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The critical state of matter

Above was explained, what to turn vapor into a liquid is necessary to increase the pressure and lowering temperature. In this way, English scientist M. Faraday was able to turn to a liquid state many substances, which were previously known only in the gaseous state. However, some gases have long been able to translate in a liquid state even at very high pressures. The theoretical explanation for these failures gave Russian scientist D. And. Mendeleev.

Border, separating liquid from the environment, is the free surface of the liquid. The presence of this surface gives us the ability to precisely specify, where is the liquid phase of matter where gaseous. Such a sharp distinction between the liquid and its vapor is mainly due to the, what, generally speaking, the density of a liquid is many times more, than its vapor. However, if you heat liquid in a hermetically closed vessel, it is due to the expansion of the liquid its density will decrease, and vapor density above the surface will increase. This means, what is the difference between a liquid and its vapor saturating in the process of such heating is more diffuse and at a sufficiently high temperature should disappear altogether.

In 1861 g. Mendeleev set, for each fluid must exist such a temperature, in which every distinction disappears between liquid and steam. Mendeleev called it "the temperature of the absolute boiling". Experimentally investigated the process of turning vapor to liquid and returning at different pressures the English scientist T. Andrews. He showed, such a temperature for each fluid really exists, and introduced her to a new term: the critical temperature, which is currently in use.

Critical temperature tToP substances called thief in the temperature, when the density of the fluid and the density of its saturating vapor become the same. Schedule of changes in the density of water and its saturating vapor depending on temperature is shown in figure. 8.8; the figure shows, for water the critical temperature tToPcorresponds to 374°C. Because not only the density, but the pressure of saturating steam is uniquely determined by its temperature, it is possible to construct a graph of pressure p against temperature t for saturating vapor (rice. 8.9).

Saturating vapor pressure of a substance at its critical temperature is called critical pressure tToP. It is the greatest possible pressure of saturating vapour of this substance. For water pCU=22,1*106 PA. When a critical temperature specific heat of vaporization of water is equal to zero. This also applies to other liquids. Therefore, when the critical temperature is lost every distinction between the liquid and its vapor, and the boundary between them disappears. This means, what if the temperature is above tкp substance can exist in only one state, which is called the gaseous, and in this case no increase of pressure to turn it into a liquid cannot.

If the substance is at critical temperature and critical pressure, the state called the critical state. Volume, occupied by a substance under critical condition, is called the critical volume tToP. It is the largest, who can take the existing mass of the substance in the liquid state. In tables are usually given the values of the critical volume for one mole of a substance. T valuesToP, gCU and VCU(for one mole) called critical parameters of substances (table. 8.3).

To observe the transition of a substance through a critical condition when heated the air in the ampoule (rice. 8.10, a — g). In the manufacture of the vial would be inserted in a mass of air, a volume in critical condition equal to the inner volume of the ampoule. When cooled, when the temperature is below a critical, the air becomes liquid (rice. 8.10, d, e).

Now you can see, what is the fundamental difference between gas and vapor is not. Usually the gas is called a substance in the gaseous state, when its temperature is above the critical. Steam also revive the substance in the gaseous state, but when its temperature is below the critical. Therefore, steam can be transferred to the liquid only pressure increase, and gas is not.

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