Question

Explain the following statements-
Most metal oxides are insoluble in water but some of these dissolve in water. What are these oxides and their solutions in water called?

Answer 1

Hint: The process of dissolution of a compound in a particular solvent depends on the nature of intermolecular interactions present between the solute-solute and solvent-solvent particles. If the interactions between solute and solvent are of similar kind then the solvent dissolves easily.

Complete answer:
Metal ions are highly electropositive and form basic compounds upon reaction with oxygen, therefore metal oxides are basic in nature.
The first step of the dissolution process of metal oxides in the dissociation of ions, where the metal ions get extensively hydrated and the oxide ions react with water to produce hydroxide ions. The high charge density on the oxide anion due to its extremely small size is mainly responsible for its highly basic character that forces the water molecules to behave as proton donors and hydroxide ions are released.
Alkali metal oxides show enough basicity to liberate sufficient amounts of hydroxide ions and dissolve completely. The reaction between sodium metal oxide and water is highly exothermic and can be written as follows:
\[N{a_2}O + {H_2}O \to 2NaOH(aq)\]
Hence, the metal oxides that dissolve in water are basic alkali metal oxides and their solution is called an alkali.

Note:
The oxides of hard metal or the transition metals oxides like oxides of copper, zinc, iron and chromium do not dissolve in water due to their limited basicity. Alkaline earth metal oxides also form hydroxides in water but these hydroxides themselves give slaked solutions and are not completely soluble.