Water is the most common solvent on planet Earth, largely determining the nature of terrestrial chemistry as a science. Most of chemistry, at its inception as a science, began precisely as the chemistry of aqueous solutions of substances. It is sometimes considered as an ampholyte - and an acid and a base at the same time (cation H + anion OH -). In the absence of foreign substances in the water, the concentration of hydroxide ions and hydrogen ions (or hydronium ions) is the same.

Water is a chemically quite active substance. It reacts with many substances in organic and inorganic chemistry.

1) Water reacts with many metals with the release of hydrogen:

2Na + 2H 2 O = H 2 + 2NaOH (violently)

2K + 2H 2 O = H 2 + 2KOH (violently)

3Fe + 4H 2 O = 4H 2 + Fe 3 O 4 (only when heated)

Not all, but only sufficiently active metals can participate in redox reactions of this type. The most easily react are alkali and alkaline earth metals of groups I and II.

From non-metals with water react, for example, carbon and its hydrogen compound (methane). These substances are much less active than metals, but they can still react with water at high temperatures:

C + H 2 O = H 2 + CO (with strong heating)

CH 4 + 2H 2 O = 4H 2 + CO 2 (with strong heating)

2) Electrolysis. Water decomposes into hydrogen and oxygen when exposed to an electric current. It is also a redox reaction, where water is both an oxidizing agent and a reducing agent.

3) Water reacts with many non-metal oxides. Unlike the previous ones, these reactions are not redox, but compound reactions:

SO 2 + H 2 O = H 2 SO 3

SO 3 + H 2 O = H 2 SO 4

CO 2 + H 2 O = H 2 CO 3

4) Some metal oxides can also react with water:

CaO + H 2 O = Ca (OH) 2

Not all metal oxides react with water. Some of them are practically insoluble in water and therefore do not react with water. We have already encountered such oxides. These are ZnO, TiO 2, Cr 2 O 3, from which, for example, water-resistant paints are prepared. Iron oxides are also insoluble in water and do not react with it.

5) Water forms numerous compounds in which its molecule is completely preserved. These are the so-called hydrates. If the hydrate is crystalline, then it is called crystal hydrate... For example:

CuSO 4 + 5H 2 O = CuSO 4 * 5H 2 O (crystalline hydrate (copper sulfate))

Here are other examples of hydrate formation:

H 2 SO 4 + H 2 O = H 2 SO 4 * H 2 O (sulfuric acid hydrate)

NaOH + H 2 O = NaOH * H 2 O (sodium hydroxide hydrate)

Compounds that bind water into hydrates and crystalline hydrates are used as desiccants. With their help, for example, water vapor is removed from humid atmospheric air.

6) Photosynthesis. A special reaction of water - the synthesis of starch by plants (C 6 H 10 O 5) n and other similar compounds (carbohydrates), occurring with the release of oxygen:

6n CO 2 + 5n H 2 O = (C 6 H 10 O 5) n + 6n O 2 (under the action of light)

7) Hydration reactions in oragic chemistry. (The addition of water to hydrocarbon molecules.) For example:

C 2 H 4 + H 2 O = C 2 H 5 OH

11.1. Physical dissolution

When a substance enters water, it can:
a) dissolve in water, that is, mix with it at the atomic-molecular level;
b) enter into a chemical reaction with water;
c) do not dissolve and do not react.
What does the result of the interaction of a substance with water depend on? Naturally, on the characteristics of the substance and on the characteristics of water.
Let's start with dissolution and consider what characteristics of water and substances interacting with it are of greatest importance in these processes.
Place in two test tubes a small portion of C 10 H 8 naphthalene. Pour water into one of the test tubes, and into the other - heptane С 7 Н 16 (you can use gasoline instead of pure heptane). Naphthalene will dissolve in heptane, but not in water. Let's check whether the naphthalene really dissolved in heptane or reacted with it. To do this, put a few drops of the solution on a glass and wait until the heptane evaporates - colorless lamellar crystals form on the glass. The fact that it is naphthalene can be seen by its characteristic smell.

One of the differences between heptane and water is that its molecules are non-polar, while water molecules are polar. In addition, there are hydrogen bonds between water molecules, but there are no hydrogen bonds between heptane molecules.

To dissolve naphthalene in heptane, it is required to break weak intermolecular bonds between naphthalene molecules and weak intermolecular bonds between heptane molecules. Upon dissolution, equally weak intermolecular bonds are formed between the molecules of naphthalene and heptane. The thermal effect of such a process is practically zero.
How does naphthalene dissolve in heptane? Only due to the entropy factor (disorder grows in the naphthalene - heptane system).

To dissolve naphthalene in water, it is necessary, in addition to weak bonds between its molecules, to break hydrogen bonds between water molecules. In this case, hydrogen bonds between naphthalene and water molecules are not formed. The process turns out to be endothermic and so energetically unfavorable that the entropy factor is not able to help here.
And if instead of naphthalene we take another substance, the molecules of which are capable of forming hydrogen bonds with water molecules, will such a substance dissolve in water?
If there are no other obstacles, then there will be. For example, you know that sugar (sucrose C 12 H 22 O 11) is perfectly soluble in water. Looking at the structural formula of sucrose, you will see that its molecule contains –O – H groups capable of forming hydrogen bonds with water molecules.
Make sure experimentally that sucrose is slightly soluble in heptane, and try to explain yourself why the properties of naphthalene and sucrose differ so much.
The dissolution of naphthalene in heptane and sucrose in water is called physical dissolution.

Only molecular substances can physically dissolve.

The other components of the solution are called solutes.

The patterns we have identified also apply to the cases of dissolution in water (and in most other solvents) of liquid and gaseous substances. If all the substances forming the solution were in the same state of aggregation before dissolution, then the substance that is larger in the solution is usually called the solvent. The exception to this rule is water: it is usually called a solvent, even if it is less than the solute.
The reason for the physical dissolution of a substance in water can be not only the formation of hydrogen bonds between the molecules of the dissolved substance and water, but also the formation of other types of intermolecular bonds. This happens primarily in the case of dissolution of gaseous substances in water (for example, carbon dioxide or chlorine), in which the molecules are not bound to each other at all, as well as some liquids with very weak intermolecular bonds (for example, bromine). An energy gain is achieved here due to the orientation of dipoles (water molecules) around polar molecules or polar bonds in a solute, and in the case of chlorine or bromine, it is caused by the tendency to attach electrons of chlorine and bromine atoms, which is also preserved in the molecules of these simple substances (for more details - in § 11.4).
In all these cases, substances are much less soluble in water than during the formation of hydrogen bonds.
If you remove the solvent from the solution (for example, as you did in the case of a solution of naphthalene in heptane), then the solute will be released in a chemically unchanged form.

PHYSICAL SOLUTION, SOLVENT.
1 explain why heptane is insoluble in water
2. Prompt the sign of the thermal effect of dissolution of ethyl alcohol (ethanol) in water.
3. Why is ammonia highly soluble in water, and oxygen poorly?
4. Which substance is better soluble in water - ammonia or phosphine (PH 3)?
5. Explain the reason for the better solubility of ozone in water than oxygen.
6. Determine the mass fraction of glucose (grape sugar, C 6 H 12 O 6) in an aqueous solution if 120 ml of water and 30 g of glucose were used for its preparation (take the density of water equal to 1 g / ml). What is the concentration of glucose in this solution if the density of the solution is 1.15 g / ml?
7. How much sugar (sucrose) can be isolated from 250 g of syrup with a mass fraction of water equal to 35%?

