Which salt solutions are alkaline? Water. Neutral, acidic and alkaline environment. Strong protoliths. Salt hydrolysis. Environment of aqueous solutions: acidic, neutral, alkaline

26.11.2019

Chemically, the pH of a solution can be determined using acid-base indicators.

Acid-base indicators are organic substances whose color depends on the acidity of the medium.

The most common indicators are litmus, methyl orange, phenolphthalein. Litmus turns red in an acidic environment and blue in an alkaline environment. Phenolphthalein is colorless in an acidic medium, but turns crimson in an alkaline medium. Methyl orange turns red in an acidic environment and yellow in an alkaline environment.

In laboratory practice, a number of indicators are often mixed, selected in such a way that the color of the mixture varies over a wide range of pH values. With their help, you can determine the pH of the solution with an accuracy of up to one. These mixtures are called universal indicators.

There are special devices - pH meters, with which you can determine the pH of solutions in the range from 0 to 14 with an accuracy of 0.01 pH units.

Salt hydrolysis

When some salts are dissolved in water, the equilibrium of the water dissociation process is disturbed and, accordingly, the pH of the medium changes. This is because salts react with water.

Salt hydrolysis – chemical exchange interaction of dissolved salt ions with water, leading to the formation of weakly dissociating products (molecules of weak acids or bases, anions of acid salts or cations of basic salts) and accompanied by a change in the pH of the medium.

Consider the process of hydrolysis, depending on the nature of the bases and acids that form the salt.

Salts formed by strong acids and strong bases (NaCl, kno3, Na2so4, etc.).

Let's say that when sodium chloride reacts with water, a hydrolysis reaction occurs with the formation of an acid and a base:

NaCl + H 2 O ↔ NaOH + HCl

For a correct understanding of the nature of this interaction, we write the reaction equation in ionic form, taking into account that the only weakly dissociating compound in this system is water:

Na + + Cl - + HOH ↔ Na + + OH - + H + + Cl -

With the reduction of identical ions, the water dissociation equation remains on the left and right sides of the equation:

H 2 O ↔ H + + OH -

As can be seen, there are no excess H + or OH - ions in the solution compared to their content in water. In addition, no other weakly dissociating or hardly soluble compounds are formed. Hence we conclude that salts formed by strong acids and bases do not undergo hydrolysis, and the reaction of solutions of these salts is the same as in water, neutral (pH = 7).

When compiling ion-molecular equations for hydrolysis reactions, it is necessary:

1) write down the salt dissociation equation;

2) determine the nature of the cation and anion (find the cation of a weak base or the anion of a weak acid);

3) write down the ion-molecular equation of the reaction, given that water is a weak electrolyte and that the sum of the charges must be the same in both parts of the equation.

Salts formed from a weak acid and a strong base

(Na 2 CO 3 , K 2 S, CH 3 COONa and others .)

Consider the hydrolysis reaction of sodium acetate. This salt in solution decomposes into ions: CH 3 COONa ↔ CH 3 COO - + Na + ;

Na + is a cation of a strong base, CH 3 COO - is an anion of a weak acid.

Na + cations cannot bind water ions, since NaOH, a strong base, completely decomposes into ions. Anions of weak acetic acid CH 3 COO - bind hydrogen ions to form slightly dissociated acetic acid:

CH 3 COO - + HOH ↔ CH 3 COOH + OH -

It can be seen that, as a result of the hydrolysis of CH 3 COONa, an excess of hydroxide ions formed in the solution, and the reaction of the medium became alkaline (рН > 7).

Thus, it can be concluded that salts formed by a weak acid and a strong base are hydrolyzed at the anion ( An n - ). In this case, salt anions bind H ions + , and OH ions accumulate in the solution - , which causes an alkaline environment (pH> 7):

An n - + HOH ↔ Han (n -1) - + OH -, (at n = 1, HAn is formed - a weak acid).

Hydrolysis of salts formed by dibasic and tribasic weak acids and strong bases proceeds stepwise

Consider the hydrolysis of potassium sulfide. K 2 S dissociates in solution:

K 2 S ↔ 2K + + S 2-;

K + is a cation of a strong base, S 2 is an anion of a weak acid.

