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. 

Explanation

A is calcium (Ca), B is calcium hydroxide `[Ca(OH)_(2)` C is calcium oxide (CaO)]
`Ca(s) + 2H_(2)O rarrCa(OH)_(2)(aq.) + H_(2)`
`Ca(OH)_(2) overset("Heat")rarr CaO + H_(2)O` (Dehydration)
`CaO + H_(2)O rarrCa(OH)_(2)` (Hydration)
`CaO + H_(2) rarr Ca(OH)_(2)`