Aqueous solutions of some substances are conductors of electric current. These substances are classified as electrolytes. Electrolytes are acids, bases and salts, melts of some substances.
DEFINITION
The process of electrolyte decomposition into ions in aqueous solutions and melts under the influence of electric current is called electrolytic dissociation.
Solutions of some substances in water do not conduct electricity. Such substances are called non-electrolytes. These include many organic compounds, such as sugars and alcohols.
Electrolytic dissociation theory
The theory of electrolytic dissociation was formulated by the Swedish scientist S. Arrhenius (1887). The main provisions of the theory of S. Arrhenius:
— electrolytes, when dissolved in water, break up (dissociate) into positively and negatively charged ions;
— under the influence of electric current, positively charged ions move to the cathode (cations), and negatively charged ones move to the anode (anions);
— dissociation is a reversible process
KA ↔ K + + A −
The mechanism of electrolytic dissociation is the ion-dipole interaction between ions and water dipoles (Fig. 1).
Rice. 1. Electrolytic dissociation of sodium chloride solution
Substances with ionic bonds dissociate most easily. Dissociation occurs similarly in molecules formed according to the type of polar covalent bond (the nature of the interaction is dipole-dipole).
Dissociation of acids, bases, salts
When acids dissociate, hydrogen ions (H +) are always formed, or more precisely hydronium (H 3 O +), which are responsible for the properties of acids (sour taste, action of indicators, interaction with bases, etc.).
HNO 3 ↔ H + + NO 3 −
When bases dissociate, hydrogen hydroxide ions (OH −) are always formed, which are responsible for the properties of bases (changes in the color of indicators, interaction with acids, etc.).
NaOH ↔ Na + + OH −
Salts are electrolytes, upon dissociation of which metal cations (or ammonium cation NH 4 +) and anions of acid residues are formed.
CaCl 2 ↔ Ca 2+ + 2Cl −
Polybasic acids and bases dissociate stepwise.
H 2 SO 4 ↔ H + + HSO 4 − (I stage)
HSO 4 − ↔ H + + SO 4 2- (II stage)
Ca(OH) 2 ↔ + + OH − (I stage)
+ ↔ Ca 2+ + OH −
Degree of dissociation
Electrolytes are divided into weak and strong solutions. To characterize this measure, there is the concept and value of the degree of dissociation (). The degree of dissociation is the ratio of the number of molecules dissociated into ions to the total number of molecules. often expressed in %.
Weak electrolytes include substances whose degree of dissociation in a decimolar solution (0.1 mol/l) is less than 3%. Strong electrolytes include substances whose degree of dissociation in a decimolar solution (0.1 mol/l) is greater than 3%. Solutions of strong electrolytes do not contain undissociated molecules, and the process of association (combination) leads to the formation of hydrated ions and ion pairs.
The degree of dissociation is particularly influenced by the nature of the solvent, the nature of the dissolved substance, temperature (for strong electrolytes, the degree of dissociation decreases with increasing temperature, and for weak electrolytes it passes through a maximum in the temperature range of 60 o C), the concentration of solutions, and the introduction of ions of the same name into the solution.
Amphoteric electrolytes
There are electrolytes that, upon dissociation, form both H + and OH − ions. Such electrolytes are called amphoteric, for example: Be(OH) 2, Zn(OH) 2, Sn(OH) 2, Al(OH) 3, Cr(OH) 3, etc.
H + +RO − ↔ ROH ↔ R + +OH −
Ionic reaction equations
Reactions in aqueous solutions of electrolytes are reactions between ions - ionic reactions, which are written using ionic equations in molecular, full ionic and abbreviated ionic forms. For example:
BaCl 2 + Na 2 SO 4 = BaSO 4 ↓ + 2NaCl (molecular form)
Ba 2+ + 2 Cl − + 2 Na+ + SO 4 2- = BaSO 4 ↓ + 2 Na + + 2 Cl− (full ionic form)
Ba 2+ + SO 4 2- = BaSO 4 ↓ (short ionic form)
pH value
Water is a weak electrolyte, so the dissociation process occurs to an insignificant extent.