Potassium cations do not take part in the hydrolysis reaction; only anions of weak hydrosulphuric acid interact with water. In this reaction, weakly dissociating HS - ions are formed in the first stage, and weak acid H 2 S is formed in the second stage:

1st stage: S 2- + HOH ↔ HS - + OH -;

2nd stage: HS - + HOH ↔ H 2 S + OH -.

The OH ions formed in the first stage of hydrolysis significantly reduce the likelihood of hydrolysis in the next stage. As a result, the process that proceeds only through the first stage is usually of practical importance, which, as a rule, is limited when assessing the hydrolysis of salts under normal conditions.

Hydrolysis is the interaction of substances with water, as a result of which the medium of the solution changes.

Cations and anions of weak electrolytes are able to interact with water to form stable low-dissociation compounds or ions, as a result of which the solution medium changes. Water formulas in hydrolysis equations are usually written as H-OH. When reacting with water, the cations of weak bases take away the hydroxyl ion from the water, and an excess of H + is formed in the solution. The solution becomes acidic. Anions of weak acids attract H + from water, and the reaction of the medium becomes alkaline.

In inorganic chemistry, most often one has to deal with the hydrolysis of salts, i.e. with the exchange interaction of salt ions with water molecules in the process of their dissolution. There are 4 variants of hydrolysis.

1. Salt is formed by a strong base and a strong acid.

Such a salt is practically not subjected to hydrolysis. At the same time, the equilibrium of water dissociation in the presence of salt ions is almost not disturbed, therefore pH = 7, the medium is neutral.

Na + + H 2 O Cl - + H 2 O

2. If the salt is formed by a cation of a strong base and an anion of a weak acid, then hydrolysis occurs at the anion.

Na 2 CO 3 + HOH NaHCO 3 + NaOH

Since OH - ions accumulate in the solution, the medium is alkaline, pH> 7.

3. If the salt is formed by a cation of a weak base and an anion of a strong acid, then hydrolysis proceeds along the cation.

Cu 2+ + HOH CuOH + + H +

СuCl 2 + HOH CuOHCl + HCl

Since H + ions accumulate in the solution, the medium is acidic, pH<7.

4. A salt formed by a cation of a weak base and an anion of a weak acid undergoes hydrolysis both at the cation and at the anion.

CH 3 COONH 4 + HOH NH 4 OH + CH 3 COOH

CH 3 COO - +
+ HOH NH 4 OH + CH 3 COOH

Solutions of such salts have either a slightly acidic or slightly alkaline environment, i.e. the pH value is close to 7. The reaction of the medium depends on the ratio of the acid and base dissociation constants. The hydrolysis of salts formed by very weak acids and bases is practically irreversible. These are mainly sulfides and carbonates of aluminum, chromium, and iron.

Al 2 S 3 + 3HOH 2Al(OH) 3 + 3H 2 S

When determining the medium of a salt solution, it must be taken into account that the medium of the solution is determined by the strong component. If the salt is formed by an acid that is a strong electrolyte, then the medium of the solution is acidic. If the base is a strong electrolyte, then it is alkaline.

Example. Solution has an alkaline environment

1) Pb(NO 3) 2 ; 2) Na 2 CO 3 ; 3) NaCl; 4) NaNO 3

1) Pb (NO 3) 2 lead (II) nitrate. Salt is made up of a weak base and strong acid, means the solution medium sour.

2) Na 2 CO 3 sodium carbonate. Salt formed strong base and a weak acid, then the solution medium alkaline.

3) NaCl; 4) NaNO 3 Salts are formed by the strong base NaOH and the strong acids HCl and HNO 3 . The medium of the solution is neutral.

Correct answer 2) Na2CO3

An indicator paper was dipped into the salt solutions. In NaCl and NaNO 3 solutions, it did not change color, which means the solution medium neutral. In a solution of Pb (NO 3) 2 turned red, the solution medium sour. In a solution of Na 2 CO 3 turned blue, the solution medium alkaline.

Remember:

A neutralization reaction is a reaction between an acid and a base that produces salt and water;

By pure water, chemists understand chemically pure water that does not contain any impurities and dissolved salts, that is, distilled water.

Acidity of the environment

For various chemical, industrial and biological processes, a very important characteristic is the acidity of solutions, which characterizes the content of acids or alkalis in solutions. Since acids and alkalis are electrolytes, the content of H + or OH - ions is used to characterize the acidity of the medium.

