Pure Substances and Chemical Compounds

Study Notes

A pure substance has distinct properties and a composition that does not vary from sample to sample.
Examples of pure substances include water and table salt (sodium chloride).

Elements are substances that cannot be decomposed into simpler substances.
Each element is composed of only one kind of atom.
Compounds are substances composed of two or more elements; they contain two or more kinds of atoms.
Examples of compounds include water (composed of hydrogen and oxygen).

Mixtures are combinations of two or more substances in which each substance retains its chemical identity.
Mixtures do not have the same composition, properties, and appearance throughout.
Heterogeneous mixtures vary in texture and appearance in any typical sample (e.g., rocks and wood).
Homogeneous mixtures are uniform throughout (e.g., air, salt, and sugar dissolved in water).
Air is a homogeneous mixture of nitrogen, oxygen, and smaller amounts of other gases.

Physical properties can be observed without changing the identity and composition of the substance.
Examples of physical properties include color, odor, density, melting point, boiling point, and hardness.
Chemical properties describe the way a substance may change, or react, to form other substances.
Intensive properties do not depend on the amount of sample being examined (e.g., temperature and melting point).
Extensive properties depend on the amount of sample (e.g., mass and volume).

Physical change: a substance changes its physical appearance but not its composition (e.g., evaporation of water).
Chemical change (or chemical reaction): a substance is transformed into a chemically different substance (e.g., hydrogen burning in air).
Examples of physical changes include changes of state (e.g., from liquid to gas or from liquid to solid).
Examples of chemical changes include combustion reactions (e.g., burning wood) and decomposition reactions (e.g., rotting banana).

Distillation is an important method of separating the components of a homogeneous mixture.
Distillation depends on the different abilities of substances to form gases.