Advanced Placement (AP) Chemistry Practice Exam

Compared to ionic substances, the melting and boiling points of covalent substances are usually:

1. Higher

2. Lower

3. The same

4. Variable

The correct answer is: Lower

Ionic substances typically possess strong electrostatic forces between their charged ions, leading to considerably high melting and boiling points. The lattice structure that these ions form must be overcome, requiring significant energy. In contrast, covalent substances, particularly those formed by nonmetals, tend to have comparatively lower melting and boiling points. This is primarily due to the presence of weaker intermolecular forces, such as London dispersion forces, dipole-dipole interactions, or hydrogen bonding, which need to be overcome to change state. In many covalent molecular compounds, the actual bonds holding the atoms together within the molecules are strong, but the forces between the molecules are relatively weak. Thus, when comparing covalent substances to ionic substances, it is observed that the melting and boiling points of covalent substances are usually lower, affirming that option B accurately reflects this trend.