. It began to be studied since the time of Archimedes (2200 years ago). However, an analysis of how molecules of a liquid substance behave is still one of the most difficult areas of applied science. There is still no universally recognized and complete theory of liquids. However, we can say something about their behavior quite definitely.

Behavior of molecules in a liquid

A liquid 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 towards them becomes increasingly less orderly, and then the order disappears altogether. Liquid substances consist of molecules that move much more freely than in solids (and in gases - even more freely). Within a certain time, each of them rushes from one side to the other, not moving away from its neighbors. However, the fluid molecule breaks out of the environment from time to time. She falls into a new one, moving to another place. Here again, for a certain time, she performs similar motion fluctuations.

Y. I. Frenkel's contribution to the study of liquids

I. I. I. Frenkel, a Soviet scientist, has great merit in developing a whole series of problems on a topic such as liquid substances. Chemistry has made great progress thanks to its discoveries. He believed that in liquids the thermal motion has the following character. Within 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 the molecule itself. In other words, molecules move inside the liquid, but slowly. Part 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 movements. Oscillations in one place after a while are replaced by a free transition from place to place.

Pressure in a liquid

Some properties of liquid matter are known to us through constant interaction with them. So, from the experience of everyday life, we know that it acts on the surface of solids that come into contact with it, with known forces. They are called the forces of fluid pressure.

For example, by opening the tap hole with your finger and including water, we feel how it presses on the finger. A swimmer who dived to great depth, not accidentally experiencing pain in the 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 water temperature at sea depth, very strong thermometers should be used to prevent them from crushing the pressure of the liquid.

This pressure is caused by compression, that is, by changing the volume of the liquid. It has elasticity with respect to this change. Forces of pressure - this is the force of elasticity. Consequently, if the liquid acts on the bodies in contact with it, then it is compressed. Since the density of matter under compression increases, it can be assumed that the fluids with respect to density change have elasticity.

Evaporation

Continuing to consider the properties of liquid matter, we proceed to evaporation. Near the surface of it, and also directly in the surface layer, there are forces that ensure the very existence of this layer. They do not allow the molecules in it to leave the volume of liquid. However, some of them, thanks to the thermal motion, develop rather high speeds, with 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, but with its increase the evaporation rate increases.

Condensation

If the molecules that left the liquid are removed from the space near its surface, then all of it eventually evaporates. If the molecules that leave it are not removed, they form steam. Falling into the region near the surface of the liquid, the vapor molecules are drawn into it by the forces of attraction. This process is called condensation.

Consequently, if the molecules are not removed, the rate of evaporation decreases with time. If the vapor density further increases, a situation is achieved in which the number of molecules leaving the liquid in a certain time will equal the number of molecules that return to it in the same time. So there is a state of dynamic equilibrium. The vapor in it is called saturated. The pressure and density of it increase with increasing temperature. The higher it is, the more molecules of the liquid have sufficient energy for evaporation and the more dense the vapor must have in order that condensation can equalize with evaporation.

Boiling

When a temperature is reached during the heating of liquid substances, at which the saturated vapor has the same pressure as the external medium, equilibrium is established between the saturated vapor and the liquid. If the liquid informs an additional amount of heat, the corresponding mass of liquid immediately turns into steam. This process is called boiling.

Boiling is an intense evaporation of the liquid. It occurs not only from the surface, but concerns its entire volume. Inside the liquid, bubbles of steam appear. In order to convert to vapor from a liquid, molecules need to acquire energy. It is needed to overcome the forces of attraction, through which they are retained in the liquid.

Boiling temperature

The boiling point is the one at which two pressures are observed-external and saturated vapors. It increases with increasing pressure and decreases when it decreases. Because the pressure in the liquid changes with the height of the column, boiling occurs in it at different levels at different temperatures. Only saturated steam, which is above the surface of the liquid during boiling, 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 in different liquids, which is widely used in engineering, in particular, in the distillation of petroleum products.

The latent heat of vaporization is the amount of heat necessary to turn an isothermally defined quantity 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 a foam by dissolving soap in water. This is nothing more than a lot of bubbles, which are limited by a thin film consisting of liquid. However, a separate film can also be obtained from the foam forming liquid. Its properties are very interesting. These films can be very thin: their thickness in the thinnest parts does not exceed a one-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 this stability? In order for the film to appear, it is necessary to add to the pure liquid substances that dissolve in it. But not any, but such that significantly reduce the surface tension.

Liquid films in nature and technology

In technology and nature, we meet mainly not with individual films, but with foam, which is their totality. It can often be observed in streams, where small trickles fall into calm water. The ability of water to foams in this case is related to the presence in it of an organic substance, which is extracted from the roots of plants. This is an example of how natural liquid substances foam. But what about technology? In the construction, for example, use special materials that have a cellular structure resembling foam. They are light, cheap, strong enough, poorly conduct sounds and warmth. To obtain them in special solutions add foaming agents.

Conclusion

So, we learned what substances belong to the liquid, found out that the liquid is an intermediate state of matter between gaseous and solid. Therefore, it has properties that are characteristic of both. Liquid crystals, which are now widely used in engineering 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 kind of substances liquid science will invent in the future. However, it is clear that in this state of matter there is a great potential that can be used for the benefit of mankind.

Particular interest in the consideration of physico-chemical processes occurring in the liquid state is due to the fact that the person himself is 90% water, which is the most abundant liquid on the Earth. It is in it that all vital processes occur both in the plant and in the animal kingdom. Therefore, for all of us, it is important to study the liquid state of matter.