Question

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

Answer 1

A is calcium, it reacts with water to form calcium hydroxide which is used in white washing. So, B is Ca(OH)₂.
Ca(s) + 2H₂O(l) ------> Ca(OH)₂(aq) + H₂(g)
                                             (B)
On heating compound B, i.e., calcium hydroxide, it forms calcium oxide, i.e., C.
Ca(OH)₂ +Heat ----> CaO + H₂O

                                    (C)