A water soluble compound of a posisonous element M when heated with `Zn//H_(2)SO_(4)` gives a colourless and extremely posisonous gaseous compound N which when passed through a heated tube gives a silvery mirror of element M. Identify M and N.

Explanation

The psisonous element M is likely to be arenic (As) and the water soluble compound is `AsCl_(3)`. On the basis of available data, the following reactions are feasible.
`underset(("Soluble compound"))underset((M))(AsCl_(3))+6H overset(Zn//HCl)to underset(("Poisonous gas"))underset((N))(AsH_(3))+3HCl`
`2AsH_(3)overset("heat")to underset((M))(2As)+3H_(2)`