Question

Which one of the following bases is insoluble in water?

• A. $\mathrm{NaOH}$
• B. $\mathrm{KOH}$
• C. $\mathrm{MgO}$
• D. ${\mathrm{NH}}_3$

Answer 1

The correct option is C

$\mathrm{MgO}$

Explanation for correct option

(C) $\mathbf{MgO}$

Solubility of Ionic Compound

• Ionic compounds dissolve readily in any liquid capable of dissolving their ionic connection.
• Since water has a stronger ionic bond and is polar, it breaks the ionic bond through hydrogen bonding.
• Many other solvents, like kerosene and petrol, are unable to break the ionic bond.

Solubility of $\mathbf{MgO}$ in water

• There is a significant attraction between the ions ${\mathrm{Mg}}^{2+}$ and ${\mathrm{O}}^{2-}$ in the molecule magnesium oxide $\left(\mathrm{MgO}\right)$.
• Magnesium and oxygen, which are both doubly charged, have a strong affinity.
• $\mathrm{MgO}$ is insoluble in water because it takes a lot of energy to separate the ions.

Explanation for incorrect options

(A) $\mathbf{NaOH}$

• Sodium hydroxide $\left(\mathrm{NaOH}\right)$ is broken down into positively and negatively charged sodium $\left({\mathrm{Na}}^{+}\right)$ and hydroxide $\left({\mathrm{OH}}^{-}\right)$ ions.
• It has high water solubility.

(B) $\mathbf{KOH}$

• Potassium hydroxide $\left(\mathrm{KOH}\right)$ dissociates completely into its component ions, potassium $\left({\mathrm{K}}^{+}\right)$ and hydroxide $\left({\mathrm{OH}}^{-}\right)$, in water.
• To balance their charges, any of these ions is then completely solvated (i.e. surrounded by water molecules).
• It is water soluble.

(D) ${\mathbf{NH}}_{\mathbf{3}}$

• Ammonia gas $\left({\mathrm{NH}}_3\right)$ is extremely soluble in water.
• The relatively high solubility is due to the hydrogen bonding that occurs between the ammonia and water molecules.

Therefore, the correct option is (C) $\mathbf{MgO}$.