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

Element A is calcium (Ca). When it reacts with water, it forms calcium hydroxide. Thus. comF30und B is calcium hydroxide `[Ca(OH)_(2)]`, which is used in white washing.
Compound 8 on heating gives CaO.
`underset((B))(Ca(OH)_(2))overset("Heat")tounderset((C))(CaO)+H_(2)O`
Thus,C is calcium oxide(CaO).
The reactions involved are `underset((A))(Ca)+2H_(2)Oto underset((B))(Ca(OH)_(2)H_(2)uparrow)`
`underset((B))(Ca(OH_(2)))overset(Delta)tounderset((C))(CaO)+H_(2)O`
`underset((C))(CaO)+H_(2)Otounderset((B))(Ca(OH)_(2))`