The high boiling point of hydrogen fluoride can be explained in terms of

Step 1: Analyze the question We are asked why HF has an unusually high boiling point (about 19.5 °C) compared with the other hydrogen halides (HCl, HBr, HI), which boil at much lower temperatures than you would expect from HF's small size.

Step 2: Explain the concept Normally, boiling points in a group rise as molecular mass increases because of stronger van der Waals forces. HF breaks this trend. Fluorine is the most electronegative element, so the H-F bond is highly polar. The H atom carries a strong partial positive charge (δ+), and F carries a strong partial negative charge (δ−) and has lone pairs.

The δ+ hydrogen of one HF molecule is strongly attracted to the lone pair on the fluorine of a neighbouring molecule. This is a hydrogen bond, an intermolecular force that occurs when H is bonded to F, O, or N. Hydrogen bonds are much stronger than ordinary dipole-dipole attractions, so a lot of extra energy is needed to separate the molecules, and the boiling point is high.

Step 3: Check the options

• A. Covalent bonds: these are within the molecule and are not broken when HF boils.
• B. Dative bonds: not relevant here.
• C. Dipole-dipole bonds: HF does have dipole-dipole forces, but they are weak and do not explain the unusually high boiling point. The special, stronger interaction is hydrogen bonding.
• D. Ionic bonds: HF is a covalent molecule.
• E. Hydrogen bonds: correct.

Answer: E, hydrogen bonds.

Exam tip: Remember "FON": when hydrogen is bonded to F, O, or N and the question mentions an unusually high boiling point (HF, H₂O, NH₃), the answer is almost always hydrogen bonding.