A three-substance table appeared in all five years: identify the forces, then explain a difference in boiling point or ΔvapH°. ⚠
Identify ALL the forces, not just the strongest
The question says "identify all the types of attractive forces". A hydrogen-bonding molecule also has permanent dipole–dipole and temporary dipole forces. Listing only hydrogen bonding loses the mark.
The three forces, in the order to list them
Force
Present when
Relative strength
Temporary dipole (dispersion)
Always, in every substance
Grows with number of electrons / molecular size
Permanent dipole–dipole
The molecule is polar
Moderate
Hydrogen bonding
H bonded directly to N, O or F
Strongest of the three
Worked example Why does Br₂ have the highest ΔvapH°? (2025) HBr 17.3, HF 25.2, Br₂ 29.6 kJ mol⁻¹. Justify why Br₂ is highest, despite being non-polar. Br₂ is non-polar, so it has only temporary dipole (dispersion) forces. But Br₂ has far more electrons than HF or HBr, so its electron cloud is much more polarisable. That makes its temporary dipoles substantially stronger. Here dispersion outweighs even the hydrogen bonding in HF. ✓ Br₂ has the most electrons, so its temporary dipole forces are strong enough to exceed HF’s hydrogen bonding. ✕
“Hydrogen bonding always wins”
It does not. Compare like with like: hydrogen bonding beats dipole–dipole at similar molecular size, but a much larger non-polar molecule can beat a small hydrogen-bonded one. This exact trap was set in 2025 (Br₂ vs HF) and 2022 (BF₃ vs NOCl). ℹ
The chain-length version
2023 asked why decane boils higher than chloroethane. Answer with electron count and surface contact: decane is a much longer chain with many more electrons, so its dispersion forces are far stronger, outweighing chloroethane’s permanent dipole.