Question

Which salt on heating does not give brown coloured gas?

• A. $LiN{O}_3$
• B. $KN{O}_3$
• C. $PbN{O}_3$
• D. $AgN{O}_3$

Answer 1

The correct option is B $KN{O}_3$

The nitrates of lithium, silver and most divalent and trivalent metals on heating decompose to give reddish brown fumes of nitrogen dioxide and oxygen as the gaseous products.

$4$ $LiN{O}_3$ $\to$ $2$ $L{i}_{2}O$ + $4$ $N{O}_2$ + $O_2$

$2$ $AgN{O}_3$ $\to$ $2$ $Ag$ + $2$ $N{O}_2$ + $O_2$

$Hg(N{O}_3{)}_2$ $\to$ $Hg$ + $2$ $N{O}_2$ + $O_2$

$2$ $Mg(N{O}_3{)}_2$ $\to$ $2$ $MgO$ + $4N{O}_2$ + $O_2$

$2$ $Pb(N{O}_3{)}_2$ $\to$ $2$ $PbO$ + $4$ $N{O}_2$ + $O2$

$4$ $Al(N{O}_3{)}_3$ $\to$ $2$ $A{l}_2{O}_3$ + $12$ $N{O}_2$ + $3$ $O_2$

But the nitrates of Sodium and other heavier alkali metals - Potassium, Rubidium and Caesium - do not liberate $N{O}_2$ on heating. They give only oxygen as the gaseous product and the respective nitrite is left as the solid residue.

$2$ $NaN{O}_3$ $\to$ $2$ $NaN{O}_2$ + $O_2$

$2$ $KN{O}_3$ $\to$ $2$ $KN{O}_2$ + $O_2$

Hence the correct option is 'B'.