An element A reacts with water to form a compound B which is used in white washing. On exposure to air compound B forms C. The compound C on heating forms an oxide D which on treatment with water gives back B. Identify A, B and C and D, write the balanced chemical equations for the reactions involved.

A- Ca

B- Ca(OH)2

C-CaCO3

D- CaO

Explanation:

Element A is calcium. Calcium reacts with water to form calcium hydroxide (compound B) as follows; Ca(s) + 2H2O(l) > Ca(OH)2(s) + H2(g).

When exposed to air, calcium hydroxide (compound B) forms calcium trioxocarbonate IV (CaCO3) which is compound C as follows;

Ca(OH)2(s) + CO2(g) > CaCO3(s) + H2O(l)

On heating compound C, the oxide, CaO (D) is formed as follows;

CaCO3(s) > CaO(s) + CO2(g)

This oxide reacts with water to yield B again as follows;

CaO(s) + H2O(l) > Ca(OH)2(s)

we can conclude that the melting point of table salt is above room temperature (it's actually 801 deg c). the melting point of mercury must be below room temperature (- 39 deg c).