H 2 O ↔ H + + OH −
The law of mass action can be applied to any equilibrium and the expression for the equilibrium constant can be written:
K = /
The equilibrium concentration of water is a constant value, therefore.
K = = K W
It is convenient to express the acidity (basicity) of an aqueous solution through the decimal logarithm of the molar concentration of hydrogen ions, taken with the opposite sign. This value is called the pH value.
Unified State Exam. Electrolytic dissociation of salts, acids, alkalis. Ion exchange reactions. Hydrolysis of salts
Solutions and their concentration, dispersed systems, electrolytic dissociation, hydrolysis
During the lesson you will be able to test your knowledge on the topic “Unified State Exam. Electrolytic dissociation of salts, acids, alkalis. Ion exchange reactions. Hydrolysis of salts." You will consider solving problems from the Unified State Exam of groups A, B and C on various topics: “Solutions and their concentrations”, “Electrolytic dissociation”, “Ion exchange reactions and hydrolysis”. To solve these problems, in addition to knowledge of the topics under consideration, you also need to be able to use the solubility table of substances, know the electron balance method, and have an understanding of the reversibility and irreversibility of reactions.
Topic: Solutions and their concentration, dispersed systems, electrolytic dissociation
Lesson: Unified State Exam. Electrolytic dissociation of salts, acids, alkalis. Ion exchange reactions. Hydrolysis of salts
I. Select one correct option from 4 offered.
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Question |
A comment |
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A1. Strong electrolytes are: |
By definition, strong electrolytes are substances that completely disintegrate into ions in an aqueous solution. CO 2 and O 2 cannot be strong electrolytes. H 2 S is a weak electrolyte. The correct answer is 4. |
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A2. Substances that dissociate only into metal ions and hydroxide ions are: 1. acids 2. alkalis 4. amphoteric hydroxides |
By definition, a compound that, when dissociated in an aqueous solution, produces only hydroxide anions is called a base. Only alkali and amphoteric hydroxide fit this definition. But the question says that the compound should dissociate only into metal cations and hydroxide anions. Amphoteric hydroxide dissociates stepwise, and therefore hydroxometal ions are in solution. Correct answer 2. |
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A3. The exchange reaction occurs to completion with the formation of a water-insoluble substance between: 1. NaOH and MgCl 2 2. NaCl and CuSO 4 3. CaCO 3 and HCl (solution) |
To answer, you need to write these equations and look in the solubility table to see if there are any insoluble substances among the products. This is in the first reaction magnesium hydroxide Mg(OH) 2 Correct answer 1. |
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A4. The sum of all coefficients in full and reduced ionic form in the reaction betweenFe(NO 3 ) 2 +2 NaOHis equal to: |
Fe(NO 3) 2 +2NaOH Fe(OH) 2 ↓ +2Na NO 3 molecular Fe 2+ +2NO 3 - +2Na+2OH - Fe(OH) 2 ↓ +2Na + +2 NO 3 - complete ionic equation, the sum of the coefficients is 12 Fe 2+ + 2OH - Fe(OH) 2 ↓ abbreviated ionic, the sum of the coefficients is 4 The correct answer is 4. |
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A5. The abbreviated ionic equation for the reaction H + +OH - →H 2 O corresponds to the interaction: 2. NaOH (PP) +HNO 3 3. Cu(OH) 2 + HCl 4. CuO + H 2 SO 4 |
This shorthand equation reflects the interaction between a strong base and a strong acid. The base is available in versions 2 and 3, but Cu(OH) 2 is an insoluble base Correct answer 2. |
