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  4. When conc. H2SO4 is added to dry KNO3, brown fumes are evolved. These fumes are of

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Q. When conc. $H_{2}SO_{4}$ is added to dry $KNO_{3}$, brown fumes are evolved. These fumes are of

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Solution:

When heated with conc. $H_{2}SO_{4}$ potassium nitrate undergoes double decomposition reaction forming free nitric add. Some of the nitric acid gets decomposed to give brown fumes of nitrogen dioxide.
$KNO_{3}+H_{2}SO_{4} \to KHSO_{4}+HNO_{3}$
$4HNO_{3} \to 4NO_{2}+2H_{2}O+O_{2}$