In pure water and in any solution, along with particles of dissolved substances, there are also H + and OH - ions. This is due to the dissociation of the water itself. And although we consider water to be a non-electrolyte, nevertheless it can dissociate: H 2 O ^ H + + OH -. But this process occurs to a very small extent: in 1 liter of water, only 1 decomposes into ions. 10 -7 mol molecules.

In acid solutions, as a result of their dissociation, additional H+ ions appear. In such solutions, there are much more H + ions than OH - ions formed during a slight dissociation of water, therefore these solutions are called acidic (Fig. 11.1, left). It is customary to say that in such solutions an acidic environment. The more H+ ions are contained in the solution, the greater the acidity of the medium.

In alkali solutions, as a result of dissociation, on the contrary, OH - ions predominate, and H + cations are almost absent due to the insignificant dissociation of water. The environment of such solutions is alkaline (Fig. 11.1, right). The higher the concentration of OH - ions, the more alkaline the solution medium is.

In a solution of table salt, the number of H + and OH ions is the same and equal to 1. 10 -7 mol in 1 liter of solution. Such an environment is called neutral (Fig. 11.1, center). In fact, this means that the solution contains neither acid nor alkali. A neutral environment is characteristic of solutions of some salts (formed by alkali and strong acid) and many organic substances. Pure water also has a neutral environment.

Hydrogen indicator

If we compare the taste of kefir and lemon juice, then we can safely say that lemon juice is much more acidic, that is, the acidity of these solutions is different. You already know that pure water also contains H+ ions, but the water does not taste sour. This is due to the too low concentration of H+ ions. Often it is not enough to say that the environment is acidic or alkaline, but it is necessary to characterize it quantitatively.

The acidity of the environment is quantitatively characterized by the hydrogen indicator pH (pronounced "p-ash"), associated with the concentration

hydrogen ions. The pH value corresponds to a certain content of hydrogen cations in 1 liter of solution. In pure water and in neutral solutions, 1 liter contains 1. 10 7 mol of H + ions, and the pH value is 7. In acid solutions, the concentration of H + cations is greater than in pure water, and less in alkaline solutions. In accordance with this, the pH value also changes: in an acidic environment, it ranges from 0 to 7, and in alkaline environments, from 7 to 14. For the first time, the Danish chemist Peder Sørensen suggested using the pH value.

You may have noticed that the pH value is related to the concentration of H+ ions. Determining pH is directly related to calculating the logarithm of a number, which you will study in math lessons in grade 11. But the relationship between the content of ions in a solution and the pH value can be traced according to the following scheme:

The pH value of aqueous solutions of most substances and natural solutions is in the range from 1 to 13 (Fig. 11.2).

Rice. 11.2. pH value of various natural and artificial solutions

Søren Peder Lauritz Sørensen

Danish physical chemist and biochemist, President of the Royal Danish Society. Graduated from the University of Copenhagen. At 31, he became a professor at the Danish Polytechnic Institute. He headed the prestigious physical and chemical laboratory at the Carlsberg brewery in Copenhagen, where he made his main scientific discoveries. His main scientific activity is devoted to the theory of solutions: he introduced the concept of hydrogen index (pH), studied the dependence of enzyme activity on the acidity of solutions. For scientific achievements, Sørensen is included in the list of "100 outstanding chemists of the 20th century", but in the history of science he remained primarily as a scientist who introduced the concepts of "pH" and "pH-metry".

Determination of the acidity of the medium

To determine the acidity of a solution in laboratories, a universal indicator is most often used (Fig. 11.3). By its color, one can determine not only the presence of acid or alkali, but also the pH value of the solution with an accuracy of 0.5. For a more accurate measurement of pH, there are special devices - pH meters (Fig. 11.4). They allow you to determine the pH of the solution with an accuracy of 0.001-0.01.

Using indicators or pH meters, you can monitor the progress of chemical reactions. For example, if hydrochloric acid is added to a solution of sodium hydroxide, then a neutralization reaction will occur:

Rice. 11.3. A universal indicator determines the approximate pH value

Rice. 11.4. To measure the pH of solutions, special devices are used - pH meters: a - laboratory (stationary); b - portable

In this case, the solutions of the reactants and reaction products are colorless. If, however, the electrode of a pH meter is placed in the initial alkali solution, then the complete neutralization of the alkali with acid can be judged by the pH value of the resulting solution.

