SOLUTIONS

General information

Solutions - components.

"solvent" And "solute" polar non-polar

hydrophilic(attracting water) and hydrophobic diphilic

Theories of solutions

Physical theory of solutions.

ideal

Chemical theory of solutions.

The chemical, or solvate, theory of solutions was proposed in 1887 by D.I. Mendeleev, who found that real solution contains not only individual components, but also products of their interaction. Studies of aqueous solutions of sulfuric acid and ethyl alcohol, carried out by D.I. Mendeleev, formed the basis of the theory, the essence of which is that interactions occur between particles of a solute and solvent molecules, as a result of which unstable compounds of variable composition are formed, called solvates or hydrates if the solvent is water. The main role in the formation of solvates is played by unstable intermolecular forces, in particular, hydrogen bonding.

In this regard, the following interpretation of the concept of "solution" should be adopted:

A solution is a homogeneous system of variable composition, consisting of two or more components and products of their interaction.

From this definition it follows that solutions occupy an intermediate position between chemical compounds and mixtures. On the one hand, the solutions are homogeneous, which allows us to consider them as chemical compounds. On the other hand, there is no strict stoichiometric ratio between the components in solutions. In addition, solutions can be divided into component parts (for example, when NaCl solution is evaporated, salt can be isolated individually).

Communication between different ways

Acids and bases

Despite the fact that the concepts of "acid" and "base" are widely used to describe chemical processes, there is no single approach to the classification of substances in terms of classifying them as acids or bases. Current theories ( ionic theory S. Arrhenius, protolytic theory I. Bronsted and T. Lowry And electronic theory G. Lewis) have certain limitations and are therefore applicable only in particular cases. Let's take a closer look at each of these theories.

Arrhenius theory.

In the ionic theory of Arrhenius, the concepts of "acid" and "base" are closely related to the process of electrolytic dissociation:

An acid is an electrolyte that dissociates in solutions to form H + ions;

The base is an electrolyte that dissociates in solutions to form OH - ions;

Ampholyte (amphoteric electrolyte) is an electrolyte that dissociates in solutions to form both H + ions and OH - ions.

For example:

ON ⇄ H + + A - nH + + MeO n n - ⇄Me (OH) n ⇄Me n + + nOH -

In accordance with the ionic theory, both neutral molecules and ions can be acids, for example:

HF⇄H++F-

H 2 PO 4 - ⇄ H + + HPO 4 2 -

NH 4 + ⇄H + + NH 3

Similar examples can be given for the grounds:

KOH K + + OH -

- ⇄Al(OH) 3 + OH -

+ ⇄Fe 2+ + OH -

Ampholytes include hydroxides of zinc, aluminum, chromium and some others, as well as amino acids, proteins, nucleic acids.

In general, the acid-base interaction in solution is reduced to a neutralization reaction:

H + + OH - H 2 O

However, a number of experimental data show the limitations of the ionic theory. So, ammonia, organic amines, metal oxides such as Na 2 O, CaO, anions of weak acids, etc. in the absence of water, they exhibit the properties of typical bases, although they do not contain hydroxide ions.

On the other hand, many oxides (SO 2 , SO 3 , P 2 O 5 , etc.), halides, acid halides, without hydrogen ions in their composition, even in the absence of water, exhibit acidic properties, i.e. bases are neutralized.

In addition, the behavior of an electrolyte in an aqueous solution and in a non-aqueous medium can be opposite.

So, CH 3 COOH in water is a weak acid:

CH 3 COOH⇄CH 3 COO - + H +,

and in liquid hydrogen fluoride it exhibits the properties of a base:

HF + CH 3 COOH⇄CH 3 COOH 2 + +F -

Studies of these types of reactions, and especially those occurring in non-aqueous solvents, have led to more general theories of acids and bases.

Theory of Bronsted and Lowry.

