Explain why silicon dioxide has a much higher melting point than carbon dioxide, even though both substances contain covalent bonds.

Carbon dioxide is a simple molecular substance, consisting of discrete CO₂ molecules. The forces between these molecules are weak London dispersion forces, so only a small amount of energy is required to overcome them and the substance remains a gas at room temperature.

In contrast, silicon dioxide has a giant covalent network structure. Each Si atom is covalently bonded to four O atoms and each O to two Si atoms, forming a continuous three‑dimensional lattice. To melt SiO₂ a large number of strong covalent bonds must be broken throughout the network, which requires a great deal of energy. Consequently its melting point is above 1600 °C.