Background: Changes of state from solid to liquid and from liquid to gas occur when matter gains energy. The energy allows individual molecules to separate and move apart from one another. It takes more energy to bring about these changes of state for polar molecules. Although hydrogen bonds are weak, they add to the energy needed for molecules to move apart from one another, so it takes higher temperatures for these changes of state to occur in polar compounds. This explains why polar compounds have relatively high melting and boiling points. The Table below compares melting and boiling points for some polar and nonpolar covalent compounds.

Situation: Students in a chemistry class are performing experiments using two different compounds. The first compound, compound A, has a high number of hydrogen bonds between molecules of the compound. The second compound, compound B, has a low number of hydrogen bonds between the molecules of the compound. Aside from this difference, the molecular structure between the two compounds is similar.

Question: Which compound will have a higher melting point?
Answer: compound A

Situation: Students in a chemistry class are performing experiments using two different compounds. The first compound, compound A, has a high number of hydrogen bonds between molecules of the compound. The second compound, compound B, has a low number of hydrogen bonds between the molecules of the compound. Aside from this difference, the molecular structure between the two compounds is similar.

Question: Which compound will have a lower melting point?
Answer: compound B

Situation: Students in a chemistry class are performing experiments using two different compounds. The first compound, compound A, has a high number of hydrogen bonds between molecules of the compound. The second compound, compound B, has a low number of hydrogen bonds between the molecules of the compound. Aside from this difference, the molecular structure between the two compounds is similar.

Question: Which compound will have a higher boiling point?
Answer: compound A

Situation: Students in a chemistry class are performing experiments using two different compounds. The first compound, compound A, has a high number of hydrogen bonds between molecules of the compound. The second compound, compound B, has a low number of hydrogen bonds between the molecules of the compound. Aside from this difference, the molecular structure between the two compounds is similar.

Question: Which compound will have a lower boiling point?
Answer: compound B

Situation: Students in a chemistry class are performing experiments using two different compounds. The first compound, compound A, has a high number of hydrogen bonds between molecules of the compound. The second compound, compound B, has a low number of hydrogen bonds between the molecules of the compound. Aside from this difference, the molecular structure between the two compounds is similar.

Question: Which compound will require more energy to undergo state changes?
Answer: compound A

Situation: Students in a chemistry class are performing experiments using two different compounds. The first compound, compound A, has a high number of hydrogen bonds between molecules of the compound. The second compound, compound B, has a low number of hydrogen bonds between the molecules of the compound. Aside from this difference, the molecular structure between the two compounds is similar.

Question: Which compound will require less energy to undergo state changes?
Answer: compound B