A double salt which contains fourth period alkali metal (A) on heating at 500 K 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 alum
`K_(2)SO_(4).Al_(2)(SO_(4))_(3).24H_(2)O`
(ii) On heating potash alum (A) 500 kgive anhydrous potash alum (or) burnt alum (B)
`K_(2)SO_(4).underset((A))underset(("Potash alumn"))(Al_(2)(SO_(4))_(3).24H_(2)O)overset(500 K)(to) underset((B))underset(("Burnt alum"))(K_(2)SO_(4).Al_(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)toBaSO_(4)+2Cl^(-)`
Aluminium ion reacts with alizarin solution to give a red colour compound.