Which one of the following pairs is an example of polar molecular solids?

🧠 Polar molecular solids consist of polar molecules (net dipole ≠ 0) held by dipole-dipole interactions and/or hydrogen bonds. They are distinct from ionic solids, covalent network solids, and non-polar molecular solids.

🗺️ Analyse the options:

(a) $\mathrm{SO_2(s)}$ and $\mathrm{CO_2(s)}$:

• $\mathrm{SO_2}$: bent, polar ✓
• $\mathrm{CO_2}$: linear, symmetric → μ = 0 → non-polar molecular solid ✗

(b) $\mathrm{SO_2(s)}$ and $\mathrm{NH_3(s)}$:

• $\mathrm{SO_2}$: bent, polar ✓
• $\mathrm{NH_3}$: pyramidal, polar ✓ → both polar molecular solids ✓

(c) $\mathrm{MgO(s)}$ and $\mathrm{SO_2(s)}$:

• MgO: ionic solid ✗

(d) $\mathrm{HCl(s)}$ and AlN(s):

• HCl: polar molecular solid ✓
• AlN: covalent network solid ✗

⚡ Only option (b) has both species as polar molecular solids.

$\boxed{\text{Answer: } \mathrm{SO_2(s)} \text{ and } \mathrm{NH_3(s)}}$