It is important to know and understand how transitions between aggregate states of substances are carried out. The scheme of such transitions is depicted in Figure 4.

5 - sublimation (sublimation) - transition from solid state into the gaseous, bypassing the liquid;

6 - desublimation - transition from gaseous state into the solid without going through the liquid.

B. 2 Melting of ice and freezing of water (crystallization)
If you put ice in a flask and start heating it with a burner, you will notice that its temperature will begin to rise until it reaches its melting point (0 o C). Then the melting process will begin, but the temperature of the ice will not rise, and only after the end of the melting process of all the ice, the temperature of the formed water will begin to rise.

Definition. Melting- the process of transition from a solid to a liquid state. This process takes place at a constant temperature.

The temperature at which a substance melts is called the melting point and is a measured value for many solids and is therefore a tabular value. For example, the melting point of ice is 0 o C, and the melting point of gold is 1100 o C.

The reverse process of melting - the process of crystallization - is also conveniently considered by the example of freezing water and turning it into ice. If you take a test tube with water and begin to cool it, then at first there will be a decrease in the temperature of the water until it reaches 0 o C, and then it will freeze at a constant temperature), and after complete freezing, further cooling of the formed ice.
If the described processes are considered from the point of view internal energy bodies, then during melting, all the energy received by the body is spent on the destruction of the crystal lattice and the weakening of intermolecular bonds, thus, the energy is spent not on changing the temperature, but on changing the structure of the substance and the interaction of its particles. In the process of crystallization, the energy exchange takes place in reverse direction: the body gives off heat environment, and its internal energy decreases, which leads to a decrease in the mobility of particles, an increase in the interaction between them and solidification of the body.

Melting and crystallization chart

It is useful to be able to graphically depict the processes of melting and crystallization of a substance on a graph. Along the axes of the graph are located: the abscissa axis - time, the ordinate axis - the temperature of the substance. As the substance under study, we will take ice at a negative temperature, i.e., one that, upon receiving heat, will not immediately begin to melt, but will be heated to the melting point. Let us describe the sections on the graph, which represent separate thermal processes:
Initial state - a: heating ice to a melting temperature of 0 o C;
a - b: melting process at a constant temperature of 0 o C;
b - point with a certain temperature: heating the water formed from ice to a certain temperature;
Point with a certain temperature - c: cooling water to freezing point 0 o C;
c - d: the process of freezing water at a constant temperature of 0 o C;
d - final state: ice cooling down to some negative temperature.

What is a "triple point" and how to determine its coordinates? Experiments show that for each substance there are conditions (pressure and temperature) under which vapor, liquid and crystal can coexist simultaneously for an arbitrarily long time. For example, if you place water with floating ice in a closed vessel at zero degrees, then in free space both water and ice will evaporate. However, at a vapor pressure of 0.006 atm. (this is their “own” pressure, without taking into account the pressure created by air) and a temperature of 0.01 ° C, the increase in the mass of steam will stop. From now on, ice, water and steam will retain their masses indefinitely. This is the triple point for water (left diagram). If water or steam is placed in the conditions of the left region, they will become ice. If a liquid or a solid body is introduced into the "lower region", then steam will be obtained. In the right area, water will condense and ice will melt.

A similar diagram can be constructed for any substance. The purpose of such diagrams is to answer the question: what state of matter will be stable at such and such a pressure and such and such a temperature. For example, the diagram on the right is for carbon dioxide. The triple point for this substance has a “pressure” coordinate of 5.11 atm, that is, much more than normal Atmosphere pressure. Therefore, under normal conditions (pressure 1 atm), we can only observe transitions "below triple point”, that is, the independent transformation of a solid into a gas. At a pressure of 1 atm, this will occur at a temperature of -78 °C (see dotted coordinate lines below the triple point).

We all live "near" the values ​​of "normal conditions", that is, primarily at a pressure close to one atmosphere. Therefore, if the atmospheric pressure is lower than the pressure corresponding to the triple point, when the body is heated, we will not see liquid, the solid will immediately turn into vapor. This is exactly how “dry ice” behaves, which is very convenient for ice cream sellers. Ice cream briquettes can be shifted with pieces of "dry ice" and not be afraid that the ice cream will get wet. If the pressure corresponding to the triple point is less than atmospheric, then the substance belongs to the "melting" - when the temperature rises, it first turns into a liquid, and then boils.

