The same substance in the real world, depending on the surrounding conditions, can be in different states. For example, water can be in the form of a liquid, in the idea of ​​a solid body - ice, in the form of a gas - water vapor.

• These states are called aggregate states of matter.

Molecules of a substance in different states of aggregation do not differ from each other. A specific state of aggregation is determined by the arrangement of molecules, as well as the nature of their movement and interaction with each other.

Gas - the distance between molecules is much larger than the size of the molecules themselves. Molecules in a liquid and in a solid are quite close to each other. In solids even closer.

To change the aggregate state of the body, he needs to give some energy. For example, in order to convert water into steam, it must be heated. In order for steam to become water again, it must give up energy.

The transition from solid to liquid

The transition of a substance from a solid to a liquid state is called melting. In order for the body to begin to melt, it must be heated to a certain temperature. The temperature at which a substance melts is called the melting point of the substance.

Each substance has its own melting point. For some bodies it is very low, for example, for ice. And some bodies have a very high melting point, for example, iron. In general, the melting of a crystalline body is a complex process.

ice melt chart

The figure below shows a graph of the melting of a crystalline body, in this case ice.

• The graph shows the dependence of the temperature of the ice on the time that it is heated. Temperature is plotted on the vertical axis, time is plotted on the horizontal axis.

From the graph, the initial temperature of the ice was -20 degrees. Then they started to heat it up. The temperature started to rise. Section AB is the section of ice heating. Over time, the temperature increased to 0 degrees. This temperature is considered the melting point of ice. At this temperature, the ice began to melt, but at the same time its temperature ceased to increase, although the ice also continued to heat up. The melting area corresponds to the BC section on the graph.

Then, when all the ice melted and turned into a liquid, the temperature of the water began to increase again. This is shown on the graph by ray C. That is, we conclude that during melting, the body temperature does not change, All incoming energy is used for heating.

The transition of a substance from a solid crystalline state to a liquid state is called melting. To melt a solid crystalline body, it must be heated to a certain temperature, that is, heat must be supplied.The temperature at which a substance melts is calledthe melting point of the substance.

The reverse process - the transition from a liquid to a solid state - occurs when the temperature drops, that is, heat is removed. The transition of a substance from a liquid to a solid state is calledhardening , or crystallysis . The temperature at which a substance crystallizes is calledcrystal temperaturetions .

Experience shows that any substance crystallizes and melts at the same temperature.

The figure shows a graph of the dependence of the temperature of a crystalline body (ice) on the heating time (from the point BUT to the point D) and cooling time (from point D to the point K). It shows time on the horizontal axis and temperature on the vertical axis.

It can be seen from the graph that the observation of the process began from the moment when the temperature of the ice was -40 °C, or, as they say, the temperature at the initial moment of time tearly= -40 °С (point BUT on the chart). With further heating, the temperature of the ice increases (on the graph, this is the area AB). The temperature rises to 0 °C, the melting point of ice. At 0°C, ice begins to melt and its temperature stops rising. During the entire melting time (i.e., until all the ice has melted), the temperature of the ice does not change, although the burner continues to burn and heat is therefore supplied. The melting process corresponds to the horizontal section of the graph Sun . Only after all the ice has melted and turned into water does the temperature begin to rise again (section CD). After the water temperature reaches +40 ° C, the burner is extinguished and the water begins to cool, i.e. heat is removed (for this, a vessel with water can be placed in another, larger vessel with ice). The water temperature begins to drop (section DE). When the temperature reaches 0 °C, the temperature of the water stops decreasing, despite the fact that heat is still removed. This is the process of crystallization of water - the formation of ice (horizontal section EF). Until all the water turns to ice, the temperature will not change. Only after this does the temperature of the ice begin to decrease (section FK).

The view of the considered graph is explained as follows. Location on AB due to the heat input, the average kinetic energy of the ice molecules increases, and its temperature rises. Location on Sun all the energy received by the contents of the flask is spent on the destruction of the crystal lattice of ice: the ordered spatial arrangement of its molecules is replaced by disordered, the distance between the molecules changes, i.e. molecules are rearranged in such a way that the substance becomes liquid. The average kinetic energy of the molecules does not change, so the temperature remains unchanged. A further increase in the temperature of molten ice-water (in the area CD) means an increase in the kinetic energy of water molecules due to the heat supplied by the burner.

When cooling water (section DE) part of the energy is taken away from it, water molecules move at lower speeds, their average kinetic energy drops - the temperature decreases, the water cools. At 0°C (horizontal section EF) molecules begin to line up in a certain order, forming a crystal lattice. Until this process is completed, the temperature of the substance will not change, despite the heat removed, which means that when solidifying, the liquid (water) releases energy. This is exactly the energy that the ice absorbed, turning into a liquid (section Sun). The internal energy of a liquid is greater than that of a solid. During melting (and crystallization), the internal energy of the body changes abruptly.