A further development of the theory of acids and bases was the protolytic (proton) theory proposed by I. Bronsted and T. Lowry. According to this theory:

An acid is any substance whose molecules (or ions) are capable of donating a proton, i.e. be a proton donor;

A base is any substance whose molecules (or ions) are capable of attaching a proton, i.e. be a proton acceptor;

Thus, the concept of the basis is significantly expanded, which is confirmed by the following reactions:

OH - + H + H 2 O

NH 3 + H + NH 4 +

H 2 N-NH 3 + + H + H 3 N + -NH 3 +

According to the theory of I. Bronsted and T. Lowry, an acid and a base form a conjugated pair and are connected by equilibrium:

ACID ⇄ PROTON + BASE

Since the proton transfer reaction (protolytic reaction) is reversible, and a proton is also transferred in the reverse process, the reaction products are acid and base in relation to each other. This can be written as an equilibrium process:

ON + B ⇄ VN + + A -,

where HA is an acid, B is a base, BH + is an acid conjugated with base B, A - is a base conjugated with acid HA.

Examples.

1) in reaction:

HCl + OH - ⇄Cl - + H 2 O,

HCl and H 2 O are acids, Cl - and OH - are the corresponding conjugate bases;

2) in reaction:

HSO 4 - + H 2 O⇄SO 4 2 - + H 3 O +,

HSO 4 - and H 3 O + - acids, SO 4 2 - and H 2 O - bases;

3) in reaction:

NH 4 + + NH 2 - ⇄ 2NH 3,

NH 4 + is an acid, NH 2 - is a base, and NH 3 acts as both an acid (one molecule) and a base (another molecule), i.e. shows signs of amphotericity - the ability to exhibit the properties of an acid and a base.

Water also has this ability:

2H 2 O ⇄ H 3 O + + OH -

Here, one H 2 O molecule adds a proton (base), forming a conjugate acid - a hydroxonium ion H 3 O +, the other gives a proton (acid), forming a conjugate base OH -. This process is called autoprotolysis.

It can be seen from the above examples that, in contrast to the ideas of Arrhenius, in the theory of Brönsted and Lowry, the reactions of acids with bases do not lead to mutual neutralization, but are accompanied by the formation of new acids and bases.

It should also be noted that the protolytic theory considers the concepts of "acid" and "base" not as a property, but as a function that the compound in question performs in the protolytic reaction. The same compound can react as an acid under certain conditions and as a base under others. So, in an aqueous solution of CH 3 COOH exhibits the properties of an acid, and in 100% H 2 SO 4 - a base.

However, despite its merits, the protolytic theory, like the Arrhenius theory, is not applicable to substances that do not contain hydrogen atoms, but, at the same time, exhibit the function of an acid: boron, aluminum, silicon, and tin halides.

Lewis theory.

A different approach to the classification of substances in terms of classifying them as acids and bases was the electronic theory of Lewis. Within the electronic theory:

an acid is a particle (molecule or ion) capable of attaching an electron pair (electron acceptor);

A base is a particle (molecule or ion) capable of donating an electron pair (electron donor).

According to Lewis, an acid and a base interact with each other to form a donor-acceptor bond. As a result of the addition of a pair of electrons, an electron-deficient atom has a complete electronic configuration - an octet of electrons. For example:

The reaction between neutral molecules can be represented in a similar way:

The neutralization reaction in terms of the Lewis theory is considered as the addition of an electron pair of a hydroxide ion to a hydrogen ion, which provides a free orbital to accommodate this pair:

Thus, the proton itself, which easily attaches an electron pair, from the point of view of the Lewis theory, performs the function of an acid. In this regard, Bronsted acids can be considered as reaction products between Lewis acids and bases. So, HCl is the product of neutralization of the acid H + with the base Cl -, and the H 3 O + ion is formed as a result of the neutralization of the acid H + with the base H 2 O.