As you can see, the features of aggregate transformations of substances directly depend on how the current values ​​of pressure and temperature correlate with the coordinates of the "triple point" on the "pressure-temperature" diagram.

And in conclusion, let us name the substances known to you, which always sublimate under normal conditions. This is iodine, graphite, "dry ice". At pressures and temperatures other than normal, these substances can be observed both in a liquid and even in a boiling state.

(C) 2013. Physics.ru with the participation of A.V. Kuznetsova (Samara)

Depending on the conditions, bodies can be in a liquid, solid or gaseous state. These states are called aggregate states of matter .

In gases, the distance between molecules is much more sizes molecules. If the walls of the vessel do not interfere with the gas, its molecules fly apart.

In liquids and solids, the molecules are closer together and therefore cannot move far apart.

The transition from one aggregate state to another is called phase transition .

The transition of a substance from a solid to a liquid state is called melting , and the temperature at which this occurs is melting point . The transfer of matter from liquid state into a solid called crystallization , and the transition temperature is crystallization temperature .

The amount of heat that is released during the crystallization of a body or absorbed by the body during melting, per unit mass of the body, is called specific heat of fusion (crystallization) λ:

During crystallization, the same amount of heat is released as is absorbed during melting.

The transition of a substance from a liquid state to a gaseous state is called vaporization . The transition of a substance from a gaseous state to a liquid state is called condensation . The amount of heat required for vaporization (released during condensation):

Vaporization from the surface of a liquid is called evaporation . Evaporation can take place at any temperature. The transition of liquid to vapor, which occurs throughout the volume of the body, is called boiling , and the temperature at which the liquid boils is boiling point .

Finally, sublimation - this is the transition of a substance from a solid state directly to a gaseous state, bypassing the liquid stage.

If other parameters external environment(in particular, pressure) remain constant, then the temperature of the body in the process of melting (crystallization) and boiling does not change.

If the number of molecules leaving the liquid is equal to the number of molecules returning to the liquid, then they say that a dynamic equilibrium has come between the liquid and its vapor. A vapor in dynamic equilibrium with its liquid is called

In this section, we will look at aggregate states, in which the matter surrounding us resides and the forces of interaction between particles of matter, characteristic of each of the aggregate states.

1. Solid State,

2. liquid state and

3. gaseous state.

Often there is a fourth state of aggregation – plasma.

Sometimes, the plasma state is considered one of the types of gaseous state.

Plasma - partially or fully ionized gas, most often found in high temperatures Oh.

Plasma is the most common state of matter in the universe, since the matter of stars is in this state.

For everybody state of aggregation characteristic features in the nature of the interaction between the particles of a substance, which affects its physical and chemical properties.

Each substance can be in different states of aggregation. At sufficiently low temperatures, all substances are in solid state. But as they heat up, they become liquids, then gases. Upon further heating, they ionize (the atoms lose some of their electrons) and pass into the state plasma.

Gas

gaseous state(from Dutch. gas, goes back to other Greek. Χάος ) characterized by very weak bonds between its constituent particles.

The molecules or atoms that form the gas move randomly and, at the same time, they are at large (in comparison with their sizes) distances from each other for the majority of the time. Thereby interaction forces between gas particles are negligible.

The main feature of the gas is that it fills all available space without forming a surface. Gases always mix. Gas is an isotropic substance, that is, its properties do not depend on direction.

In the absence of gravity pressure the same at all points in the gas. In the field of gravitational forces, density and pressure are not the same at each point, decreasing with height. Accordingly, in the field of gravity, the mixture of gases becomes inhomogeneous. heavy gases tend to settle lower and more lungs- to go up.

The gas has a high compressibility- when the pressure increases, its density increases. As the temperature rises, they expand.

When compressed, a gas can turn into a liquid., but condensation does not occur at any temperature, but at a temperature below the critical temperature. The critical temperature is a characteristic of a particular gas and depends on the forces of interaction between its molecules. So, for example, gas helium can only be liquefied at temperatures below 4.2K.

