Let's return to the graph of melting and crystallization of naphthalene (Fig. 198) and consider that part of it that relates to the cooling of naphthalene.

When molten naphthalene is cooled, its temperature decreases. But how as soon as the naphthalene begins to harden, the decrease in temperature stops, although naphthalene continues to give up its internal energy to the bodies surrounding it. After all, its temperature is higher than the temperature of these bodies. And until all the naphthalene hardens, its temperature does not change. But as soon as it all hardens, the temperature will begin to drop again.

This phenomenon is seen in every crystalline body. Why does the temperature of a crystalline body not decrease during its solidification?

We already know that at the solidification temperature, the internal energy of a body in a liquid state is greater than its internal energy in solid state. During the entire curing process, excess internal energy is released and replenishes the energy lost due to cooling. Therefore, the average energy of molecules, and, consequently, the temperature of the body remain unchanged until until the curing process is completed. From this moment, the temperature of the solid body will begin to decrease, since the loss of internal energy in it will no longer be replenished.

Carefully conducted experiments show that during the solidification of a crystalline substance exactly the same amount of heat is released that is absorbed during its melting. So, when solidifying water weighing 1 kg at a temperature of 0 ° C 3.4 10 6 J is released. But exactly the same amount of heat is required for the melting of ice weighing 1 kg at a temperature of 0 °C.

Questions.

1. How to explain that in the process of solidification of a substance its temperature 0 remains constant?
2. How much energy is released when 1 kg of water solidifies?

Exercises.

1. Melting ice was brought into a room with a temperature of 0°C. Will the ice in this room continue to melt?
2. Pieces of ice float in a bucket of water. The total temperature of water and ice is 0°C. Will the ice melt or the water freeze? What does it depend on?
3. How much energy does it take to melt 4 kg of ice at 0°C?
4. How much energy is required to melt 20 kg of lead at the melting point? How much energy is needed for this if initial temperature lead 27°C?

Tasks.

1. Place two identical cans on the stove. Pour 0.5 kg of water into one, put 0.5 kg of snow into the other. Note how long it takes for the water to boiled in both banks. Write a short account of your experience and explain the results.
2. Prepare a report on the topic "Metal Casting" by reading such a paragraph at the end of the textbook.

Much attention has been paid to the mutual transformations of liquids and gases. Now consider the transformation of solids into liquids and liquids into solids.

Melting of crystalline bodies

Melting is the transformation of a substance from a solid to a liquid state.

There is a significant difference between the melting of crystalline and amorphous bodies. In order for a crystalline body to begin to melt, it must be heated to a temperature that is quite specific for each substance, called the melting point.

For example, under normal atmospheric pressure the melting point of ice is 0 °C, naphthalene - 80 °C, copper - 1083 °C, tungsten - 3380 °C.

For the body to melt, it is not enough to heat it to the melting point; it is necessary to continue to supply heat to it, i.e., to increase its internal energy. During melting, the temperature of the crystalline body does not change.

If the body continues to be heated after it has melted, the temperature of its melt will rise. The above can be illustrated by a graph of the dependence of body temperature on the time of its heating (Fig. 8.27). Plot AB corresponds to the heating of a solid body, the horizontal section sun- melting process and plot CD - heating the melt. Curvature and slope of plot sections AB and CD depend on the process conditions (mass of the heated body, heater power, etc.).

The transition of a crystalline body from a solid to a liquid state occurs abruptly, abruptly - either a liquid or a solid body.

Melting of amorphous bodies

Amorphous bodies behave differently at all. When heated, they gradually, as the temperature rises, soften and eventually become liquid, remaining homogeneous during the entire time of heating. There is no definite transition temperature from solid to liquid. Figure 8.28 shows a plot of temperature versus time during the transition of an amorphous body from a solid to a liquid state.

Solidification of crystalline and amorphous bodies

The transfer of matter from liquid state into a solid is called solidification or crystallization(for crystalline bodies).

There is also a significant difference between the solidification of crystalline and amorphous bodies. When a molten crystalline body (melt) is cooled, it continues to remain in a liquid state until its temperature drops to a certain value. At this temperature, called the crystallization temperature, the body begins to crystallize. The temperature of the crystalline body does not change during solidification. Numerous observations have shown that crystalline bodies melt and solidify at the same temperature determined for each substance. With further cooling of the body, when the entire melt solidifies, the body temperature will decrease again. The foregoing is illustrated by a graph of the dependence of body temperature on the time of its cooling (Fig. 8.29). Plot BUT 1 AT 1 corresponds to liquid cooling, horizontal section AT 1 FROM 1 - crystallization process and plot C 1 D 1 - cooling the solid body resulting from crystallization.

