Sulphuric acid forms well defined hydrates with various amounts of water. That is the reason it absorbs voluminous amount of water from any given ‘source’ of water or moisture. Considerable amount of heat is evolved in its combination with water. Following is an account of various hydrates that H2SO4 forms.
When 4 parts of the acid is combined with 1 part water, the first hydrate is formed.
Two parts of the acid, when combines with 1 part water, another hydrate is obtained.
Two parts of the acid, when combines with 1 part water, oil of vitriol (a highly concentrated form of sulphuric acid) is obtained.
Acid of specific gravity 1.78 is another hydrate. Here 1 part of the acid is combined with 2 parts of water.
Sulphuric acid still forms yet another hydrate of specific gravity 1.632, when 1 part of the acid is combined with 3 parts of water.
Other concentrated acids capable of forming similar hydrates, too absorb little amount of water, albeit with much reduced vigour. Concentrated nitric acid is an example.
why not other acids absorbs water????