Chemical A is used for the preparation of washing soda to recover ammonia. When CO2 is bubbled through an aqueous solution of A, the solution turns milky. It is used in whitewashing due to its disinfectant nature. The chemical formula of A is-

1. Ca(HCO3)2

2. CaO

3. Ca(OH)2

4. CaCO3

Hint: CO2 turns calcium hydroxide solution milky
To recover NH3 in the Solvay ammonia soda process Ca(OH)2 is used. The reaction between calcium hydroxide and ammonium chloride is :
On passing CO2 through Ca(OH)2, it turns milky due to the formation of CaCO3. The reaction is 
Ca(OH)2 is used for whitewashing.