1. Experiments on the dissolution of various substances in various solvents.
2. Preparation of solutions.

In the first paragraph, we considered the cases of dissolution of substances in which the chemical bonds remained unchanged. But this is not always the case.
Place some sodium chloride crystals in a test tube and add water. After a while, the crystals will dissolve. What happened?
Sodium chloride is a non-molecular substance. A NaCl crystal is composed of Na and Cl ions. When such a crystal enters the water, these ions pass into it. In this case, ionic bonds in the crystal and hydrogen bonds between water molecules are broken. Ions trapped in water interact with water molecules. In the case of chloride ions, this interaction is limited by the electrostatic attraction of dipole water molecules to the anion, and in the case of sodium cations, it approaches donor-acceptor in nature. Either way, the ions are covered hydration shell(fig.11.1).

In the form of a reaction equation, this can be written as follows:

NaCl cr + ( n + m) H 2 O = + A

or abbreviated where index aq means that ion hydrated... Such an equation is called ionic equation.

You can also write down the "molecular" equation of this process: (this name has been preserved since the time when it was assumed that all substances consist of molecules)

Hydrated ions are less attracted to each other, and the energy of thermal motion is sufficient to prevent these ions from sticking together into a crystal.

In practice, the presence of ions in a solution can be easily confirmed by studying the electrical conductivity of sodium chloride, water, and the resulting solution. You already know that sodium chloride crystals do not conduct electric current, because although they contain charged particles - ions, they are "fixed" in the crystal and cannot move. Water conducts electric current very poorly, because although oxonium ions and hydroxide ions are formed in it due to autoprotolysis, they are very few. A sodium chloride solution, on the other hand, conducts an electric current well, because there are many ions in it, and they can move freely, including under the influence of an electric voltage.
Energy must be expended to break ionic bonds in a crystal and hydrogen bonds in water. When the ions are hydrated, energy is released. If the energy consumption for breaking bonds exceeds the energy released during the hydration of ions, then dissolution endothermic, and if vice versa, then - exothermic.
Sodium chloride dissolves in water with practically zero thermal effect, therefore, the dissolution of this salt occurs only due to an increase in entropy. But usually dissolution is accompanied by a noticeable release of heat (Na 2 CO 3, CaCl 2, NaOH, etc.) or its absorption (KNO 3, NH 4 Cl, etc.), for example:

When water is evaporated from solutions obtained by chemical dissolution, dissolved substances are again released from them in a chemically unchanged form.

Both with physical and chemical dissolution, a solution of the substance that we dissolved is formed, for example, a solution of sugar in water or a solution of sodium chloride in water. In other words, the solute can be separated from the solution by removing the water.

HYDRATE SHELL, HYDRATION, CHEMICAL DISSOLUTION.
Give three examples of substances well known to you: a) soluble in water or reacting with it, b) insoluble in water and not reacting with it.
2. What is a solvent and what is a dissolved substance (or substances) in the following solutions: a) soapy water, b) table vinegar, c) vodka d) hydrochloric acid, e) fuel for a motorcycle, f) pharmacy "hydrogen peroxide", g) sparkling water, i) "brilliant green", j) cologne?
In case of difficulty, consult with your parents.
3. List the methods by which you can remove the solvent from the liquid solution.
4. How do you understand the expression "chemically unchanged" in the last paragraph of the first paragraph of this chapter? What changes can occur to a substance as a result of its dissolution and subsequent separation from solution?
5. It is known that fats are insoluble in water, but dissolve well in gasoline. Based on this, what can be said about the structure of fat molecules?
6. Write down the equations of chemical dissolution in water of the following ionic substances:
a) silver nitrate, b) calcium hydroxide, c) cesium iodide, d) potassium carbonate, e) sodium nitrite, f) ammonium sulfate.
7. Write down the equations of crystallization of substances from the solutions listed in task 6, when removing water.
8. What is the difference between solutions obtained by physical dissolution of substances from solutions obtained by chemical dissolution? What do these solutions have in common?
9. Determine the mass of salt that must be dissolved in 300 ml of water to obtain a solution with a mass fraction of this salt equal to 0.1. The density of the water is 1 g / ml and the density of the solution is 1.05 g / ml. What is the concentration of salt in the resulting solution if its formula weight is 101 days?
10. How much water and barium nitrate should be taken to prepare 0.5 l of a 0.1 M solution of this substance (solution density 1.02 g / ml)?
Experiments on the dissolution of ionic substances in water.

Any portion of sodium chloride (or other similar substance) placed in water would always dissolve completely if, in addition to the dissolution process

the reverse process would not take place - the process of crystallization of the initial substance from solution:

At the moment the crystal is placed in water, the rate of the crystallization process is zero, but as the concentration of ions in the solution increases, it increases and at some point becomes equal to the rate of dissolution. A state of equilibrium sets in:

the resulting solution is called saturated.

As such a characteristic, the mass fraction of a solute, its concentration, or another physical quantity characterizing the composition of the solution can be used.
According to their solubility in a given solvent, all substances are divided into soluble, slightly soluble and practically insoluble. Usually, practically insoluble substances are simply called insoluble. For the conditional boundary between soluble and poorly soluble substances, the solubility is taken equal to 1 g in 100 g of H 2 O ( w 1%), and for the conditional boundary between poorly soluble and insoluble substances - solubility equal to 0.1 g in 100 g H 2 O ( w 0,1%).
The solubility of a substance is temperature dependent. Since solubility is a characteristic of equilibrium, its change with a change in temperature occurs in full accordance with Le Chatelier's principle, that is, with exothermic dissolution of a substance, its solubility decreases with increasing temperature, and with endothermic dissolution, it increases.
Solutions in which, under the same conditions, the solute is less than saturated, are called unsaturated.

SATURATED SOLUTION; UNSATURATED SOLUTION; SOLUBILITY OF SUBSTANCE; SOLUBLE, Slightly Soluble and Insoluble Substances.

1. Write down the equilibrium equations in the saturated solution - precipitate system for a) potassium carbonate, b) silver nitrate and c) calcium hydroxide.
2. Determine the mass fraction of potassium nitrate in an aqueous solution of this salt saturated at 20 ° C, if during the preparation of such a solution 100 g of potassium nitrate was added to 200 g of water, and at the same time, after the end of the preparation of the solution, 36.8 g of potassium nitrate did not dissolve.
3. Is it possible to prepare an aqueous solution of potassium chromate K 2 CrO 4 with a mass fraction of a dissolved substance equal to 45% at 20 ° C, if at this temperature no more than 63.9 g of this salt dissolves in 100 g of water.
4. The mass fraction of potassium bromide in a saturated aqueous solution at 0 ° C is 34.5%, and at 80 ° C - 48.8%. Determine the mass of potassium bromide released upon cooling to 0 ° C 250 g of an aqueous solution of this salt saturated at 80 ° C.
5. The mass fraction of calcium hydroxide in a saturated aqueous solution at 20 ° C is 0.12%. How many liters of calcium hydroxide solution (lime water) saturated at this temperature can be obtained with 100 g of calcium hydroxide? Take the density of the solution equal to 1 g / ml.
6. At 25 ° C, the mass fraction of barium sulfate in a saturated aqueous solution is 2.33 · 10 –2%. Determine the minimum volume of water required to completely dissolve 1 g of this salt.
preparation of saturated solutions.