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A6. The ion exchange reaction proceeds to completion when the solutions are drained: 1. sodium nitrate and potassium sulfate 2. potassium sulfate and hydrochloric acid 3. calcium chloride and silver nitrate 4. sodium sulfate and potassium chloride |
Let's write how the ion exchange reactions between each pair of substances should take place. NaNO 3 +K 2 SO 4 →Na 2 SO 4 +KNO 3 K 2 SO 4 +HCl→H 2 SO 4 +KCl CaCl 2 +2AgNO 3 → 2AgCl↓ + Ca(NO 3) 2 Na 2 SO 4 + KCl → K 2 SO 4 + NaCl From the solubility table we see that AgCl↓ Correct answer 3. |
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A7. In an aqueous solution it dissociates stepwise: |
Polybasic acids undergo stepwise dissociation in an aqueous solution. Among these substances, only H2S is an acid. Correct answer 3. |
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A8. Reaction equation CuCl 2 +2 KOH→ Cu(OH) 2 ↓+2 KClcorresponds to the abbreviated ionic equation: 1. CuCl 2 +2OH - →Cu 2+ +2OH - +2Cl - 2. Cu 2+ +KOH→Cu(OH) 2 ↓+K + 3. Cl - +K + →KCl 4. Cu 2+ +2OH - →Cu(OH) 2 ↓ |
Let's write the complete ionic equation: Cu 2+ +2Cl - +2K + +2OH - → Cu(OH) 2 ↓+2K + +2Cl - Eliminating unbound ions, we get the abbreviated ionic equation Сu 2+ +2OH - →Cu(OH) 2 ↓ The correct answer is 4. |
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A9. The reaction is almost complete: 1. Na 2 SO 4 + KCl→ 2. H 2 SO 4 + BaCl 2 → 3. KNO 3 + NaOH → 4. Na 2 SO 4 + CuCl 2 → |
Let's write the hypothetical ion exchange reactions: Na 2 SO 4 + KCl → K 2 SO 4 + Na Cl H 2 SO 4 + BaCl 2 → BaSO 4 ↓ + 2HCl KNO 3 + NaOH → NaNO 3 + KOH Na 2 SO 4 + CuCl 2 → CuSO 4 + 2NaCl According to the solubility table we see BaSO 4 ↓ Correct answer 2. |
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A10. The solution has a neutral environment: 2. (NH 4) 2 SO 4 |
Only aqueous solutions of salts formed by a strong base and a strong acid have a neutral environment. NaNO3 is a salt formed by the strong base NaOH and the strong acid HNO3. Correct answer 1. |
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A11. Soil acidity can be increased by introducing a solution: |
It is necessary to determine which salt will give an acidic reaction to the medium. It must be a salt formed by a strong acid and a weak base. This is NH 4 NO 3. Correct answer 1. |
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A12. Hydrolysis occurs when dissolved in water: |
Only salts formed by a strong base and a strong acid do not undergo hydrolysis. All of the above salts contain strong acid anions. Only AlCl 3 contains a weak base cation. The correct answer is 4. |
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A 13. Does not undergo hydrolysis: 1. acetic acid 2. ethyl acetic acid 3. starch |
Hydrolysis is of great importance in organic chemistry. Esters, starch and protein undergo hydrolysis. Correct answer 1. |
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A14. What number indicates a fragment of the molecular equation of a chemical reaction corresponding to the multiple ionic equation C u 2+ +2 OH - → Cu(OH) 2 ↓? 1. Cu(OH) 2 + HCl→ 2. CuCO 3 + H 2 SO 4 → 3. CuO + HNO 3 → 4. CuSO 4 +KOH→ |
According to the abbreviated equation, it follows that you need to take any soluble compound containing a copper ion and a hydroxide ion. Of all the copper compounds listed, only CuSO 4 is soluble, and only in the aqueous reaction is OH - . The correct answer is 4. |
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A15.When what substances interact will sulfur oxide be released?: 1. Na 2 SO 3 and HCl 2. AgNO 3 and K 2 SO 4 3. BaCO 3 and HNO 3 4. Na 2 S and HCl |
The first reaction produces unstable acid H 2 SO 3, which decomposes into water and sulfur oxide (IV) Correct answer1.
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II. Short answer and matching tasks.
