Final answer:
Covalent network solids have high melting points because melting them requires breaking many strong covalent bonds throughout the three-dimensional network, which is energy-intensive.
Step-by-step explanation:
The high melting points of large covalently bonded networks, otherwise known as covalent network solids, are due to the extensive web of strong covalent bonds that hold the atoms together. These solids, like diamond and silicon dioxide (SiO2), have a structure where each atom is covalently bonded to its neighbors, creating a large three-dimensional network.
To melt such a solid, one must break a significant number of covalent bonds, a process that requires a substantial amount of energy. This explains why covalent network solids like diamond have melting points exceeding 3,500°C, which is much higher than most other types of solids.