The main classes of inorganic substances, in addition to oxides, acids and salts, include a group of compounds called bases or hydroxides. All of them have a single plan for the structure of the molecule: they necessarily contain one or several hydroxyl groups connected to a metal ion. The main hydroxides are genetically bound with metal oxides and salts, this determines not only their chemical properties, but also methods of preparation in the laboratory and industry.
There are several forms of classification of bases, which are based both on the characteristics of the metal that is part of the molecule, and on the ability of a substance to dissolve in water. In our article, we will consider these features of hydroxides, and also get acquainted with their chemical properties, on which the use of bases in industry and everyday life depends.
Physical properties
All bases formed by active or typical metals are solids having a wide range of melting points. In relation to water, they are divided into highly soluble - alkalis and insoluble in water. For example, basic hydroxides containing elements of group IA as cations are readily soluble in water and are strong electrolytes. They are soapy to the touch, corrode tissue, skin and are called alkalis. When they dissociate in solution, OH - ions are detected, which are determined using indicators. For example, colorless phenolphthalein in an alkaline environment becomes raspberry. Both solutions and melts of sodium, potassium, barium, and calcium hydroxides are electrolytes, i.e. conduct electric current and are considered conductors of the second kind. The soluble bases most commonly used in industry include about 11 compounds, for example, such as basic hydroxides of sodium, potassium, ammonium, etc.
The structure of the base molecule
An ionic bond forms between the metal cation and the anions of the hydroxyl groups in the molecule of the substance. It is strong enough for water-insoluble hydroxides, so the polar molecules of water are not able to destroy the crystal lattice of such a compound. Alkalis are persistent substances and practically do not form oxide and water when heated. So, the main hydroxides of potassium and sodium boil at a temperature of more than 1000 ° C, while they do not decompose. In the graphic formulas of all the bases, it is clearly seen that the oxygen atom of the hydroxyl group is bound by one covalent bond to the metal atom, and the other to the hydrogen atom. The structure of the molecule and the type of chemical bond determine not only the physical, but also all the chemical characteristics of the substances. Let us dwell on them in more detail.
Calcium and magnesium and features of the properties of their compounds
Both elements are typical representatives of active metals and can interact with oxygen and water. The product of the first reaction is the basic oxide. Hydroxide is formed as a result of an exothermic process involving the release of a large amount of heat. Calcium and magnesium bases are sparingly soluble white powdery substances. The following names are often used for calcium compounds: milk of lime (if it is a suspension in water) and lime water. Being a typical basic hydroxide, Ca (OH) 2 interacts with acidic and amphoteric oxides, acids and amphoteric bases, for example, with aluminum and zinc hydroxides. Unlike typical alkalis that are resistant to heat, magnesium and calcium compounds decompose under the influence of temperature into oxide and water. Both bases, especially Ca (OH) 2 , are widely used in industry, agriculture, and household needs. Consider their application below.
Applications for Calcium and Magnesium Compounds
It is well known that a chemical material called cannon or slaked lime is used in construction. This is the base of calcium. Most often it is obtained by the reaction of water with basic calcium oxide. The chemical properties of basic hydroxides make it possible to widely use them in various sectors of the national economy. Napimer, for the purification of impurities in the production of raw sugar, for the production of bleach, in the bleaching of cotton and linen yarn. Before the invention of ion exchangers - cation exchangers, calcium and magnesium bases were used in water softening technologies, which made it possible to get rid of bicarbonates that worsen its quality. To do this, water was boiled with a small amount of soda ash or slaked lime. An aqueous suspension of magnesium hydroxide can be used as a remedy for patients with gastritis to reduce the acidity of gastric juice.
Properties of basic oxides and hydroxides
The groups most important for substances are reactions with acid oxides, acids, amphoteric bases and salts. Interestingly, insoluble bases, such as, for example, hydroxides of copper, iron or nickel, cannot be obtained by direct reaction of the oxide with water. In this case, the laboratory uses the reaction between the corresponding salt and alkali. As a result, bases are formed which precipitate. For example, a blue precipitate of copper hydroxide, a green precipitate of the base of ferrous iron, is obtained. Subsequently, they are evaporated to solid powdery substances related to water-insoluble hydroxides. A distinctive feature of these compounds is that under the action of high temperatures they decompose into the corresponding oxide and water, which cannot be said about alkalis. After all, water-soluble bases are thermally stable.
Electrolysis ability
Continuing to study the basic properties of hydroxides, let us dwell on another feature , according to which it is possible to distinguish the bases of alkaline and alkaline-earth metals from water-insoluble compounds. This is the inability of the latter to dissociate into ions under the influence of an electric current. On the contrary, melts and solutions of potassium, sodium, barium, and strontium hydroxides are easily electrolyzed and are second-order conductors.
Getting the grounds
Speaking about the properties of this class of inorganic substances, we partially enumerated the chemical reactions that underlie their production in laboratory and industrial conditions. The most affordable and economically advantageous method can be considered the thermal decomposition of natural limestone, which results in quicklime. If you carry out a reaction with water, then it forms a basic hydroxide - Ca (OH) 2 . A mixture of this substance with sand and water is called a mortar. It continues to be used for plastering walls, for bundling bricks and in other types of construction work. Alkalis can also be obtained by reacting the corresponding oxides with water. For example: K 2 O + H 2 O = 2KOH. The process is exothermic with the release of a large amount of heat.
The interaction of alkalis with acid and amphoteric oxides
The characteristic chemical properties of water-soluble bases include their ability to form salts in reactions with oxides containing non-metal atoms in the molecules, such as carbon dioxide, sulfur dioxide or silicon oxide. In particular, calcium hydroxide is used to dry gases, and sodium and potassium hydroxides to produce the corresponding carbonates. Zinc and aluminum oxides related to amphoteric substances can interact with both acids and alkalis. In the latter case, complex compounds can be formed, for example, such as sodium hydroxide.
Neutralization reaction
One of the most important properties of bases, both insoluble in water and alkalis, is their ability to react with inorganic or organic acids. This reaction reduces to the interaction between two types of ions: hydrogen and hydroxyl groups. It leads to the formation of water molecules: HCI + KOH = KCI + H 2 O. From the point of view of the theory of electrolytic dissociation, the whole reaction is reduced to the formation of a weak, slightly dissociated electrolyte - water.
In the above example, an average salt formed - potassium chloride. If, for the reaction, hydroxides of a basic nature are taken in an amount less than necessary for the complete neutralization of the polybasic acid, then, upon evaporation of the resulting product, acid salt crystals are detected. The neutralization reaction plays an important role in the metabolic processes occurring in living systems - cells and allows them to neutralize the excess amount of hydrogen ions accumulating in the dissimilation reactions using their own buffer complexes.