When concentrated sulphuric acid was added to an unknown salt present in a test tube a brown gas (A) was evolved. This gas intensified when copper turnings were added to this test tube. On cooling, the gas (A) changed into a colourless solid (B). Identify (A) and (B).

The brown gas (A) is nitrogen dioxide (NO2). Salt reacts with conc. sulphuric acid to form brown fumes of nitrogen dioxide.
MNO3+H2SO4 → MHSO4+HNO3
4HNO3 → 4NO2↑+O2+2H2O
The fumes intensify when copper turnings are added.
3Cu+8HNO3 → 3Cu(NO3)2+2NO↑+4H2O
2NO+O2 → 2NO2
At low temperature, odd electron, brown coloured NO2 gas dimerises to colourless N2O4. NO2 is paramagnetic and N2O4 is diamagnetic.
2NO2 ⇌ N2O4