Which of the following dissolves in water to form alkali?

Sodium oxide (Na₂O) and potassium oxide (K₂O) dissolve in water to form an alkali because they react with water molecules to form sodium hydroxide (NaOH), and Potassium hydroxide (KOH) which are strong bases.

The reaction between sodium oxide and water is as follows:

$$\begin{gathered} {\mathrm{Na}}_2\mathrm{O} + {\mathrm{H}}_2\mathrm{O} \to 2\mathrm{NaOH} \\ {\mathrm{K}}_2\mathrm{O}+ {\mathrm{H}}_2\mathrm{O}\to 2\mathrm{KOH} \end{gathered}$$

The sodium oxide and potassium oxide molecules donate an electron pair to the water molecule, forming two hydroxide ions (OH^-) and metallic ions (Na⁺, K⁺). The hydroxide ions increase the concentration of hydroxide ions in the solution, making it basic.

Hence, Sodium hydroxide and potassium hydroxide are strong bases because they dissociate completely in water to release hydroxide ions. This makes them highly reactive with acids and neutralises them.