Which of the following is a covalent solid?

SiC

The correct answer is option 3. SiC.

To determine which of the given substances is a covalent solid, let's briefly analyze each option:

1. Dry Ice (Solid \(CO_2\)):

Dry ice is solid carbon dioxide. In its solid state, \(CO_2\) molecules are held together by intermolecular forces (van der Waals forces), not covalent bonds between molecules. Therefore, dry ice is considered a molecular solid, not a covalent solid.

2. \(P_4O_{10}\) (Phosphorus Pentoxide):

\(P_4O_{10}\) is composed of discrete P4O10 molecules held together by van der Waals forces. It is also a molecular solid, not a covalent solid.

3. \(SiC\) (Silicon Carbide):

\(SiC\) is a covalent network solid. In \(SiC\), each silicon atom is covalently bonded to four carbon atoms in a tetrahedral arrangement, forming a continuous three-dimensional network of strong covalent bonds. This makes silicon carbide a covalent solid.

4. \(NaCl\) (Sodium Chloride):

\(NaCl\) is an ionic solid. It consists of a lattice of sodium ions \((Na^+)\) and chloride ions \((Cl^-)\) held together by ionic bonds. It is not a covalent solid.

Based on the analysis, the correct answer is \(SiC\) (Silicon Carbide)

\(SiC\) is a covalent solid because it consists of a continuous network of covalent bonds.