Water is an inorganic compound made up of oxygen and hydrogen. Under normal conditions, it is a colorless, transparent liquid that is odorless and tasteless. In solid form, water is called snow, ice or hoarfrost, in gaseous form - steam. Approximately 71% of the entire surface of the planet is covered with water. Approximately 96% of water reserves fall on the oceans, lakes, glaciers, swamps and groundwater fall on the remaining 4%. By its nature, water is an excellent solvent and always contains dissolved substances or gases in its composition, with the exception of distilled water. Water is the most important source of life on the entire planet. Therefore, in our article we will try to tell you everything about this amazing substance, and most importantly, what kind of substance is water in nature and what are its chemical and physical properties.

Physical properties of water

• Under normal atmospheric conditions, water remains a liquid state, while other hydrogen compounds of a similar plan are gases. This phenomenon is explained by the special properties of the addition of water molecules and atoms, and the bonds present between them. The oxygen atoms are attached to the hydrogen atoms in an angle of almost 105 degrees, and this configuration is always preserved. Due to the large difference in the electronegativity of oxygen and hydrogen atoms, electron clouds are strongly shifted towards oxygen. In connection with this reason, the water molecule is considered an active dipole, in which the hydrogen side has a positive charge, and the oxygen side has a negative charge. As a result, the water molecule forms bonds, which are quite difficult to break and require a lot of energy.
• Water is practically incompressible. So, with an increase in atmospheric pressure by one bar, water is compressed by only 0.00005 of its original volume.
• The structure of ice and water is very similar. In both ice and water, the molecules try to arrange themselves in some particular order - they want to form a structure, but thermal motion prevents this. When water passes into a solid state, the thermal rotation of the molecules no longer prevents structural formation, after which the molecules are ordered, and the voids between them increase, from which, consequently, the density decreases. This explains the moment that water is a very anomalous substance. The solid state of aggregation of water - ice, can safely float on the surface of the liquid state of aggregation of water. When evaporation occurs, on the contrary, all bonds are immediately broken. A considerable amount of energy is required to break these bonds, which explains the highest heat capacity of water among all substances. To heat a liter of water by 1 degree, you need to spend about 4 kJ of energy. Due to this property, water is often used as a heat carrier.
• Water has a high surface tension, second only to mercury in this indicator. The high viscosity of water is explained by its hydrogen bonds, which prevent molecules from moving at different speeds.
• Water is a good solvent. The solute molecules are immediately surrounded by water molecules. Positive solute particles are attracted to oxygen atoms, and negative particles are attracted to hydrogen atoms. Since the sizes of water molecules are quite small, each molecule of the dissolved substance can be immediately surrounded by a large number of water molecules.
• Water is a substance that has a negative electrical surface potential.
• In its pure form, water is a good insulator, but since certain substances, salts or acids are often dissolved in it, negative and positive ions are always found in water. Because of these properties, water can conduct electricity.
• The refractive index of water is n=1.33. But water perfectly absorbs infrared radiation, and in connection with this property, water, or rather water vapor, is a greenhouse gas. Also, water is able to absorb microwave radiation, on which the action of microwave ovens is based.

Chemical properties

Those who think that water is organic matter are greatly mistaken. Water is made up of two elements, oxygen and hydrogen. Next, consider the basic chemical properties of water.

Electrolytes and non-electrolytes

It is known from the lessons of physics that solutions of some substances are capable of conducting electric current, while others are not.

Substances whose solutions conduct electricity are called electrolytes.

Substances whose solutions do not conduct electricity are called non-electrolytes. For example, solutions of sugar, alcohol, glucose and some other substances do not conduct electricity.

Electrolytic dissociation and association

Why do electrolyte solutions conduct electricity?

The Swedish scientist S. Arrhenius, studying the electrical conductivity of various substances, came in 1877 to the conclusion that the cause of electrical conductivity is the presence in solution ions formed when an electrolyte is dissolved in water.

The process by which an electrolyte breaks down into ions is called electrolytic dissociation.

S. Arrhenius, who adhered to the physical theory of solutions, did not take into account the interaction of electrolyte with water and believed that free ions were present in solutions. In contrast, the Russian chemists I. A. Kablukov and V. A. Kistyakovsky applied the chemical theory of D. I. Mendeleev to explain electrolytic dissociation and proved that when the electrolyte is dissolved, the chemical interaction of the solute with water occurs, which leads to the formation hydrates, and then they dissociate into ions. They believed that in solutions there are not free, not "naked" ions, but hydrated ones, that is, "dressed in a fur coat" of water molecules.

Water molecules are dipoles(two poles), since the hydrogen atoms are located at an angle of 104.5 °, due to which the molecule has an angular shape. The water molecule is shown schematically below.

As a rule, substances dissociate most easily with ionic bond and, accordingly, with an ionic crystal lattice, since they already consist of ready-made ions. When they dissolve, the dipoles of water are oriented with oppositely charged ends around the positive and negative ions of the electrolyte.

Forces of mutual attraction arise between electrolyte ions and water dipoles. As a result, the bond between the ions weakens, and the transition of ions from the crystal to the solution occurs. Obviously, the sequence of processes occurring during the dissociation of substances with an ionic bond (salts and alkalis) will be as follows:

1) orientation of water molecules (dipoles) near crystal ions;

2) hydration (interaction) of water molecules with ions of the surface layer of the crystal;

3) dissociation (decay) of the electrolyte crystal into hydrated ions.