Substances from a liquid state to a solid state during crystallization also pass abruptly without intermediate states.

The solidification of an amorphous body, such as resin, occurs gradually and equally in all its parts; the resin remains homogeneous, i.e. hardening amorphous bodies- it's just a gradual thickening of them. There is no specific curing temperature. Figure 8.30 shows a plot of curing resin temperature versus time.

In this way, amorphous substances do not have a certain temperature, melting and solidification.

Melting - the transition of the body from the crystalline solid state into liquid. Melting occurs with the absorption of the specific heat of fusion and is phase transition first kind.

The ability to melt refers to physical properties substances

At normal pressure, tungsten has the highest melting point among metals (3422 ° C), simple substances in general - carbon (according to various sources 3500 - 4500 ° C) and among arbitrary substances - hafnium carbide HfC (3890 ° C). We can assume that helium has the lowest melting point: at normal pressure, it remains liquid at arbitrarily low temperatures.

Many substances at normal pressure do not have a liquid phase. When heated, they immediately pass through sublimation into gaseous state.

Figure 9 - Melting ice

Crystallization is the process of a phase transition of a substance from a liquid state to a solid crystalline state with the formation of crystals.

A phase is a homogeneous part of a thermodynamic system separated from other parts of the system (other phases) by an interface, upon passing through which chemical composition, the structure and properties of matter change in jumps.

Figure 10 - Crystallization of water with the formation of ice

Crystallization is the process of separating a solid phase in the form of crystals from solutions or melts; in the chemical industry, the crystallization process is used to obtain substances in a pure form.

Crystallization begins when a certain limiting condition is reached, for example, liquid supercooling or vapor supersaturation, when many small crystals appear almost instantly - crystallization centers. Crystals grow by attaching atoms or molecules from a liquid or vapor. The growth of crystal faces occurs layer by layer, the edges of incomplete atomic layers (steps) move along the face during growth. The dependence of the growth rate on crystallization conditions leads to a variety of growth forms and crystal structures (polyhedral, lamellar, acicular, skeletal, dendritic and other forms, pencil structures, etc.). In the process of crystallization, various defects inevitably arise.

The number of crystallization centers and the growth rate are significantly affected by the degree of supercooling.

The degree of supercooling is the level of cooling of a liquid metal below the temperature of its transition into a crystalline (solid) modification. It is necessary to compensate for energy latent heat crystallization. Primary crystallization is the formation of crystals in metals (and alloys) during the transition from a liquid to a solid state.

Specific heat melting (also: enthalpy of melting; there is also an equivalent concept of specific heat of crystallization) - the amount of heat that must be imparted to one unit of mass of a crystalline substance in an equilibrium isobaric-isothermal process in order to transfer it from a solid (crystalline) state to a liquid (the same amount heat is released during the crystallization of a substance).

The amount of heat during melting or crystallization: Q=ml

> Evaporation and boiling. Specific heat of vaporization

Evaporation is the process of transition of a substance from a liquid state to a gaseous state (steam). The evaporation process is the reverse of the condensation process (transition from a vapor to a liquid state. Evaporation (vaporization), the transition of a substance from a condensed (solid or liquid) phase to a gaseous (steam); first-order phase transition.

There is a more detailed concept of evaporation in higher physics

Evaporation is a process in which the surface of a liquid or solid body particles (molecules, atoms) fly out (detach), while Ek > Ep.

Figure 11 - Evaporation over a mug of tea

Specific heat of vaporization (vaporization) (L) -- physical quantity, showing the amount of heat that must be imparted to 1 kg of a substance taken at the boiling point in order to transfer it from a liquid state to a gaseous state. The specific heat of vaporization is measured in J/kg.

Boiling is the process of vaporization in a liquid (the transition of a substance from a liquid to a gaseous state), with the appearance of phase separation boundaries. The boiling point at atmospheric pressure is usually given as one of the main physicochemical characteristics of a chemically pure substance.

Boiling is a first-order phase transition. Boiling occurs much more intensively than evaporation from the surface, due to the formation of foci of vaporization, due to both reached temperature boiling, and the presence of impurities.

The process of bubble formation can be influenced by pressure, sound waves, ionization. In particular, it is on the principle of boiling up microvolumes of liquid from ionization during the passage of charged particles that the bubble chamber operates.

Figure 12 - Boiling water

The amount of heat during boiling, liquid evaporation and vapor condensation: Q=mL