In everyday life, we are constantly confronted withthree 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 a 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.
By its properties the liquid state of matterespecially difficult, which is due to its intermediate position. It began to be studied from 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 liquid 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, it performs similar motion fluctuations.
Y. I. Frenkel's contribution to the study of liquids
ME AND.Frenkel, a Soviet scientist, has great merit in developing a whole series of problems on a subject 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 near the equilibrium position. However, it changes its place from time to time, moving abruptly to a new position, which from the previous one is 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. The oscillations in one place after a while are replaced by a free transition from place to place.
Pressure in a liquid
Some properties of a liquid substance are known to usdue to 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, opening a tap holetap the finger and turn on the 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 of 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 ischange in the volume of the liquid. It has elasticity with respect to this change. Forces of pressure - this is the force of elasticity. Therefore, 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 liquidsubstance, 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, however with its increase the evaporation rate increases.
Condensation
If the molecules that left the liquid are removed from thespace, located near its surface, then all of it, in the end, 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.
Therefore, if the molecules are not removed,the rate of evaporation decreases. If the vapor density further increases, a situation is achieved in which the number of molecules leaving the liquid for a certain time will equal the number of molecules that return during this time to it. 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 in the process of heating liquid substancesa temperature is reached at which saturated vapor has the same pressure as the external medium, an equilibrium is established between the saturated vapor and the liquid. If the liquid reports an additional amount of heat, the corresponding mass of liquid is immediately converted to steam. This process is called boiling.
Boiling is intense evaporationliquid. It occurs not only from the surface, but concerns its entire volume. Inside the liquid, bubbles of steam appear. In order to switch 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 whichthere is an equality of two pressures - external and saturated vapors. It increases with increasing pressure and decreases as it decreases. Due to the fact that 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 amountof the heat necessary to convert an isothermally determined 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,dissolving in water soap. This is nothing more than a lot of bubbles, which are limited to 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. 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 mainlynot 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 due 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? During 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 found out which substances belong toliquid, it was found 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 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 physical and chemicalof the processes taking place 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 the 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.