Simplified, the ongoing processes can be reflected using the following equation:

Similarly, electrolytes dissociate, in the molecules of which there is a covalent bond (for example, molecules of hydrogen chloride HCl, see below); only in this case, under the influence of water dipoles, does the covalent polar bond transform into an ionic one; the sequence of processes occurring in this case will be as follows:

1) orientation of water molecules around the poles of electrolyte molecules;

2) hydration (interaction) of water molecules with electrolyte molecules;

3) ionization of electrolyte molecules (transformation of a covalent polar bond into an ionic one);

4) dissociation (decay) of electrolyte molecules into hydrated ions.

Simplified, the process of dissociation of hydrochloric acid can be reflected using the following equation:

It should be taken into account that randomly moving hydrated ions in electrolyte solutions can collide and reunite with each other. This reverse process is called association. Association in solutions occurs in parallel with dissociation, therefore, the sign of reversibility is put in the reaction equations.

The properties of hydrated ions differ from those of non-hydrated ones. For example, the unhydrated copper ion Cu 2+ is white in anhydrous copper(II) sulfate crystals and is blue when hydrated, i.e. bound to water molecules Cu 2+ nH 2 O. Hydrated ions have both constant and variable the number of water molecules.

Degree of electrolytic dissociation

In electrolyte solutions, along with ions, molecules are also present. Therefore, electrolyte solutions are characterized degree of dissociation, which is denoted by the Greek letter a ("alpha").

This is the ratio of the number of particles decomposed into ions (N g) to the total number of dissolved particles (N p).

The degree of electrolyte dissociation is determined empirically and is expressed in fractions or percentages. If a \u003d 0, then there is no dissociation, and if a \u003d 1, or 100%, then the electrolyte completely decomposes into ions. Different electrolytes have different degrees of dissociation, i.e., the degree of dissociation depends on the nature of the electrolyte. It also depends on the concentration: with the dilution of the solution, the degree of dissociation increases.

According to the degree of electrolytic dissociation, electrolytes are divided into strong and weak.

Strong electrolytes- these are electrolytes, which, when dissolved in water, almost completely dissociate into ions. For such electrolytes, the value of the degree of dissociation tends to unity.

Strong electrolytes include:

1) all soluble salts;

2) strong acids, for example: H 2 SO 4, HCl, HNO 3;

3) all alkalis, for example: NaOH, KOH.

Weak electrolytes- these are electrolytes that, when dissolved in water, almost do not dissociate into ions. For such electrolytes, the value of the degree of dissociation tends to zero.

Weak electrolytes include:

1) weak acids - H 2 S, H 2 CO 3, HNO 2;

2) an aqueous solution of ammonia NH 3 H 2 O;

4) some salts.

Dissociation constant

In solutions of weak electrolytes, due to their incomplete dissociation, dynamic equilibrium between non-dissociated molecules and ions. For example, for acetic acid:

You can apply the law of mass action to this equilibrium and write the expression for the equilibrium constant:

The equilibrium constant characterizing the process of dissociation of a weak electrolyte is called dissociation constant.

The dissociation constant characterizes the ability of an electrolyte (acid, base, water) dissociate into ions. The larger the constant, the easier the electrolyte decomposes into ions, therefore, the stronger it is. The values ​​of dissociation constants for weak electrolytes are given in reference books.

The main provisions of the theory of electrolytic dissociation

1. When dissolved in water, electrolytes dissociate (decompose) into positive and negative ions.

ions- this is one of the forms of existence of a chemical element. For example, sodium metal atoms Na 0 interact vigorously with water, forming an alkali (NaOH) and hydrogen H 2, while sodium ions Na + do not form such products. Chlorine Cl 2 has a yellow-green color and a pungent odor, poisonous, and chlorine ions Cl are colorless, non-toxic, odorless.

ions- These are positively or negatively charged particles into which atoms or groups of atoms of one or more chemical elements are converted as a result of the transfer or addition of electrons.

In solutions, ions move randomly in different directions.

According to their composition, ions are divided into simple- Cl - , Na + and complex- NH 4 +, SO 2 -.

2. The reason for the dissociation of the electrolyte in aqueous solutions is its hydration, i.e., the interaction of the electrolyte with water molecules and the breaking of the chemical bond in it.

As a result of this interaction, hydrated, i.e., associated with water molecules, ions are formed. Therefore, according to the presence of a water shell, ions are divided into hydrated(in solution and crystalline hydrates) and non-hydrated(in anhydrous salts).

3. Under the action of an electric current, positively charged ions move towards the negative pole of the current source - the cathode and therefore are called cations, and negatively charged ions move towards the positive pole of the current source - the anode and therefore are called anions.

Therefore, there is another classification of ions - by the sign of their charge.

The sum of the charges of the cations (H +, Na +, NH 4 +, Cu 2+) is equal to the sum of the charges of the anions (Cl -, OH -, SO 4 2-), as a result of which electrolyte solutions (HCl, (NH 4) 2 SO 4, NaOH, CuSO 4) remain electrically neutral.

4. Electrolytic dissociation is a reversible process for weak electrolytes.

Along with the process of dissociation (decomposition of the electrolyte into ions), the reverse process also proceeds - association(connection of ions). Therefore, in the equations of electrolytic dissociation, instead of the equal sign, the sign of reversibility is put, for example:

5. Not all electrolytes dissociate into ions to the same extent.

Depends on the nature of the electrolyte and its concentration. The chemical properties of electrolyte solutions are determined by the properties of the ions that they form during dissociation.