Metals that melt at temperatures above 1650 ºС are called refractory(titanium, chromium, molybdenum, etc.). Tungsten has the highest melting point among them - about 3400 ° C. Refractory metals and their compounds are used as heat-resistant materials in aircraft construction, rocketry and space technology, and nuclear energy.

We emphasize once again that during melting, the substance absorbs energy. During crystallization, on the contrary, it gives it to the environment. Receiving a certain amount of heat released during crystallization, the medium heats up. This is well known to many birds. No wonder they can be seen in winter in frosty weather sitting on the ice that covers rivers and lakes. Due to the release of energy during the formation of ice, the air above it turns out to be several degrees warmer than in the forest on the trees, and the birds take advantage of this.

Melting of amorphous substances.

The presence of a certain melting points is an important feature of crystalline substances. It is on this basis that they can be easily distinguished from amorphous bodies, which are also classified as solids. These include, in particular, glass, very viscous resins, and plastics.

Amorphous substances(unlike crystalline) do not have a specific melting point - they do not melt, but soften. When heated, a piece of glass, for example, first becomes soft from hard, it can be easily bent or stretched; at a higher temperature, the piece begins to change its shape under the influence of its own gravity. As it heats up, the thick viscous mass takes the shape of the vessel in which it lies. This mass is at first thick, like honey, then like sour cream, and, finally, it becomes almost as low-viscosity liquid as water. However, it is impossible to indicate a specific temperature for the transition of a solid to a liquid here, since it does not exist.

The reasons for this lie in the fundamental difference between the structure of amorphous bodies and the structure of crystalline ones. Atoms in amorphous bodies are arranged randomly. Amorphous bodies in their structure resemble liquids. Already in solid glass, the atoms are arranged randomly. This means that an increase in the temperature of glass only increases the range of vibrations of its molecules, giving them gradually more and more freedom of movement. Therefore, the glass softens gradually and does not exhibit the sharp "solid-liquid" transition characteristic of the transition from an arrangement of molecules in a strict order to a disorderly one.

Melting heat.

Melting heat- this is the amount of heat that must be imparted to a substance at constant pressure and a constant temperature equal to the melting point in order to completely transfer it from a solid crystalline state to a liquid one. The heat of fusion is equal to the amount of heat that is released during the crystallization of a substance from a liquid state. During melting, all the heat supplied to the substance goes to increase the potential energy of its molecules. The kinetic energy does not change because melting occurs at a constant temperature.

Studying experimentally the melting of various substances of the same mass, one can notice that different amounts of heat are required to convert them into a liquid. For example, in order to melt one kilogram of ice, you need to expend 332 J of energy, and in order to melt 1 kg of lead - 25 kJ.

The amount of heat released by the body is considered negative. Therefore, when calculating the amount of heat released during the crystallization of a substance with a mass m, you should use the same formula, but with a minus sign:

Heat of combustion.

Heat of combustion(or calorific value, calories) is the amount of heat released during the complete combustion of fuel.

To heat bodies, the energy released during the combustion of fuel is often used. Conventional fuel (coal, oil, gasoline) contains carbon. During combustion, carbon atoms combine with oxygen atoms in the air, resulting in the formation of carbon dioxide molecules. The kinetic energy of these molecules turns out to be greater than that of the initial particles. The increase in the kinetic energy of molecules during combustion is called the release of energy. The energy released during the complete combustion of fuel is the heat of combustion of this fuel.

The heat of combustion of fuel depends on the type of fuel and its mass. The greater the mass of the fuel, the greater the amount of heat released during its complete combustion.

The physical quantity showing how much heat is released during the complete combustion of fuel weighing 1 kg is called specific heat of combustion of fuel.The specific heat of combustion is denoted by the letterqand is measured in joules per kilogram (J/kg).

Quantity of heat Q released during combustion m kg of fuel is determined by the formula:

To find the amount of heat released during the complete combustion of a fuel of arbitrary mass, it is necessary to multiply the specific heat of combustion of this fuel by its mass.

Melting

Melting It is the process of changing a substance from a solid to a liquid state.

Observations show that if crushed ice, having, for example, a temperature of 10 ° C, is left in a warm room, then its temperature will rise. At 0 °C, the ice will begin to melt, and the temperature will not change until all the ice has turned into a liquid. After that, the temperature of the water formed from the ice will rise.

This means that crystalline bodies, which include ice, melt at a certain temperature, which is called melting point. It is important that during the melting process the temperature of the crystalline substance and the liquid formed during its melting remains unchanged.

