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Question

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?

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Solution

The brown gas (A) is nitrogen dioxide (NO2). Salt reacts with conc. sulphuric acid to form brown fumes of nitrogen dioxide.
MNO3+H2SO4MHSO4+HNO3
4HNO34NO2+O2+2H2O
The fumes intensify when copper turnings are added.
3Cu+8HNO33Cu(NO3)2+2NO+4H2O
2NO+O22NO2
At low temperature, odd electron, brown coloured NO2 gas dimerises to colourless N2O4.
NO2 is paramagnetic and N2O4 is diamagnetic.
2NO211degCN2O4
792664_807419_ans_3ba411cc76ff4ac2bcfae64e87c6f213.png

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