There are gases that, when cooled, pass into a solid body, bypassing the liquid phase. The transformation of a liquid into a gas is called evaporation, and the direct transformation solid body into the gas sublimation.

Solid

Solid State in comparison with other states of aggregation characterized by shape stability.

Distinguish crystalline and amorphous solids.

Crystalline state of matter

The stability of the shape of solids is due to the fact that most of the solids have crystalline structure.

In this case, the distances between the particles of the substance are small, and the interaction forces between them are large, which determines the stability of the form.

It is easy to verify the crystalline structure of many solids by splitting a piece of matter and examining the resulting fracture. Usually, at a break (for example, in sugar, sulfur, metals, etc.), small crystal faces located at different angles are clearly visible, gleaming due to the different reflection of light by them.

When the crystals are very small, crystal structure substances can be identified using a microscope.

Crystal forms

Each substance forms crystals perfectly defined form.

The variety of crystalline forms can be summarized in seven groups:

1. Triclinic(parallelepiped),

2.Monoclinic(prism with a parallelogram at the base),

3. Rhombic (cuboid),

4. tetragonal(rectangular parallelepiped with a square at the base),

5. Trigonal,

6. Hexagonal(prism with the base of the right centered
hexagon),

7. cubic(cube).

Many substances, in particular iron, copper, diamond, sodium chloride, crystallize in cubic system. The simplest forms of this system are cube, octahedron, tetrahedron.

Magnesium, zinc, ice, quartz crystallize in hexagonal system. The main forms of this system are hexagonal prisms and bipyramid.

Natural crystals, as well as crystals obtained artificially, rarely correspond exactly to theoretical forms. Usually, when the molten substance solidifies, the crystals grow together and therefore the shape of each of them is not quite correct.

However, no matter how unevenly the crystal develops, no matter how distorted its shape, the angles at which the crystal faces converge in the same substance remain constant.

Anisotropy

Peculiarities crystalline bodies are not limited to the shape of the crystals. Although the substance in a crystal is perfectly homogeneous, many of its physical properties- strength, thermal conductivity, relation to light, etc. - are not always the same in different directions inside the crystal. This important feature of crystalline substances is called anisotropy.

Internal structure of crystals. Crystal lattices.

The outer shape of the crystal reflects it internal structure and is due to the correct arrangement of the particles that make up the crystal - molecules, atoms or ions.

This arrangement can be represented as crystal lattice- a spatial frame formed by intersecting straight lines. At the points of intersection of the lines - lattice nodes are the centers of the particles.

Depending on the nature of the particles located at the nodes of the crystal lattice, and on what forces of interaction between them prevail in a given crystal, the following types are distinguished crystal lattices:

1. molecular,

2. atomic,

3. ionic and

4. metal.

Molecular and atomic lattices are inherent in substances with covalent bond, ionic - to ionic compounds, metallic - to metals and their alloys.

At the nodes of atomic lattices are atoms. They are connected to each other covalent bond.

There are relatively few substances that have atomic lattices. They belong to diamond, silicon and some inorganic compounds.

These substances are characterized by high hardness, they are refractory and practically insoluble in any solvents. These properties are due to their durability. covalent bond.

Molecules are located at the nodes of molecular lattices. They are connected to each other intermolecular forces.

There are a lot of substances with a molecular lattice. They belong to nonmetals, with the exception of carbon and silicon, all organic compounds with non-ionic bond and many inorganic compounds.

The forces of intermolecular interaction are much weaker than the forces of covalent bonds, therefore molecular crystals have low hardness, fusible and volatile.

In the nodes of ionic lattices, positively and negatively charged ions are located, alternating. They are connected to each other by forces electrostatic attraction.

Ionic compounds that form ionic lattices include most salts and a small number of oxides.

By strength ionic lattices inferior to atomic, but exceed molecular.

Ionic compounds have relatively high melting points. Their volatility in most cases is not great.

At the nodes of metal lattices there are metal atoms, between which electrons common to these atoms move freely.

The presence of free electrons in the crystal lattices of metals can explain many of their properties: plasticity, malleability, metallic luster, high electrical and thermal conductivity.

There are substances in whose crystals two kinds of interactions between particles play a significant role. So, in graphite, carbon atoms are connected to each other in the same directions. covalent bond, and in others metallic. Therefore, the graphite lattice can also be considered as nuclear, And How metal.

