Q: Which one of the following forms a molecular solid when solidified?

Explanation:

Molecular Solids

Methane forms a molecular solid when solidified because it consists of discrete CH₄ molecules held together by weak intermolecular forces known as van der Waals forces. In a molecular solid, the individual molecules are arranged in a regular pattern, but the forces between them are much weaker compared to bonds within the molecules. This allows the solid to have lower melting and boiling points.

Let’s examine the other options:

Silicon carbide is not a molecular solid. It forms a covalent network solid where each silicon atom is bonded to carbon atoms in a continuous network, resulting in a very hard and high melting material.

Calcium fluoride is an ionic solid. It consists of calcium ions (Ca²⁺) and fluoride ions (F⁻) arranged in a repeating lattice structure held together by strong electrostatic forces.

Rock salt is also an ionic solid, similar to calcium fluoride. It typically refers to sodium chloride (NaCl), where sodium and chloride ions form a crystalline lattice through strong ionic bonds.

Among the given options, only methane consists of discrete molecules held together by weak intermolecular forces, making it a molecular solid when solidified.