Which of the following pairs of molecules form hydrogen bonds?

Step 1: Analyze the question We need the pair in which the molecules can form hydrogen bonds with each other.

Step 2: The concept A hydrogen bond forms when a hydrogen atom covalently bonded to a very electronegative atom (F, O or N) is attracted to a lone pair on another F, O or N atom. So, for a pair to hydrogen bond, at least one molecule must have an H attached to F/O/N, and the other must have a lone pair on F/O/N.

Step 3: Check each option

• A. C₂H₅OH and CH₃OH: Both are alcohols with an –OH group. Each has an O–H (donor) and lone pairs on O (acceptor). Hydrogen bonds form between them. ✔
• B. CH₃OH and H₂: H₂ has no electronegative atom and no lone pairs, so it cannot take part. ✘
• C. H₂S and CH₄: Sulphur is not electronegative enough, and C–H in methane is nonpolar. No hydrogen bonding. ✘
• D. NH₃ and SO₂: NH₃ has N–H, but SO₂ has no H and its oxygen atoms are bonded to S, not H. NH₃ could in principle be attracted to SO₂ by dipole interactions, but this is not the classic hydrogen bonding pair expected here. ✘

Step 4: Answer The correct option is A, since both molecules contain the –OH group, which allows hydrogen bonding between them (this is also why alcohols have relatively high boiling points and dissolve well in water).

Exam tip: Remember "FON" + H. Look for H directly attached to F, O or N. If the molecule pair has an –OH or –NH group, hydrogen bonding is likely. Molecules like H₂S and CH₄ are common distractors.