Many substances in contact with water enter into chemical reactions with it. As a result of this interaction, with an excess of water, as with dissolution, a solution is obtained. But if we remove water from this solution, we will not get the original substance.

What products are formed when a substance reacts with water? It depends on the type of chemical bond in the substance; if the bonds are covalent, then on the degree of polarity of these bonds. In addition, other factors also have an impact, some of which we will get acquainted with.

a) Compounds with ionic bond

Most ionic compounds are either chemically soluble in water or insoluble. Ionic hydrides and oxides stand apart, that is, compounds containing the same elements as water itself, and some other substances. Let us consider the behavior of ionic oxides in contact with water using calcium oxide as an example.
Calcium oxide, being an ionic substance, could dissolve chemically in water. In this case, calcium ions and oxide ions would pass into the solution. But a doubly charged anion is not the most stable valence state of an oxygen atom (if only because the energy of affinity for the second electron is always negative, and the radius of the oxide ion is relatively small). Therefore, oxygen atoms tend to lower their formal charge. In the presence of water, this is possible. Oxide ions caught on the surface of the crystal interact with water molecules. This reaction can be represented as a diagram showing its mechanism ( mechanism diagrams).

For a better understanding of what is happening, we conditionally divide this process into stages:
1. The water molecule turns to the oxide ion by the hydrogen atom (oppositely charged).
2. An oxide ion is shared with a hydrogen atom by a lone pair of electrons; a covalent bond is formed between them (formed by the donor-acceptor mechanism).
3. At a hydrogen atom in a single valence orbital (1 s) there are four electrons (two "old" and two "new"), which contradicts the Pauli principle. Therefore, the hydrogen atom gives up a pair of bond electrons ("old" electrons) to the oxygen atom, which is part of the water molecule, especially since this pair of electrons was already largely displaced to the oxygen atom. The bond between the hydrogen atom and the oxygen atom is broken.
4. Due to the formation of a bond by the donor-acceptor mechanism, the formal charge on the former oxide ion becomes equal to –1 e; on the oxygen atom, which was previously part of the water molecule, a charge appears, also equal to –1 e... Thus, two hydroxide ions are formed.
5. Calcium ions, now not bound by ionic bonds with oxide ions, pass into the solution and are hydrated:

The positive charge of calcium ions is, as it were, "blurred" over the entire hydrated ion.
6. The formed hydroxide ions are also hydrated:

In this case, the negative charge of the hydroxide ion is also "washed out".
Total ionic equation of the reaction of calcium oxide with water
CaO cr + H 2 O Ca 2 aq+ 2OH aq .

Calcium ions and hydroxide ions appear in the solution in a ratio of 1: 2. The same would happen if calcium hydroxide was dissolved in water. Indeed, by evaporating the water and drying the residue, we can obtain crystalline calcium hydroxide from this solution (but by no means an oxide!). Therefore, the equation for this reaction is often written as follows:

CaO cr + H 2 O = Ca (OH) 2p

and called " molecular"the equation of this reaction. In those and in other equations, letter indices are sometimes not given, which often greatly complicates the understanding of the processes occurring, or is simply misleading. At the same time, the absence of letter indices in the equations is permissible, for example, when solving calculation tasks
In addition to calcium oxide, the following oxides also interact with water: Li 2 O, Na 2 O, K 2 O, Rb 2 O, Cs 2 O, SrO, BaO - that is, oxides of those metals that themselves react with water. All of these oxides are basic oxides. The rest of the ionic oxides do not react with water.
Ionic hydrides, for example, sodium hydride NaH, react in exactly the same way with water. The sodium ion is only hydrated, while the hydride ion reacts with a water molecule:

As a result, sodium hydroxide remains in the solution.
The ionic equation of this reaction

NaH cr + H 2 O = Na aq+ OH aq+ H 2,

and the "molecular" equation is NaH cr + H 2 O = NaOH p + H 2.

b) Substances with a metallic bond

As an example, consider interaction with sodium water.

On the diagrams, the half-arrow curve means the transfer or movement of one electric

The sodium atom is prone to giving up its only valence electron. Once in water, it easily gives it up to the hydrogen atom of the water molecule (there is a significant + on it) and turns into a sodium cation (Na). The hydrogen atom, having received an electron, becomes neutral (Н · ) and can no longer hold a pair of electrons connecting it to an oxygen atom (remember Pauli's principle). This pair of electrons completely goes to the oxygen atom (in the water molecule it has already been displaced in its direction, but only partially). The oxygen atom acquires the formal charge A, the bond between the hydrogen and oxygen atoms is broken, and a hydroxide ion (O – H) is formed.
The fate of the resulting particles is different: the sodium ion interacts with other water molecules and, naturally, is hydrated

just like the sodium ion, the hydroxide ion is hydrated, and the hydrogen atom, "waiting" for the appearance of another similar hydrogen atom, forms with it a hydrogen molecule 2H · = H 2.
Due to the non-polarity of its molecules, hydrogen is practically insoluble in water and is released from solution in the form of a gas. The ionic equation of this reaction

2Na cr + 2H 2 O = 2Na aq+ 2OH aq+ H 2

a "molecular" -

2Na cr + 2H 2 O = 2NaOH p + H 2

Just like sodium, Li, K, Rb, Cs, Ca, Sr, Ba react violently with water at room temperature. When heated, Mg also reacts with it, as well as some other metals.

c) Substances with covalent bonds

Of the substances with covalent bonds with water, only those substances can react
a) bonds in which are strongly polar, which gives these substances some similarity with ionic compounds, or
b) which include atoms with a very high tendency to attach electrons.
Thus, they do not react with water and are insoluble in it (or very slightly soluble):
a) diamond, graphite, silicon, red phosphorus and other simple non-molecular substances;
b) silicon dioxide, silicon carbide and other complex non-molecular substances;
c) methane, heptane and other molecular substances with low polarity bonds;
d) hydrogen, sulfur, white phosphorus and other simple molecular substances, the atoms of which are not very inclined to attach electrons, as well as nitrogen, the molecules of which are very strong.
Of greatest importance is the interaction with water of molecular oxides, hydrides and hydroxides, and among simple substances - halogens.
We will consider how molecular oxides react with water using the example of sulfur trioxide:

The water molecule, due to one of the lone pairs of electrons of the oxygen atom, attacks a positively charged sulfur atom (+) and joins it with the O – S bond, and a formal charge B arises on the oxygen atom. Having received extra electrons, the sulfur atom ceases to hold the electron pair of one of -bonds, which is completely transferred to the corresponding oxygen atom, on which, due to this, a formal charge A arises. Then the lone pair of electrons of this oxygen atom is accepted by one of the hydrogen atoms that was part of the water molecule, which thus passes from one oxygen atom to another ... As a result, a sulfuric acid molecule is formed. Reaction equation:

SO 3 + H 2 O = H 2 SO 4.

