What it covers: The largest unit on the exam — intermolecular forces, states of matter, gas laws, solutions, solubility, and spectroscopy.
Exam weight: About 18–22% of the AP Chemistry exam — nearly a quarter of all questions.
The big question: How does the strength and type of intermolecular force between particles determine the physical properties and behavior of a substance?
CED topics covered: 3.1–3.13, from intermolecular and interparticle forces through the Beer-Lambert law.
Key topics at a glance
Intermolecular Forces
London dispersion (all molecules) < dipole-dipole (polar molecules) < hydrogen bonding (N–H, O–H, F–H). Stronger IMF = higher boiling point.
Properties of Solids
Ionic, metallic, covalent network, molecular solid types — each with distinct melting points and conductivity based on bonding/IMF type.
States of Matter
Vapor pressure, viscosity, surface tension — all inversely or directly tied to IMF strength. Phase diagrams show solid/liquid/gas regions.
Ideal Gas Law
PV = nRT. Use to solve for any one variable given the other three.
Kinetic Molecular Theory
Gas particles: negligible volume, no IMFs, constant random motion. Average KE ∝ absolute temperature (K).
Real Gas Deviation
Deviates most at high pressure (molecular volume matters) and low temperature (IMFs matter).
Solutions & Solubility
Molarity = mol solute / L solution. "Like dissolves like" — match solute and solvent polarity.
Spectroscopy
Beer-Lambert law: A = εlc. Absorbance is directly proportional to concentration — the basis for finding unknown concentrations.
The key terms you must know
London dispersion force — temporary dipole attraction present in ALL molecules; the only IMF in nonpolar substances.
Dipole-dipole force / hydrogen bonding — stronger IMFs found only in polar molecules (hydrogen bonding requires N–H, O–H, or F–H).
Vapor pressure, viscosity, surface tension — macroscopic properties directly explained by IMF strength.
Ideal gas law (PV = nRT) — the core equation relating pressure, volume, moles, and temperature.
Kinetic molecular theory — the model behind ideal gas behavior; average KE is proportional to temperature in Kelvin.
Real gas deviation — occurs at high pressure (volume) and low temperature (IMFs).
Molarity — mol solute per liter of solution; the standard concentration unit.
Beer-Lambert law (A = εlc) — absorbance is proportional to concentration.
Phase diagram — pressure vs. temperature graph showing solid/liquid/gas regions, triple point, and critical point.
Key themes to remember
IMFs are everything in this unit. Nearly every property — boiling point, viscosity, solubility, vapor pressure — traces back to IMF type and strength.
The structure-property chain runs from Unit 2 into Unit 3. Molecular shape and polarity (Unit 2) determine which IMFs apply (Unit 3), which determine physical properties.
"Ideal" gas behavior is a useful fiction. Real gases only deviate when molecular volume or IMFs actually start to matter — high pressure and low temperature.
Dissolving is a competition between IMFs. A solute dissolves only if new solute-solvent IMFs are strong enough to replace the ones being broken.
Spectroscopy connects light to concentration. The Beer-Lambert law is the mathematical bridge between an absorbance reading and an actual concentration value.
Common exam traps
Every molecule has London dispersion forces — even polar ones. Don't say a polar molecule has "no London dispersion forces" just because it also has dipole-dipole forces.
Hydrogen bonding requires a specific setup. It's not "any molecule with hydrogen" — it requires H bonded directly to N, O, or F, attracted to a lone pair on a neighboring N, O, or F.
Higher molar mass usually (not always) means stronger London dispersion. Molecular shape and surface area matter too — long, skinny molecules have more surface contact than compact, round ones of similar mass.
Real gas deviation isn't random — name the cause. High pressure → molecular volume becomes significant. Low temperature → intermolecular attraction becomes significant. Always specify which.
Don't confuse molarity with molality or mass percent. Molarity is specifically moles of solute per liter of solution — know the units cold.