In the experiment described above, the ice received a certain amount of heat, its internal energy increased due to an increase in the average kinetic energy of the movement of molecules. Then the ice melted, its temperature did not change, although the ice received a certain amount of heat. Consequently, its internal energy increased, but not due to the kinetic, but due to the potential energy of the interaction of molecules. The energy received from the outside is spent on the destruction of the crystal lattice. Similarly, the melting of any crystalline body occurs.

Amorphous bodies do not have a specific melting point. As the temperature rises, they gradually soften until they turn into a liquid.

Crystallization

Crystallization is the process by which a substance changes from a liquid state to a solid state. Cooling, the liquid will give off a certain amount of heat to the surrounding air. In this case, its internal energy will decrease due to a decrease in the average kinetic energy of its molecules. At a certain temperature, the process of crystallization will begin, during this process the temperature of the substance will not change until the entire substance passes into a solid state. This transition is accompanied by the release of a certain amount of heat and, accordingly, a decrease in the internal energy of the substance due to a decrease in the potential energy of interaction of its molecules.

Thus, the transition of a substance from a liquid state to a solid state occurs at a certain temperature, called the crystallization temperature. This temperature remains constant throughout the melting process. It is equal to the melting point of this substance.

The figure shows a graph of the dependence of the temperature of a solid crystalline substance on time in the process of heating it from room temperature to the melting point, melting, heating the substance in the liquid state, cooling the liquid substance, crystallization and subsequent cooling of the substance in the solid state.

Specific heat of fusion

Different crystalline substances have different structures. Accordingly, in order to destroy the crystal lattice of a solid at its melting point, it is necessary to inform it of a different amount of heat.

Specific heat of fusion- this is the amount of heat that must be imparted to 1 kg of a crystalline substance in order to turn it into a liquid at the melting point. Experience shows that the specific heat of fusion is specific heat of crystallization .

The specific heat of fusion is denoted by the letter λ . Unit of specific heat of fusion - [λ] = 1 J/kg.

The values ​​of the specific heat of fusion of crystalline substances are given in the table. The specific heat of melting of aluminum is 3.9 * 10 5 J / kg. This means that for the melting of 1 kg of aluminum at the melting temperature, it is necessary to spend an amount of heat of 3.9 * 10 5 J. The increase in internal energy of 1 kg of aluminum is equal to the same value.

To calculate the amount of heat Q, required to melt a substance with a mass m, taken at the melting point, follows the specific heat of fusion λ multiply by the mass of the substance: Q = λm.

Traffic. Warmth Kitaygorodsky Alexander Isaakovich

The influence of pressure on the melting point

If the pressure is changed, the melting point will also change. We met with the same regularity when we talked about boiling. The higher the pressure, the higher the boiling point. As a rule, this is also true for melting. However, there are a small number of substances that behave anomalously: their melting point decreases with increasing pressure.

The fact is that the vast majority of solids are denser than their liquids. The exception to this rule is precisely those substances whose melting point does not change quite normally with a change in pressure - for example, water. Ice is lighter than water, and the melting point of ice decreases as pressure increases.

Compression promotes the formation of a denser state. If a solid is denser than a liquid, then compression helps solidify and prevents melting. But if melting is hindered by compression, then this means that the substance remains solid, whereas earlier at this temperature it would have already melted, i.e. as the pressure increases, the melting point rises. In the anomalous case, the liquid is denser than the solid, and the pressure helps the formation of the liquid, i.e. lowers the melting point.

The effect of pressure on the melting point is much less than that of boiling. An increase in pressure by more than 100 kg/cm 2 lowers the melting point of ice by 1 °C.

From this, by the way, one can see how naive is the often encountered explanation of the sliding of skates on ice by a decrease in the melting point due to pressure. The pressure on the blade of the skate, in any case, does not exceed 100 kg/cm 2 , and for this reason a decrease in the melting point cannot play a role for skaters.

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The increase in the volume of water when it freezes is of great importance in nature. Due to the lower density of ice compared to the density of water (at 0 ° C, the density of ice is 900 kg / m 3, and water 1000 kg / m 3), ice floats on water. Possessing poor thermal conductivity, the layer of ice protects the water under it from cooling and freezing. Therefore, fish and other living creatures in the water do not die during frosts. If the ice sank, then not very deep reservoirs would freeze through during the winter.

When freezing water expands in a closed vessel, huge forces arise that can break a thick-walled cast-iron ball. Such an experiment is easy to carry out with a bottle filled with water up to the neck and exposed to frost. An ice plug forms on the surface of the water, clogging the bottle, and when the freezing water expands, the bottle will break.

Freezing of water in the cracks of rocks leads to their destruction.

