Chemistry · Lesson
Dispersion, dipole forces and hydrogen bonding
Explain why molecules can remain intact while a substance changes state. Compare the interactions between particles without turning their names into a universal strength ranking.
Back to the Chemistry mapBy the end, you can
- Separate bonds within molecules from interactions between molecules.
- Recognize dispersion in both polar and nonpolar substances.
- Identify common hydrogen-bond donors and acceptors while allowing the broader definition.
A molecule can keep its bonds while its neighbours change
Imagine water evaporating from a clean surface. Ordinary vaporization separates water molecules from their neighbours; it does not normally turn each H₂O molecule into separate hydrogen and oxygen atoms. The covalent O–H bonds remain within the molecule, while the arrangement and interactions among molecules change.
This distinction works well for molecular substances. An ionic crystal or a covalent network needs its own structural description rather than a picture of independent little molecules. First identify the entities in the material, then ask which interactions change during the process.
Fluctuating electron density produces dispersion
Even a nonpolar molecule has an electron distribution that fluctuates. A fluctuation can influence a neighbouring distribution, producing correlated interactions called London dispersion. Polar molecules have dispersion too: having a permanent molecular dipole does not remove this contribution.
A more easily distorted electron distribution is more polarizable. Size, shape and available contact geometry can affect dispersion and the total attraction among molecules. Molar mass sometimes follows useful patterns within a comparable series, but mass alone is not a universal predictor of boiling point.
Permanent dipoles and hydrogen bonds add other possibilities
Polar molecules can interact through their permanent dipoles as well as through dispersion. Their orientations fluctuate, so a liquid is not a rigid drawing in which every neighbouring dipole stays perfectly aligned. Temperature and molecular arrangement influence the combined behaviour.
A hydrogen bond involves hydrogen attached to a donor atom interacting with an appropriate acceptor. O–H, N–H and F–H donors and suitable electron-rich acceptors are common introductory examples. Hydrogen bonds can join different molecules or occur within one molecule. The common N/O/F examples are useful patterns rather than an exhaustive definition.
Compare all the interactions in the stated setting
Water molecules can donate and accept hydrogen bonds and also have dipole interactions and dispersion. A methane molecule has no permanent molecular dipole in its symmetric structure, yet methane molecules still interact through dispersion. These cases help explain why “nonpolar” does not mean “no attraction”.
Do not assign one universal order such as dispersion always weaker than every hydrogen bond. An individual interaction and the total cohesion of a material are different questions. Compare the actual species, their shape, possible contacts, phase and conditions before explaining a property.
| Molecular case | Dispersion | Permanent-dipole interactions | Common hydrogen-bond role |
|---|---|---|---|
| CH₄, symmetric isolated molecule | Present | No permanent molecular dipole | No ordinary N/O/F donor |
| HCl, polar molecule | Present | Possible between molecules | Not an introductory O–H/N–H/F–H donor example |
| H₂O | Present | Possible between molecules | O–H donor and oxygen acceptor |
| CH₃OCH₃, dimethyl ether | Present | Possible between molecules | Oxygen acceptor; no O–H donor |
A qualitative comparison of selected molecular possibilities. Entries identify mechanisms; they are not a numerical ranking of strength or boiling point.
Worked examples
Compare a donor and an acceptor
Can dimethyl ether accept a hydrogen bond from water even though it has no O–H bond?
- Water supplies an O–H donor.
- The ether oxygen can provide an appropriate acceptor site.
- A molecule need not donate a hydrogen bond in order to accept one.
Yes. Water can donate to the ether oxygen. This identifies a possible interaction, without predicting the entire mixture’s behaviour from that one contact.
Try it yourself
During ordinary water vaporization, which changes first: the H₂O molecular identity or its neighbouring arrangement?
Show the answer and reasoning
The neighbouring arrangement.
Water molecules separate while their covalent O–H bonds normally remain intact.
Does a nonpolar molecular liquid have no intermolecular attractions?
Show the answer and reasoning
No.
Dispersion is present even without a permanent molecular dipole.
Must every hydrogen bond connect different molecules?
Show the answer and reasoning
No.
Hydrogen bonds can also occur within one molecule when donor and acceptor geometry permits.
Can an interaction label alone establish a universal boiling-point ranking?
Show the answer and reasoning
No.
Total interactions, molecular shape and conditions affect the comparison.
What to carry forward
- Keep molecular identity separate from arrangement among molecules.
- Dispersion occurs in polar and nonpolar species.
- Hydrogen bonds can be between or within molecules.
- Explain a material’s properties through the whole system.
References
Teaching resources and reference material supporting this lesson’s explanations and examples.