Give a reason to support that sulphuric acid is a dehydrating agent.
A double salt which contains fourth period alkali metal (A) on heating at 500K gives (B). Aqueous solution of (B) gives white precipitate with `BaCl_(2)` and gives a red colour compound with alizarin. Identify A and B.

Explanation

(i) A double salt which contains fourth period alkali metal (A) is potash a alum
`"K"_(2)"SO"_(4)cdot"Al"_(2)(SO_(4))_(3)cdot24"H"_(2)O`
(ii) On heating potash alum (A) 500 K give anhydrous potash alum (or) burnt alum (B).
`underset((A))underset(("Potash alum"))("K"_(2)"SO"_(4)cdotAl_(2)(SO_(4))_(3)cdot24"H"_(2)"O")overset(500K)to underset((B))underset(("Burnt alum"))("K"_(2)"SO"_(4)cdotAl_(2))(SO_(4))_(3)+24H_(2)"O"`
(iii) Aqueous solution of burnt alum, has sulphates ion, potassium ion and aluminium ion. Sulphate ion reacts with `BaCl_(2)` to form white precipitate of Barium Sulphate:
`(SO_(4))^(2-)+BaCl_(2)to BaSO_(4)+2Cl^(-)`
Aluminium ion reacts with alizarin solution to give a red colour compound.