When Conc. H2SO4 was added to an unknown salt in a test tube, and a brown gas (A) was evolved. This gas intensified when copper turnings were added to the test tube. On cooling gas (A) changed into a colorless gas (B). Identify the gases ‘A’ and ‘B’. Write the equations for the reactions involved.

Que: When Conc. H₂SO₄ was added to an unknown salt in a test tube, and a brown gas (A) was evolved. This gas intensified when copper turnings were added to the test tube. On cooling gas (A) changed into a colorless gas (B). Identify the gases ‘A’ and ‘B’. Write the equations for the reactions involved.
Ans: The brown gas (A) is nitrogen dioxide (NO₂​). Salt reacts with conc. Sulphuric acid forms brown fumes of nitrogen dioxide.
MNO₃​ + H₂​SO₄ ​→ MHSO₄ ​+ HNO₃​
4HNO₃​ → 4NO_2​↑ + O_2​ + 2H₂​O
The fumes intensify when copper turnings are added.
3Cu + 8HNO₃ ​→ 3Cu(NO₃​)₂ ​+ 2NO↑ + 4H₂​O
2NO + O_2​ → 2NO_2​
At low temperatures, odd electron, brown-colored NO₂​ gas dimerizes to colorless gas N₂​O₄​.
NO₂​ is paramagnetic and N₂​O₄​ is diamagnetic.
2NO₂​ ⇌​ N₂​O₄​