The properties of solutions of weak electrolytes are due to the molecules and ions formed in the process of dissociation, which are in dynamic equilibrium with each other.

The smell of acetic acid is due to the presence of CH 3 COOH molecules, the sour taste and color change of the indicators are associated with the presence of H + ions in the solution.

The properties of solutions of strong electrolytes are determined by the properties of the ions that are formed during their dissociation.

For example, the general properties of acids, such as sour taste, discoloration of indicators, etc., are due to the presence of hydrogen cations in their solutions (more precisely, oxonium ions H 3 O +). The general properties of alkalis, such as soapiness to the touch, discoloration of indicators, etc., are associated with the presence of OH - hydroxide ions in their solutions, and the properties of salts are associated with their decomposition in solution into metal (or ammonium) cations and anions of acid residues.

According to the theory of electrolytic dissociation all reactions in aqueous electrolyte solutions are reactions between ions. This is the reason for the high rate of many chemical reactions in electrolyte solutions.

The reactions that take place between ions are called ionic reactions, and the equations of these reactions - ionic equations.

Ion exchange reactions in aqueous solutions can proceed:

1. irreversibly, to end.

2. reversible i.e. flow in two opposite directions at the same time. Exchange reactions between strong electrolytes in solutions proceed to the end or are practically irreversible, when ions, combining with each other, form substances:

a) insoluble;

b) low dissociating (weak electrolytes);

c) gaseous.

Here are some examples of molecular and reduced ionic equations:

The reaction is irreversible, since one of its products is an insoluble substance.

The neutralization reaction is irreversible, since a low-dissociating substance is formed - water.

The reaction is irreversible, since CO 2 gas is formed and a low-dissociating substance is water.

If among the starting materials and among the products of the reaction there are weak electrolytes or poorly soluble substances, then such reactions are reversible, that is, they do not proceed to the end.

In reversible reactions, the equilibrium shifts towards the formation of the least soluble or least dissociated substances.

For example:

The equilibrium shifts towards the formation of a weaker electrolyte - H 2 O. However, such a reaction will not proceed to the end: undissociated molecules of acetic acid and hydroxide ions remain in the solution.

If the starting materials are strong electrolytes that, when interacting, do not form insoluble or low-dissociating substances or gases, then such reactions do not proceed: when the solutions are mixed, a mixture of ions is formed.

Reference material for passing the test:

periodic table

Solubility table

A solution is a homogeneous system consisting of two or more substances, the content of which can be changed within certain limits without violating homogeneity.

Aquatic solutions are made up of water(solvent) and solute. The state of substances in an aqueous solution, if necessary, is indicated by a subscript (p), for example, KNO 3 in solution - KNO 3 (p) .

Solutions that contain a small amount of solute are often referred to as diluted while solutions with high solute content concentrated. A solution in which further dissolution of a substance is possible is called unsaturated and a solution in which a substance ceases to dissolve under given conditions is saturated. The last solution is always in contact (in heterogeneous equilibrium) with the undissolved substance (one or more crystals).

Under special conditions, such as gentle (without stirring) cooling of a hot unsaturated solution solid substances can form supersaturated solution. When a crystal of a substance is introduced, such a solution is separated into a saturated solution and a precipitate of the substance.

In accordance with chemical theory of solutions D. I. Mendeleev, the dissolution of a substance in water is accompanied, firstly, destruction chemical bonds between molecules (intermolecular bonds in covalent substances) or between ions (in ionic substances), and thus the particles of the substance mix with water (in which some of the hydrogen bonds between molecules are also destroyed). Chemical bonds are broken due to the thermal energy of the movement of water molecules, and in this case cost energy in the form of heat.

Secondly, once in the water, the particles (molecules or ions) of the substance are subjected to hydration. As a result, hydrates- compounds of indeterminate composition between particles of a substance and water molecules (the internal composition of the particles of a substance itself does not change when dissolved). This process is accompanied highlighting energy in the form of heat due to the formation of new chemical bonds in hydrates.

In general, a solution cools down(if the cost of heat exceeds its release), or heats up (otherwise); sometimes - if the cost of heat and its release are equal - the temperature of the solution remains unchanged.

Many hydrates are so stable that they do not break down even when the solution is completely evaporated. So, solid crystal hydrates of salts CuSO 4 5H 2 O, Na 2 CO 3 10H 2 O, KAl (SO 4) 2 12H 2 O, etc. are known.

The content of a substance in a saturated solution at T= const quantifies solubility this substance. Solubility is usually expressed as the mass of solute per 100 g of water, for example 65.2 g KBr/100 g H 2 O at 20 °C. Therefore, if 70 g of solid potassium bromide is introduced into 100 g of water at 20 °C, then 65.2 g of salt will go into solution (which will be saturated), and 4.8 g of solid KBr (excess) will remain at the bottom of the beaker.

It should be remembered that the solute content in rich solution equals, in unsaturated solution less and in supersaturated solution more its solubility at a given temperature. So, a solution prepared at 20 ° C from 100 g of water and sodium sulfate Na 2 SO 4 (solubility 19.2 g / 100 g H 2 O), with a content

15.7 g of salt - unsaturated;

19.2 g salt - saturated;

2O.3 g of salt is supersaturated.

