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When concentrated sulphuric acid was add...

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, gas A changed into a colourless solid B .
Why does gas A change to solid on cooling?

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In gaseous state `NO_(2)` exists as monomer and it is paramagnetic. On cooling it dimerises to `N_(2)O_(4)` which is solid and diamagnetic.
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