Advanced Placement (AP) Chemistry Practice Exam

What is a common property of network covalent solids?

1. High electrical conductivity

2. Low melting and boiling points

3. Very hard and high melting/boiling points

4. High solubility in water

The correct answer is: Very hard and high melting/boiling points

Network covalent solids are characterized by a three-dimensional lattice structure where atoms are linked by covalent bonds in a continuous network. This type of bonding results in the formation of very strong interactions throughout the entire solid. One of the defining characteristics of network covalent solids is their high melting and boiling points. The strength of the covalent bonds requires a significant amount of energy to break them apart, leading to these elevated temperature thresholds. For example, diamond, a well-known network covalent solid, exhibits exceptional hardness and a very high melting point due to the robust bonding between carbon atoms. Furthermore, the hardness of network covalent solids stems from the rigidity of their extended structures. Unlike metals or ionic compounds, which can have a range of hardness due to their bonding types and structural arrangements, the consistent nature of covalent bonding in these solids contributes to their significant toughness. This property distinguishes them from other types of solids, such as ionic compounds, which may have lower melting points or metallic solids that can conduct electricity. Additionally, network covalent solids typically do not dissolve well in water, eliminating high solubility as a characteristic. Thus, the combination of very hard structures and high melting and boiling points is a hallmark of network covalent solids.