The big idea: Because each water molecule is polar (a δ− oxygen and two δ+ hydrogens), the molecules attract one another.
A slightly positive (δ+) hydrogen on one molecule is pulled towards a slightly negative (δ−) oxygen on a neighbouring molecule.
This weak attraction between water molecules is called a hydrogen bond.
Each water molecule is polar: oxygen is δ−, the two hydrogens are δ+. This is what lets molecules attract each other.
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A hydrogen bond (dashed, green) runs from a δ+ hydrogen of one molecule to the δ− oxygen of another. The covalent O–H bonds stay WITHIN each molecule.
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Within vs between: Don't mix these up:
- The covalent bond holds the atoms WITHIN one water molecule (O joined to its H). - The hydrogen bond acts BETWEEN separate water molecules.
A hydrogen bond is much weaker than a covalent bond — but there are a huge number of them.
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A hydrogen bond forms when the δ+ hydrogen of one water molecule is attracted to the δ− oxygen of a nearby water molecule.
It is an attraction between molecules, not a sharing of electrons, so it is weak compared with a covalent bond. But each water molecule can hydrogen-bond to several neighbours at once, so across a whole sample there are an enormous number of them holding the water together.
- Hydrogen bond
- A weak attraction between a δ+ hydrogen of one molecule and a δ− atom (here, oxygen) of another molecule.
- Covalent bond
- A strong bond WITHIN a molecule, where atoms share a pair of electrons (the O–H bonds inside one water molecule).
- Polarity
- The uneven spread of charge in a molecule (δ− and δ+ ends) — the property that makes hydrogen bonds possible.
| Covalent bond | Hydrogen bond | |
|---|---|---|
| Where it acts | within one molecule (O–H) | between separate molecules |
| What causes it | sharing electrons | δ+ H attracted to δ− O |
| Strength | strong | weak (but very many of them) |
Why so many bonds matter: Any one hydrogen bond is easy to break.
But because there are so many of them, you need to put in a lot of energy to pull all the water molecules apart.
This is the reason behind nearly all of water's special properties — its high boiling point, the way it sticks together, and its ability to absorb a lot of heat.
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Answer exam-style questions and get AI feedback that shows you exactly what examiners want to see in a full-marks response.
How this is tested: On Paper 1A (multiple choice) you may have to explain why water stays liquid over a wider temperature range than a non-polar molecule of similar mass, or identify a biological consequence of water forming many hydrogen bonds.
On Paper 2 a 4-mark Draw question asks you to draw two water molecules and show the hydrogen bond between them.
IB-style question — draw two water molecules interacting
Draw a labelled diagram to show how two water molecules interact with each other. Show the atoms, the partial charges and the hydrogen bond. [4]
How to score all four marks
- Draw two complete water molecules. Each is bent, with one oxygen (O) joined to two hydrogens (H).
- Add the partial charges. Mark δ− on each oxygen and δ+ on each hydrogen.
- Show the hydrogen bond. Draw a dashed line from a δ+ hydrogen of one molecule to the δ− oxygen of the other molecule, and label it 'hydrogen bond'.
- Answer the command term (Draw): the marks are for (1) two correct bent molecules, (2) the δ−/δ+ charges, (3) the dashed line between a δ+ H and a δ− O, and (4) labelling it as a hydrogen bond.
Final answer
Two bent H–O–H molecules, each with δ− on its O and δ+ on its H atoms, joined by a dashed line (the hydrogen bond) running from a δ+ hydrogen of one molecule to the δ− oxygen of the other.
What your diagram must contain: Check yours has all four features: two bent molecules, δ− on each oxygen, δ+ on each hydrogen, and a dashed hydrogen bond drawn from a δ+ H of one molecule to the δ− O of the other (labelled 'hydrogen bond').
Here is what a full-mark answer looks like — the dashed bond joins a δ+ H of one molecule to the δ− O of the next:
A hydrogen bond (dashed, green) runs from a δ+ hydrogen of one molecule to the δ− oxygen of another. The covalent O–H bonds stay WITHIN each molecule.
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Explore the labelled diagram, charts and maps for this topic in full study mode.