A pure substance is a substance that has a definite and constant composition. It cannot be separated into simpler substances by physical means. Examples of pure substances include water, sugar, and salt.

A mixture is a combination of two or more substances that do not have a definite and constant composition. The composition of a mixture can vary depending on the proportions of the substances present. Examples of mixtures include air, seawater, and gasoline.

When two or more pure substances are mixed together, the resulting substance is usually a mixture. However, in some cases, the substances may react with each other to form a new pure substance. For example, when hydrogen and oxygen are mixed together and ignited, they react to form water.