N 2 O 5, P 4 O 10 and some other molecular oxides react in a similar, but somewhat more complicated manner with water. They are all acidic oxides.
N 2 O 5 + H 2 O = 2HNO 3;
P 4 O 10 + 6H 2 O = 4H 3 PO 4.

In all these reactions, acids are formed, which, in the presence of an excess of water, react with it. But, before considering the mechanism of these reactions, let's see how hydrogen chloride, a molecular substance with strongly polar covalent bonds between hydrogen and chlorine atoms, reacts with water:

A polar molecule of hydrogen chloride, once in water, is oriented as shown in the diagram (opposite charges of the dipoles are attracted). The electron shell sparse due to polarization (1 s-EO) of the hydrogen atom accepts the lone pair sp 3 -hybrid electrons of the oxygen atom, and hydrogen attaches to the water molecule, completely donating a pair of electrons to the chlorine atom, which bound these atoms in the hydrogen chloride molecule. As a result, a chlorine atom is converted into a chloride ion, and a water molecule is converted into an oxonium ion. Reaction equation:

HCl g + H 2 O = H 3 O aq+ Cl aq .

At low temperatures, crystalline oxonium chloride (H 3 O) Cl ( t pl = -15 ° C).

The interaction of HCl and H 2 O can be imagined in another way:

that is, as a result of the transfer of a proton from a hydrogen chloride molecule to a water molecule. Therefore, this is an acid-base reaction.
The interaction of nitric acid with water takes place in a similar way.

which can also be represented as a proton transfer:

Acids, in the molecules of which there are several hydroxyls (OH-groups), react with water in several stages (stepwise). An example is sulfuric acid.

The second proton is split off much more difficult than the first, so the second stage of this process is reversible. Having compared the magnitude and distribution of charges in the sulfuric acid molecule and in the hydrosulfate ion, try to explain this phenomenon yourself.
Upon cooling, individual substances can be isolated from sulfuric acid solutions: (H 3 O) HSO 4 (t pl = 8.5 ° C) and (H 3 O) 2 SO 4 (t pl = - 40 ° C).
Anions formed from acid molecules after the abstraction of one or more protons are called acidic residues.
Of the molecular simple substances, only F 2, Cl 2, Br 2 and, to an extremely insignificant extent, I 2 react with water under normal conditions. Fluorine reacts violently with water, completely oxidizing it:

2F 2 + H 2 O = 2HF + OF 2.

In this case, other reactions also occur.
The reaction of chlorine with water is much more important. Having a high tendency to attach electrons (the molar energy of affinity for an electron of a chlorine atom is 349 kJ / mol), chlorine atoms partially retain it in the molecule (the molar energy of affinity for an electron of a chlorine molecule is 230 kJ / mol). Therefore, dissolving, chlorine molecules are hydrated, attracting oxygen atoms of water molecules to themselves. On some of these oxygen atoms, the chlorine atoms can accept a lone pair of electrons. The rest is shown in the diagram of the mechanism:

The overall equation for this reaction

Cl 2 + 2H 2 O = HClO + H 3 O + Cl.

But the reaction is reversible, so an equilibrium is established:

Cl 2 + 2H 2 O HClO + H 3 O + Cl.

The resulting solution is called "chlorine water". Due to the presence of hypochlorous acid in it, it has strong oxidizing properties and is used as a bleaching and disinfectant.
Remembering that Cl and H 3 O are formed during the interaction ("dissolution") of hydrogen chloride in water, we can write down the "molecular" equation:

Cl 2 + H 2 O HClO p + HCl p.

Bromine reacts similarly with water, only the equilibrium in this case is strongly shifted to the left. Iodine practically does not react with water.

To imagine the extent to which chlorine and bromine physically dissolve in water, and to what extent they react with it, we use the quantitative characteristics of solubility and chemical equilibrium.

The mole fraction of chlorine in an aqueous solution saturated at 20 ° C and atmospheric pressure is 0.0018, that is, for every 1000 water molecules there are about 2 chlorine molecules. For comparison, in a nitrogen solution saturated under the same conditions, the molar fraction of nitrogen is 0.000012, that is, one nitrogen molecule accounts for approximately 100,000 water molecules. And to obtain a hydrogen chloride solution saturated under the same conditions, for every 100 water molecules, you need to take about 35 molecules of hydrogen chloride. Hence we can conclude that, although chlorine is soluble in water, it is insignificantly. The solubility of bromine is slightly higher - about 4 molecules per 1000 water molecules.

5. Give the reaction equations allowing to carry out the following transformations:

During the chemical dissolution of ionic substances, the ions passing into the solution are hydrated. Both cations and anions are hydrated. As a rule, hydrated cations are stronger than anions, and hydrated simple cations are stronger than complex ones. This is due to the fact that simple cations have free valence orbitals that can partially accept lone electron pairs of oxygen atoms that make up water molecules.
When an attempt is made to isolate the starting material from a solution by removing water, it often fails to obtain it. For example, if we dissolve colorless copper sulfate CuSO 4 in water, we get a blue solution, which is given to it by hydrated copper ions:

After evaporation of the solution (removal of water) and cooling, blue crystals with the composition CuSO 4 5H 2 O will stand out (the point between the formulas of copper sulfate and water means that for each formula unit of copper sulfate there is the number of water molecules indicated in the formula). The original copper sulfate can be obtained from this compound by heating it to 250 ° C. In this case, the reaction occurs:

CuSO 4 5H 2 O = CuSO 4 + 5H 2 O.

The study of the structure of CuSO 4 · 5H 2 O crystals showed that in its formula unit, four water molecules are bound to the copper atom, and the fifth - to sulfate ions. Thus, the formula of this substance is SO 4 · H 2 O, and it is called tetraaquamated (II) sulfate monohydrate, or simply "copper sulfate".
Four water molecules bound to the copper atom are the remainder of the hydration shell of the Cu 2 ion aq, and the fifth water molecule is the remainder of the hydration shell of the sulfate ion.
A similar structure has a compound SO 4 · H 2 O - monohydrate of hexaaquat iron (II) sulfate, or "iron vitriol".
Other examples:
Cl is hexaaquacalcium chloride;
Cl 2 - hexaaquamagnesium chloride.
These and similar substances are called crystal hydrates, and the water contained in them is crystallization water.
Often the structure of the crystalline hydrate is unknown, or it is impossible to express it using conventional formulas. In these cases, the above-mentioned "dot formulas" and simplified names are used for crystalline hydrates, for example:
CuSO 4 · 5H 2 O - copper sulfate pentahydrate;
Na 2 CO 3 · 10H 2 O - sodium carbonate decahydrate;
AlCl 3 · 6H 2 O - aluminum chloride hexahydrate.

When crystalline hydrates are formed from the initial substances and water, the O-H bonds do not break in water molecules.

If water of crystallization is held in crystalline hydrate by weak intermolecular bonds, then it is easily removed by heating:
Na 2 CO 3 10H 2 O = Na 2 CO 3 + 10H 2 O (at 120 ° C);
K 2 SO 3 2H 2 O = K 2 SO 3 + 2H 2 O (at 200 ° C);
CaCl 2 6H 2 O = CaCl 2 + 6H 2 O (at 250 ° C).