Reactions between Lewis acids and bases are also illustrated by the following examples:

Lewis bases also include halide ions, ammonia, aliphatic and aromatic amines, oxygen-containing organic compounds of the type R 2 CO, (where R is an organic radical).

Lewis acids include halides of boron, aluminum, silicon, tin and other elements.

Obviously, in the theory of Lewis, the concept of "acid" includes a wider range of chemical compounds. This is explained by the fact that, according to Lewis, the assignment of a substance to the class of acids is due solely to the structure of its molecule, which determines the electron-acceptor properties, and is not necessarily associated with the presence of hydrogen atoms. Lewis acids that do not contain hydrogen atoms are called aprotic.

SOLUTIONS

General information

Solutions - These are homogeneous systems of variable composition, consisting of two or more substances, called components. According to the state of aggregation, solutions can be gaseous (air), liquid (blood, lymph) and solid (alloys). In medicine, liquid solutions, which play an exceptional role in the life of living organisms, are of the greatest importance. The processes of assimilation of food and excretion of waste products from the body are associated with the formation of solutions. A large number of drugs are administered in the form of solutions.

For the qualitative and quantitative description of liquid solutions, the terms "solvent" And "solute", although in some cases such a division is rather conditional. So, medical alcohol (96% solution of ethanol in water) should rather be considered as a solution of water in alcohol. All solvents are divided into inorganic and organic. The most important inorganic solvent (and in the case of biological systems, the only one) is water. This is due to such properties of water as polarity, low viscosity, the tendency of molecules to associate, and relatively high boiling and melting points. Organic solvents are divided into polar(alcohols, aldehydes, ketones, acids) and non-polar(hexane, benzene, carbon tetrachloride).

The process of dissolution equally depends on the nature of the solvent and on the properties of the solute. Obviously, the ability to form solutions is expressed in different substances in different ways. Some substances can be mixed with each other in any quantities (water and ethanol), others - in limited quantities (water and phenol). However, it should be remembered: absolutely insoluble substances do not exist!

The propensity of a substance to dissolve in a particular solvent can be determined using a simple rule of thumb: like dissolves into like. Indeed, substances with an ionic (salts, alkalis) or polar (alcohols, aldehydes) type of bond are highly soluble in polar solvents, for example, in water. Conversely, the solubility of oxygen in benzene is an order of magnitude higher than in water, since the O 2 and C 6 H 6 molecules are nonpolar.

The degree of affinity of a compound for a certain type of solvent can be assessed by analyzing the nature and quantitative ratio of its constituent functional groups, among which are hydrophilic(attracting water) and hydrophobic(repel water). Hydrophilic include polar groups, such as hydroxyl (-OH), carboxyl (-COOH), thiol (-SH), amino (-NH 2). Non-polar groups are considered hydrophobic: hydrocarbon radicals of the aliphatic (-CH 3, -C 2 H 5) and aromatic (-C 6 H 5) series. Compounds containing both hydrophilic and hydrophobic groups are called diphilic. Such compounds include amino acids, proteins, nucleic acids.

Theories of solutions

Currently, two main theories of solutions are known: physical and chemical.

Physical theory of solutions.

The physical theory of solutions was proposed by S. Arrhenius (1883) and J. G. van't Hoff (1885). In this theory, the solvent is considered as a chemically inert medium in which particles (molecules, ions) of the solute are uniformly distributed. In this case, it is assumed that there is no intermolecular interaction both between the particles of the solute and between the molecules of the solvent and the particles of the solute. However, later it turned out that the conditions of this model are satisfied by the behavior of only a small group of solutions, which were named ideal. In particular, gas mixtures and very dilute solutions of non-electrolytes can be considered ideal solutions.