The solubility of solids (Table 14) usually increases with increasing temperature (KBr, NaCl), and only for some substances (CaSO 4 , Li 2 CO 3) is the opposite observed.

The solubility of gases decreases with increasing temperature, and increases with increasing pressure; for example, at a pressure of 1 atm, the solubility of ammonia is 52.6 (20 ° C) and 15.4 g / 100 g H 2 O (80 ° C), and at 20 ° C and 9 atm it is 93.5 g / 100 g H 2 O.

In accordance with the solubility values, substances are distinguished:

– well soluble, the mass of which in a saturated solution is commensurate with the mass of water (for example, KBr - at 20 ° C the solubility is 65.2 g / 100 g H 2 O; 4.6 M solution), they form saturated solutions with a molarity of more than 0.1 M;

– sparingly soluble, the mass of which in a saturated solution is much less than the mass of water (for example, CaSO 4 - at 20 ° C, the solubility is 0.206 g / 100 g H 2 O; 0.015 M solution), they form saturated solutions with a molarity of 0.1–0.001 M;

– practically insoluble the mass of which in a saturated solution is negligible compared to the mass of the solvent (for example, AgCl - at 20 ° C, the solubility is 0.00019 g per 100 g of H 2 O; 0.0000134 M solution), they form saturated solutions with a molarity of less than 0.001 M.

Compiled according to reference data solubility table common acids, bases and salts (Table 15), in which the type of solubility is indicated, substances are noted that are not known to science (not obtained) or completely decomposed by water.

Conventions used in the table:

"r" is a highly soluble substance

"m" - poorly soluble substance

"n" - practically insoluble substance

"-" - the substance is not received (does not exist)

"" - the substance is mixed with water indefinitely

Note. This table corresponds to the preparation of a saturated solution at room temperature by adding a substance (in the appropriate state of aggregation) to water. It should be noted that it is not always possible to obtain precipitates of poorly soluble substances using ion exchange reactions (for details, see 13.4).

13.2. Electrolytic dissociation

The dissolution of any substance in water is accompanied by the formation of hydrates. If at the same time there are no formula changes in the particles of the dissolved substance in the solution, then such substances are classified as non-electrolytes. They are, for example, gas nitrogen N 2 liquid chloroform CHCl 3 , solid sucrose C 12 H 22 O 11, which exist in an aqueous solution in the form of hydrates of their molecules.

Many substances are known (in the general form MA), which, after dissolution in water and the formation of hydrates of the MA nH 2 O molecules, undergo significant formula changes. As a result, hydrated ions appear in the solution - M + nH 2 O cations and A nH 2 O anions:

Such substances are electrolytes.

The process of the appearance of hydrated ions in an aqueous solution called electrolytic dissociation(S. Arrhenius, 1887).

Electrolytic dissociation ionic crystalline substances (M +) (A -) in water is irreversible reaction:

Such substances are strong electrolytes, they are many bases and salts, for example:

Electrolytic dissociation of MA substances consisting of polar covalent molecules, is reversible reaction:

Such substances are classified as weak electrolytes, they are many acids and some bases, for example:

In dilute aqueous solutions of weak electrolytes, we will always find both the original molecules and the products of their dissociation - hydrated ions.

The quantitative characteristic of the dissociation of electrolytes is called degree of dissociation and marked? , always? > 0.

For strong electrolytes? = 1 by definition (the dissociation of such electrolytes is complete).

For weak electrolytes, the degree of dissociation is the ratio of the molar concentration of the dissociated substance (s d) to the total concentration of the substance in solution (s):

The degree of dissociation is a fraction of unity or 100%. For weak electrolytes? « From 1 (100%).

For weak acids H n A, the degree of dissociation for each next step decreases sharply compared to the previous one:

The degree of dissociation depends on the nature and concentration of the electrolyte, as well as on the temperature of the solution; it grows with decrease the concentration of a substance in a solution (i.e., when the solution is diluted) and when heating.

AT diluted solutions strong acids H n A their hydroanions H n-1 A do not exist, for example:

B concentrated solutions, the content of hydroanions (and even initial molecules) becomes noticeable:

(it is impossible to sum the equations of the stages of reversible dissociation!). When heated value? 1 and? 2 increase, which promotes reactions involving concentrated acids.

Acids are electrolytes that, when dissociated, supply hydrogen cations to an aqueous solution and do not form any other positive ions:

Common strong acids:

In a dilute aqueous solution (conditionally up to 10% or 0.1 molar), these acids dissociate completely. For strong acids H n A, the list includes them hydroanions(anions of acid salts), which also dissociate completely under these conditions.

Common weak acids:

Bases are electrolytes that, when dissociated, supply hydroxide ions to an aqueous solution and do not form any other negative ions:

Dissociation sparingly soluble bases Mg (OH) 2, Cu (OH) 2, Mn (OH) 2, Fe (OH) 2 and others have no practical significance.

To strong grounds ( alkalis) include NaOH, KOH, Ba(OH) 2 and some others. The best known weak base is ammonia hydrate NH 3 H 2 O.

Medium salts are electrolytes that, upon dissociation, supply any cations, except H +, and any anions, except OH -, to an aqueous solution:

We are talking only about highly soluble salts. Dissociation sparingly soluble and practically insoluble salt doesn't matter.

