1. Solid state
2. liquid state
3. gaseous state
4. Change in the state of matter

Chemistry is the study of matter. What is a "substance"? Matter is anything that has mass and volume. A substance can be in one of three aggregate states: solid, liquid, gaseous.

1. Solid State

Particles (molecules) in a solid body are combined into a rigid repeating structure - crystal lattice. Particles in the crystal lattice make small vibrations around the centers of equilibrium. The solid has form and volume.

2. Liquid state

Unlike solids, a liquid does not have a definite shape, but has a volume. This is explained by the fact that in liquids the particles are at a greater distance from each other than in solids and move more actively.

Since the particles in liquids are less dense than in solids, they cannot form a crystal lattice, therefore liquids do not have a definite shape.

3. Gaseous state

In a gas, particles are still at greater distances than in liquids. Moreover, the particles are constantly in chaotic (random) motion. Therefore, gases tend to uniformly fill the volume provided to them (hence the fact that gases do not have a definite shape).

4. Change in the state of matter

Let's take a banal example and follow the process of changing the state of water.

In its solid state, water is ice. The temperature of the ice is less than 0 ° C. When heated, the ice begins to melt and turn into water. This is due to the fact that ice particles in the crystal lattice begin to move when heated, as a result of which the lattice is destroyed. The temperature at which a substance melts is called "melting point" substances. The melting point of water is 0 o C.

It should be noted that until the ice is completely melted, the temperature of the ice will be 0 o C.

After the ice has completely turned into water, we will continue heating. The temperature of the water will rise, and the movement of particles under the influence of heat will be accelerated more and more. This happens until the water reaches its next state change point - boiling.

This moment comes when the bonds of water particles are completely broken and their movement becomes free: water turns into steam.

The temperature at which a liquid boils is called "boiling point".

Note that the boiling point is pressure dependent. At normal pressure (760 mm Hg), the boiling point of water is 100 o C.

By analogy with melting: until the water completely turns into steam, the temperature will be constant.

Summarize. As a result of heating, we obtained different phase states of water:

Ice → water → steam or H 2 0 (t) → H 2 0 (g) → H 2 0 (g)

What happens if we start to cool the water vapor? You don't have to be "seven spans in the forehead" to guess - the reverse process of phase changes in water will start:

Steam → water → ice

There are some substances that go directly from a solid state to a gaseous state, bypassing the liquid phase. Such a process is called sublimation or sublimation. So, for example, behaves "dry ice" (nitrogen dioxide CO 2). When it is heated, you will not see a drop of water - the "dry ice" will seem to evaporate before your eyes.

The process, the reverse of sublimation (the transition of a substance from gas to solid state), is called desublimation.

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The gaseous state of a substance is characterized mainly by very small molecular cohesive forces, as a result of which the gas tends to occupy the maximum volume.

The gaseous state of matter is the most accessible for understanding; the liquid state is already much less understood, and the solid state can, apparently, be considered the most complex. Powders are often referred to as the fourth state of matter. In addition, phenomena at the solid-solid and solid-gas interfaces are among the least studied aspects of the solid state.

The gaseous state of a substance is mainly characterized by very small intermolecular cohesive forces.

The gaseous state of matter is characterized by the fact that the smallest particles of matter - atoms or molecules - most of the time are relatively far from each other. The forces of interaction between them have a noticeable effect only during very short periods of time when the particles of the gas collide with each other. Therefore, the action of molecular forces is expressed only in the exchange of energies during collisions. The lower the gas density, the greater the free path of its molecules and, consequently, the less influence molecular forces have on general behavior gas with certain changes in its state.

The gaseous state of matter is very common. Gases are involved in the most important chemical reactions, are coolants and sources of energy. He extended the law of conservation of energy to thermal phenomena, assuming that the particles of gases are in continuous chaotic motion, collide and repel each other in random reciprocity. Later, the theory of gases was developed on the basis of the following provisions: 1) a gas consists of a huge number of molecules in continuous thermal motion; 2) molecules obey the laws of mechanics, there is no interaction between them; 3) constantly occurring collisions between molecules are similar to collisions between absolutely elastic balls and occur without loss of speed. Molecules only change the direction of movement, and their total kinetic energy remains constant.

The gaseous state of a substance is characterized by a small interaction between its particles and large distances between them. Therefore, gases are mixed in any ratio. At very high pressures, when the density of a gas approaches that of a liquid and the gas cannot be considered ideal even approximately, limited solubility can be observed.

The gaseous state of matter (gas) - state of aggregation a substance in which its particles are not bound or very weakly bound by interaction forces and move freely, uniformly filling in the absence of external fields the entire volume provided to them.

The gaseous state of matter is characterized by random thermal motion of molecules. The latter collide with each other and with the walls of the vessel in which the gas is located. Impacts of molecules on the walls of the vessel create pressure, which is numerically equal to the force of impacts per unit wall surface.

