Mistake Master
Intermolecular and interparticle forces
Boiling water breaks the forces between molecules, not the bonds inside them. Those intermolecular forces come in a strength ladder, and where a substance sits on it decides how it melts, boils, and dissolves.
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The forces between molecules.
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Intermolecular forces (IMFs) are attractions between molecules, far weaker than the bonds within them. They set physical properties: melting and boiling points, viscosity, and solubility.
They form a strength ladder. London dispersion forces (LDF) act between all particles and grow with size; dipole-dipole forces add for polar molecules; hydrogen bonding is a strong special case when H is bonded to N, O, or F; and ion-dipole forces (ions in a polar solvent) are stronger still.
Every molecule has dispersion forces. Polar molecules have those plus dipole-dipole; molecules with H–N, H–O, or H–F add hydrogen bonding. Stronger IMFs mean more energy to separate the molecules, so higher boiling points.
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Identifying and ranking IMFs.
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Find every force a molecule has, then rank by the strongest present.
- Every substance has dispersion. London dispersion is always present; for nonpolar molecules it is the only IMF, and it grows with molar mass.
- Add dipole-dipole if polar. A polar molecule (net dipole) also has dipole-dipole attractions on top of dispersion.
- Check for hydrogen bonding. If hydrogen is bonded directly to N, O, or F, the molecule can hydrogen-bond — a strong dipole-dipole special case.
- Rank by the strongest force. Compare substances by their strongest IMF; that dominates the boiling point (for similar sizes).
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The pieces you'll meet.
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Know the ladder and what triggers each rung.
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Worked example: why does water boil so much higher than methane?
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Question. Methane (CH₄) boils at −162 °C, water (H₂O) at 100 °C, despite similar molar masses. Why the huge gap?
Methane. It is nonpolar, so its only IMF is weak London dispersion. Little energy separates the molecules, so it boils very low.
Water. It is polar and has H bonded to O, so it hydrogen-bonds — a strong IMF. Pulling water molecules apart takes far more energy, so it boils far higher.
Key point. Boiling breaks the IMFs between molecules, not the O–H bonds within them. The difference in boiling points is a difference in intermolecular forces, not in bond strength.
§5
Mistakes that cost real points.
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"Boiling a substance breaks its chemical bonds."
Boiling separates molecules by overcoming intermolecular forces; it does not break the covalent bonds inside the molecules. Steam is still H₂O molecules, with their O–H bonds intact — only the attractions between molecules have been overcome.
Fix. Read a phase change as overcoming IMFs, not breaking bonds. The molecules survive; only their attractions to each other are broken.
"Only polar molecules have intermolecular forces."
Every substance has London dispersion forces, including nonpolar ones — that is why even noble gases and nonpolar molecules can be liquefied. Polar molecules simply have additional (dipole-dipole, maybe hydrogen-bonding) forces.
Fix. Start every analysis with dispersion (always present), then add dipole-dipole and hydrogen bonding where the structure warrants.
"Any molecule containing hydrogen can hydrogen-bond."
Hydrogen bonding requires H bonded directly to N, O, or F. A C–H bond does not hydrogen-bond, so methane and other hydrocarbons cannot, despite being full of hydrogen.
Fix. Check what the hydrogen is attached to. Only H–N, H–O, and H–F enable hydrogen bonding.
§6
Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.