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.

$$
\mathrm{Zn}+\underset{\text { Hot and conc. }}{4 \mathrm{HNO}_3} \longrightarrow \underset{\text { Zinc nitrate }}{\mathrm{Zn}\left(\mathrm{NO}_3\right)_2}
$$
$+\underset{\substack{\text { Nitrogen } \\ \text { dioxide }}}{2 \mathrm{NO}_2}+2 \mathrm{H}_2 \mathrm{O}$