Dissociate similarly double salts:

Acid salts(most of them are soluble in water) dissociate completely according to the type of medium salts:

The resulting hydroanions are, in turn, exposed to water:

a) if the hydroanion belongs strong acid, then it itself also dissociates completely:

and the full dissociation equation can be written as:

(solutions of such salts will necessarily be acidic, as well as solutions of the corresponding acids);

b) if the hydroanion belongs weak acid, then its behavior in water is dual - either incomplete dissociation as a weak acid:

or interaction with water (called reversible hydrolysis):

At? 1 > ? 2 dissociation predominates (and the salt solution will be acidic), and when? 1 > ? 2 - hydrolysis (and the salt solution will be alkaline). So, solutions of salts with anions HSO 3 -, H 2 PO 4 -, H 2 AsO 4 - and HSeO 3 - will be acidic, solutions of salts with other anions (most of them) will be alkaline. In other words, the name "acidic" for salts with the majority of hydroanions does not imply that these anions will behave like acids in solution (hydrolysis of hydroanions and the calculation of the ratio between α 1 and a 2 are studied only in higher education).

Basic salts of MgCl(OH), Cu 2 CO 3 (OH) 2 and others are mostly practically insoluble in water, and it is impossible to discuss their behavior in an aqueous solution.

13.3. dissociation of water. Solution medium

The water itself is very weak electrolyte:

The concentrations of the H + cation and the OH anion - in pure water are very small and amount to 1 10 -7 mol / l at 25 °C.

The hydrogen cation H + is the simplest nucleus - the proton p+(the electron shell of the H + cation is empty, 1s 0). A free proton has high mobility and penetrating power; surrounded by polar H 2 O molecules, it cannot remain free. The proton immediately attaches to the water molecule:

In the future, for simplicity, the notation H + is left (but H 3 O + is implied).

Types media of aqueous solutions:

For water at room temperature we have:

Therefore, in pure water:

This equality is also valid for aqueous solutions:

The practical pH scale corresponds to the interval 1-13 (dilute solutions of acids and bases):

In a practically neutral medium with pH = 6–7 and pH = 7–8, the concentration of H + and OH - is very low (1 10 -6 - 1 10 -7 mol / l) and is almost equal to the concentration of these ions in pure water. Such solutions of acids and bases are considered extremely diluted (contain very little substance).

For the practical establishment of the type of medium of aqueous solutions, indicators Substances that give a characteristic color to neutral, acidic and/or alkaline solutions.

Common indicators in the laboratory are litmus, methyl orange, and phenolphthalein.

Methyl orange (an indicator for an acidic environment) becomes pink in a strongly acidic solution (Table 16), phenolphthalein (an indicator for an alkaline environment) - raspberry in a strongly alkaline solution, and litmus is used in all environments.

13.4. Ion exchange reactions

In dilute solutions of electrolytes (acids, bases, salts), chemical reactions usually proceed with the participation of ions. In this case, all elements of the reagents can retain their oxidation states ( exchange reactions) or change them redox reactions). The examples given below refer to exchange reactions (for the occurrence of redox reactions, see Section 14).

In accordance with Berthollet's ruleionic reactions proceed almost irreversibly if poorly soluble solid substances are formed(they fall out) volatile substances(they are released as gases) or soluble substances are weak electrolytes(including water). Ionic reactions are represented by a system of equations - molecular, full and short ionic. The full ionic equations are omitted below (the reader is invited to make up their own).

When writing the equations of ionic reactions, it is necessary to be guided by the solubility table (see Table 8).

Examples precipitation reactions:

Attention! The slightly soluble (“m”) and practically insoluble (“n”) salts indicated in the solubility table (see Table 15) precipitate exactly as they are presented in the table (СаF 2 v, PbI 2 v, Ag 2 SO 4 v, AlPO 4 v, etc.).

In table. 15 not listed carbonates- medium salts with the anion CO 3 2-. It should be borne in mind that:

1) K 2 CO 3, (NH 4) 2 CO 3 and Na 2 CO 3 are soluble in water;

2) Ag 2 CO 3, BaCO 3 and CaCO 3 are practically insoluble in water and precipitate as such, for example:

3) salts of other cations, such as MgCO 3 , CuCO 3 , FeCO 3 , ZnCO 3 and others, although insoluble in water, do not precipitate from an aqueous solution during ionic reactions (i.e., they cannot be obtained by this method).

For example, iron (II) carbonate FeCO 3 obtained "dry" or taken in the form of a mineral siderite, when introduced into water, it precipitates without visible interaction. However, when trying to obtain it by an exchange reaction in a solution between FeSO 4 and K 2 CO 3, a precipitate of the basic salt precipitates (a conditional composition is given, in practice the composition is more complex) and carbon dioxide is released:

Similar to FeCO 3 , sulfide chromium (III) Cr 2 S 3 (insoluble in water) does not precipitate from solution:

In table. 15 also does not indicate the salts that decompose water - sulfide aluminum Al 2 S 3 (as well as BeS) and acetate chromium (III) Cr (CH 3 COO) 3:

Consequently, these salts also cannot be obtained by the exchange reaction in solution:

(in the last reaction, the composition of the precipitate is more complex; such reactions are studied in more detail in higher education).

