AP Chem · Unit 3 Progress Check

AP Chemistry Unit 3 Progress Check Intermolecular Forces and Properties walkthrough.

AP Chemistry Unit 3 covers intermolecular forces and the properties they explain, and the Unit 3 Progress Check MCQ tests whether you can connect structure to behaviour. This walkthrough explains what the questions ask, the traps, and how to reason from molecules to properties. Explanations only — no AP Classroom answer keys.

Updated August 2026Written by Mahmudul HasanFree · No signup

What Unit 3 covers

Unit 3 is Intermolecular Forces and Properties. Intermolecular forces and properties is roughly 18–22% of the AP Chemistry exam — one of the heaviest units.

Types of intermolecular force

London dispersion, dipole-dipole, and hydrogen bonding, in order of typical strength.

Structure to property

How IMFs explain boiling point, viscosity, surface tension, and vapour pressure.

States of matter

Solids, liquids, gases, and the particle-level differences between them.

Gas laws

The ideal gas law, partial pressures, and where real gases deviate.

Solutions and concentration

Molarity, dissolution, and “like dissolves like”.

Spectroscopy and Beer’s law

Absorbance, concentration, and photon energy.

The Progress Check MCQ: what each question type tests

These are the question patterns that recur on this Progress Check, and what each one is really asking.

Rank the boiling points

Identify the strongest IMF present in each substance, then compare. Size matters within the same force type.

Identify the dominant force

Look for O–H, N–H, or F–H bonds for hydrogen bonding; check molecular polarity otherwise.

Gas law reasoning

Often conceptual: what happens to pressure if temperature rises at fixed volume. Reason from the relationship.

Solubility prediction

Polar dissolves polar. Justify with the specific interaction, not the slogan.

Beer’s law graphs

Absorbance is proportional to concentration — read the line, do not compute.

The Progress Check FRQ

Unit 3 free response typically asks you to explain a physical property using particle-level reasoning. Points come from naming the specific intermolecular force, comparing it correctly between substances, and linking it explicitly to the property in question — “stronger hydrogen bonding requires more energy to separate the molecules, so the boiling point is higher.”

Where students lose the most points

Confusing intermolecular with intramolecular

Boiling breaks forces between molecules, not the covalent bonds inside them. This is the classic Unit 3 trap.

Forgetting London forces exist in everything

All molecules have dispersion forces; larger, more polarisable molecules have stronger ones.

Calling any H atom a hydrogen bond

Hydrogen bonding requires H attached to N, O, or F.

Explaining a property without a mechanism

Saying “it has stronger IMFs” is only half the answer — say why that raises the boiling point.

Ignoring molecular size

Between two nonpolar molecules, the bigger one usually boils higher.

How to work through this unit

Every Unit 3 explanation should follow one chain: structure → dominant intermolecular force → energy needed to separate particles → the property. Skip a link and you lose the point.

Once you have finished the Progress Check, put your raw score into our AP Chemistry Calculator to see roughly where that pace puts you on the 1–5 scale, and use the AP Chemistry Review for the full exam format and study plan.

Frequently asked questions

Quick answers — written by humans, not a chatbot.

What does the AP Chemistry Unit 3 Progress Check cover?

Intermolecular forces — London dispersion, dipole-dipole, and hydrogen bonding — and the properties they explain, plus states of matter, gas laws, solutions, and Beer’s law.

What is the most common Unit 3 mistake?

Confusing intermolecular forces with covalent bonds. Boiling separates molecules from each other; it does not break the bonds within them.

How do I rank boiling points?

Identify the strongest intermolecular force in each substance, compare those, and use molecular size as the tiebreaker when the force type is the same.

Do you publish AP Chemistry Unit 3 answers?

No. We publish walkthroughs of the reasoning and the traps, never raw AP Classroom answer keys.