How does a pure substance differ from a mixture?

A pure substance is made up of only one type of particle — either a single element or a single compound. It has a fixed composition and definite properties like boiling point, melting point, and density. For example, pure water (H₂O) always contains hydrogen and oxygen in a 2:1 ratio, and gold (Au) is made entirely of gold atoms.

A mixture, on the other hand, contains two or more substances combined physically, not chemically. The components of a mixture can vary in proportion and can be separated by simple physical methods such as filtration, evaporation, or distillation. Examples include air (a mix of gases) and saltwater (salt + water).

In simple terms, a pure substance is uniform and consistent, while a mixture can change in composition.

• Pure substance: Fixed composition, cannot be separated by physical means.
• Mixture: Variable composition, can be separated by physical means.

Thus, the main difference lies in uniformity - pure substances are chemically identical, whereas mixtures are combinations of different materials that keep their own properties.