It is shown above that the reaction of pure water is neutral (pH = 7). Aqueous solutions of acids and bases have, respectively, acidic (pH< 7) и щелочную (рН >7) reaction. Practice, however, shows that not only acids and bases, but also salts can have an alkaline or acidic reaction - the reason for this is the hydrolysis of salts. The interaction of salts with water, which results in the formation of an acid (or acid salt) and a base (or basic salt), is called salt hydrolysis. Consider the hydrolysis of salts of the following main types: 1. Salts of a strong base and a strong acid (for example, KBr, NaNO3) do not hydrolyze when dissolved in water, and the salt solution has a neutral reaction ....

It is well known that some substances in a dissolved or molten state conduct electric current, while others do not conduct current under the same conditions. This can be observed with a simple instrument. It consists of carbon rods (electrodes) connected by wires to an electrical network. An electric bulb is included in the circuit, which indicates the presence or absence of current in the circuit. If the electrodes are immersed in a sugar solution, the lamp does not light up. But it will light up brightly if they are lowered into a solution of sodium chloride. Substances that decompose into ions in solutions or melts and therefore conduct electricity are called electrolytes. Substances that do not decompose into ions under the same conditions and do not conduct electric current are called non-electrolytes. Electrolytes include acids, bases and almost all salts, non-electrolytes - most organic compounds, ...

To explain the features of aqueous solutions of electrolytes, the Swedish scientist S. Arrhenius in 1887 proposed the theory of electrolytic dissociation. Later it was developed by many scientists on the basis of the theory of the structure of atoms and chemical bonding. The current content of this theory can be reduced to the following three propositions: 1. When dissolved in water, electrolytes decompose (dissociate) into positive and negative ions. Ions are in more stable electronic states than atoms. They can consist of one atom - these are simple ions (Na +, Mg2 +, Al3 +, etc.) - or of several atoms - these are complex ions (NO3-, SO2-4, ROZ-4, etc.). 2. Under the action of an electric current, the ions acquire a directed movement: positively charged ions move towards the cathode, negatively charged ones move towards the anode. Therefore, the former are called cations, the latter anions. The directed movement of ions occurs as a result of their attraction by oppositely charged electrodes. 3. Dissociation is a reversible process: in parallel with the disintegration of molecules into ions (dissociation), the process of combining ions (association) proceeds. Therefore, in the equations of electrolytic dissociation, instead of the equal sign, the sign of reversibility is put. For example,…

The question of the mechanism of electrolytic dissociation is essential. Substances with an ionic bond dissociate most easily. As you know, these substances are composed of ions. When they dissolve, the dipoles of water orient themselves around the positive and negative ions. Forces of mutual attraction arise between the ions and dipoles of water. As a result, the bond between the ions weakens, and the transition of ions from the crystal to the solution occurs. At…

Using the theory of electrolytic dissociation, definitions are given and the properties of acids, bases and salts are described. Electrolytes are called acids, during the dissociation of which only hydrogen cations are formed as cations H3PO4 H+ + H2PO-4 (first stage) H2PO-4 H+ + HPO2-4 (second stage) HPO2-4 H+ PO3-4 (third stage) The dissociation of a polybasic acid proceeds mainly through the first stage, to a lesser extent through the second, and only to a small extent through the third. Therefore, in an aqueous solution of, for example, phosphoric acid, along with H3PO4 molecules, there are ions (in successively decreasing amounts) H2PO2-4, HPO2-4 and PO3-4. Bases are called electrolytes, during the dissociation of which only hydroxide ions are formed as anions. For example: KOH K+ + OH—;…

Since electrolytic dissociation is a reversible process, electrolyte solutions contain molecules along with their ions. Therefore, electrolyte solutions are characterized by the degree of dissociation (denoted by the Greek letter alpha α). The degree of dissociation is the ratio of the number of molecules N 'decayed into ions to the total number of dissolved molecules N: The degree of dissociation of the electrolyte is determined empirically and is expressed in fractions of a unit or in percent. If α = 0, then there is no dissociation, and if α = 1 or 100%, then the electrolyte completely decomposes into ions. If α = 20%, then this means that out of 100 molecules of this electrolyte, 20 decomposed into ions. Different electrolytes have different degrees of dissociation. Experience shows that it depends on the concentration of the electrolyte and on the temperature. With decreasing electrolyte concentration, ...