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IN 1. The total sum of all coefficients in the full and reduced ionic equation for the reaction between silver nitrate and sodium hydroxide is... |
Let's write the reaction equation: 2AgNO 3 +2NaOH→Ag 2 O↓+ 2NaNO 3 +H 2 O Full ionic equation: 2Ag + +2NO 3 - +2Na + +2OH - →Ag 2 O↓+ 2Na + +2NO 3 - +H 2 O Abbreviated ionic equation: 2Ag + +2OH - →Ag 2 O↓+H 2 O Correct answer: 20 |
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AT 2. Write a complete ionic equation for the interaction of 1 mol of potassium hydroxide with 1 mol of aluminum hydroxide. Give the number of ions in the equation. |
KOH + Al(OH) 3 ↓→ K Full ionic equation: K + +OH - + Al(OH) 3 ↓ → K + + - Correct answer: 4 ions. |
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AT 3. Match the name of the salt with its relationship to hydrolysis: A) ammonium acetate 1. does not hydrolyze B) barium sulfide 2. by cation B) ammonium sulfide 3. by anion D) sodium carbonate 4. by cation and anion |
To answer the question, you need to analyze what strength of base and acid these salts are formed with. Correct answer A4 B3 C4 D3 |
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AT 4. A solution of one mole of sodium sulfate contains 6.02sodium ions. Calculate the degree of dissociation of the salt. |
Let's write the equation for the electrolytic dissociation of sodium sulfate: Na 2 SO 4 ↔ 2Na + +SO 4 2- 0.5 mol of sodium sulfate disintegrated into ions. |
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AT 5. Match the reagents with the abbreviated ionic equations: 1. Ca(OH) 2 +HCl → A)NH 4 + +OH - →NH 3 +H 2 O 2. NH 4 Cl + NaOH → B) Al 3+ + OH - → Al(OH) 3 ↓ 3. AlCl 3 +KOH → B) H + +OH - →H 2 O 4. BaCl 2 +Na 2 SO 4 → D) Ba 2+ +SO 4 2- → BaSO 4 ↓ Correct answer: B1 A2 B3 D4 |
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AT 6. Write the complete ionic equation corresponding to the abbreviated one: WITHO 3 2- +2 H + → CO 2 + H 2 O. State the sum of the coefficients in the molecular and total ionic equations. |
You need to take any soluble carbonate and any soluble strong acid. Molecular: Na 2 CO 3 +2HCl → CO 2 +H 2 O +2NaCl; Full Ionic: 2Na + +CO 3 2- +2H + +2Cl - → CO 2 +H 2 O +2Na + +2Cl - ; |
III.Tasks with detailed answers
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Question |
(1887) to explain the properties of aqueous solutions of electrolytes. Subsequently, it was developed by many scientists on the basis of the doctrine of the structure of the atom and chemical bonds. The modern content of this theory can be reduced to the following three provisions:
Scheme for dissolving a crystal of table salt. Sodium and chlorine ions in solution.
1. Electrolytes, when dissolved in water, dissociate (break up) into ions - positively and negatively charged. (“Ion” is Greek for “wandering.” In a solution, ions move randomly in different directions.)
2. Under the influence of electric current, ions acquire directional movement: positively charged ones move towards the cathode, negatively charged ones move towards the anode. Therefore, the former are called cations, the latter - anions. The directional movement of ions occurs as a result of the attraction of their oppositely charged electrodes.
3. Dissociation is a reversible process. This means that a state of equilibrium occurs in which as many molecules break up into ions (dissociation), so many of them are formed again from ions (association). Therefore, in the equations of electrolytic dissociation, instead of the equal sign, the reversibility sign is used.
For example:
KA ↔ K + + A − ,
where KA is an electrolyte molecule, K + is a cation, A − is an anion.
The doctrine of chemical bonding helps answer the question of why electrolytes dissociate into ions. Substances with ionic bonds dissociate most easily, since they already consist of ions (see Chemical bonding). When they dissolve, the water dipoles are oriented around the positive and negative ions. Mutual attractive forces arise between the ions and dipoles of water. As a result, the bond between the ions weakens, and the ions move from the crystal to the solution. Electrolytes, whose molecules are formed according to the type of covalent polar bond, dissociate similarly. The dissociation of polar molecules can be complete or partial - it all depends on the degree of polarity of the bonds. In both cases (during the dissociation of compounds with ionic and polar bonds), hydrated ions are formed, that is, ions chemically bonded to water molecules.