Examples reactions with evolution of gases:

Examples reactions with the formation of weak electrolytes:

If the reactants and products of the exchange reaction are not strong electrolytes, there is no ionic form of the equation, for example:

13.5. Salt hydrolysis

Salt hydrolysis is the interaction of its ions with water, leading to the appearance of an acidic or alkaline environment, but not accompanied by the formation of a precipitate or gas (we are talking about medium salts below).

The hydrolysis process proceeds only with the participation soluble salt and consists of two stages:

1) dissociation salt in solution irreversible reaction (degree of dissociation? = 1, or 100%);

2) actually hydrolysis, i.e. the interaction of salt ions with water, - reversible reaction (degree of hydrolysis?< 1, или 100 %).

The equations of the 1st and 2nd stages - the first of them is irreversible, the second is reversible - cannot be added!

Note that salts formed by cations alkalis and anions strong acids do not undergo hydrolysis, they only dissociate when dissolved in water. In solutions of KCl, NaNO 3, Na 2 SO 4 and BaI 2 salts, the medium neutral.

In case of interaction anion salt hydrolysis at the anion.

The dissociation of the KNO 2 salt proceeds completely, the hydrolysis of the NO 2 anion - to a very small extent (for a 0.1 M solution - by 0.0014%), but this is enough for the solution to become alkaline(among the hydrolysis products there is an OH - ion), it has pH = 8.14.

Anions undergo hydrolysis only weak acids (in this example, the nitrite ion NO 2 - corresponding to the weak nitrous acid HNO 2). The anion of a weak acid attracts the hydrogen cation present in water to itself and forms a molecule of this acid, while the hydroxide ion remains free:

List of hydrolyzable anions:

Please note that in examples (c - e) it is impossible to increase the number of water molecules and instead of hydroanions (HCO 3 -, HPO 4 2-, HS -) write the formulas of the corresponding acids (H 2 CO 3, H 3 PO 4, H 2 S ). Hydrolysis is a reversible reaction, and it cannot proceed “to the end” (until the formation of acid H n A).

If such an unstable acid as H 2 CO 3 were formed in a solution of its salt Na 2 CO 3, then CO 2 gas would be released from the solution (H 2 CO 3 \u003d CO 2 v + H 2 O). However, when soda is dissolved in water, a transparent solution is formed without gas evolution, which is evidence of the incompleteness of the hydrolysis of the CO| anion. with the appearance in the solution of only the hydroanion of carbonic acid HCOg.

The degree of salt hydrolysis by anion depends on the degree of dissociation of the hydrolysis product - acid (HNO 2, HClO, HCN) or its hydroanion (HCO 3 -, HPO 4 2-, HS -); the weaker the acid, the higher the degree of hydrolysis. For example, ions CO 3 2-, PO 4 3- and S 2- undergo hydrolysis to a greater extent (in 0.1 M solutions ~ 5%, 37% and 58%, respectively) than the NO 2 ion, since the dissociation of H 2 CO 3 and H 2 S in the 2nd stage, and H 3 PO 4 in the 3rd stage (i.e., the dissociation of HCO 3 -, HS - and HPO 4 2- ions) proceeds much less than the dissociation of acid HNO 2 . Therefore, solutions, for example, Na 2 CO 3, K 3 PO 4 and BaS will highly alkaline(which is easy to verify by the soapiness of the soda solution to the touch). An excess of OH ions in a solution is easy to detect with an indicator or measure with special instruments (pH meters).

If aluminum is introduced into a concentrated solution of a salt that is highly hydrolyzed by anion, for example Na 2 CO 3 , then the latter (due to amphotericity) will react with OH -

and hydrogen evolution will be observed. This is additional evidence of the hydrolysis of the CO 3 2- ion (after all, we did not add NaOH alkali to the Na 2 CO 3 solution!).

In case of interaction cation dissolved salt with water the process is called salt hydrolysis by cation:

The dissociation of the Ni(NO 3) 2 salt proceeds completely, the hydrolysis of the Ni 2+ cation proceeds to a very small extent (for a 0.1 M solution, by 0.001%), but this is enough for the solution to become sour(among the hydrolysis products there is an H + ion), it has pH = 5.96.

Only cations of poorly soluble basic and amphoteric hydroxides and the ammonium cation NH 4 + undergo hydrolysis. The hydrolysable cation attracts the OH - anion present in the water and forms the corresponding hydroxocation, while the H + cation remains free:

The ammonium cation in this case forms a weak base - ammonia hydrate:

List of hydrolyzable cations:

Examples:

Please note that in examples (a - c) it is impossible to increase the number of water molecules and instead of hydroxocations FeOH 2+, CrOH 2+, ZnOH + write the formulas of FeO (OH), Cr (OH) 3, Zn (OH) 2 hydroxides. If hydroxides were formed, then precipitates would fall out of solutions of FeCl 3, Cr 2 (SO 4) 3 and ZnBr 2 salts, which is not observed (these salts form transparent solutions).

An excess of H + cations is easy to detect with an indicator or measure with special instruments. You can also

do such an experience. In a concentrated solution of a salt that is highly hydrolyzed by cation, for example AlCl 3:

magnesium or zinc is added. The latter will react with H +:

and hydrogen evolution will be observed. This experiment is additional evidence of the hydrolysis of the Al 3+ cation (because we did not add acid to the AlCl 3 solution!).

