An element $A$ reacts with water to form a compound $B$ which is used in white washing. The compound $B$ on heating forms an oxide $C$ which on treatment with water gives back $B$. Identify $A$,$B$ and $C$ and give the reactions involved.

(A) The element $A$ is Calcium $(Ca)$.
When $Ca$ reacts with water,it forms Calcium hydroxide $(Ca(OH)_2)$,which is compound $B$ and is used for white washing.
Reaction: $Ca(s) + 2H_2O(l) \rightarrow Ca(OH)_2(aq) + H_2(g)$
When $Ca(OH)_2$ is heated,it decomposes to form Calcium oxide $(CaO)$,which is oxide $C$.
Reaction: $Ca(OH)_2(s) \xrightarrow{\text{Heat}} CaO(s) + H_2O(g)$
When $CaO$ is treated with water,it gives back $Ca(OH)_2$.
Reaction: $CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)$
Therefore,$A = Ca$,$B = Ca(OH)_2$,and $C = CaO$.