According to the theory of electrolytic dissociation, all reactions in aqueous electrolyte solutions are reactions between ions. They are called ionic reactions, and the equations of these reactions are called ionic equations. They are simpler than reaction equations written in molecular form and are more general. When compiling ionic reaction equations, one should be guided by the fact that poorly dissociated, slightly soluble (precipitating) and gaseous substances are written in molecular form. The sign ↓, standing at the formula of a substance, means that this substance leaves the reaction sphere in the form of a precipitate, the sign means that the substance is removed from the reaction sphere in the form of a gas. Strong electrolytes, being completely dissociated, are recorded as ions. The sum of the electric charges on the left side of the equation must be equal to the sum of the electric charges on the right side. To consolidate these provisions, consider two examples. Example 1. Write the reaction equations between solutions of iron (III) chloride and sodium hydroxide in molecular and ionic forms. Let's break the solution of the problem into four stages. one….

KH2O = 1.10-4 This constant for water is called the ionic product of water, which depends only on temperature. During the dissociation of water, one OH– ion is formed for each H+ ion, therefore, in pure water, the concentrations of these ions are the same: [H+] = [OH–]. Using the value of the ionic product of water, we find: \u003d [OH -] \u003d mol / l. These are the concentrations of H+ and OH- ions…

1.2 MAIN DIRECTIONS IN THE DEVELOPMENT OF THE THEORY OF SOLUTIONS

Physical theory of solutions. The development of views on the nature of solutions since ancient times has been associated with the general course of development of science and production, as well as with philosophical ideas about the causes of chemical affinity between different substances. In the 17th and in the first half of the 18th century. The corpuscular theory of solutions has become widespread in the field of natural sciences and philosophy. In this theory, the dissolution process was considered as a mechanical process, when the corpuscles of the solvent enter the pores of the bodies and tear off the particles of the dissolved substance, which occupy the pores of the solvent forming a single solution. Such ideas initially satisfactorily explained the fact that a given solvent can dissolve not all substances, but only some.

At the beginning of the 19th century prerequisites are being created for the development of a physical theory of solutions, which was a generalization of a number of studies. The physical theory of solutions, which arose mainly on the basis of the works of J. Van't Hoff, S. Arrhenius and W. Ostwald, was based on an experimental study of the properties of dilute solutions (osmotic pressure, an increase in the boiling point, a decrease in the freezing point of a solution, a decrease in vapor pressure over a solution) , depending mainly on the concentration of the solute, and not on its nature. Osmosis is the spontaneous penetration of a solvent into a solution that is separated from it by a semi-permeable partition through which the solvent can enter, cannot, the solute passes.

Solution and solvent separated by a semi-permeable partition can be considered as two phases. The equilibrium of the solvent on both sides of the partition is expressed by the equality of its chemical potential in solution (to which additional pressure is applied) and the chemical potential of a pure solvent.

Quantitative laws (van't Hoff, Raoult) were interpreted in the continuation that in dilute solutions the molecules of the solute are similar to the molecules of an ideal gas. Deviations from these laws, observed for electrolyte solutions, were explained on the basis of the theory of electrolytic dissociation by S. Arrhenius.

The analogy between highly dilute solutions and gases seemed so convincing to many scientists that they began to consider the process of dissolution as a physical act. From the point of view of these scientists, the solvent is only a medium into which solute particles can diffuse. The simplicity of the representations of the physical theory of solutions and its successful application to explain many properties of solutions ensured the rapid success of this theory.

