In everyday life, we are constantly confronted with three states of matter - liquid, gaseous and solid. What solids and gases are, we have a pretty clear idea. Gas is a collection of molecules that move randomly in all directions. All molecules of a solid body maintain a relative position. They make only minor fluctuations.
Features of a liquid substance
But 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 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 liquid is a ball. Gravity usually prevents her from taking this form, so the liquid either takes the shape of a vessel or spreads out on the surface with a thin layer.
By its properties, the liquid state of a substance is especially difficult, due to its intermediate position. It began to be studied since the time of Archimedes (2200 years ago). However, an analysis of how liquid matter molecules behave is still one of the most difficult areas of applied science. There is still no universally recognized and completely finished theory of liquids. However, we can definitely say something about their behavior.
The behavior of molecules in a liquid
Fluid is something that can flow. The short-range order is observed in the arrangement of its particles. This means that the arrangement of the neighbors closest to it with respect to any particle is ordered. However, as she moves away from others, her position in relation to them becomes less orderly, and then the order disappears altogether. Liquid substances are composed of molecules that move much more freely than in solids (and in gases - even freer). Over a period of time, each of them rushes one way, then the other, without moving away from its neighbors. However, the liquid molecule from time to time breaks out of the environment. She gets into a new one, moving to another place. Here again, over a period of time, she commits oscillatory movements.
Contribution of Ya. I. Frenkel to the study of liquids
Ya. I. Frenkel, a Soviet scientist, owes great merit to the development of a whole series of problems devoted to such a topic as liquid substances. Chemistry has made great progress thanks to his discoveries. He believed that in liquids the thermal motion has the following character. Over a period of time, each molecule oscillates around an equilibrium position. However, it changes its place from time to time, moving abruptly to a new position, which is at a distance from the previous one, which is approximately the size of this molecule itself. In other words, inside the fluid, the molecules move, but slowly. Part of the time they stay near certain places. Therefore, their movement is something like a mixture of movements in a gas and in a solid. Oscillations in one place after some time are replaced by a free transition from place to place.
Fluid pressure
Some properties of liquid substances are known to us due to the 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 fluid pressure forces .
For example, when we open the tap opening with a finger and turn on the water, we feel how it presses on the finger. But a swimmer who dives to a great depth does not accidentally experience pain in his ears. It is explained by the fact that pressure forces act on the eardrum of the ear. Water is a liquid substance, therefore, it has all its properties. In order to measure the temperature of water at sea depth, very robust thermometers should be used so that they cannot be crushed by fluid pressure.
This pressure is due to compression, that is, a change in the volume of the liquid. It has elasticity with respect to this change. Pressure forces - this is the force of elasticity. Therefore, if a fluid acts on bodies in contact with it, then it is compressed. Since the density of a substance increases upon compression, it can be assumed that liquids have elasticity with respect to a change in density.
Evaporation
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 liquid volume. However, some of them, thanks to thermal motion, develop rather high speeds, 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, but with its increase the evaporation rate increases.
Condensation
If the molecules that leave the liquid are removed from the space located near its surface, then all of it, in the end, evaporates. If the molecules that leave it are not removed, they form steam. Trapped in a region near the surface of a liquid, vapor molecules are drawn into it by attractive forces. This process is called condensation.
Therefore, if molecules are not removed, the rate of evaporation decreases over 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 be equal to the number of molecules that return to it during the same time. So there is a state of dynamic equilibrium. The steam 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 higher the vapor density must be in order for condensation to equal the evaporation.
Boiling
When, during the heating of liquid substances, a temperature is reached at which the saturated vapor has the same pressure as the external environment, an equilibrium is established between the saturated vapor and the liquid. If the liquid provides an additional amount of heat, the corresponding mass of liquid is immediately converted into steam. 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 pass into the vapor from the liquid, the molecules must acquire energy. It is needed to overcome the forces of attraction, thanks to which they are held in liquid.
Boiling temperature
The boiling point is the one at which the equality of two pressures is observed - external and saturated vapors. It increases with increasing pressure and decreases with decreasing pressure. 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 located above the surface of the liquid during boiling has a certain temperature. It is determined only by external pressure. That is what we mean when we talk about the boiling point. It differs in different liquids, which is widely used in technology, in particular, in the distillation of petroleum products.
The latent heat of vaporization is the amount of heat needed to convert an isothermally determined amount of liquid into steam if the external pressure is the same as the saturated vapor pressure.
Liquid Film Properties
We all know how to get foam by dissolving soap in water. This is nothing but a multitude of bubbles, which are limited by a thin film consisting of a 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 one hundredth of a millimeter. However, they are sometimes very stable, despite this. The soap film can be subjected to deformation and stretching, a stream 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 clean liquid. But not any, but those that significantly reduce 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 in it of an organic substance that the roots of plants secrete. 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 lightweight, cheap, strong enough, and poorly conduct sounds and warmth. To obtain them in special solutions add foaming substances.
Output
So, we learned what substances are liquid, found out that liquid is an intermediate state of a substance 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 vivid 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 in this state of matter there is great potential that can be used for the benefit of mankind.
Of particular interest in the consideration of physicochemical processes occurring in a liquid state is due to the fact that man himself consists of 90% of water, which is the most common liquid on Earth. It is in it that all vital processes occur in both the plant and animal world. Therefore, it is important for all of us to study the liquid state of matter.