In many inorganic compounds, for example, in BeO, ZnS, CuCl, the connection between the particles located at the lattice sites is partially ionic, and partly covalent. Therefore, lattices of such compounds can be considered as intermediate between ionic and atomic.

Amorphous state of matter

Properties of amorphous substances

Among solid bodies there are those in which no signs of crystals can be found in the fracture. For example, if you break a piece of ordinary glass, then its break will be smooth and, unlike the breaks of crystals, it is limited not by flat, but by oval surfaces.

A similar picture is observed when splitting pieces of resin, glue and some other substances. This state of matter is called amorphous.

Difference between crystalline and amorphous bodies is particularly pronounced in their relation to heating.

While the crystals of each substance melt at a strictly defined temperature and at the same temperature a transition from a liquid state to a solid occurs, amorphous bodies do not have constant temperature melting. When heated, the amorphous body gradually softens, begins to spread and, finally, becomes completely liquid. When cooled, it also gradually hardens.

Due to the lack of a specific melting point, amorphous bodies have a different ability: many of them flow like liquids, i.e. with prolonged action of relatively small forces, they gradually change their shape. For example, a piece of resin placed on a flat surface spreads in a warm room for several weeks, taking the form of a disk.

The structure of amorphous substances

Difference between crystalline and amorphous state of matter is as follows.

Ordered arrangement of particles in a crystal, reflected by the unit cell, is preserved in large areas of crystals, and in the case of well-formed crystals - in their entirety.

AT amorphous bodies order in the arrangement of particles is observed only in very small areas. Moreover, in a number of amorphous bodies even this local ordering is only approximate.

This difference can be summarized as follows:

• crystal structure is characterized by long-range order,
• structure of amorphous bodies - near.

Examples of amorphous substances.

Stable amorphous substances include glass(artificial and volcanic), natural and artificial resins, glues, paraffin, wax and etc.

Transition from an amorphous state to a crystalline one.

Some substances can be in both crystalline and amorphous states. Silicon dioxide SiO 2 occurs in nature in the form of well-formed quartz crystals, as well as in the amorphous state ( flint mineral).

Wherein the crystalline state is always more stable. Therefore, a spontaneous transition from crystalline substance into an amorphous state is impossible, and the reverse transformation - a spontaneous transition from an amorphous state to a crystalline one - is possible and sometimes observed.

An example of such a transformation is devitrification- spontaneous crystallization of glass at elevated temperatures, accompanied by its destruction.

amorphous state many substances is obtained at a high rate of solidification (cooling) of the liquid melt.

For metals and alloys amorphous state is formed, as a rule, if the melt is cooled for a time on the order of fractions or tens of milliseconds. For glasses, a much lower cooling rate is sufficient.

Quartz (SiO2) also has a low crystallization rate. Therefore, the products cast from it are amorphous. However, natural quartz, which had hundreds and thousands of years to crystallize when the earth's crust or deep layers of volcanoes cooled, has a coarse-grained structure, in contrast to volcanic glass, which has frozen on the surface and is therefore amorphous.

Liquids

Liquid is an intermediate state between a solid and a gas.

liquid state is intermediate between gaseous and crystalline. According to some properties, liquids are close to gases, according to others - to solid bodies.

With gases, liquids are brought together, first of all, by their isotropy and fluidity. The latter determines the ability of the liquid to easily change its shape.

However high density and low compressibility liquids brings them closer to solid bodies.

The ability of liquids to easily change their shape indicates the absence of hard forces of intermolecular interaction in them.

At the same time, the low compressibility of liquids, which determines the ability to maintain a constant volume at a given temperature, indicates the presence, although not rigid, but still significant forces of interaction between particles.

The ratio of potential and kinetic energy.

Each state of aggregation is characterized by its own ratio between the potential and kinetic energies of the particles of matter.

For solids, the average potential energy particles is greater than their average kinetic energy. Therefore, in solids, particles occupy certain positions relative to each other and only oscillate relative to these positions.

For gases, the energy ratio is reversed, as a result of which gas molecules are always in a state of chaotic motion and there are practically no cohesive forces between molecules, so that the gas always occupies the entire volume provided to it.

