Mixtures vs. Compounds — What's the Difference?

Mixtures and compounds both consist of two or more elements or compounds, but mixtures are formed by physical blending without chemical bonding. Each substance in a mixture retains its own chemical properties and can be separated by physical means. Compounds, however, result from a chemical reaction where elements bond in a fixed, definite ratio, and the resulting compound has properties different from its constituent elements.

In a mixture, components can be present in any proportion and can be homogeneous or heterogeneous, like salt mixed with pepper or air, which is a homogeneous mixture of gases. Compounds, such as water (H2O), always have a homogeneous composition with elements present in a fixed ratio and cannot be separated into their elements by physical means, but rather require chemical reactions.

The properties of a mixture reflect the individual properties of its components. For example, a mixture of iron filings and sulfur can exhibit both the magnetic properties of iron and the yellow color of sulfur. In a compound, properties are unique and distinct from the properties of the elements that constitute it; sodium chloride (table salt) is neither metallic like sodium nor poisonous like chlorine gas.

Mixtures can be made and unmade through physical processes without a change in energy states. Mixing oil and water creates a mixture that can be separated by decanting. Conversely, compounds require a chemical reaction involving a change in energy; forming water releases energy, and breaking water into hydrogen and oxygen requires energy input.

The formation of mixtures is typically reversible and non-reactive, such as mixing alcohol and water, which can be separated again by distillation. The formation and decomposition of compounds, like the rusting of iron (Fe2O3), involve reactions that are often irreversible and result in new chemical structures, which necessitates a change in the identity of the elements involved.