The founder of this view of electrolytic dissociation was honorary academician I. A. Kablukov. In contrast to the Arrhenius theory, which did not take into account the interaction of the solute with the solvent, I. A. Kablukov applied the chemical theory of solutions of D. I. Mendeleev to explain electrolytic dissociation. He showed that during dissolution, a chemical interaction of the solute with water occurs, which leads to the formation of hydrates, and then they dissociate into ions. I. A. Kablukov believed that an aqueous solution contains only hydrated ions. Currently, this idea is generally accepted. So, ion hydration is the main cause of dissociation. In other, non-aqueous electrolyte solutions, the chemical bond between the particles (molecules, ions) of the solute and the solvent particles is called solvation.
Hydrated ions have both a constant and variable number of water molecules. A hydrate of constant composition forms hydrogen ions H + that hold one molecule of water - this is a hydrated proton H + (H 2 O). In the scientific literature, it is usually represented by the formula H 3 O + (or OH 3 +) and called the hydronium ion.
Since electrolytic dissociation is a reversible process, in solutions of electrolytes, along with their ions, there are also molecules. Therefore, electrolyte solutions are characterized by the degree of dissociation (denoted by the Greek letter a). The degree of dissociation is the ratio of the number of molecules disintegrated into ions, n, to the total number of dissolved molecules N:
The degree of electrolyte dissociation is determined experimentally and is expressed in fractions of a unit or as a percentage. If α = 0, then there is no dissociation, and if α = 1, or 100%, then the electrolyte completely disintegrates into ions. Different electrolytes have different degrees of dissociation. With dilution of the solution it increases, and with the addition of ions of the same name (the same as the electrolyte ions) it decreases.
However, to characterize the ability of an electrolyte to dissociate into ions, the degree of dissociation is not a very convenient value, since it... depends on the electrolyte concentration. A more general characteristic is the dissociation constant K. It can be easily derived by applying the law of mass action to the electrolyte dissociation equilibrium (1):
K = () / ,
where KA is the equilibrium concentration of the electrolyte, and are the equilibrium concentrations of its ions (see Chemical equilibrium). K does not depend on concentration. It depends on the nature of the electrolyte, solvent and temperature. For weak electrolytes, the higher K (dissociation constant), the stronger the electrolyte, the more ions in the solution.
Strong electrolytes do not have dissociation constants. Formally, they can be calculated, but they will not be constant as the concentration changes.
Polybasic acids and polyacid bases dissociate stepwise. Each dissociation step has its own dissociation constant. For example, for the dissociation of phosphoric acid:
The decrease in the constant from the first stage to the third is due to the fact that it becomes increasingly difficult to remove a proton as the negative charge of the resulting particle increases.
The total dissociation constant is equal to the product of the constants corresponding to the individual stages of dissociation. For example, in the case of phosphoric acid for the process:
To assess the degree of dissociation of weak electrolytes, it is sufficient to take into account only the first stage of dissociation – it, first of all, determines the concentration of ions in the solution.
Acidic and basic salts also dissociate in steps, for example:
It is easy to notice that the dissociation of a hydroanion or hydroxocation is identical to the second or third stage of dissociation of the corresponding acid or base and therefore obeys the same laws that have been formulated for the stepwise dissociation of acids and bases. In particular, if the basic salt corresponds to a weak base, and the acid salt – weak acid, then the dissociation of the hydroanion or hydroxocation (i.e., the second or third stage of salt dissociation) occurs to an insignificant extent.
Every oxygen-containing acid and every base (meaning acids and bases in the traditional sense) contain hydroxo groups. The difference between an acid and a base is that in the first case, dissociation occurs at the EO-H bond, and in the second – via E-ON connection.
