Which of the following oxides are acidic ?
$\text{CuO,}{\text{Mn}}_{\text{2}}{\text{O}}_{\text{7}}\text{,}\text{CO,}{\text{SO}}_{\text{2}}\text{,}{\text{CrO}}_{\text{3}}$

Observe the following chemical equations:

(i) ${\mathrm{Al}}_2{\mathrm{O}}_3+2\mathrm{NaOH}\to 2{\mathrm{NaAlO}}_2+{\mathrm{H}}_2\mathrm{O}$

(ii) ${\mathrm{Al}}_2{\mathrm{O}}_3+6\mathrm{HCl}\to 2{\mathrm{AlCl}}_3+3{\mathrm{H}}_2\mathrm{O}$

What is the conclusion that you take about the nature of aluminium oxide with the help of these equations? Give a reason for your conclusion.

Which of the following is amphoteric oxide?

${\mathrm{Mn}}_2{\mathrm{O}}_7, {\mathrm{CrO}}_3, {\mathrm{Cr}}_2{\mathrm{O}}_3, \mathrm{CrO}, {\mathrm{V}}_2{\mathrm{O}}_5, {\mathrm{V}}_2{\mathrm{O}}_4$