Introduction
It is well known that a base is a substance that combines with an acid to form salt and water only.
In the equations given below, the metal oxides behave as bases.
Action of Alkalis on Solution of Salts
Alkalis react with aqueous solutions of salts producing the precipitates of the respective metals as hydroxides. These precipitates are usually colored. From the color and texture of the precipitate, it is possible to identify the metal ions present in the solution. In some cases, the precipitate formed may dissolve in excess of the alkali, to form a solution.
Action of Alkalis on Certain Metals
Metals like zinc, lead, aluminium etc. react with concentrated sodium hydroxide and potassium hydroxide to produce salt and liberate hydrogen gas.
Action of Alkalis on Metal Oxides
The oxides that are amphoteric by nature, such as oxides of zinc, lead and aluminium, dissolve in concentrated alkalis to yield salt and water.
Summary
Certain metal oxides like of zinc, lead and aluminium dissolve both in acid or alkali to yield salt and water. They are known as amphoteric substances.
