When conc. `H_(2)SO_(4)` was added to an unknown salt present in a test which was heated, a brown gas (A) was evolved. The gas intenstified when copper turning were also added into the test tube. On cooling, the gas (A) changed into a colourless liquid (B).
(a) Identify the gases A and B
(b) Write equations for the reactions involved.

Explanation

The available data suggests that the salt is some nitrate of a metal and the chemical reactions involved are listed :
`MNO_(3)+H_(2)O("conc.") overset("Heat")to MHSO_(4)+HNO_(3)`
`4HNO_(3)overset("Heat")to underset((A))underset("Brown gas") (4NO_(2)+2H_(2)O+O_(2))`
Upon adding some copper turning, more fumes of gas (A) were evolved and therefore, the brown colour was intestified.
`Cu+4HNO_(3) overset("Heat")to Cu(NO_(3))_(2)+2H_(2)O+2NO_(2)`
`underset((B))underset("Brown gas")(2NO_(2))underset("Heat")overset("Cool")harr underset((B))underset("Colourless liquid")(N_(2)O_(4))`