When conc. H_(2)SO_(4) was added into an unknown nitrate salt present in a lest-tube, a brown gas (A) was evolved. This gas intensified when copper turnings were also added into this test-tube. On cooling, the gas (A) changed into a colourless gas (B). (a) Identify the gases (A) and (B). (b) Write the equations for the reactions involved.

When conc. H_(2)SO_(4) was added into an unknown nitrate salt present

1.	When conc. H_(2)SO_(4) was added into an unknown nitrate salt present in a lest-tube, a brown gas (A) was evolved. This gas intensified when copper turnings were also added into this test-tube. On cooling, the gas (A) changed into a colourless gas (B). (a) Identify the gases (A) and (B). (b) Write the equations for the reactions involved.
Answer» Solution :When a nitrate salt is heated with conc. `H_(2)SO_(4)`, vapours of `NHO_(3)` are first produced which undergo decomposition to produce nitrogen dioxide `(NO_(2))` which is a brown GAS (A). When Cu turnings are added, the gas (A) intensified due to the reduction of `HNO_(3)` by Cu. `underset("Sod. nitrate")(2NaNO_(3))+H_(2)SO_(4)overset("Heat")tounderset(("Colourless"))underset("Nitcid acide")(2HNO_(3))+Na_(2)SO_(4)` `4HNO_(3)overset("Heat")tounderset("Brown gas A")underset("Nitrogen dioxide")(4NO_(2))+O_(2)+_2H_(2) O` `underset("Copper turnings")(Cu)+4HNO_(3)overset("Heat")toCu(NO_(3))_(2)+underset("Nitrogen dioxide(A)")(2NO_(2))+2H_(2)O` `NO_(2)` is an ODD electron species (7+8+8=23 electrons). In the valence shell, it has only 7 elecrons and hene is less stable. To become more stable by having 8 electrons in the valence shell, it undergoes dimerization to form a colourless gas, `N_(2)O_(4)`. THUS, (A) is nitrogen dioxide `(NO_(2))` and (B) is dinitrogen tetroxide `(N_(2)O_(4))`.