Unit 3: Properties of Substances and Mixtures
Test AP Chemistry Unit 3 — intermolecular forces, states of matter, solubility, and colligative properties. Practice connecting IMFs to physical properties.
What Unit 3 Covers in AP Chemistry
Unit 3 connects molecular structure to bulk physical properties. Understanding why substances have particular melting points, solubilities, or vapor pressures requires understanding the intermolecular forces at work — a skill AP Chemistry tests extensively in both MCQ and FRQ.
Core Topics in Unit 3
- Intermolecular forces (IMFs) — London dispersion forces, dipole-dipole interactions, and hydrogen bonding; identifying the dominant IMF in a substance and ranking compounds by relative strength of IMFs
- States of matter — Solid, liquid, and gas phase properties; phase diagrams including triple point and critical point; heating and cooling curves
- Solutions and solubility — The 'like dissolves like' principle, factors affecting solubility, Henry's law for gas solubility, and the dissolution process at the particulate level
- Colligative properties — Boiling point elevation, freezing point depression, osmotic pressure, and vapor pressure lowering; calculations using molality and the van't Hoff factor
Connecting IMFs to Physical Properties
The central AP Chemistry skill in Unit 3 is using IMF strength to explain physical properties. Stronger IMFs lead to higher boiling points, greater viscosity, lower vapor pressure, and higher surface tension. Practice ranking a series of molecules by boiling point using IMF analysis — this is a classic AP FRQ and MCQ task.
Key AP FRQ Patterns in Unit 3
- Comparing two substances and justifying which has the higher boiling point by identifying and ranking their IMFs
- Explaining why a solute dissolves or does not dissolve in a given solvent at the particulate level
- Calculating the freezing point depression or boiling point elevation of a solution given molality and van't Hoff factor
Common Mistakes in Unit 3
- Overlooking London dispersion forces — even polar molecules experience LDF, and in large nonpolar molecules LDF can dominate
- Forgetting that hydrogen bonding requires H bonded directly to N, O, or F — not just any polar bond involving H
- Using molarity instead of molality in colligative property calculations
- Ignoring the van't Hoff factor for ionic solutes when calculating colligative properties