$\mathrm{SnO}_2$ is an amphoteric oxide because it reacts with acids as well as bases to form corresponding salts.
$\begin{aligned}
& \mathrm{SnO}_2+4 \mathrm{HCl} \longrightarrow \mathrm{SnCl}_4+2 \mathrm{H}_2 \mathrm{O} \\
& \mathrm{SnO}_2+2 \mathrm{NaOH} \longrightarrow \begin{array}{c}
\mathrm{Na}_2 \mathrm{SnO}_3+\mathrm{H}_2 \mathrm{O} \\
\text { sod. stannate }
\end{array}
\end{aligned}$