From the given compounds, number of compounds acidic in water is ______.

$\mathrm{CaO}, {\mathrm{SO}}_2, {\mathrm{SO}}_3, {\mathrm{Fe}}_2{\mathrm{O}}_3, {\mathrm{Cl}}_2{\mathrm{O}}_7, {\mathrm{CO}}_2, {\mathrm{Na}}_2\mathrm{O}$.

Metallic oxides are known to show basic nature. On the other hand, non-metallic oxides are observed to show acidic properties.

${\mathrm{SO}}_2, {\mathrm{SO}}_3, {\mathrm{Cl}}_2{\mathrm{O}}_7, {\mathrm{CO}}_2$ are acidic in nature, so total 4.

$\mathrm{CaO}, {\mathrm{Na}}_2\mathrm{O}$ are basic in nature.

${\mathrm{Fe}}_2{\mathrm{O}}_3$ is amphoteric in nature.