If in crystalline hydrate the bonds between water molecules and other particles are close to chemical ones, then such crystalline hydrate either dehydrates (loses water) at a higher temperature, for example:
Al 2 (SO 4) 3 * 18H 2 O = Al 2 (SO 4) 3 + 18H 2 O (at 420 ° C);
CoSO 4 7H 2 O = CoSO 4 + 7H 2 O (at 410 ° C);

or decomposes when heated to form other chemicals, for example:
2 (FeCl 3 6H 2 O) = Fe 2 O 3 + 6HCl + 9H 2 O (above 250 ° C);
2 (AlCl 3 6H 2 O) = Al 2 O 3 + 6HCl + 9H 2 O (200 - 450 ° C).

Thus, the interaction with water of anhydrous substances that form crystalline hydrates can be either chemical dissolution or a chemical reaction.

CRYSTAL HYDRATES
Determine the mass fraction of water in a) copper sulfate pentahydrate, b) sodium hydroxide dihydrate, c) KAl (SO 4) 2 · 12H 2 O (potassium alum).
2. Determine the composition of the crystalline magnesium sulfate hydrate if the mass fraction of water in it is 51.2%. (3) What is the mass of water released during the calcination of sodium sulfate decahydrate (Na 2 SO 4 10H 2 O) weighing 644 g?
4. How much anhydrous calcium chloride can be obtained by calcining 329 g of calcium chloride hexahydrate?
5. Calcium sulfate dihydrate CaSO 4 2H 2 O when heated to 150 ° C loses 3/4 of its water. Make a formula for the resulting crystalline hydrate (alabaster) and write down the equation for the transformation of gypsum into alabaster.
6. Determine the mass of copper sulfate and water that you need to take to prepare 10 kg of a 5% solution of copper sulfate.
7. Determine the mass fraction of iron (II) sulfate in a solution obtained by mixing 100 g of ferrous sulfate (FeSO 4 7H 2 O) with 9900 g of water.
Obtaining and decomposition of crystalline hydrates.

Water (hydrogen oxide) is a binary inorganic compound with the chemical formula Н 2 O. A water molecule consists of two hydrogen atoms and one oxygen atoms, which are connected by a covalent bond.

Hydrogen peroxide.

Physical and chemical properties

The physical and chemical properties of water are determined by the chemical, electronic and spatial structure of H 2 O molecules.

The H and O atoms in the H 2 0 molecule are in their stable oxidation states, +1 and -2, respectively; therefore, water does not exhibit pronounced oxidizing or reducing properties. Please note: in metal hydrides, hydrogen is in the -1 oxidation state.

The Н 2 O molecule has an angular structure. The Н-O bonds are very polar. On the O atom there is an excess negative charge, on the H atoms - excess positive charges. On the whole, the Н 2 O molecule is polar, i.e. dipole. This explains the fact that water is a good solvent for ionic and polar substances.

The presence of excess charges on the H and O atoms, as well as the lone electron pairs of the O atoms, causes the formation of hydrogen bonds between water molecules, as a result of which they combine into associates. The existence of these associates explains the abnormally high values ​​of mp. etc. kip. water.

Along with the formation of hydrogen bonds, the result of the mutual influence of H2O molecules on each other is their self-ionization:
in one molecule, a heterolytic cleavage of the polar O-H bond occurs, and the released proton is attached to the oxygen atom of another molecule. The resulting hydronium ion H 3 O + is essentially a hydrated hydrogen ion H + H 2 O, therefore, the simplified equation of water self-ionization is written as follows:

H 2 O ↔ H + + OH -

The dissociation constant of water is extremely small:

This indicates that water very slightly dissociates into ions, and therefore the concentration of undissociated H2O molecules is practically constant:

In pure water [H +] = [OH -] = 10 -7 mol / l. This means that water is a very weak amphoteric electrolyte, exhibiting no noticeable acidic or basic properties.
However, water has a strong ionizing effect on the electrolytes dissolved in it. Under the action of water dipoles, polar covalent bonds in the molecules of solutes are converted into ionic ones, the ions are hydrated, the bonds between them are weakened, as a result of which electrolytic dissociation occurs. For example:
HCl + Н 2 O - Н 3 O + + Сl -

(strong electrolyte)

(or excluding hydration: HCl → H + + Cl -)

CH 3 COOH + H 2 O ↔ CH 3 COO - + H + (weak electrolyte)

(or CH 3 COOH ↔ CH 3 COO - + H +)

According to the Brønsted-Lowry theory of acids and bases, in these processes water exhibits the properties of a base (proton acceptor). According to the same theory, water acts as an acid (proton donor) in reactions, for example, with ammonia and amines:

NH 3 + H 2 O ↔ NH 4 + + OH -

CH 3 NH 2 + H 2 O ↔ CH 3 NH 3 + + OH -

Redox reactions involving water

I. Reactions in which water plays the role of an oxidizing agent

These reactions are possible only with strong reducing agents, which are able to reduce the hydrogen ions that are part of water molecules to free hydrogen.

1) Interaction with metals

a) Under normal conditions, H 2 O interacts only with the gap. and shch.-zem. metals:

2Na + 2Н + 2 О = 2NaOH + H 0 2

Ca + 2Н + 2 О = Ca (OH) 2 + H 0 2

b) At high temperatures, H 2 O reacts with some other metals, for example:

Mg + 2Н + 2 О = Mg (OH) 2 + H 0 2

3Fe + 4Н + 2 О = Fe 2 O 4 + 4H 0 2

c) Al and Zn displace Н 2 from water in the presence of alkalis:

2Al + 6Н + 2 О + 2NaOH = 2Na + 3H 0 2

2) Interaction with low EO non-metals (reactions occur under harsh conditions)

C + H + 2 O = CO + H 0 2 ("water gas")

2P + 6H + 2 O = 2HPO 3 + 5H 0 2

In the presence of alkalis, silicon displaces hydrogen from water:

Si + H + 2 O + 2NaOH = Na 2 SiO 3 + 2H 0 2

3) Interaction with metal hydrides

NaH + H + 2 O = NaOH + H 0 2

CaH 2 + 2H + 2 O = Ca (OH) 2 + 2H 0 2

4) Interaction with carbon monoxide and methane

CO + H + 2 O = CO 2 + H 0 2

2CH 4 + O 2 + 2H + 2 O = 2CO 2 + 6H 0 2

Reactions are used industrially to produce hydrogen.

II. Reactions in which water plays the role of a reducing agent

These reactions are possible only with very strong oxidants, which are capable of oxidizing the oxygen CO CO 2, which is part of water, to free oxygen O 2 or to peroxide anions 2-. In an exceptional case (in the reaction with F 2), oxygen is formed with c o. +2.

1) Interaction with fluorine

2F 2 + 2H 2 O -2 = O 0 2 + 4HF

2F 2 + H 2 O -2 = O +2 F 2 + 2HF

2) Interaction with atomic oxygen

H 2 O -2 + O = H 2 O - 2

3) Interaction with chlorine

At high T, a reversible reaction occurs

2Cl 2 + 2H 2 O -2 = O 0 2 + 4HCl

III. Reactions of intramolecular oxidation - reduction of water.

Under the action of an electric current or high temperature, water can decompose into hydrogen and oxygen:

2H + 2 O -2 = 2H 0 2 + O 0 2

Thermal decomposition is a reversible process; the degree of thermal decomposition of water is low.