The use of the pH indicator

Determining the acidity of solutions is of great practical importance in many areas of science, industry and other areas of human life.

Environmentalists regularly measure the pH of rainwater, rivers and lakes. A sharp increase in the acidity of natural waters may be the result of atmospheric pollution or the ingress of waste from industrial enterprises into water bodies (Fig. 11.5). Such changes entail the death of plants, fish and other inhabitants of water bodies.

The hydrogen index is very important for studying and observing the processes occurring in living organisms, since numerous chemical reactions take place in cells. In clinical diagnostics, the pH of blood plasma, urine, gastric juice, etc. is determined (Fig. 11.6). Normal blood pH is between 7.35 and 7.45. Even a small change in the pH of human blood causes serious illness, and at pH = 7.1 and below, irreversible changes begin that can lead to death.

For most plants, soil acidity is important, so agronomists analyze soils in advance, determining their pH (Fig. 11.7). If the acidity is too high for a particular crop, the soil is limed - chalk or lime is added.

In the food industry, with the help of acid-base indicators, food quality control is carried out (Fig. 11.8). For example, the normal pH for milk is 6.8. A deviation from this value indicates either the presence of impurities or its souring.

Rice. 11.5. The influence of the pH level of water in reservoirs on the vital activity of plants in them

The pH value of cosmetic products that we use in everyday life is important. The average pH for human skin is 5.5. If the skin comes into contact with agents whose acidity differs significantly from this value, then this leads to premature aging of the skin, its damage or inflammation. It was noticed that laundresses who used regular laundry soap (pH = 8-10) or washing soda (Na 2 CO 3 , pH = 12-13) for washing for a long time, the skin of the hands became very dry and cracked. Therefore, it is very important to use various cosmetic products (gels, creams, shampoos, etc.) with a pH that is close to the natural pH of the skin.

LABORATORY EXPERIMENTS No. 1-3

Equipment: stand with test tubes, pipette.

Reagents: water, hydrochloric acid, NaCl, NaOH solutions, table vinegar, universal indicator (solution or indicator paper), food and cosmetic products (e.g. lemon, shampoo, toothpaste, washing powder, carbonated drinks, juices, etc.) .).

Safety regulations:

For experiments, use small amounts of reagents;

Be careful not to get reagents on the skin, in the eyes; in case of contact with a corrosive substance, wash it off with plenty of water.

Determination of hydrogen ions and hydroxide ions in solutions. Establishing the approximate pH value of water, alkaline and acidic solutions

1. Pour 1-2 ml into five test tubes: into test tube No. 1 - water, No. 2 - perchloric acid, No. 3 - sodium chloride solution, No. 4 - sodium hydroxide solution and No. 5 - table vinegar.

2. Add 2-3 drops of universal indicator solution to each tube, or omit indicator paper. Determine the pH of solutions by comparing the color of the indicator against a reference scale. Draw conclusions about the presence of Hydrogen cations or hydroxide ions in each test tube. Write the dissociation equations for these compounds.

pH testing of food and cosmetic products

Test samples of food and cosmetic products with a universal indicator. To study dry substances, for example, washing powder, they must be dissolved in a small amount of water (1 spatula of dry matter per 0.5-1 ml of water). Determine the pH of the solutions. Draw conclusions about the acidity of the environment in each of the studied products.

Key Idea

test questions

130. The presence of what ions in a solution determines its acidity?

131. What ions are found in excess in acid solutions? in alkaline?

132. What indicator quantitatively describes the acidity of solutions?

133. What is the pH value and the content of H+ ions in solutions: a) neutral; b) slightly acidic; c) slightly alkaline; d) strongly acidic; e) strongly alkaline?

Tasks for mastering the material

134. An aqueous solution of some substance has an alkaline environment. Which ions are more in this solution: H + or OH -?

135. Two test tubes contain solutions of nitrate acid and potassium nitrate. What indicators can be used to determine which tube contains a salt solution?

136. Three test tubes contain solutions of barium hydroxide, nitrate acid and calcium nitrate. How to recognize these solutions using one reagent?

