Question

select the correct answer. which property is typical of a covalent compound? a. they are extremely hard solids. b. they have low melting and boiling points. c. they conduct electricity. d. they have rigid crystal structures. e. they exist only as gases and liquids.

Explanation:

• Option A: Ionic compounds (like salts) are often extremely hard solids due to strong ionic bonds in a lattice. Covalent compounds (especially molecular covalent) have weaker intermolecular forces, so they aren't typically extremely hard. So A is incorrect.
• Option B: Covalent compounds (molecular) have weak intermolecular forces (like London dispersion, dipole - dipole, or hydrogen bonds). These forces are easy to overcome, leading to low melting and boiling points. For example, water (a covalent compound with hydrogen bonding) has a relatively low boiling point compared to ionic compounds. This is a typical property of covalent compounds.
• Option C: Covalent compounds don't have free ions or electrons (in the case of molecular covalent) to conduct electricity. Ionic compounds conduct when dissolved or molten as ions are free. So C is incorrect.
• Option D: Rigid crystal structures are characteristic of ionic compounds (ionic lattices) or network covalent solids (like diamond, but network covalent is a special case). Most covalent compounds are molecular and don't have rigid crystal structures. So D is incorrect.
• Option E: Covalent compounds can exist as solids (e.g., sugar, which is a covalent compound, is a solid). So the statement that they exist only as gases and liquids is false.

Answer:

B. They have low melting and boiling points.