HF has the strongest hydrogen bond but water boils way higher than HF, why?
Diya Menon11th Sample
4 Sept 2026 · 1 min read
omg okay so this is bugging me since last week. NCERT says the strength of hydrogen bond follows F–H···F > O–H···O > N–H···N. So HF should have the strongest hydrogen bonding, right?
But boiling points: water is 373 K and HF is only around 293 K. Even NH3 is about 240 K. If HF's hydrogen bonds are the strongest then HF should boil higher than water!! Is NCERT lying to me 😭
I've also been thinking about ice. I know it floats and that the structure is "open", but my teacher said "because of hydrogen bonding" and moved on. I want to actually see why hydrogen bonding makes ice less dense when I would expect stronger attraction to pack things closer.
And lastly o-nitrophenol and p-nitrophenol: one is steam volatile and one is not. I memorised which, but I can't explain why it depends on where the NO2 is.
I tried drawing HF and H2O side by side and counting bonds, which I think is the key, but I couldn't finish it. Can someone walk me through the counting?
7 answers & replies
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[+][–]Zoya Khan12thSample· 1mo ago
Your instinct about counting is exactly right. Strength of one bond and the number of bonds per molecule are two different things, and boiling point depends on the total.
Water has two H atoms and two lone pairs on oxygen, so each molecule can donate two hydrogen bonds and accept two, up to four contacts, and that builds a three-dimensional network. HF has only one hydrogen but three lone pairs on fluorine, so each molecule can donate only one hydrogen bond. That makes zigzag chains, roughly one hydrogen bond per molecule on average compared to two for water. So even though each F–H···F is stronger than each O–H···O, water has the greater total, and it boils higher.
Same reasoning for NH3: three hydrogens but only one lone pair, and the N–H···N bond is weaker, so its boiling point is lower again.
[+][–]Diya Menon11thSample· 1mo agoAuthor
ahhh the donate/accept counting finally works! drawing the chains on my sheet right now, thank you zoya 💛
[+][–]Farhan ShaikhDropperSample· 1mo ago
Ice part. In ice every oxygen is hydrogen-bonded to four others in a tetrahedral arrangement, which forces an open hexagonal cage structure with a lot of empty space. When ice melts, some bonds break and the molecules collapse into those gaps, so liquid water is denser. That's why ice floats and why lakes freeze from the top down so fish survive underneath. Water is densest at about 4 °C (277 K).
[+][–]Aman YadavDropperSample· 1mo ago
Order of boiling points: . Water wins on number of H-bonds, not strength. Remember that and move on.
[+][–]Ishaan Bose11thSample· 1mo ago
isn't hydrogen bond basically just a weak covalent bond? like H is sharing electrons with the F of the next molecule, that's why it's so strong. i just thought of it as a third bond
[+][–]Nisha Verma12thSample· 1mo ago
Ishaan no. Hydrogen bond is an electrostatic attraction between the partly positive H and a lone pair on a very electronegative atom of another molecule. No electron sharing, no new bond in the covalent sense. Strength is only some tens of kJ/mol, covalent bonds are hundreds. NCERT puts it as stronger than van der Waals but much weaker than covalent.
[+][–]Lakshmi TripathiDropperSample· 1mo ago
Nitrophenols. In ortho-nitrophenol the OH and NO2 are neighbours, so the H of OH bonds to an O of NO2 inside the same molecule (intramolecular, a chelate ring). The molecule is "self-satisfied", does not link to other molecules, so it has a lower boiling point and is steam volatile.
In para-nitrophenol they are too far apart. The OH bonds with other molecules, giving intermolecular hydrogen bonds and a chain, so higher boiling point, not steam volatile. Ortho also dissolves less in water because the OH is tied up internally.
