The gases liberated when zinc reacts with dilute and conc. $\mathrm{HNO}_3$ respectively are

When zinc reacts with dilute $\mathrm{HNO}_3$, it will produce nitrous oxide gas. At room temperature, it is a colourless, non-flammable gas, with a pleasant, slightly sweet odor. $\begin{aligned} & 4 \mathrm{Zn}+\underset{\text { Warm dilute }}{10 \mathrm{HNO}_3} \longrightarrow \underset{\text { Zinc nitrate }}{4 \mathrm{Zn}\left(\mathrm{NO}_3\right)_2} \\ &+\mathrm{NH}_4 \mathrm{NO}_3+3 \mathrm{H}_2 \mathrm{O}\end{aligned}$ $4 \mathrm{Zn}+\underset{\text { Cold dilute }}{10 \mathrm{HNO}_3} \longrightarrow \underset{\text { Zinc nitrate }}{4 \mathrm{Zn}\left(\mathrm{NO}_3\right)_2}+\underset{\text { Nitrous oxide }}{\mathrm{N}_2 \mathrm{O}}$ $+5 \mathrm{H}_2 \mathrm{O}$ Similarly, when zinc repcts with concentrated $\mathrm{HNO}_3$, it will produce nitrogen dioxide gas $\left(\mathrm{NO}_2\right)$. It is used as the indicator for the larger group of nitrogen oxides. $$ \mathrm{Zn}+\underset{\text { Hot and conc. }}{4 \mathrm{HNO}_3} \longrightarrow \underset{\text { Zinc nitrate }}{\mathrm{Zn}\left(\mathrm{NO}_3\right)_2} $$ $+\underset{\substack{\text { Nitrogen } \\ \text { dioxide }}}{2 \mathrm{NO}_2}+2 \mathrm{H}_2 \mathrm{O}$