137. From the above list, write out separately the formulas of substances whose solutions have an environment: a) acidic; b) alkaline; c) neutral. NaCl, HCl, NaOH, HNO 3 , H 3 PO 4 , H 2 SO 4 , Ba(OH) 2 , H 2 S, KNO 3 .

138. Rain water has pH = 5.6. What does this mean? What substance contained in the air, when dissolved in water, determines such an acidity of the environment?

139. What medium (acidic or alkaline): a) in a shampoo solution (pH = 5.5);

b) in the blood of a healthy person (pH = 7.4); c) in human gastric juice (рН = 1.5); d) in saliva (pH = 7.0)?

140. The composition of coal used in thermal power plants contains Nitrogen and Sulfur compounds. The emission of coal combustion products into the atmosphere leads to the formation of so-called acid rain, containing small amounts of nitrate or sulfite acids. What pH values are typical for such rainwater: more than 7 or less than 7?

141. Does the pH of a strong acid solution depend on its concentration? Justify the answer.

142. A solution of phenolphthalein was added to a solution containing 1 mol of potassium hydroxide. Will the color of this solution change if chloride acid is added to it with the amount of the substance: a) 0.5 mol; b) 1 mol;

c) 1.5 mol?

143. In three test tubes without inscriptions there are colorless solutions of sodium sulfate, sodium hydroxide and sulfate acid. For all solutions, the pH value was measured: in the first tube - 2.3, in the second - 12.6, in the third - 6.9. Which tube contains which substance?

144. A student bought distilled water in a pharmacy. The pH meter showed that the pH value of this water is 6.0. The student then boiled this water for a long time, filled the container to the top with hot water, and closed the lid. When the water cooled to room temperature, the pH meter read 7.0. After that, the student passed air through the water with a tube, and the pH meter again showed 6.0. How can the results of these pH measurements be explained?

145. Why do you think two bottles of vinegar from the same manufacturer may contain solutions with slightly different pH values?

This is textbook material.

We study the effect of a universal indicator on solutions of some salts

As we can see, the environment of the first solution is neutral (pH=7), the second one is acidic (pH< 7), третьего щелочная (рН >7). How to explain such an interesting fact? 🙂

First, let's remember what pH is and what it depends on.

pH is a hydrogen indicator, a measure of the concentration of hydrogen ions in a solution (according to the first letters of the Latin words potentia hydrogeni - the strength of hydrogen).

pH is calculated as the negative decimal logarithm of the concentration of hydrogen ions, expressed in moles per liter:

In pure water at 25 °C, the concentrations of hydrogen ions and hydroxide ions are the same and amount to 10 -7 mol/l (pH=7).

When the concentrations of both types of ions in a solution are the same, the solution is neutral. When > the solution is acidic, and when > - alkaline.

Due to what, in some aqueous solutions of salts, is there a violation of the equality of the concentrations of hydrogen ions and hydroxide ions?

The fact is that there is a shift in the equilibrium of water dissociation due to the binding of one of its ions (or) with salt ions with the formation of a poorly dissociated, hardly soluble or volatile product. This is the essence of hydrolysis.

- this is the chemical interaction of salt ions with water ions, leading to the formation of a weak electrolyte - an acid (or acid salt), or a base (or basic salt).

The word "hydrolysis" means decomposition by water ("hydro" - water, "lysis" - decomposition).

Depending on which salt ion interacts with water, there are three types of hydrolysis:

žhydrolysis by cation (only cation reacts with water);
žanion hydrolysis (only anion reacts with water);
ž joint hydrolysis - hydrolysis by cation and anion (both cation and anion react with water).

Any salt can be considered as a product formed by the interaction of a base and an acid:

Salt hydrolysis - 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.
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, or 100%);
2) actually , i.e. interaction of salt ions with water reversible reaction (degree of hydrolysis ˂ 1, or 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 salts KCl, NaNO 3 , NaSO 4 and BaI, the medium neutral.

