Question 7
Which covalent substances break the rule that covalent compounds have low melting and boiling points?
The right answer is the one marked above.
Diamond and graphite. Their atoms form one giant covalent lattice with no separate molecules, so melting them means breaking covalent bonds throughout the solid.
Water, ammonia, oxygen and sulfur dioxide are all ordinary molecular covalent substances and follow the rule. Sodium chloride and potassium chloride are ionic, so the rule never applied to them.