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) changes into a colourless solid (B). Why does gas (A) change to solid on cooling?

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