Examples of tasks of parts A, B

1. A strong electrolyte is

1) C 6 H 5 OH

2) CH 3 COOH

3) C 2 H 4 (OH) 2

2. Weak electrolyte is

1) hydrogen iodide

2) hydrogen fluoride

3) ammonium sulfate

4) barium hydroxide

3. In an aqueous solution of every 100 molecules, 100 hydrogen cations are formed for an acid

1) coal

2) nitrogenous

3) nitrogen

4-7. In the equation for the dissociation of a weak acid over all possible steps

the sum of the coefficients is

8-11. For the equations of dissociation in a solution of two alkalis of the set

8. NaOH, Ba (OH) 2

9. Sr (OH) 2, Ca (OH) 2

10. KOH, LiOH

11. CsOH, Ca (OH) 2

the total sum of the coefficients is

12. Lime water contains a set of particles

1) CaOH +, Ca 2+, OH -

2) Ca 2+, OH -, H 2 O

3) Ca 2+, H 2 O, O 2-

4) CaOH +, O 2-, H +

13-16. With the dissociation of one formula unit of salt

14. K 2 Cr 2 O 7

16. Cr 2 (SO 4) 3

the number of ions formed is

17. Greatest the amount of PO 4 -3 ion can be found in a solution containing 0.1 mol

18. Precipitation reaction is

1) MgSO 4 + H 2 SO 4 >…

2) AgF + HNO 3 >…

3) Na 2 HPO 4 + NaOH >…

4) Na 2 SiO 3 + HCl >…

19. The reaction with the release of gas is

1) NaOH + CH 3 COOH >…

2) FeSO 4 + KOH >…

3) NaHCO 3 + HBr >…

4) Pl(NO 3) 2 + Na 2 S>…

20. Brief ionic equation OH - + H + = H 2 O corresponds to the interaction

1) Fe(OH) 2 + HCl >…

2) NaOH + HNO 2 >…

3) NaOH + HNO 3 >…

4) Ba (OH) 2 + KHSO 4 > ...

21. In the ionic reaction equation

SO 2 + 2OH = SO 3 2- + H 2 O

OH ion - can respond to the reagent

4) C 6 H 5 OH

22-23. Ionic equation

22. ZCa 2+ + 2PO 4 3- \u003d Ca 3 (PO 4) 2 v

23. Ca 2+ + HPO 4 2- \u003d CaHRO 4 v

corresponds to the reaction between

1) Ca (OH) 2 and K 3 PO 4

2) CaCl 2 and NaH 2 PO 4

3) Ca (OH) 2 and H 3 RO 4

4) CaCl and K 2 HPO 4

24-27. In the molecular reaction equation

24. Na 3 PO 4 + AgNO 3 >…

25. Na 2 S + Cu (NO 3) 2 > ...

26. Ca(HSO 3) 2 >…

27. K 2 SO 3 + 2HBr >… the sum of the coefficients is

28-29. For a complete neutralization reaction

28. Fe(OH) 2 + HI >…

29. Ba (OH) 2 + H 2 S > ...

the sum of the coefficients in the full ionic equation is

30-33. In the short ionic reaction equation

30. NaF + AlCl 3 >…

31. K 2 CO 3 + Sr (NO 3) 2 > ...

32. Mgl 2 + K 3 PO 4 > ...

33. Na 2 S + H 2 SO 4 > ...

the sum of the coefficients is

34-36. In an aqueous solution of salt

34. Ca(ClO 4) 2

36. Fe 2 (SO 4) 3

environment is formed

1) acidic

2) neutral

3) alkaline

37. The concentration of hydroxide ion increases after salt is dissolved in water.

38. The neutral medium will be in the final solution after mixing the solutions of the initial salts in the sets

1) BaCl 2, Fe (NO 3) 3

2) Na 2 CO 3, SrS

4) MgCl 2 , RbNO 3

39. Establish a correspondence between salt and its ability to hydrolyze.

40. Establish a correspondence between salt and solution medium.

41. Establish a correspondence between salt and the concentration of the hydrogen cation after the salt is dissolved in water.

Water (hydrogen oxide)- a binary inorganic compound with the chemical formula H 2 O. The water molecule consists of two hydrogen atoms and one oxygen, which are interconnected by a covalent bond. Under normal conditions, it is a transparent liquid that has no color (with a small layer thickness), odor and taste. In the solid state it is called ice (ice crystals can form snow or frost), and in the gaseous state it is called water vapor. Water can also exist as liquid crystals (on hydrophilic surfaces). It is approximately 0.05% of the Earth's mass.

Water solution A type of solution in which water is the solvent. Being an excellent solvent, it is water that is used to prepare most solutions in chemistry.

Substances that dissolve poorly in water are called hydrophobic (“water-afraid”), and those that dissolve well in it are called hydrophilic (“water-loving”). An example of a typical hydrophilic compound is sodium chloride (common salt).

If a substance forms an aqueous solution that conducts electricity well, then it is called a strong electrolyte; otherwise, weak. Strong electrolytes in solution almost completely decompose into ions (α→1), while weak ones practically do not decompose (α→0).

Substances that dissolve in water, but do not decompose into ions (that is, they are in solution in a molecular state), are called non-electrolytes (an example is sugar).

When performing calculations in reaction equations where one or more aqueous solutions interact, it is often necessary to know the molar concentration of the solute.