Chemical theory of solutions. DI. Mendeleev and his followers considered the process of solution formation as a kind of chemical process, which is characterized by the interaction between the particles of the components. DI. Mendeleev considered solutions as systems formed by particles of a solvent, a solute and unstable chemical compounds that form between them and are in a state of partial dissociation. DI. Mendeleev noted that the processes occurring in a solution are dynamic in nature and the need to use the entire amount of physical and chemical information about the properties of the particles that form the solution, emphasized that all components of the solution are equal and without taking into account the properties and states of each of them it is impossible to give a complete characterization systems as a whole. The scientist attached great importance to the study of the properties of solutions as a function of temperature, pressure, concentration; he was the first to express the idea of ​​the need to study the properties of solutions in mixed solvents. Developing the teachings of D.I. Mendeleev, supporters of the chemical view of the nature of solutions pointed out that the particles of the dissolved substance do not move in a vacuum, but in the space occupied by the particles of the solvent, with which they interact, forming complex compounds with different stability. The development of the theory of D.I. Mendeleev is the polyhedral theory of the formation of solutions, according to which elementary space groups-polyhedra are created in a liquid from homogeneous and heterogeneous molecules. However, the chemical theory cannot explain the mechanism of formation of ideal solutions, deviations in the properties of real solutions from the properties of ideal solutions.

The development of the chemical theory of solutions proceeded in several directions united by a single idea of ​​the interaction of a solvent with a solute. These studies concerned finding certain compounds in solution based on the study of property-composition diagrams, the study of vapor pressure over solutions, the distribution of substances between two solvents, and the study of the thermochemistry of solutions. Work on the determination of compounds in solutions was associated with great difficulties, since it was impossible to prove the existence of complex compounds (hydrates) in aqueous solutions by direct experiment, since they are in a state of dissociation, and attempts to isolate them from solutions in an undecomposed form ended in failure. Thermodynamic studies were of great importance for confirming the chemical theory of solutions. On many systems, it was shown that during the formation of a solution, cooling or heating of the system is observed, which was explained by the chemical interaction between the components. The chemical nature of the dissolution process was confirmed both by studies of the vapor pressure over the solution and by the study of the distribution of substances between two solvents.

By the beginning of the 20th century extensive experimental material has been accumulated showing that solutions are complex systems in which the phenomenon of association, dissociation, complex formation is observed, and in their study it is necessary to take into account all types of interaction between particles present and formed in a solution.

Due to the wide variety of solutions, both the physical and chemical theory of solutions are used to explain their nature and properties.

Adsorption in chemistry

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Topic 7. Solutions. Disperse systems

Lectures 15-17 (6 hours)

The purpose of the lectures: to study the main provisions of the solvate (hydrate) theory of dissolution; general properties of solutions (laws of Raoult, van't Hoff, osmotic pressure, Arrhenius equation); types of liquid solutions, define solubility; consider the properties of weak electrolytes (solubility constant, Ostwald's dilution law, ionic product of water, pH of the medium, solubility product); properties of strong electrolytes (Debye-Hückel theory, ionic strength of the solution); give a classification of dispersed systems; consider the stability of colloidal solutions, coagulation, peptization, obtaining colloid-dispersed systems and properties of colloid-dispersed systems (molecular-kinetic, optical and electro-kinetic).

Issues under study:

7.1. Solvate (hydrate) theory of dissolution.

7.2. General properties of solutions.

7.3. Types of liquid solutions. Solubility.

7.4. Properties of weak electrolytes.

7.5. Properties of strong electrolytes.

7.6. Classification of disperse systems.

7.7. Obtaining colloid-dispersed systems.

7.8. Stability of colloidal solutions. Coagulation. Peptization.

7.9. Properties of colloid-disperse systems.

Solutions homogeneous systems are called, consisting of two or more substances, the composition of which can vary within a fairly wide range allowed by solubility. Any solution consists of several components: a solvent ( BUT) and a solute of one or more ( IN).

Component- this is a part of a thermodynamic system that is homogeneous in chemical properties, which can be isolated from it and exist in a free form for an arbitrarily long time.

