Which of the following processes are reversible?

I. Vaporisation of a liquid at its boiling point.

II. Expansion of gas into vacuum.

III. Transformation of a solid substance into liquid at its melting point.

At $T(\mathrm{~K})$ in a saturated solution of $\mathrm{MgCO}_3$ and $\mathrm{Ag}_2 \mathrm{CO}_3$, if the concentration of $\mathrm{Mg}^{2+}$ ion is $3.2 \times 10^{-5} \mathrm{M}$, then the concentration of $\mathrm{Ag}^{+}$ion in the solution will be [Given, $K_{\mathrm{sp}}\left(\mathrm{MgCO}_3\right)=1.6 \times 10^{-6}$ and $K_{\mathrm{sp}}\left(\mathrm{Ag}_2 \mathrm{CO}_3\right)=8.0 \times 10^{-12}$ at $T(\mathrm{~K})$ ]

Temperature of maximum density of $\mathrm{H}_2 \mathrm{O}$ is $y \mathrm{~K}$ and $\mathrm{D}_2 \mathrm{O}$ is $x \mathrm{~K} .(x-y)($ in K$)$ is nearly

How many of the following metals give oxides and nitrides when burnt in air?

Be, Na, Mg, Ba, Sr, Li, K