Question

Which of the following reaction(s) will give brown coloured gas?

• A. $Zn+\text{dil.}HN{O}_3$
• B. $Cu+\text{dil.}HN{O}_3$
• C. Both B and C
• D. $Zn+\text{conc.}HN{O}_3$

Answer 1

The correct option is D $Zn+\text{conc.}HN{O}_3$

(1) $Zn+\text{dil.}HN{O}_3:$
$4Zn+10HN{O}_3\left(\text{dil.}\right)\to 4Zn(N{O}_3{)}_2+N{H}_{4}N{O}_3+3H_{2}O$
Here, no gas is formed.

(2) $Zn+\text{conc.}HN{O}_3:$
$Zn+4HN{O}_3\left(\text{conc.}\right)\to Zn(N{O}_3{)}_2+2N{O}_2+2H_{2}O$
Here, $N{O}_2$ (nitrogen dioxide) is formed which is brown in colour.

(3) $Cu+\text{dil.}HN{O}_3:$
$3Cu+8HN{O}_3\left(\text{dil.}\right)\to 3Cu(N{O}_3{)}_2+2NO+4H_{2}O$
Here, $NO$ a colourless gas is formed.
Therefore, option (B) $Zn+\text{conc.}HN{O}_3$ will be the correct option.