Hydration reactions

I. Hydration of ions. The ions formed during the dissociation of electrolytes in aqueous solutions attach a certain number of water molecules and exist in the form of hydrated ions. Some ions form such strong bonds with water molecules that their hydrates can exist not only in solution, but also in a solid state. This explains the formation of crystalline hydrates such as CuSO4 5H 2 O, FeSO 4 7H 2 O, etc., as well as aqua complexes: CI 3, Br 4, etc.

II. Hydration of oxides

III. Hydration of organic compounds containing multiple bonds

Hydrolysis reactions

I. Hydrolysis of salts

Reversible hydrolysis:

a) by salt cation

Fe 3+ + H 2 O = FeOH 2+ + H +; (acidic medium. pH

b) by salt anion

CO 3 2 - + H 2 O = HCO 3 - + OH -; (alkaline medium. pH> 7)

c) by cation and by salt anion

NH 4 + + CH 3 COO - + H 2 O = NH 4 OH + CH 3 COOH (medium close to neutral)

Irreversible hydrolysis:

Al 2 S 3 + 6H 2 O = 2Аl (OH) 3 ↓ + 3H 2 S

II. Hydrolysis of metal carbides

Al 4 C 3 + 12Н 2 O = 4Аl (OH) 3 ↓ + 3CH 4 netane

CaC 2 + 2H 2 O = Ca (OH) 2 + C 2 H 2 acetylene

III. Hydrolysis of silicides, nitrides, phosphides

Mg 2 Si + 4H 2 O = 2Mg (OH) 2 ↓ + SiH 4 silane

Ca 3 N 2 + 6Н 2 O = ЗСа (ОН) 2 + 2NH 3 ammonia

Cu 3 P 2 + 6Н 2 O = ЗСu (ОН) 2 + 2РН 3 phosphine

IV. Hydrolysis of halogens

Cl 2 + H 2 O = HCl + HClO

Br 2 + H 2 O = HBr + HBrO

V. Hydrolysis of organic compounds

WATER

A water molecule consists of an oxygen atom and two hydrogen atoms attached to it at an angle of 104.5 °.

Physical properties

WATER, ICE AND STEAM,respectively, the liquid, solid and gaseous states of a chemical compound of molecular formula H 2 O.

Due to the strong attraction between the molecules, water has high melting (0C) and boiling points (100C). A thick layer of water is blue, which is due not only to its physical properties, but also to the presence of suspended particles of impurities. The water of mountain rivers is greenish due to the suspended particles of calcium carbonate contained in it. Clean water is a poor conductor of electricity. The density of water is maximum at 4C, it is equal to 1 g / cm 3. Ice has a lower density than liquid water and floats to its surface, which is very important for inhabitants of reservoirs in winter.

Water has an extremely high thermal capacity, so it heats up slowly and cools slowly. Thanks to this, water pools regulate the temperature of our planet.

Chemical properties of water

Water is a highly reactive substance. Under normal conditions, it interacts with many basic and acidic oxides, as well as with alkali and alkaline earth metals. Water forms numerous compounds - crystalline hydrates.

Under the action of an electric current, water decomposes into hydrogen and oxygen:

2 H 2 O electricity= 2 H 2 + O 2

Video "Water electrolysis"

Mg + 2H 2 O = Mg (OH) 2 + H 2

• Beryllium with water forms an amphoteric oxide: Be + H 2 O = BeO + H 2

1. Active metals are:

Li, Na, K, Rb, Cs, Fr- 1 group "A"

Ca, Sr, Ba, Ra- 2 group "A"

2. A series of metal activities

3. Alkali is a water-soluble base, a complex substance that contains an active metal and a hydroxyl group OH ( I).

4. Metals of medium activity in the series of voltages are from MgbeforePb(aluminum in special position)

Video "Interaction of sodium with water"

Remember !!!

Aluminum reacts with water like active metals to form a base:

2Al + 6H 2 O = 2Al( OH) 3 + 3H 2

Using a sample, write down the interaction reaction equations:

WITHO 2 + H 2 O =

SO 3 + H 2 O =

Cl 2 O 7 + H 2 O =

P 2 O 5 + H 2 O (hot) =

N 2 O 5 + H 2 O =

The most important derivatives of oxygen are its compounds with hydrogen - water Н2О and Н2О2.
Let's consider both compounds and first of all the most common of them - water.

The structure of the water molecule and the polar nature of the bond between hydrogen and oxygen atoms were discussed in. water is 18. In the gaseous state (in the form of vapor), water is lighter than air, the average of which is 29. However, under normal conditions, water is a liquid that has a much higher density. This is due to the fact that water molecules are combined (associated) with each other by an additional special type of bond - hydrogen bond.

The hydrogen bond is so named because it necessarily requires the presence of a hydrogen ion. In a water molecule, where common electron pairs are strongly displaced towards oxygen, hydrogen atoms are practically devoid of electrons and represent a bare nucleus. Such a nucleus (for hydrogen, this is a proton) is attracted by the electron shells of oxygen atoms of neighboring molecules, and a bond is formed between the molecules. Unlike other types of chemical bonds, denoted by dashes in structural formulas, hydrogen bonds are denoted by a dotted line

A hydrogen bond is different from a chemical bond. It is much weaker than the latter. However, the hydrogen bond cannot be considered simply intermolecular bonding, it is much stronger.
A hydrogen bond can arise not only between water molecules. It is often found in organic matter.

■ 30. Explain the mechanism of hydrogen bond formation.
31. List the types of chemical bonds known to you.
32. What type of chemical bond is the water molecule built on?
33. What is the reason for the association of the water molecule?

According to its physical properties, water is a liquid that has no color, taste or smell.
Water has the highest density (1 g / cm3) at 4 °. As the temperature rises and falls, the density of the water decreases (so ice floats on water). The melting point of ice 0 ° and the boiling point of water 100 ° are the main points of the centigrade temperature scale. Water is an excellent solvent for liquids, gases and solids. Water conducts electricity very poorly. The specific heat capacity of water is the mostthe largest among all solids and liquids.

Water in nature

Water is very widespread in nature. About 3/4 of the earth's surface is occupied by water. These are oceans, seas, surface flowing fresh waters, lakes, fresh and salt, glaciers, underground waters, water vapor; constantly present in the atmosphere in greater or lesser amounts, as well as crystallization water, which is part of the crystalline hydrates.

Since water is a good solvent, natural waters always contain a variety of dissolved substances. Seawater contains in a dissolved state many different salts, including NaCl, magnesium sulfate MgSO4, etc., which give it a bitter salty. Underground waters flowing through rocks dissolve various, and these solutions emerging on the surface are called mineral springs.

There are especially many mineral springs in the Caucasus. From the water of carbon dioxide sources they improve and. Carbon dioxide is dissolved under pressure in these waters. Sulphurous waters in Matsesta and Pyatigorsk are cold and hot, contain and. Hydrogen sulfide baths lower blood pressure and improve heart function. The ferrous waters of Zheleznovodsk and Lipetsk are recommended for oral administration with anemia. The lime waters of Kislovodsk are used for kidney diseases, The waters of the warm springs of Transbaikalia and Turkestan are used in their natural form for baths with general weakness of the body, nervous diseases, skin diseases, etc.