Solvent is a component whose concentration is higher than the concentration of other components in the solution. It retains its phase state during the formation of solutions.

Any solution is characterized by such properties as density, boiling point, freezing point, viscosity, surface tension, solvent pressure over the solution, osmotic pressure, etc. These properties change smoothly with changes in pressure, temperature, composition (concentration). The concentration of a solution indicates the amount of a substance that is contained in a certain weight of a solution or solvent, or in a certain volume of solution. In chemistry, various methods are used to express the concentration of solutions:

Mass fraction of solute (percentage concentration (w)) shows the number of grams of a solute ( m in) in 100 g of solution ( m p), expressed in %:

Molar concentration (C) shows the number of moles of a solute (n) in 1 dm³ of a solution (V):

Expressed in mol / dm³, for example, C (1 / 1H 2 SO 4) \u003d 0.1 mol / dm³.

Molar equivalent concentration is the number of mole equivalents of a solute in 1 dm³ of solution (V):

Expressed in mol/dm³. For example, C (1 / 2H 2 SO 4) \u003d 0.1 mol / dm³; C (1/5 KMnO 4) \u003d 0.02 mol / dm³.

The concepts of equivalent, equivalence factor (for example, f equiv (HCl) \u003d 1/1; f equiv (H 2 SO 4) \u003d 1/2; f equiv (Na 2 CO 3) \u003d 1/2; f equiv (KMnO 4) = 1/5) and the molar mass equivalent (for example, for sodium carbonate: M(1/2 Na 2 CO 3) = f eq M(Na 2 CO 3) = 1/2 M(Na 2 CO 3)) were considered in the introduction (paragraph 2).

Molality (C m) shows the number of moles (n) of a solute in 1000 g of solvent (m p-la):

Expressed in mol/kg of solvent, for example C m (NaCl) = 0.05 mol/kg.

Mole fraction is the ratio of the number of moles of a substance to the sum of the numbers of moles in a solution:

where N A and N B are the mole fraction of the solvent and the solute, respectively. The mole fraction multiplied by 100% is the mole percentage, so

N A + N B = 1. (7.6)

In practical work, it is important to be able to quickly move from one concentration unit to another, so it is important to remember that

m r-ra = V r-ra ρ, (7.7)

where m r-ra is the mass of the solution, g; V p-ra - the volume of the solution, cm 3; ρ is the density of the solution, g/cm3.

The dissolution process is a complex physical and chemical process in which the interaction between particles (molecules or ions) of various chemical nature is most clearly manifested.

The processes of dissolution of many substances in different states of aggregation are greatly influenced by the polarity of the molecules of the solvent and the solute. It should be noted that like dissolves like. Polar solvents (water, glycerin) dissolve polar molecules (KCl, NH 4 Cl, etc.); non-polar solvents (toluene, gasoline, etc.) dissolve non-polar molecules (hydrocarbons, fats, etc.).

Modern dissolution theory based on the physical theory of Van't Hoff and S. Arrhenius and the chemical theory of D. I. Mendeleev. According to this theory, the dissolution process consists of three stages:

1) mechanical destruction of bonds between particles of a dissolved substance, for example, the destruction of the crystal lattice of salt (this is a physical phenomenon);

2) education solvates (hydrates), i.e., unstable compounds of solute particles with solvent molecules (this is a chemical phenomenon);

3) a spontaneous process of diffusion of solvated (hydrated) ions throughout the volume of the solvent (this is a physical process). In solution, any charged particle (ion or polar molecule) is surrounded by solvation shell , which consists of solvent molecules oriented in an appropriate way. If the solvent is water, then the term hydration shell , and the phenomenon itself is called hydration .