In the case of liquids, the kinetic and potential energies of particles are approximately the same, i.e. particles are connected to each other, but not rigidly. Therefore, liquids are fluid, but have a constant volume at a given temperature.

The structures of liquids and amorphous bodies are similar.

As a result of applying methods to liquids structural analysis found that the structure liquids are like amorphous bodies. Most liquids have short range order is the number of nearest neighbors for each molecule and their mutual arrangement approximately the same throughout the volume of the liquid.

The degree of ordering of particles in different liquids is different. In addition, it changes with temperature.

At low temperatures slightly above the melting point of a given substance, the degree of order in the arrangement of the particles of a given liquid is high.

As the temperature rises, it decreases and as the liquid heats up, the properties of the liquid more and more approach the properties of the gas. When the critical temperature is reached, the distinction between liquid and gas disappears.

Due to the similarity in the internal structure of liquids and amorphous bodies, the latter are often considered as liquids with a very high viscosity, and only substances in the crystalline state are classified as solids.

Likening amorphous bodies liquids, however, it should be remembered that in amorphous bodies, unlike ordinary liquids, particles have a slight mobility - the same as in crystals.

Any body can be in different states of aggregation at certain temperatures and pressures - in solid, liquid, gaseous and plasma states.

For the transition from one state of aggregation to another occurs under the condition that the heating of the body from the outside occurs faster than its cooling. And vice versa, if the cooling of the body from the outside occurs faster than the heating of the body due to its internal energy.

During the transition to another state of aggregation, the substance remains the same, the same molecules will remain, only their relative position, speed of movement and forces of interaction with each other will change.

Those. a change in the internal energy of the particles of the body transfers it from one phase of the state to another. Moreover, this state can be maintained in a large temperature range of the external environment.

When a state of aggregation changes, a certain amount of energy is needed. And in the process of transition, energy is spent not on changing the temperature of the body, but on changing the internal energy of the body.

Let us display on the graph the dependence of body temperature T (at constant pressure) on the amount of heat Q supplied to the body during the transition from one state of aggregation to another.

Consider a body of mass m, which is in a solid state with a temperature T1.

The body does not go instantly from one state to another. First, energy is needed to change the internal energy, and this takes time. The rate of transition depends on the mass of the body and its heat capacity.

Let's start heating the body. Formulas can be written like this:

Q = c⋅m⋅(T 2 -T 1)

This is how much heat the body must absorb in order to warm up from temperature T 1 to T 2 .

The transition of a solid to a liquid

Further, at the critical temperature T 2 , which is different for each body, intermolecular bonds begin to break down and the body passes into another state of aggregation - liquid, i.e. intermolecular bonds weaken, molecules begin to move with a greater amplitude with greater speed and greater kinetic energy. Therefore, the temperature of the same body in the liquid state is higher than in the solid state.

In order for the whole body to pass from a solid to a liquid state, it takes time to accumulate internal energy. At this time, all the energy goes not to heat the body, but to the destruction of old intermolecular bonds and the creation of new ones. The amount of energy you need:

λ - specific heat melting and crystallization of a substance in J / kg, for each substance its own.

After the whole body has passed into a liquid state, this liquid again begins to heat up according to the formula: Q = c⋅m⋅(T-T 2); [J].

The transition of a body from a liquid state to a gaseous state

When a new critical temperature T 3 is reached, a new process of transition from liquid to vapor begins. To move further from liquid to vapor, you need to expend energy:

r - specific heat of gas formation and condensation of a substance in J / kg, each substance has its own.

Note that the transition from the solid state to the gaseous state is possible, bypassing the liquid phase. Such a process is called sublimation, and the reverse process is desublimation.

The transition of a body from a gaseous state to a plasma state

Plasma- a partially or fully ionized gas in which the densities of positive and negative charges practically the same.

Plasma usually occurs at high temperatures, from several thousand °C and above. According to the method of formation, two types of plasma are distinguished: thermal, which occurs when a gas is heated to high temperatures, and gaseous, which forms during electrical discharges in a gaseous medium.

This process is very complex and has a simple description, and even in everyday life it is not achievable for us. Therefore, we will not dwell on this issue in detail.