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 $(N O_{2})$ . Salt reacts with conc. sulphuric acid to form brown fumes of nitrogen dioxide.
$M N O_{3} + H_{2} S O_{4} \to M H S O_{4} + H N O_{3}$
$4 H N O_{3} \to 4 N O_{2} ↑ + O_{2} + 2 H_{2} O$
The fumes intensify when copper turnings are added.
$3 C u + 8 H N O_{3} \to 3 C u (N O_{3})_{2} + 2 N O ↑ + 4 H_{2} O$
$2 N O + O_{2} \to 2 N O_{2}$
At low temperature, odd electron, brown coloured $N O_{2}$ gas dimerises to colourless $N_{2} O_{4}$ .
$N O_{2}$ is paramagnetic and $N_{2} O_{4}$ is diamagnetic.
$2 N O_{2} ⇌ - 11 d e g C N_{2} O_{4}$

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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?

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?