Composition of Mixtures (College Board AP® Chemistry)

Composition of Mixtures

Classification of matter

• Matter is classified in 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) or hydrogen gas (H₂), sodium chloride (NaCl) or calcium carbonate (CaCO₃) are pure substances

• A mixture is matter whose composition is not fixed. Therefore, it contains two or more types of atoms, molecules or formula units

• The molecules or formula units that are part of a mixture are called components

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

  • g. Water and oil is a mixture between two liquids

Types of Mixtures

• Homogeneous mixtures have the same composition throughout

  • The composition is uniform because the components are mixed uniformly in the space

  • g. Air is an homogenous mixture

• Heterogeneous mixtures varies from one region of the mixture to another

  • The composition is made from distinct regions because the components separate

  • g. Water and oil is an heterogeneous mixture

Molecules in a pure substance and mixtures

A pure substance, an homogeneous mixture and a heterogeneous mixture at the molecular level