The gaseous state of a substance is the simplest in its properties, especially when not too high pressures and not too low temperatures. If, for example, at high pressures (more than 100 atm), gases such as O2, N2 and H2, taken at the same initial temperatures and pressures will have noticeable differences in compressibility and thermal expansion, then at pressures close to one atmosphere, the individual differences between these and other gases are smoothed out.

The gaseous state of matter is characterized by the fact that the smallest particles of matter - atoms or molecules - most of the time are relatively far from each other. The forces of interaction between them have a noticeable effect only during very short periods of time when the particles of the gas collide with each other. Therefore, the action of molecular forces is expressed only in the exchange of energies during collisions. The lower the density of the gas, the greater the free path of its molecules and, consequently, the less influence molecular forces have on the general behavior of the gas during certain changes in its state.

The gaseous state of matter is characterized by the fact that the smallest particles of matter - - atoms or molecules - most of the time are relatively far from each other. The forces of interaction between them have their effect only during very short periods of time, when the particles of the gas collide with each other. Therefore, the action of molecular forces is expressed only in the exchange of energies during collisions. The lower the density of the gas, the greater the free path of its molecules and, consequently, the less influence molecular forces have on the general behavior of the gas during certain changes in its state.

The gaseous state of a substance is characterized by negligible forces acting between the molecules of this substance, and the dimensions of the molecules themselves are also small compared to the average distances between them. The movement of gas molecules in intermolecular spaces before their collision occurs uniformly, rectilinearly and randomly.

The gaseous state of matter corresponds to complete molecular disorder.

The gaseous state of matter corresponds to complete molecular disorder. Such a distribution of molecules (or atoms) corresponds to a very large number of possible rearrangements of molecules in space. However physical properties substances remain unchanged during all these rearrangements. Therefore, they all correspond to one gaseous state.

There are drop-liquid and gaseous states of matter.

Back to main gas law refers to the equation of state of the Mendeleev-Claiperon gas pV=nRT, where n is the number of moles of gas, R- a constant equal to 8.314 J / (K × mol) or (l × kPa) / (K × mol). A gas that obeys this law is called ideal.

Avogadro's law states that equal volumes of all gases at the same pressure and temperature contain the same number of molecules. One mole contains 6.022×10 23 molecules. At standard conditions a mole of gas occupies a volume of 22.4 liters.

It is assumed that the existence ideal gas possible under the following conditions: the gas consists of a large number molecules in constant motion; gas molecules are not attracted to each other; the time of collision of molecules with each other is very small compared to the time between collisions; the average kinetic energy of a gas is proportional to the absolute temperature.

Due to the continuous movement of the gas molecules tend to spread throughout the volume. This propagation is called diffusion, the rate of this process is inversely proportional to the square root of the gas density.

The behavior of real gases deviates from the laws defined for ideal gases. The reason for such deviations is intermolecular interaction, as well as the fact that each molecule has its own volume. Van der Waals proposed an equation of state for a gas that takes into account these factors: (p + an 2 / V 2)×(V – nb) = nRT.

Here is a constant a takes into account intermolecular interactions, and its value grows with increasing energy of the van der Waals interaction, and the constant in takes into account the volume of molecules, and its value increases with increasing molecular size.

Liquid state of matter

As the pressure increases, the distance between the gas particles decreases and the forces of attraction of the molecules become more and more manifest. At a certain pressure, depending on the nature of the substance and temperature, the gas turns into a liquid - the gas condenses.

According to the molecular kinetic theory, the distances between the particles of a liquid are much smaller than in gases, therefore, van der Waals interactions arise between them: dispersion, dipole-dipole and induction. These interactions keep molecules close to each other and lead to some sort of ordering or association of particles. Small groups of particles united by certain forces are called clusters. In the case of identical particles, clusters in a liquid are called associates.

The degree of order increases with an increase in the polarity of the molecules, since the van der Waals forces increase in this case. Ordering is especially significant in the formation of hydrogen bonds between molecules. However, even hydrogen bonds, and even more so van der Waals forces, are relatively weak, so the molecules in liquid state are in continuous motion, called Brownian motion.

Due to the continuous movement, individual molecules can break out of the liquid and pass into the gaseous state. This process is called liquid evaporation. The tendency of a liquid to evaporate is called volatility. Due to evaporation, the partial vapor pressure of a given liquid in the gas phase above the liquid increases, i.e. steam condensation. At some partial pressure the rates of evaporation and condensation of vapor become equal, and this pressure is called pressure saturated vapors liquids.

At a partial pressure of saturated vapors of a liquid equal to atmospheric pressure, liquid gas bubbles form, and boiling begins. The temperature at which this state is reached is called the boiling point of the liquid.

Liquids are fluid. The resistance of a fluid to flow is called viscosity. Viscosity increases with increasing interaction energy of particles and depends on the structure of molecules. As the temperature increases, the viscosity decreases.

The forces of molecular interaction of molecules located on the surface are not balanced, therefore the resulting force is directed into the depth of the liquid. Under the action of this force, the liquid tends to reduce its surface. A sphere has the smallest surface with the same volume, so a drop of liquid takes the form of a sphere.

