All complex substances in reactions show the different nature of behavior: either acid, or alkaline. However there are substances which behavior nature in various reactions under various conditions changes. Such substances are called amphoteric, i.e. in reactions they show both acid, and main properties.
It is required to you
- Typical bases, for example sodium hydroxide and typical acids, sulfuric and hydrochloric acids.
1. Only difficult connections, such as oxides and hydroxides can be amphoteric. Oxides are the complex substances incorporating an element of metal and oxygen. Only the oxides formed by compound of oxygen and transitional metals which show the valency of II, III, IV are amphoteric. They react with strong acids, forming at the same time salts of these acids. For example, interaction of oxide of zinc and sulfuric acid: ZnO + H2SO4 → ZnSO4 + H2O. During this reaction a cation of hydrogen released from an acid molecule connects to the molecule of oxygen released from an oxide molecule forming thereby average salt of sulfate of sodium and water.
2. In interaction with acids, (not only with acids, but also in general in acidic environment) similar oxides show alkaline (the main properties). Acid properties are proved, on the contrary, by interaction with alkalis. So, for example, the same oxide of zinc, but already with strong alkali of sodium, gives salt dioxozincate of sodium (II): ZnO + 2NaOH → Na2ZnO2 + H2O.
3. Hydroxides are the complex substances formed by compound of metals with the ON hydroxyl group. Only hydroxides which in interaction with acids show properties of alkalis are amphoteric, and in reactions with alkalis behave like acids, t.e show dual properties.
4. Like oxides, transitional metals with a valency of II, III or IV are a part of amphoteric hydroxides. Reactions of interaction of similar hydroxides are reversible. The course of reaction depends by nature metal, rn Wednesdays and on temperature (at increase in temperature the balance is displaced towards formation of complexes). At reaction of hydroxide of zinc and oxygen-free chlorohydrogen acid there is a usual reaction of neutralization, i.e. average salt and water is as a result formed: Zn(OH)2+2HCl=ZnCl2+2H2O.
5. The characteristic sign that amphoteric connection takes part in reaction is a loss of a slightly soluble white or brown jellylike deposit which does not decay even when heating.