Explain why the reactivity of $\mathrm{Al}$ decreases if it is dipped in ${\mathrm{HNO}}_3$.

Reactivity of $\mathrm{Al}$ decreases when it is dipped in ${\mathrm{HNO}}_3$:

1. Nitric acid $\left({\mathrm{HNO}}_3\right)$ is a strong oxidizing agent.
2. When Aluminium reacts with Nitric acid, a layer of Aluminium oxide gets deposited, Nitrogen dioxide gas is evolved and Water is formed as a by-product,
3. The balanced equation for the reaction between $\mathrm{Al}$ and ${\mathrm{HNO}}_3$is

$\underset{\mathrm{Aluminium}}{\mathrm{Al}\left(\mathrm{s}\right)}+\underset{\mathrm{Nitric}\mathrm{acid}}{6{\mathrm{HNO}}_3\left(\mathrm{aq}\right)}\to \underset{\mathrm{Aluminium}\mathrm{nitrate}}{\mathrm{Al}{\left({\mathrm{NO}}_3\right)}_3\left(\mathrm{aq}\right)}+\underset{\mathrm{Nitrogen}\mathrm{dioxide}}{3{\mathrm{NO}}_2\left(\mathrm{g}\right)}+\underset{\mathrm{Water}}{3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$

5. The layer of Aluminium oxide thus formed, prevents Aluminium from further reactions and due to this, the reactivity of $\mathrm{Al}$ decreases.

6. Nitric Acid is a highly corrosive mineral acid.

7. The solution of nitric acid is strongly acidic, it can give both Hydronium and Nitrate ions.

Therefore the reactivity of Aluminium decreases due to the formation of a layer of Aluminium oxide when it is dipped in ${\mathrm{HNO}}_3$.