20. Select two substances that contain hydrogen bonds. 1) C_(3)H_(7)OH 2) HF 3) CO 4) C_(2)H_(4) 5) N_(2) Answer (score: 1):

The correct answer is 1) $C_{3}H_{7}OH$ and 2) HF.<br /><br />Hydrogen bonding is a type of intermolecular force that occurs between a hydrogen atom bonded to a highly electronegative atom (such as oxygen, nitrogen, or fluorine) and another electronegative atom. In the given options, $C_{3}H_{7}OH$ (propanol) and HF (hydrofluoric acid) both contain hydrogen atoms bonded to oxygen and fluorine, respectively, which are highly electronegative atoms. Therefore, they can form hydrogen bonds.