Question

In preparation of alum from alunite, it is roasted
(a) True
(b) False

Answer 1

Hint: Concentrated solutions of potassium or ammonium sulfate in hot solution of aluminum sulphate constitutes the alums whereas roasting is the process of heating of the ore in the presence of air to high temperature to get the pure metal. Now, you can easily answer it.

Complete step by step answer:
Alum is crystalline solid and is a hydrated potassium sulfate or ammonium sulfate salt of the aluminum and has chemical formula as ${{K}_{2}}S{{O}_{4}}.A{{l}_{2}}{{(S{{O}_{4}})}_{3}}.24{{H}_{2}}O$ and it is also called as the double salt ( double salt is that salt which is the mixture of two salts) consisting of two salts potassium sulphate and aluminum sulphate and alunite is a hydrated potassium sulphate mineral form which the potassium alum is extracted.
Alums are generally prepared by adding the concentrated hot solutions of potassium sulphate or ammonium sulphate in to the hot solution of aluminum sulphate in equimolar concentration( i.e. the concentration of both i.e. the potassium sulphate and ammonium sulphate should be the same) and after that, the reaction mixture so formed is then allowed to cool down.
alums are soluble in the water and it turns blue litmus red indicating that they are acidic in nature.
So, from the above it is clear that alum is not roasted. Roasting is the process of heating the ore to a very high temperature in the presence of the air and it mostly involves the gas-solid reactions and plays an important role in the purification of the metal from its ore. Mostly it is the process used for the extraction of Sulphur from the sulphide ore.

Note: Alums are used in various foods such as pickling, fermentation and also helps in the purification of the water and is readily soluble in water and on heating, it gives a mass known as the burnt-alum.