Which property generally decreases across period $3$?

What happens when sodium is added to water?

I. A gas is evolved.

II. The temperature of the water increases.

III. A clear, colourless solution is formed.

Which oxides produce an acidic solution when added to water?

I. ${\mathrm{P}}_4{\mathrm{O}}_{10}$   II. $\mathrm{MgO}$   III. ${\mathrm{SO}}_3$.

Which statement about the elements in group $17$ is correct?

How many of the following oxides are amphoteric?

${\mathrm{Na}}_2\mathrm{O}$, $\mathrm{MgO}$, ${\mathrm{Al}}_2{\mathrm{O}}_3$, ${\mathrm{SiO}}_2$.