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

What characteristic of silica, SiO₂, contributes to its high melting point?

1. It has weak intermolecular forces

2. It has a giant covalent structure

3. It is composed of simple molecules

4. It has metallic bonding

The correct answer is: It has a giant covalent structure

The high melting point of silica, SiO₂, is primarily attributed to its giant covalent structure. In this structure, each silicon atom is covalently bonded to four oxygen atoms in a three-dimensional network. These strong covalent bonds extend throughout the material, requiring a significant amount of energy to break. This extensive bonding network results in a substantial melting point, as all the bonds must be broken for the substance to change from solid to liquid. In contrast, options that refer to weak intermolecular forces or simple molecular compositions do not apply to silica. Weak intermolecular forces would generally lead to lower melting points, and simple molecules typically have low melting points due to minimal attractive forces between them. Additionally, metallic bonding is not applicable to silicon dioxide, as it does not exhibit metallic characteristics. Thus, the giant covalent structure is the key factor responsible for the high melting point of silica.