The process of formation of solutions is accompanied by a thermal effect, which can be both endothermic and exothermic. The first stage of dissolution always takes place with the absorption of heat, and the second can take place both with the absorption and release of heat. Therefore, the total thermal effect of dissolution depends on the thermal effect of the formation of solvates (hydrates). The combination of molecules or ions of a solute with molecules of a solvent is carried out mainly due to hydrogen bonding, or due to the electrostatic interaction of polar molecules of substances. The composition of solvates (hydrates) varies depending on the temperature and concentration of the solute. With their increase, the number of solvent molecules included in the solvate (hydrate) decreases. Thus, solutions occupy an intermediate position between mechanical mixtures and chemical compounds.

The theory of solutions does not yet make it possible in any case to predict the properties of solutions from the properties of their components. This is explained by the extremely large variety and complexity of interactions between solvent molecules and solute particles. The structure of solutions, as a rule, is much more complicated than the structure of its individual components.

According to the state of aggregation, all solutions are divided into three groups: solutions of gases in gases or gas mixtures; liquid solutions; solid solutions (metal alloys). In what follows, only liquid solutions will be considered.

At the end of the 19th century, solutions were considered physical entities in which there were no interactions between the solvent and the solute. The formation of the solution was explained by the dispersion of particles of the dissolved substance in the indifferent medium of the solvent. The founders of these views were such well-known scientists as J. van't Hoff, S. Arrenius and W. Ostwald. In 1887, the great Russian chemist D. I. Mendeleev, relying on numerous experimental data, created the chemical (hydrate) theory of solutions. The basis of this theory was the idea of ​​the chemical nature of dissolution. In solution, compounds are formed between the solute and the solvent, which change their composition with changes in temperature and concentration. These compounds were named by D. I. Mendeleev hydrates, or solvates. The resulting hydrates have different strengths. Most of them are unstable and exist only in solutions. However, some of the hydrates are so strong compounds that when a solute is released from a solution, water enters the growing crystal in a chemically bound form. Such crystals were called crystalline hydrates, and the water included in their composition was called crystallization water. Examples of crystalline hydrates are CuSO4 5H20; Na2SO4 YuN20, etc. The strength of the resulting compounds is determined by the forces acting between the solvent and the solute. The nature of these forces is now known. Solvates (hydrates) are formed due to ion-dipole, dipole-dipole, donor-acceptor interaction, due to hydrogen bonds, as well as dispersion interaction. Mendeleev did not deny the role of the physical factor in the formation of solutions. He wrote: “The two indicated aspects of dissolution (physical and chemical) and the hypotheses hitherto applied to the consideration of solutions, although they have partly different starting points, will eventually lead to a general theory of solutions, because the same general laws govern both physical and chemical phenomena. The views of D. I. Mendeleev were fully confirmed. At present, the dissolution process is considered as a physicochemical process, and solutions are considered as physicochemical systems. The chemical theory of solutions by D. I. Mendeleev made it possible to explain the presence of thermal effects that occur during the processes of dissolution of substances. The thermal effect of the dissolution process (DNsolv) can be represented as the sum of the heat required to destroy the crystal lattice of a substance (DNre1:1) and the heat released in the solvation process (DNsolvate), i.e. e. AHp^ is a significant endothermic quantity, and DNS0LV is an exothermic quantity close to it in value. Based on this, the final sign of the thermal effect of the dissolution process will be determined by the contribution of each of these parameters. When dissolved, it is endothermic. This can be observed, for example, when potassium and ammonium nitrates, potassium chloride, etc., are dissolved in water. When the dissolution process is exothermic. An example of this is the dissolution of calcium and magnesium chlorides, sodium and potassium hydroxides, etc. in water. So, the sign of the thermal effect is determined by the nature of the solute and solvent, the depth of their interaction with each other. The presence of chemical interaction between the components also explains the volumetric effects during dissolution. So, when 1 liter of ethyl alcohol is dissolved in 1 liter of water, the volume of the resulting solution is not 2 liters, but 1.93 liters. In this case, the decrease in volume is mainly due to the formation of hydrogen bonds between the hydroxyl groups of water and alcohol.