Amphoteric hydroxides dissociate both as bases and as acids (both are very weak). Thus, the ionization of zinc hydroxide can be represented by the following scheme (without taking into account the hydration of the resulting ions):
The addition of acid shifts these equilibria to the left, and the addition of alkali – to the right. Therefore, in an acidic environment, dissociation according to the type of base predominates, and in an alkaline environment – by type of acid. In both cases, the binding of ions formed during the dissociation of a poorly soluble amphoteric electrolyte into water molecules causes the transition of new portions of such ions into the solution, their binding, the transition of new ions into the solution, etc. Consequently, the dissolution of such an electrolyte occurs both in an acid solution and and in an alkali solution.
During the dissociation of acids, the role of cations is played by hydrogen ions(H +), no other cations are formed during the dissociation of acids:
HF ↔ H + + F - HNO 3 ↔ H + + NO 3 -
It is hydrogen ions that give acids their characteristic properties: sour taste, coloring of the indicator red, etc.
Negative ions (anions) split off from an acid molecule make up acid residue.
One of the characteristics of the dissociation of acids is their basicity - the number of hydrogen ions contained in an acid molecule that can be formed during dissociation:
- monobasic acids: HCl, HF, HNO 3;
- dibasic acids: H 2 SO 4, H 2 CO 3;
- tribasic acids: H 3 PO 4.
The process of elimination of hydrogen cations in polybasic acids occurs in stages: first one hydrogen ion is eliminated, then another (third).
Stepwise dissociation of a dibasic acid:
H 2 SO 4 ↔ H + + HSO 4 - HSO 4 - ↔ H + + HSO 4 2-
Stepwise dissociation of a tribasic acid:
H 3 PO 4 ↔ H + + H 2 PO 4 - H 2 PO 4 - ↔ H + + HPO 4 2- HPO 4 2- ↔ H + + PO 4 3-
When dissociating polybasic acids, the highest degree of dissociation occurs in the first step. For example, during the dissociation of phosphoric acid, the degree of first-stage dissociation is 27%; second - 0.15%; third - 0.005%.
Base dissociation
During the dissociation of bases, the role of anions is played by hydroxide ions(OH -), no other anions are formed during the dissociation of bases:
NaOH ↔ Na + + OH -
The acidity of a base is determined by the number of hydroxide ions formed during the dissociation of one molecule of the base:
- monoacid bases - KOH, NaOH;
- diacid bases - Ca(OH) 2;
- triacid bases - Al(OH) 3.
Polyacid bases, by analogy with acids, also dissociate stepwise - at each stage one hydroxide ion is split off:
Some substances, depending on the conditions, can act both as acids (dissociate with the elimination of hydrogen cations) and as bases (dissociate with the elimination of hydroxide ions). Such substances are called amphoteric(See Acid-base reactions).
Dissociation of Zn(OH) 2 as bases:
Zn(OH) 2 ↔ ZnOH + + OH - ZnOH + ↔ Zn 2+ + OH -
Dissociation of Zn(OH) 2 as an acid:
Zn(OH) 2 + 2H 2 O ↔ 2H + + 2-
Dissociation of salts
Salts dissociate in water into anions of acidic residues and cations of metals (or other compounds).
Classification of salt dissociation:
- Normal (medium) salts are obtained by complete simultaneous replacement of all hydrogen atoms in the acid with metal atoms - these are strong electrolytes, completely dissociate in water with the formation of metal catoins and a one-acid residue: NaNO 3, Fe 2 (SO 4) 3, K 3 PO 4.
- Acid salts contain in their composition, in addition to metal atoms and an acidic residue, one more (several) hydrogen atoms - they dissociate stepwise with the formation of metal cations, anions of the acidic residue and a hydrogen cation: NaHCO 3, KH 2 PO 4, NaH 2 PO 4.
- Basic salts contain in their composition, in addition to metal atoms and an acidic residue, one more (several) hydroxyl groups - they dissociate with the formation of metal cations, anions of the acidic residue and hydroxide ion: (CuOH) 2 CO 3, Mg(OH)Cl.
- Double salts are obtained by simultaneous replacement of hydrogen atoms in the acid with atoms of various metals: KAl(SO 4) 2.
- Mixed salts dissociate into metal cations and anions of several acidic residues: CaClBr.