If groundwater is located near the centers of volcanic activity, the water comes out to the surface hot in the form of so-called geysers. It is believed that there is a huge amount of hot water in the depths of the earth's crust. It can be used as a very cheap source of thermal energy.

Life on earth began with water, which we know now is the environment for the life of aquatic organisms, but it; absolutely necessary for all living organisms that cannot exist without water. The protoplasm of any cell is a colloidal solution of protein in water. The human body contains 65% water. If the human body loses 20% of water, the changes in the cells become irreversible and the person dies. A person can live without food for 30-40 days, and without water, no more than 7 days. Plant life is also impossible without water. Water for green plants is a necessary component for photosynthesis.

■ 34. In what condition and where is water found in nature? Write it down in a notebook.

35. What are mineral springs, the composition of their water, what is the use in medicine?

Chemical properties of water

Water is an indifferent oxide. Water is an extremely weak electrolyte, dissociating according to the following scheme:
H2O ⇄ H + + OH -
Some of the most active (Na, K, Ca, Ba, Al) can displace from water:
2Na + 2H2O = 2NaOH + H2
2Na + 2Н + + 2OH - = 2Na + + 2OH - + H2
2Na + 2H + = 2Na + + H2
Red-hot water decomposes with the release of hydrogen and the formation of scale:
3Fe + 4H2O = Fe3O5 + 4H2

superheated steam

Elements with stronger than oxidizing properties, for example, are displaced from water:
Cl 0 2 + H2O -2 = 2HCl -1 +
Cl 0 2 + 2 e- → 2Сl -1
O -2 - 2 e- → O 0
Hot charcoal decomposes water to form water gas, which is basically a mixture of hydrogen and carbon monoxide
C + H2O = CO + H2
Water can react with basic and acidic oxides, forming bases and acids, d The release of heat when caustic alkalis and sulfuric acid are dissolved in water is also explained by the chemical reactions of water addition occurring between water and these substances.

Water can react with salts to form crystalline hydrates. For example, copper sulfate, which has a blue color, is the products of the combination of white copper sulfate with water according to the equation:
CuSO4 + 5H2O = СuSO4 = 5H2O + Q

Water is chemically very stable, but it can decompose when exposed to electric current.

Water actively enters into hydrolysis reactions with complex inorganic and organic substances.

■ 36. Why is water classified as an indifferent oxide?
37. Is the expression "sodium dissolves in water" correct?
38. Write the reaction equations for the interaction with water of basic and acidic oxides. With which of them does the water not react?
39. For what purpose is water subjected to electrolysis?
40. Water reacts with salts to form crystalline hydrates. Write the reaction equation for the formation of crystalline hydrate. What other type of interaction is possible between water and salts?
41. In a vessel with 200 g of water was placed 9.2 g of sodium. What substance was formed in this case? Is it soluble? If it is soluble, what is its percentage in the resulting solution?
42. To 50 g of 30% sulfuric acid was added 5 g of sulfuric anhydride. What is the concentration of sulfuric acid?
43. Among those listed in the properties of water, indicate those that can be used to obtain hydrogen.
44. What volume of hydrogen can be obtained by the interaction of 5 kg of iron with superheated steam, if 10% of the weight of iron comes is reduced to scale, and 20% of the produced hydrogen is lost?
45. How much copper oxide can be reduced by hydrogen obtained in the previous problem?

The water that is part of the crystals is called crystallization water. It is chemically bound to a substance and gives the crystal the appropriate properties. For example, copper kynopoc, CuSO4 · 5H2O in the form of a crystalline hydrate with five water molecules has a bright blue color, which it loses on ignition due to the removal of crystallization water (Fig. 45). Natural CaSО4 · 2H2О, with low heating, separates one water molecule, turning into a compound of the composition 2CaSО4 · H2O, called hemihydrate gypsum. This one has the ability to "grasp", that is, when mixed with water, it attaches the missing water molecule to itself and solidifies, forming dihydrous CaSO4 · 2H2O:
2CaSО4 H2O + 3H2O = 2 (CaSO4 2H2O)
This reaction has found wide application in medicine when applying plaster casts.
However, if the gypsum is calcined until the water is completely removed
CaSO4 2H2O2 = CaSO4 + 2H2O

then the reaction becomes irreversible and water no longer adds to calcium sulfate.
Crystalline hydrates are chemical compounds of salt with water. They are referred to as complex compounds. There are many more crystalline hydrates, for example, Glauber's salt
Na2SО4 · 10H2O, iron vitriol FeSО4 · 7H2O, etc.

■ 46. ​​How much water and crystalline hydrate Na2SO4 · 10H2O should be taken to prepare 200 g of 3% sodium sulfate solution?
47. In the laboratory, anhydrous alcohol is used to absolve alcohol, with which the alcohol is boiled until it acquires a blue color. What kind of reaction occurs when this happens? How much will the weight of 25 g of copper sulfate increase if we assume that 75% of the sulfate has turned into copper sulfate?
What percentage of water was contained in alcohol, if 150 g of alcohol was subjected to absolution?
48.20 a FeSO4 · 7H2O was dissolved in 180 g of water. What is the concentration of the resulting solution?
49. What is two-water gypsum, semi-aqueous gypsum? What are their uses in medicine?
50. What water is called crystallization water?

Methods for purifying natural waters

Natural water does not always meet all the requirements that humans place on it. Therefore, water is treated differently for different purposes.
Drinking water should be frequent, clear, odorless and free of pathogenic bacteria. Natural water intended for drinking enters the water treatment plants of the city water supply system, where the oka passes through a system of treatment facilities (Fig. 46). First, it passes through metal filters to

cleaning from mechanical impurities, then enters the sedimentation tanks, where small particles polluting it gradually settle. To accelerate their sedimentation, a coagulant is usually added to the sedimentation tanks - a substance that makes suspensions and colloidal particles coagulate and settle. Aluminum chloride AlCl3 or aluminum sulfate Al2 (SO4) 3 is used as a coagulant.

Rice. 46. Wastewater treatment plant system of a water treatment plant. 1-filter; 2-settling tank; 3-mixers; 4 - pumping; 5 - water intake; 6 - chlorination; 7 - sediment; 8 - adding alum.

After settling, the water is filtered through sand, bone charcoal and fabric filters, after which soluble salts and microorganisms remain in it, among which there may be pathogenic bacteria. To destroy them, a little chlorine water is added to the water in an amount that kills bacteria, but is harmless to humans. After that, the water enters the so-called pure water reservoirs, where it is kept for some time in order for the chlorine to fully manifest itself. Purified water is supplied to consumers through the water supply system.

In rural areas, water usually does not go through such a complex treatment system, but is taken directly from wells or other natural bodies of water. Such water must be boiled, and in the case of massive gastrointestinal diseases, a small amount of bleach solution must be added to it.

Rice. 47.
1- Wurtz flask with water; Liebig 2-water refrigerator: 3 - alonge; 4- receptacle for distilled water; 5 - thermometer.

