Reviewed on June 11, 2026. The classification of matter helps chemists describe what a sample contains, whether its composition is fixed, and how its components can be separated. The central division is between pure substances and mixtures.
Short answer: A pure substance has a constant composition and characteristic properties. A mixture contains two or more substances physically combined in variable proportions.
What is matter?
Matter is anything that has mass and occupies space. Air, water, a steel spoon, soil, and the cells in the body are all matter. Light is not classified as matter because it does not have rest mass and does not occupy space in the same way.
The classification of matter chart
- Matter
- Pure substances
- Elements
- Compounds
- Mixtures
- Homogeneous mixtures
- Heterogeneous mixtures
- Pure substances
This chart classifies a sample by composition. States of matter such as solid, liquid, and gas are a different classification based on physical form.
Pure substances
A pure substance has a definite composition. Every sample has the same basic chemical identity and characteristic properties, such as density or melting point, under specified conditions.
Elements
An element contains only one kind of atom. It cannot be broken down into simpler substances by ordinary chemical reactions. Examples include copper, oxygen, carbon, and gold. An element may contain single atoms, as in helium, or bonded atoms of the same element, as in oxygen gas, O2.
Compounds
A compound contains atoms of two or more elements chemically bonded in a fixed ratio. Water is H2O, so its hydrogen-to-oxygen ratio is fixed. Sodium chloride contains sodium and chlorine ions in a definite relationship.
A compound has properties different from the elements that form it. Sodium metal is highly reactive and chlorine gas is toxic, yet sodium chloride is ordinary table salt. Separating a compound into simpler substances requires a chemical change.
Mixtures
A mixture contains substances physically combined without a fixed chemical ratio. Each component retains its chemical identity, although the mixture may have properties that depend on composition.
Homogeneous mixtures
A homogeneous mixture has a uniform composition at the scale being examined. A sample taken from one part has the same composition as a sample from another part. Solutions are homogeneous mixtures.
Examples include saltwater, air, vinegar, and many metal alloys. Saltwater may look like a pure liquid, but its salt concentration can vary, so it is a mixture.
Heterogeneous mixtures
A heterogeneous mixture is not uniform throughout. Different regions or phases can be observed directly or with suitable instruments. Examples include granite, soil, oil and water, cereal in milk, and concrete.
A heterogeneous mixture does not have to contain large visible pieces. Milk appears uniform to the eye but is a colloid with dispersed droplets, so its classification depends on the level of observation and the scientific context.
Pure substance versus homogeneous mixture
These can look similar, but composition separates them:
| Feature | Pure substance | Homogeneous mixture |
|---|---|---|
| Composition | Fixed | Variable within a uniform range |
| Components | One element or compound | Two or more substances |
| Separation | Compound needs chemical change | Physical methods can separate components |
| Example | Distilled water | Saltwater |
How mixtures are separated
The correct method depends on a physical-property difference:
- Filtration: separates an insoluble solid from a fluid.
- Distillation: separates substances with different boiling behavior.
- Evaporation or crystallization: recovers a dissolved solid.
- Chromatography: separates components by how they move between phases.
- Decanting or a separating funnel: separates phases with different densities.
- Magnetic separation: removes a magnetic material from a nonmagnetic mixture.
Physical and chemical properties
A physical property can be observed without changing chemical identity. Color, density, melting point, electrical conductivity, and state are physical properties. A chemical property describes how a substance can change into another substance, such as flammability or reactivity with acid.
Melting ice is a physical change because the substance remains water. Rusting iron is a chemical change because new iron oxide substances form.
Worked examples
- Air: a homogeneous mixture of gases whose proportions can vary.
- Carbon dioxide: a pure compound with the formula CO2.
- Copper wire: pure copper is an element, although commercial wire may contain impurities.
- Muddy water: a heterogeneous mixture that can partly separate on standing or by filtration.
- Brass: a homogeneous solid mixture, or alloy, mainly of copper and zinc.
Common questions
Is water an element, compound, or mixture?
Pure water is a compound. Tap water and seawater are mixtures because they contain dissolved substances.
Is milk homogeneous or heterogeneous?
Milk looks uniform at ordinary scale, but scientifically it is a colloid and is usually classified as a heterogeneous mixture.
Can a mixture have a chemical formula?
No single formula describes a mixture because its proportions can vary. Individual components can have formulas.