Solubility- the ability of a substance to form homogeneous systems with other substances - solutions in which the substance is in the form of individual atoms, ions, molecules or particles. Solubility is expressed by the concentration of a solute in its saturated solution, either as a percentage, or in weight or volume units, referred to 100 g or 100 cm³ (ml) of the solvent (g/100 g or cm³/100 cm³). The solubility of gases in liquids depends on temperature and pressure. The solubility of liquid and solid substances is practically only temperature dependent. All substances are soluble in solvents to some extent. When the solubility is too low to measure, the substance is said to be insoluble.

The dependence of the solubility of substances on temperature is expressed using solubility curves. Solubility curves are used to make various calculations. For example, you can determine the mass of a substance that will precipitate from a saturated solution when it is cooled.

The process of separating a solid from a saturated solution when the temperature is lowered is called crystallization. Crystallization plays a huge role in nature - leads to the formation of certain minerals, participates in the processes occurring in rocks.

The composition of any solution can be expressed both qualitatively and quantitatively. Usually, in the qualitative assessment of the solution, such concepts are used as, saturated, unsaturated, supersaturated(or oversaturated), concentrated and diluted solution.

Saturated a solution is called, which contains the maximum possible amount of a dissolved substance under given conditions (t, p). A saturated solution is often in a state of dynamic equilibrium with an excess of a solute, in which the process of dissolution and the process of crystallization (precipitation of a substance from a solution) proceed at the same rate.

To prepare a saturated solution, the dissolution of the substance must be carried out until a precipitate forms, which does not disappear during long-term storage.

unsaturated is called a solution that contains less substance than it can dissolve under given conditions.

Oversaturated solutions contain more solute by mass than can be dissolved under given conditions. Supersaturated solutions are formed upon rapid cooling of saturated solutions. They are unstable and can exist for a limited time. Very quickly, the excess solute precipitates, and the solution becomes saturated.

It should be noted that when the temperature changes, saturated and unsaturated solutions can easily reversibly transform into each other. The process by which a solid is released from a saturated solution as the temperature is lowered is called crystallization . Crystallization and dissolution play a huge role in nature: they lead to the formation of minerals, they are of great importance in atmospheric and soil phenomena. On the basis of crystallization in chemistry, a method of purification of substances is common, which is called recrystallization.

For an approximate quantitative expression of the composition of the solution, the concepts are used concentrated and dilute solutions.

concentrated a solution is called, in which the mass of the solute is commensurate with the mass of the solvent, i.e. does not differ from it by more than 10 times.

If the mass of the solute is more than ten times less than the mass of the solvent, then such solutions are called diluted .

However, it should be remembered that the division of solutions into concentrated and diluted is conditional, and there is no clear boundary between them.

The exact quantitative composition of solutions is expressed using mass fraction of solute , its molar concentration , as well as in some other ways.

Water is an amazing substance with amazing properties that are not yet fully understood. Without it, a person cannot live for a long time, so people are attentive to the conservation of the water resources of our planet.

Water as a chemical

Everyone knows the formula for water - H2O. The mass of a water molecule is 18, which is one and a half times less than the mass of air. Water is the only substance that can be vapor, liquid or solid. When solid, it turns into ice. We all know that solids are denser than liquids. Only water does not adhere to this rule! Ice is much lighter than water, which is why it floats on the surface.
Water is considered a heat-retaining substance. In order for it to heat up and turn into steam, it is necessary to expend a large amount of energy. Energy is spent on breaking hydrogen bonds. Water passes into a gaseous state, and its molecules begin to move at a great distance from each other. It is often used as a heat transfer medium because it releases heat very slowly.
In summer, you can notice that the water in the reservoirs is much warmer than the air. This is explained by hydrogen bonds. When heated, they are difficult to break. When the water cools, the molecules begin to line up on their own, releasing energy in the process.
Various substances can dissolve in water. This is due to the destruction or formation of bonds between water molecules and particles of a substance that dissolves.
Water is everywhere. In our body it is about 70%. If the human body loses about 3% of water, the person will not be able to run. With a loss of 5%, you can no longer train. 10% is an indicator that is already life-threatening. Excess water can also lead to negative consequences. Consideration should be given to both the quantity and quality of water.

Water composition

Water is a liquid that is odorless and colorless. A water molecule contains two hydrogen atoms and one oxygen atom linked by a polar covalent bond.
Water is made up of various substances. It is a rather complex solution in which there are various substances. All components of its chemical composition are divided into several groups:

1. Macrocomponents (main ions). Water gets them from the soil and rocks.


2. dissolved gases. Their number depends on the temperature of the water.


3. Biogenic elements (chemical compounds). Their source is the processes that take place inside the reservoirs. They enter water bodies along with agricultural, industrial and domestic waters.


4. Microelements. These are more than thirty substances, among which are bromine, cobalt, copper, zinc, selenium and others. There are not too many of them in the reservoirs.


5. dissolved organic matter. These are organic forms of biogenic elements.


6. Toxic substances. Among them are oil products, heavy metals, phenols, synthetic surfactants.

In natural water, there are still gas bubbles and a huge number of various solid particles. An example of solid inorganic particles that can be found in water is rust. The water also contains waste products of flora and fauna, spores, algae, bacteria, viruses and other elements.
If you want to use only high-quality water, you should pay attention to "Grafskaya". For the first time they learned about it in the 14th century! It is mined from an artesian well near the village of Stankovo. does not contain artificial additives. It contains all useful elements. Grafskaya is real clean water that takes care of your health.