Intermolecular forces (IMFs) are the Coulombic attractions between molecules, including London dispersion forces, dipole-dipole interactions, and hydrogen bonding. They are weaker than the covalent bonds inside molecules, but they control boiling point, melting point, vapor pressure, and solubility.
Intermolecular forces are the attractions between molecules, not the bonds inside them. Every IMF is really the same physics, positive charge attracting negative charge, which is why Coulomb's law explains all of them. The difference is where the charges come from. London dispersion forces come from temporary, fluctuating dipoles, and they get stronger as molecules gain more electrons, bigger electron clouds, and more contact area. Dipole-dipole interactions happen between molecules with permanent dipoles. Hydrogen bonding is an especially strong dipole-dipole interaction that happens when H is bonded directly to N, O, or F.
The AP exam cares about IMFs because they bridge the particulate level and the macroscopic level. When a liquid boils, you're not breaking covalent bonds. You're completely overcoming the intermolecular attractions, which is why boiling point and vapor pressure track IMF strength so directly (EK 3.2.A.1). One important correction to a common shortcut: dispersion forces are often the strongest net IMF between large molecules, so "London forces are always the weakest" is a trap. Strength depends on the actual molecules, not just the category.
IMFs are the backbone of Unit 3. Topic 3.1 (LO 3.1.A) asks you to connect a molecule's structure to the relative strength of its IMFs, both within one substance and between two different substances. Topic 3.2 (LO 3.2.A) then asks you to use those forces to explain macroscopic properties like boiling point, melting point, and vapor pressure. Topic 3.10 (LO 3.10.A) extends the idea to solubility, where substances with similar intermolecular interactions tend to dissolve in each other. The concept reaches backward into Unit 2, where Topic 2.2 (LO 2.2.A) gives you the potential energy versus distance picture and Coulomb's law that explain why these attractions exist at all, and forward into Unit 7, where Topic 7.14 treats dissolution as a competition between solute-solute, solvent-solvent, and solute-solvent interactions. If you can argue from structure to IMF to property, you've mastered one of the most-tested reasoning chains in AP Chem.
Keep studying AP Chemistry Unit 3
London Dispersion Forces (Unit 3)
LDFs are present in every molecule because every molecule has electrons that can fluctuate into temporary dipoles. For large or highly polarizable molecules, dispersion is often the dominant force, which is why nonpolar I₂ is a solid at room temperature while nonpolar F₂ is a gas.
Coulomb's Law (Unit 2)
Every IMF is just Coulomb's law in disguise. Bigger partial charges and shorter distances between molecules mean stronger attraction. Topic 2.2's potential energy versus distance graphs apply to molecules attracting each other, not just atoms forming bonds, only with a much shallower energy well.
Boiling Point (Unit 3)
Boiling completely overcomes intermolecular attractions, so boiling point is the cleanest experimental readout of IMF strength. If two substances have similar molar masses but different boiling points, the one that boils higher has stronger IMFs, usually a permanent dipole or hydrogen bonding.
Free Energy of Dissolution (Unit 7)
Whether something dissolves comes down to an IMF trade. You break solute-solute and solvent-solvent attractions and form new solute-solvent ones. "Like dissolves like" works because similar IMFs make that trade roughly even, so entropy can drive the mixing.
IMFs show up on both multiple choice and FRQs, almost always as a compare-and-explain task. A classic MCQ setup gives you two substances with similar molar masses but different boiling points or melting points and asks for the particulate-level explanation. Similar molar mass means similar dispersion forces, so the answer hinges on a permanent dipole or hydrogen bonding. The 2018 short FRQ did exactly this with CS₂ and COS, asking you to use their structures to explain the boiling point difference (COS is polar, CS₂ is not). The 2021 FRQ on silicon compounds and the 2019 halogens FRQ also leaned on IMF reasoning. For full credit, you need the complete chain. Name the specific force, tie it to structure (polarity, electron count, contact area), and connect it to the property. Writing "stronger IMFs" without identifying which force and why rarely earns the point. Also watch the classic trap of claiming covalent bonds break during boiling. They don't.
Intramolecular forces are the chemical bonds inside a molecule, like the C-H covalent bonds in methane. Intermolecular forces are the much weaker attractions between separate molecules. When water boils, you overcome the hydrogen bonds between H₂O molecules, but the O-H covalent bonds inside each molecule stay intact. Saying a substance boils because its 'bonds break' is one of the most common point-losing mistakes on AP Chem FRQs.
Intermolecular forces are Coulombic attractions between molecules, and they come in three main types on the AP exam: London dispersion forces, dipole-dipole interactions, and hydrogen bonding.
London dispersion forces exist in all molecules and grow stronger with more electrons, larger electron clouds, and more contact area, so they are often the strongest net IMF between large molecules.
Boiling point and vapor pressure track IMF strength directly because vaporizing a substance completely overcomes its intermolecular attractions, not its covalent bonds.
Hydrogen bonding only occurs when hydrogen is bonded directly to N, O, or F, and it explains why water boils far higher than molecules of similar size.
Substances with similar intermolecular interactions tend to dissolve in each other, which is the real reasoning behind 'like dissolves like' (EK 3.10.A.1).
On FRQs, always name the specific force, link it to molecular structure, and connect it to the property; vague answers like 'stronger forces' don't earn points.
They're the attractions between molecules, covered mainly in Topic 3.1. The three types you need are London dispersion forces (in all molecules), dipole-dipole interactions (between polar molecules), and hydrogen bonding (when H is bonded to N, O, or F).
No, and the CED explicitly warns against this. Dispersion forces are often the strongest net IMF between large molecules because they scale with electron count and contact area. That's why nonpolar I₂ is a solid while polar HCl is a gas at room temperature.
Intramolecular forces are the covalent bonds inside a molecule; intermolecular forces are the weaker attractions between molecules. Boiling water overcomes hydrogen bonds between H₂O molecules but leaves every O-H covalent bond intact.
No. Boiling only overcomes the intermolecular forces between molecules, so the molecules stay fully intact in the gas phase. Claiming bonds break during boiling is one of the most common ways to lose FRQ points.
Compare IMF strength step by step. Check for hydrogen bonding first, then permanent dipoles, then dispersion strength (molar mass, electron count, contact area). On the 2018 FRQ, COS boils higher than CS₂ because COS has a permanent dipole while CS₂ does not, even though their masses are similar.
Connect this key term to the AP exam workflow: review the course, practice questions, and check related study tools.
Review units, study guides, and course resources.
Check this vocabulary in multiple-choice context.
Apply key concepts in written AP responses.
Estimate the exam score you are working toward.
Review the highest-yield facts before practice.
Put the full course together before test day.