As said above, what state of matter is dependent on external conditions, first and foremost, from pressure and temperature. Therefore, for each substance based on experimental data can be the state chart in the coordinates g and T, which is easy to identify, in what condition will be the substance and that would happen when changing external conditions.

In figure. 12.3 schematically shows such a diagram for the substance, when in this space, in addition to this substance, nothing. The curve of the COP there are already known dependence of the saturation vapor pressure of the substance taken from the temperature, where K — critical point (rice. 8.9), and the point C corresponds To the temperature of hardening liquid under pressure to its saturation vapor (the loss of energy of that substance). The curve AC expresses the temperature dependence of saturation vapor pressure, located above the surface of a solid body. The melting point of a substance depends on the pressure, in the diagram the line BC is shown and the dependence.

Each point on the graph corresponds to equilibrium state of matter, t. e. this, in which it can remain indefinitely. Part of the diagram to the left of the line ACB corresponds to the solid state of matter; region, limited line FAC, — liquid, and the area to the right of the line ASC — gaseous state. Line the COP corresponds to the equilibrium liquid and gas phases, the line BC is the equilibrium liquid and solid phases and AC — the balance of solid and gaseous phases.

Under constant external conditions (p and T), corresponding to any point on the lines of the equilibrium phase AC, MO or the COP, two phases of a substance can be in mobile balance, in which from one phase to another moves the same number of molecules. This equilibrium can be maintained indefinitely, if energy is not supplied to the substance and not removed from it.

Point match only for the substance of value p and T, at which all three phases of that substance can be in equilibrium. The point C on the diagram of States of matter, which represents the equilibrium between all three phases of this substance, called the triple point. The water, for example, at the triple point the pressure is equal 610 PA, and the temperature equal 273,16 To (this temperature is used to determine Kelvin).

If external conditions change (R or T, or p and T at the same time), the point, according to these conditions, moves on the chart (for example, heating or cooling at constant pressure corresponds to moving the point on horizontal line). When the point on the graph moves from one area to another, there is a transition of substance from one state to another. So, when passing through the line BC is melting or crystallization, through the COP — evaporation or condensation, using AC — sublimation or desublimate. Therefore, the equilibrium line of the phases of the sun, The COP and the AU called lines of phase transitions, and a state diagram — the diagram of phase transitions.

Recall, what phase transitions are associated with changes in the internal energy of a substance and occur with absorption (or highlighting) the heat of phase transformation — heat of fusion (crystallization), of vaporization (condensation), sublimation (desublimation).

On the state diagram (rice. 12.3) see, what is sublimation and desublimation possible at temperatures and pressures less, than the triple point. So, ice can vozgonaetsa only at temperatures below 273,16 To, when the pressure of water vapor above the surface is less than the saturation pressure of water vapor.

Carbon dioxide at the triple point has a temperature, equal -56,6°C, and pressure 5,11 ATM. Therefore, at atmospheric pressure, carbon dioxide can exist only in solid or gaseous state, and "dry ice" turns directly, and gas; at normal pressure the temperature of its sublimation is equal to -78°C.

The temperature and pressure at the triple point of various substances for various. Therefore, in most cases under normal conditions of sublimation not see.

It turns out, the pressure and temperature at the triple point for the solution is always less than, than for pure solvent.

Line ST. in most cases, slightly deflected from the vertical to the right of point C, and ice, bismuth, gallium, Germany, silicon is left. The water at the point p=610Па (4,58 mm Hg. article) and T=273.16 of the To (t. e. 0,01°C), and at normal pressure (p = 1,013*10⁵ PA, or 760 mm Hg. article) the melting point of ice is 273,15 To (0°C).

Note, what is in an unstable state, the liquid may be within the scope a pair (superheated liquid) or in the solid phase (supercooled liquid). Supersaturated vapor can also be in the field of fluid or in the solid state. However, the solid phase always transforms into liquid or gaseous on the curve ACB. Thus, superheated crystals don't exist.

Important features of the state diagram of helium (rice. 12.4). This chart shows, what is the equilibrium solid phase with the liquid and the liquid phase with gaseous never intersect, t. e. in helium has no triple point. Other substances with such a feature is unknown.

The critical temperature of helium is equal to 5,25 To. Therefore, helium can be converted to liquid state, just cool it below this temperature. Experiments, made P. L. Kapitsa, showed, that at low pressures helium remains liquid even at temperatures, arbitrarily close to absolute zero. All other substances pass into the solid state at much higher temperatures. The helium passes into the solid state only under pressure in some tens atmospheres (rice. 12.4). Line sublimation in helium is missing, t. e. solid helium under no circumstances can not be in equilibrium with its vapor.

Liquid helium has an important feature. At temperatures above 2,19 It has the usual liquefied gases properties and it is called helium-I. When helium, under pressure its saturating vapour, cooled below the temperature 2,19 To, there is a sharp change in its properties, and he (remaining liquid) passes into a new state, in which it is called helium-II. In this state, helium is a mixture of two liquids, one of which is a conventional helium-I, and the other is a superfluid component, absolutely devoid of viscosity. These two components can move freely within one another without interaction between them. The superfluid component flows without friction through narrow capillaries and slits.

The chart (rice. 12.4) the field of the existence of helium-I and helium-II are separated by a dashed line. Superfluid component, formed at the transition of helium I—helium II, increases with further decreasing temperature, and at absolute zero, the entire liquid helium needs to go to a superfluid state.

The phenomenon of superfluidity of helium, open P. L. Kapitsa, it was explained on the basis of quantum mechanics, an outstanding Soviet scientist L. D. Landau. According to quantum theory the energy of molecules at absolute zero is not zero, as it follows from the classical kinetic theory of matter. Even molecules at absolute zero have so-called zero-energy — lowest possible energy for them. Helium the interaction forces between atoms are very small, and zero energy of helium is sufficient, to prevent the helium atoms form a crystal lattice. Only with the help of a large external pressure can bring the helium atoms are so, so they formed a crystal. Superfluid component in he-P, appears when temperatures, close to absolute zero, and consists of helium atoms with zero energy.