Everything around us — air, water, metals, food — is made of matter. Chemists classify matter into pure substances (elements and compounds) and mixtures (homogeneous and heterogeneous). Understanding these categories is the foundation for all further chemistry.
9.1 Classifying Matter
Definition
Pure substance
A pure substance is a substance with a fixed (constant) chemical composition throughout. A pure substance is either an element or a compound.
Definition
Element
An element is a pure substance that cannot be broken down into simpler substances by chemical means. All atoms in an element are of the same type (same atomic number).
Definition
Compound
A compound is a pure substance formed when two or more elements are chemically bonded together in a fixed ratio by mass. A compound has properties different from those of the elements from which it is made.
Definition
Mixture
A mixture is a combination of two or more substances that are not chemically bonded. The components retain their individual properties and can be separated by physical means.
Definition
Homogeneous mixture
A homogeneous mixture has a uniform composition throughout — you cannot distinguish the individual components by sight. Examples: salt water (saline solution), air, alloys such as bronze.
Definition
Heterogeneous mixture
A heterogeneous mixture has a non-uniform composition — the individual components can be distinguished. Examples: sand and iron filings, oil and water, granite.
Think of matter as a family tree. At the top level, everything is either a pure substance or a mixture. Pure substances branch into elements (e.g. copper, oxygen, gold — each made of only one kind of atom) and compounds (e.g. water H₂O, table salt NaCl, carbon dioxide CO₂ — made of two or more kinds of atoms bonded in fixed ratios). Mixtures branch into homogeneous (uniform, cannot see separate parts) and heterogeneous (non-uniform, separate parts visible or detectable). The key test for a pure substance is a sharp, fixed melting/boiling point; a mixture melts or boils over a range of temperatures.
Pure Substances vs Mixtures
| Property | Pure Substance | Mixture |
|---|---|---|
| Composition | Fixed/constant | Variable |
| Separation | Chemical change required to separate | Physical methods (filtration, distillation, etc.) |
| Properties | Definite, sharp boiling/melting point | Range of boiling/melting temperatures |
| Examples | H₂O, NaCl, O₂, Fe | Air, seawater, alloys, blood |
Practice Question
Classify each of the following as element, compound or mixture, giving a reason for each: (a) copper (b) water (c) sea water (d) carbon dioxide.
(8 marks)
Note
Air is a homogeneous mixture of N₂ (≈78%), O₂ (≈21%), Ar (≈0,9%) and CO₂ (≈0,04%). Because it is a mixture, it has no single boiling point — each component liquefies at a different temperature. Liquid air is used in industry to separate these gases by fractional distillation.