Anion hydrolysis

In case of interaction anions dissolved salt with water the process is called salt hydrolysis at the anion.
1) KNO 2 = K + + NO 2 - (dissociation)
2) NO 2 - + H 2 O ↔ HNO 2 + OH - (hydrolysis)
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 turns out to be enough for the solution to become alkaline(among the hydrolysis products there is an OH ion -), in it p H = 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:
NO 2 - + H 2 O (H +, OH -) ↔ HNO 2 + OH -
Examples:
a) NaClO \u003d Na + + ClO -
ClO - + H 2 O ↔ HClO + OH -
b) LiCN = Li + + CN -
CN - + H 2 O ↔ HCN + OH -
c) Na 2 CO 3 \u003d 2Na + + CO 3 2-
CO 3 2- + H 2 O ↔ HCO 3 - + OH -
d) K 3 PO 4 \u003d 3K + + PO 4 3-
PO 4 3- + H 2 O ↔ HPO 4 2- + OH -
e) BaS = Ba 2+ + S 2-
S 2- + H 2 O ↔ HS - + OH -
Please note that in examples (c-e) you cannot increase the number of water molecules and instead of hydroanions (HCO 3, HPO 4, 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” (before the formation of an acid).
If such an unstable acid as H 2 CO 3 were formed in a solution of its NaCO 3 salt, then CO 2 would be released from the gas solution (H 2 CO 3 \u003d CO 2 + 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 anion with the appearance in the solution of only carbonic acid hydranions HCO 3 -.
The degree of salt hydrolysis by the anion depends on the degree of dissociation of the hydrolysis product, the acid. The weaker the acid, the higher the degree of hydrolysis. For example, CO 3 2-, PO 4 3- and S 2- ions undergo hydrolysis to a greater extent 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 proceeds much less than the dissociation of the HNO 2 acid. 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 soda 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 in a concentrated solution of a salt that is strongly hydrolyzed by the anion,
for example, Na 2 CO 3, add aluminum, then the latter (due to amphoterism) will react with alkali and hydrogen evolution will be observed. This is additional evidence of hydrolysis, because we did not add NaOH alkali to the soda solution!

Pay special attention to salts of acids of medium strength - orthophosphoric and sulfurous. In the first stage, these acids dissociate quite well, so their acid salts do not undergo hydrolysis, and the medium of the solution of such salts is acidic (due to the presence of a hydrogen cation in the composition of the salt). And the average salts are hydrolyzed by the anion - the medium is alkaline. So, hydrosulfites, hydrophosphates and dihydrophosphates are not hydrolyzed by the anion, the medium is acidic. Sulfites and phosphates are hydrolyzed by the anion, the environment is alkaline.

Hydrolysis by cation

In the case of the interaction of a cation of a dissolved salt with water, the process is called
salt hydrolysis at the cation

1) Ni(NO 3) 2 = Ni 2+ + 2NO 3 - (dissociation)
2) Ni 2+ + H 2 O ↔ NiOH + + H + (hydrolysis)

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

Only cations of poorly soluble basic and amphoteric hydroxides and the ammonium cation undergo hydrolysis. NH4+. The metal cation splits off the hydroxide ion from the water molecule and releases the hydrogen cation H + .

The ammonium cation, as a result of hydrolysis, forms a weak base - ammonia hydrate and a hydrogen cation:

NH 4 + + H 2 O ↔ NH 3 H 2 O + H +

Please note that you cannot increase the number of water molecules and instead of hydroxocations (for example, NiOH +) write hydroxide formulas (for example, Ni (OH) 2). If hydroxides were formed, then precipitates would fall out of salt solutions, which is not observed (these salts form transparent solutions).
An excess of hydrogen cations is easy to detect with an indicator or measure with special instruments. Magnesium or zinc is introduced into a concentrated solution of a salt that is strongly hydrolyzed by the cation, then the latter react with the acid with the release of hydrogen.

If the salt is insoluble, then there is no hydrolysis, because the ions do not interact with water.

Hydrolysis is the interaction of substances with water, as a result of which the medium of the solution changes.

1. Salt is formed by a strong base and a strong acid.

Na + + H 2 O Cl - + H 2 O

2. If the salt is formed by a cation of a strong base and an anion of a weak acid, then hydrolysis occurs at the anion.

Na 2 CO 3 + HOH \(\leftrightarrow\) NaHCO 3 + NaOH

Since OH - ions accumulate in the solution, the medium is alkaline, pH> 7.

3. If the salt is formed by a cation of a weak base and an anion of a strong acid, then hydrolysis proceeds along the cation.

Cu 2+ + HOH \(\leftrightarrow\) CuOH + + H +

СuCl 2 + HOH \(\leftrightarrow\) CuOHCl + HCl