The main classes of inorganic substances, in addition to oxides, acids and s alts, include a group of compounds called bases or hydroxides. All of them have a single molecular structure plan: they necessarily contain one or more hydroxyl groups connected to a metal ion in its composition. Basic hydroxides are genetically related to metal oxides and s alts, which determines not only their chemical properties, but also methods of obtaining 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 the substance to dissolve in water. In our article, we will consider these features of hydroxides, as well as 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 with a wide range of melting points. In relation to water, theyare divided into highly soluble - alkali and insoluble in water. For example, basic hydroxides containing group IA elements as cations readily dissolve in water and are strong electrolytes. They are soapy to the touch, corrode fabric, skin and are called alkalis. When they dissociate in solution, OH- ions are detected, which are determined using indicators. For example, colorless phenolphthalein becomes crimson in an alkaline medium. Both solutions and melts of sodium, potassium, barium, and calcium hydroxides are electrolytes; conduct electricity and are considered conductors of the second kind. Soluble bases, most commonly used in industry, include about 11 compounds, such as basic hydroxides of sodium, potassium, ammonium, etc.
The structure of the base molecule
An ionic bond is formed between a metal cation and anions of hydroxyl groups in a substance molecule. It is strong enough for water-insoluble hydroxides, so polar water molecules are not able to destroy the crystal lattice of such a compound. Alkalis are stable substances and practically do not form oxide and water when heated. Thus, the basic hydroxides of potassium and sodium boil at temperatures above 1000 ° C, while they do not decompose. In the graphic formulas of all 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 physical, butand all chemical characteristics of substances. Let's 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 a basic oxide. The hydroxide is formed as a result of an exothermic process that releases 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, such as aluminum and zinc hydroxides. Unlike typical heat-resistant alkalis, magnesium and calcium compounds decompose into oxide and water under the influence of temperature. Both bases, especially Ca(OH)2, are widely used in industry, agriculture and domestic needs. Let's consider their application further.
Fields of application of calcium and magnesium compounds
It is well known that construction uses a chemical material called fluff or slaked lime. It is a calcium base. Most often it is obtained by the reaction of water with basic calcium oxide. The chemical properties of basic hydroxides allow them to be widely used in various branches of the national economy. For example, to clean impurities in productionraw sugar, to obtain 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 hydrocarbons that degrade 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 most important for substances of this group are reactions with acid oxides, acids, amphoteric bases and s alts. Interestingly, insoluble bases such as copper, iron or nickel hydroxides cannot be obtained by direct reaction of the oxide with water. In this case, the laboratory uses the reaction between the corresponding s alt and alkali. As a result, bases are formed that precipitate. For example, this is how a blue precipitate of copper hydroxide, a green precipitate of a ferrous base, 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's dwell on one more feature by which one can distinguish the bases of alkali and alkaline earth metals from water-insoluble compounds. This is the impossibility 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 subjected to electrolysis and are conductors of the second kind.
Getting grounds
Speaking about the properties of this class of inorganic substances, we have partially listed the chemical reactions that underlie their production in laboratory and industrial conditions. The most accessible and cost-effective method can be considered the thermal decomposition of natural limestone, as a result of which quicklime is obtained. 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 mortar. It continues to be used for plastering walls, for bonding bricks, and in other types of building work. Alkalis can also be obtained by reacting the corresponding oxides with water. For example: K2O + H2O=2KON. The process is exothermic with the release of a large amount of heat.
Interaction of alkalis with acidic and amphoteric oxides
The characteristic chemical properties of water-soluble bases include their ability to form s alts in reactions with oxides containing non-metal atoms in molecules,for example, such as carbon dioxide, sulfur dioxide or silicon oxide. In particular, calcium hydroxide is used to dry gases, and sodium and potassium hydroxides to obtain the corresponding carbonates. Oxides of zinc and aluminum, related to amphoteric substances, can interact with both acids and alkalis. In the latter case, complex compounds can be formed, such as sodium hydroxozincate.
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 is reduced to the interaction between two types of ions: hydrogen and hydroxyl groups. It leads to the formation of water molecules: HCI + KOH=KCI + H2O. 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 s alt was formed - potassium chloride. If basic hydroxides are taken for the reaction in an amount less than necessary for the complete neutralization of the polybasic acid, then upon evaporation of the resulting product, crystals of the acid s alt are found. The neutralization reaction plays an important role in the metabolic processes occurring in living systems - cells and allows them, with the help of their own buffer complexes, to neutralize the excess amount of hydrogen ions accumulated in dissimilation reactions.
