International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Why does sodium chloride exhibit a very high melting point?

1. It has a small molecular structure

2. It has strong covalent bonds

3. It has strong electrostatic forces between oppositely charged ions

4. It contains metallic bonding

The correct answer is: It has strong electrostatic forces between oppositely charged ions

Sodium chloride has a very high melting point because of the strong electrostatic forces between the oppositely charged ions it consists of, namely sodium ions (Na⁺) and chloride ions (Cl⁻). In its solid state, sodium chloride forms an extensive three-dimensional lattice structure where each sodium ion is surrounded by six chloride ions and vice versa. This arrangement maximizes the attractive forces between the positively charged sodium ions and the negatively charged chloride ions. The energy required to break these strong ionic bonds in the lattice is substantial, which translates to a high melting point for sodium chloride. When melted, the ions are freed from the lattice structure, allowing the substance to flow, but this occurs only after a significant amount of heat is provided to overcome the strong attractions. The other options do not accurately describe the reason for the high melting point of sodium chloride. A small molecular structure would not inherently lead to high melting points; in fact, many small molecular compounds have relatively low melting points. Strong covalent bonds are characteristic of pure covalent substances, not ionic ones like sodium chloride. Lastly, metallic bonding involves a different type of structure and does not pertain to ionic compounds.