AP Chemistry Unit 3: Properties of Substances and Mixtures Notes

Introduction

The arrangement, motion, and attraction of particles explain observable phase, pressure, solubility, conductivity, and separation behavior. Good comparisons control particle identity, temperature, pressure, and composition before assigning a cause.

Ideal-gas and solution equations turn particle models into quantitative predictions. Their conditions matter: kelvin temperature, conserved solute amount, a declared solution volume, and the absence—or explicit treatment—of nonideal interactions.

Spectroscopy links electromagnetic energy to quantized transitions. Beer–Lambert analysis then connects absorbance to concentration only inside a matched, calibrated linear range.

Current exam guidance: Aligned to the current 2026–27 AP Chemistry course framework and exam scope.

3.1

Intermolecular and Interparticle Forces

How does particle structure determine the attractions between separate particles?

Short answer

Charge, polarity, polarizability, distance, and orientation determine the available attractions; compare structures before using a force label to predict a property.

Know firstBond polarity · Molecular geometry · Electron-cloud size

Separate bonds from attractions between particles

London dispersion attractions arise from correlated fluctuations in electron density and occur between all particles. They generally become stronger as an electron cloud becomes more polarizable and as molecular shape permits more effective contact. Permanent polar molecules also experience orientation-dependent dipole–dipole attraction.

Hydrogen bonding is a particularly strong, directional intermolecular attraction in the AP model when H is covalently bonded to N, O, or F and interacts with a lone pair on another particle. It is not an extra covalent bond. Ions can attract polar molecules through ion–dipole forces, whose orientation reverses when the ion charge reverses.

Property comparisons must control competing variables. A higher boiling point can indicate stronger attractions, but molar mass, shape, symmetry, and the set of forces all matter. Identify every force present, then decide which structural difference best explains the observed difference.

Chemistry model

Interparticle forces connect structure to properties

Change the interaction model and compare its physical cause, molecular orientation, attraction, and likely property effect.

Interaction modelLondon dispersion

instantaneousdipole

correlated fluctuation

induceddipole

Interaction: London dispersion
Physical cause: temporary, correlated electron-cloud fluctuations
Use it to predict: present in every particle; grows with polarizability and contact area
Important limit: Do not rank substances from the interaction name alone; particle size, shape, and the full set of attractions matter.

stronger attractions require more energy to separate particles

Read this figure without the graphic

Interparticle forces connect structure to properties. The figure connects London dispersion, dipole–dipole, hydrogen bonding so the relationship can be read without relying on color alone.

Interaction	Cause	Useful prediction
London dispersion	correlated temporary dipoles	grows with polarizability and contact area
Dipole–dipole	attraction between permanent dipoles	orientation influences average attraction
Hydrogen bonding	H bonded to N, O, or F interacts with a lone pair	strong directional interaction when structural requirements are met

Force inventory

Interaction	Required feature	Direction cue	Common error
Dispersion	polarizable electron clouds	instantaneous correlated density	claiming nonpolar means no attraction
Dipole–dipole	permanent molecular dipoles	$\delta^+$ toward $\delta^-$	ignoring orientation
Hydrogen bond	H–N/O/F and a partner lone pair	directional H interaction	drawing another covalent bond
Ion–dipole	ion and polar molecule	dipole turns with ion sign	forgetting charge sign

Formula

Ion-pair scaling as a comparison

|U|\propto\frac{|q_1q_2|}{r}

Greater charge magnitude and smaller separation strengthen an electrostatic attraction in a comparable environment.

Use when: Use qualitatively; molecular orientation, medium, and finite electron density also matter.
Do not assume: The relation does not rank dispersion forces from molar mass alone.
Variables: $q_1,q_2$ : interacting charges (charge) · $r$ : separation (distance)
Common misuse: Calling an intermolecular attraction a broken intramolecular bond.

Keep these three ideas

All particles have dispersion attractions.
Permanent dipoles add orientation-dependent forces.
Control mass, shape, and structure before attributing a property difference.

Properties of Substances and Mixtures