Composition of Mixtures (College Board AP® Chemistry): Study Guide

Composition of mixtures

Classification of matter

• Matter is classified into pure substances and mixtures

• A pure substance is matter that has a fixed composition

  • They contain atoms, molecules or formula units of a single type

  • Aluminum (Al), water (H₂O), hydrogen gas (H₂), sodium chloride (NaCl) and calcium carbonate (CaCO₃) are pure substances

• A mixture contains atoms, molecules, or formula units of two or more types, whose relative proportions can vary

• The different substances in a mixture (atoms, molecules, or formula units) are called its components

  • E.g. the air we breathe is a mixture in which the components are oxygen, nitrogen and small amounts of other gases

  • E.g. water and oil form a mixture of two liquids

Types of mixtures

• Homogeneous mixtures have the same composition throughout

  • The components are spread evenly through the sample, so every part has the same composition

  • E.g. air is a homogeneous mixture

• Heterogeneous mixtures have a composition that varies from one region of the mixture to another

  • The components stay in separate regions, so the composition differs from one part of the sample to another

  • E.g. water and oil form a heterogeneous mixture

Molecules in a pure substance and mixtures

• In a particle diagram, a pure substance shows one kind of particle; a homogeneous mixture shows two or more kinds spread evenly; a heterogeneous mixture shows the kinds in separate regions

Particles show the difference: one type in a pure substance, two types evenly mixed in a homogeneous mixture, and two types in separate regions in a heterogeneous mixture

Composition by mass

• A pure compound always has the same mass percent of each element (law of definite proportions)

• In a mixture, the mass percent of each element depends on how much of each component is present, so it can vary from sample to sample

• So mass data can show whether a sample is pure, and how much of each component it contains (see the Elemental Analysis revision note)