The ability of water to expand during hardening must be taken into account when laying water and sewer pipes, as well as water heating. To avoid rupture when water freezes, underground pipes must be laid at such a depth that the temperature does not fall below 0 ° C. The outer parts of the pipes must be covered with heat-insulating materials for the winter.

Melting point versus pressure

If the melting of a substance is accompanied by an increase in its volume, then with an increase in external pressure, the melting point of the substance rises. This can be explained as follows. Compression of a substance (with an increase in external pressure) prevents an increase in the distance between molecules and, consequently, an increase in the potential energy of interaction of molecules, which is required for the transition to a liquid state. Therefore, it is necessary to heat the body to a higher temperature until the potential energy of the molecules reaches the required value.

If the melting of a substance is accompanied by a decrease in its volume, then with an increase in external pressure, the melting point of the substance decreases.

So, for example, ice at a pressure of 6 10 7 Pa melts at a temperature of -5 ° C, and at a pressure of 2.2 10 8 Pa, the melting point of ice is -22 ° C.

The decrease in the melting point of ice with increasing pressure is well illustrated by experience (Fig. 8.34). The nylon thread passes through the ice without breaking it. The fact is that due to the significant pressure of the thread on the ice, it melts under it. Water, flowing out from under the thread, immediately freezes again.

triple point

A liquid can be in equilibrium with its vapor (saturated vapor). Figure 6.5 (see § 6.3) shows the saturation vapor pressure as a function of temperature (curve AB), obtained experimentally. Since the boiling of a liquid occurs at a pressure equal to the pressure of its saturated vapors, the same curve gives the dependence of the boiling point on pressure. Area below the curve AB, corresponds to the gas state, and above - to the liquid state.

Crystalline bodies melt at a certain temperature, at which the solid phase is in equilibrium with the liquid. The melting point depends on the pressure. This dependence can be shown in the same figure, which shows the dependence of the boiling point on pressure.

In Figure 8.35, the curve TC characterizes the dependence of boiling point on pressure. It ends at the point TO, corresponding to the critical temperature, since above this temperature the liquid cannot exist. To the left of the curve TC the experimental points plotted the curve TS the dependence of the melting point on pressure (to the left, since the solid phase corresponds to lower temperatures than the liquid phase). Both curves intersect at point T.

What will happen to the substance at a temperature below the temperature t t p , corresponding point T? The liquid phase at this temperature can no longer exist. The substance will either be in a solid or gaseous state. Curve FROM(see Fig. 8.35) corresponds to the equilibrium states of a solid body - a gas that arise during the sublimation of solid bodies.

three curves CT, TS and FROM divide the phase plane into three regions in which the substance can be in one of the three phases. The curves themselves describe the equilibrium states of liquid - vapor, liquid - solid and solid - vapor. There is only one point T, in which all three phases are in equilibrium. This is the triple point.

The triple point corresponds to the only values ​​of temperature and pressure. It can be accurately reproduced, and it serves as one of the most important reference points in the construction of an absolute temperature scale. For water, the absolute temperature of the triple point is assumed to be Ttr = 273.16 K, or t t p = 0.01°C.

Figure 8.35 shows the phase diagram of water, in which the melting point decreases with increasing pressure. For ordinary substances, the curve TS inclined in the opposite direction with respect to the vertical passing through the point T.

For example, the phase diagram of carbon monoxide CO 2 will have this form. CO 2 triple point temperature t tr \u003d -56.6 ° С, and pressure p tr \u003d 5.1 atm. Therefore, at normal atmospheric pressure and temperature close to room temperature, carbon dioxide cannot be in a liquid state. The solid phase of CO 2 is usually called dry ice. It has a very low temperature and does not melt, but immediately evaporates (sublimation).

The change in volume during melting and solidification is directly related to the dependence of the melting temperature on pressure. For the vast majority of substances, the melting point increases with pressure. In water and some other substances, on the contrary, it decreases. For the inhabitants of the Earth at high geographical latitudes, this is a great boon.

There is a single point on the diagram p-T (triple point), at which all three phases of matter are in equilibrium.

In conclusion, we note the great importance of solid state physics for the development of technology and civilization in general.

Mankind has always used and will continue to use solid bodies. But if earlier solid state physics did not keep pace with the development of technology based on direct experience, now the situation has changed. Theoretical research is beginning to lead to the creation of solids, the properties of which are completely unusual and which would be impossible to obtain by trial and error. The invention of the transistor, which will be discussed later, is a prime example of how understanding the structure of solids led to a revolution in all radio engineering.

The creation of materials with specified mechanical, magnetic and other properties is one of the main areas of solid state physics. Approximately half of the world's physicists are now working in the field of solid state physics.