For the formation of a new surface, additional energy is required, which is called surface tension s, J/m2.

Solids

When the liquid is cooled, there is a further decrease kinetic energy particles. At a certain temperature or temperature range, the liquid passes into a solid state, in which the particles practically lose their forward movement and keep mostly fluctuations around their position. Unlike gases, the carriers of the properties of a liquid are molecules, the carrier of the properties solid body is the phase. Solids can be in amorphous or crystalline states.

The vast majority of solids (including all metals without exception) are in a crystalline state, therefore, they are characterized by a long-range order, i.e. three-dimensional periodicity over the entire volume of the solid. The regular arrangement of particles in a solid is depicted as a lattice, at the nodes of which certain particles are located.

Single crystals are characterized by anisotropy, i.e. dependence of properties on direction in space. However, it should be noted that real solids(metals including) polycrystalline, i.e. consist of many crystals oriented along different coordinate axes; therefore, anisotropy does not appear in polycrystalline bodies.

Crystalline bodies melt at a certain temperature, called the melting point. Crystals are characterized by the energy of the crystal lattice constant and the coordination number (the number of particles directly adjacent to a given particle in the crystal). The lattice constant characterizes the distances between the centers of particles occupying nodes in the crystal in the direction of the axes coinciding with the directions of the main faces. The energy of the crystal lattice is called the energy required to destroy one mole of the crystal and remove particles beyond the limits of their interaction. Energy is the main contributor to energy chemical bond between particles in the lattice, kJ/mol.

The smallest structural unit of a crystal, which expresses all the properties of its symmetry, is the elementary cell. With repeated repetition of the cell in three dimensions, the entire crystal lattice is obtained. Metals are characterized by two types of crystal lattice - cubic and hexagonal (Fig. 2.2).

Rice. 2.2. Types of elementary cells

crystal lattice of metals:

a– hexagonal; b- cubic;

in– cubic centered

Many substances can exist in two or more crystal structures. This phenomenon is called polymorphism. So, a-iron has a body-centered cubic cell, and g-iron has a face-centered one, etc.

According to the nature of the particles in the nodes of the crystal lattice and the chemical bonds between them, all crystals can be divided into molecular, atomic-covalent, ionic and metallic. In addition, there are crystals with mixed chemical bonds.

In molecular crystals, there are molecules at the lattice sites, between which van der Waals forces act, which have high energy and determine the properties of these crystals. Substances with spherical molecules have a close-packed structure. Crystals with polar molecules at the nodes have a higher strength and melting point than crystals with non-polar molecules. Significant strengthening of crystals is due to hydrogen bonds.

In atomic covalent crystals, atoms are located at the nodes, forming strong covalent bonds with each other. This determines the high energy of the lattice and, accordingly, the physical properties of substances. Due to the directionality of covalent bonds, the coordination numbers and packing density in atomic covalent crystals are low.

In ionic crystals, the structural units are positively and negatively charged ions, between which an electrostatic interaction occurs, characterized by a sufficiently high energy. This explains the properties of substances with ionic crystals. Due to the non-directionality and unsaturation of the bonds and the spherical shape of the particles, the coordination numbers of ions can be high. In compounds with complex ions, the shape of the crystal lattice is distorted.

Metal crystals are characterized by a number of special properties: high electrical conductivity, thermal conductivity, malleability, ductility, metallic luster and high reflectivity. These specific properties of metals are explained by a special type of chemical bond, called metallic.

Most metals on the outer electron shell there is a significant number of vacant orbitals and a small number of electrons; therefore, it is energetically more favorable that the electrons are not localized, but belong to the entire metal. Between positively charged metal ions and non-localized electrons, there is an electrostatic interaction that ensures the stability of the substance. The energy of this interaction is intermediate between the energies of covalent and molecular crystals. The presence of electrons, which can freely move around the volume of the crystal, provides high electrical conductivity and thermal conductivity, as well as malleability and ductility of metals.

One or another type of chemical bond or interaction in its pure form in crystals is rare. Usually, there are complex interactions between particles, which are described by the imposition of two or more types of bonds on top of each other. These are the so-called crystals with mixed bonds. Thus, in some crystals, along with van der Waals forces, hydrogen bonds arise, which significantly strengthen the crystals. The ionic bond in its pure form is practically absent, since a covalent bond also acts between the particles in ionic crystals. At a- or f-metals, along with a non-localized metallic bond, covalent bonds between neighboring atoms can act. In atomic crystals, along with covalent bond van der Waals forces can exist with the formation of two-dimensional flat (layered) structures. Such compounds are called intercalates. This is especially true for crystals with inclusions of graphite.

Layered compounds are a type of a special class of compounds called clathrates or inclusion compounds, which are formed by the inclusion of "guest" molecules in the cavities of a crystalline framework consisting of particles of a different kind - "hosts".

When pumping hydrocarbon gases under pressure, solid gas clathrates are formed, which, deposited on the internal surfaces of pipelines and fittings, clog them and thereby disrupt the pumping process.