Distilled water is used in chemical laboratories and medicine. To completely remove salts, water is distilled in so-called distillation stills. The principle of distillation of water can be observed in a laboratory setup (Fig. 47). The water is boiling in a flask. The resulting steam through the gas outlet pipe enters the Liebig water cooler 2, where the steam condenses and flows through the drain 3 into the receiving vessel 4. The resulting water is called distilled. It is completely salt-free and harmful to drink. The distiller works in the same way (fig. 48).

■ 51. What is distillation and for what purposes is distilled water used? 52. What are the requirements for drinking water? 53. How can water be purified: a) from mechanical impurities; b) from dissolved salts; c) from colloidal particles?

§ 54. Hydrogen peroxide

- an oxide richer in oxygen compared to water. The peroxide formula is H2O2, but this does not mean that it is monovalent in this compound. In a hydrogen peroxide molecule, there is one common electron pair between two oxygen atoms. Oxygen atoms connected in this way are contained not only in hydrogen peroxide, but also in any other peroxide and are called the "peroxide chain"

The presence of a peroxide chain makes the molecule unstable. Indeed, under the most insignificant influences - storage in a lighted room, heating, the action of the MnO2 catalyst - hydrogen peroxide decomposes, turning into water, with the release of oxygen
2H2O2 = 2H2O + O2
This reaction can be accompanied by an explosion.
A 30% hydrogen peroxide solution is called perhydrol.

It can cause severe burns if it comes into contact with the skin. Net has a density of 1.46 g / s3 and a freezing point of -1.7 °. A solution of hydrogen peroxide has an acidic reaction, which gives reason to consider it as a very weak diacid.
Some metal peroxides such as Na2O2; ВаО2, can be considered not only as, but also as a kind of salt of hydrogen peroxide. Hydrogen peroxide can be obtained from these compounds by the action of a stronger acid:
ВаО2 + H2SO4 = BaSO4 + H2O2

The behavior of hydrogen peroxide in redox reactions is discussed in § 32. When interacting with organic substances, hydrogen peroxide behaves like. Anhydrous hydrogen peroxide causes burns and spontaneous combustion of combustible materials. With hydrogen peroxide burns, a characteristic white spot appears on the skin, "and then an ulcer may form. A first aid measure, as with acid burns, is to rinse with plenty of water.

Hydrogen peroxide is used as a disinfectant in medicine for rinsing, rinsing and as a hemostatic agent in the form of a 3% solution. In addition, it is used to bleach hair, wool, silk, horns, etc. Hydrogen peroxide is also used to restore paintings painted with lead whitewash, which gradually darken in air, since under the action of air hydrogen sulfide, black lead sulfide is formed in the paint ... Hydrogen peroxide oxidizes lead sulfide to sulfate according to the following scheme:
PbS + H2O2 → PbSO4 + H2O
Such pictures are wiped with a weak solution of hydrogen peroxide.
Store hydrogen peroxide in dark glass flasks in a cool room, in the dark to slow down the ongoing decay.

■ 54. Give examples of reactions in which hydrogen peroxide would exhibit the properties of an oxidizing agent.

55, Give examples of reactions in which hydrogen peroxide would exhibit the properties of a reducing agent.

56. Where and how should hydrogen peroxide be stored in laboratories? Why?
57. What are the first aid measures for burns with hydrogen peroxide?

58. In the presence of manganese dioxide, oxygen can be obtained from hydrogen peroxide. Draw a device that can use this process.

59. How many grams of barium peroxide is required to obtain 5 moles of pure hydrogen peroxide?
60. Hydrogen peroxide dissociates as acids. Write the equation for the two-step dissociation of this acid.

61. Where and how is hydrogen peroxide used and how is it related to its properties?

Air

Our planet is surrounded by air, which is necessary for breathing for all creatures living on earth. A person passes through his lungs about 13,000 liters of air per day.
The air shell of the earth is called the atmosphere (from the words "atmos" - air, "sefira" - a ball). The air contains 78% (by volume) nitrogen, 21% oxygen, 0.96%

Inert gases, mainly argon and neon, as well as helium, krypton and xenon, 0.03-0.04% carbon dioxide and 0.01% hydrogen. The air composition is shown in Fig. 49. Average air is 29 cu. e.
In addition, the composition of the atmosphere includes accidental impurities, as well as variable components - water vapor, nitrogen, ozone, as well as dust and local air pollution that sometimes occur during intensive work of enterprises in a certain area, as well as during the operation of transport.

The amount of dust in the air can be very high, especially in large cities. Dust disturbs the transparency of the air and contributes to the formation of fogs, since water droplets condense on the dust particles. Various microorganisms can be in the air. Among them there may be disease-causing ones. Hence, it is clear how important air purification in cities is, how important it is to ensure that the air does not pollute Industrial enterprises and transport.
To clean the air inside the premises, special air conditioning devices are used: it is filtered, humidified to the desired state, removed from dust and bacteria, and maintained at the most favorable temperature.
1 m3 of air at 0 ° weighs 1.293 kg, with an increase in altitude, the density of air becomes less. At -193 °, the air turns into a liquid state. Since air is a mixture of gases with different boiling points, it can be divided into its constituent parts according to boiling points, or, as they say, subjected to fractional distillation.

The energy of compressed air is widely used, which is obtained by increasing the pressure of atmospheric air using compressors. When compressed air is blown into the blast furnace, the oxygen supply increases and the combustion becomes more intense.
Liquid air is a bluish cloudy liquid. Liquid oxygen gives it a blue color, and it can be cloudy because at the temperature of liquid air, carbon dioxide becomes solid. If it is filtered, then the air will be transparent.
Under the influence of the low temperature of liquid air, some bodies acquire special, completely new properties. For example, it acquires the elasticity of steel, becomes so hard that it is possible to hammer nails with a hammer made of it, rubber becomes fragile, like, and breaks apart from the impact. Many at the temperature of liquid air acquire the properties of superconductivity. If an electric current is excited in a metal ring, then the galvanometer connected to it will show the presence of an electric current for a very long time.

Interestingly, most of the bacteria in liquid air do not die, but plunges into a state of suspended animation.
If a combustible material is impregnated with liquid air, which either does not ignite or burns very weakly in ordinary air, for example sawdust or coal powder, then when ignited, they instantly burn out with the release of a large amount of gases, therefore, liquid air is widely used in blasting operations. For this, cardboard cartridges are stuffed with sawdust, placed in explosion chambers, impregnated with liquid air and set on fire. A violent explosion occurs. If the explosion did not occur, then after a while the air from the cartridge evaporates, and it becomes safe again, unlike any other explosive.

Liquid air is obtained at high pressure and low temperature.
Compressed air is used in pneumatic devices and various pneumatic equipment, as well as in caisson work. The caisson is a huge air-tight and waterproof concrete box, inside which several people can be. On one side, the caisson is open. It is lowered with the open side into the water to the very bottom, reinforced with a load so that it does not float, and water is forced out of it with compressed air. To displace water, the air pressure in the caisson is brought to 4 atm. At this pressure, air dissolves in large quantities in the blood. With a sharp decrease in pressure, for example, when rising to the surface, its excess quickly leaves the blood in the form of bubbles, which can clog the blood vessels and even reach the heart. In severe cases, this so-called decompression sickness can be fatal. Therefore, the rise from the caisson is carried out gradually so that the dissolved air comes out in small portions.