Final answer:
A pure substance has a constant composition and properties, while a mixture is a physical combination of two or more substances. Mixtures can be homogeneous or heterogeneous, depending on the uniformity of their composition. Examples include saltwater as a mixture and pure salt as a pure substance.
Step-by-step explanation:
A pure substance is a form of matter that has a constant composition and properties that are constant throughout the sample. It can be a single element or a single compound. Mixtures, on the other hand, are physical combinations of two or more elements and/or compounds. Mixtures can be classified as homogeneous or heterogeneous based on the uniformity of their composition. In a homogeneous mixture, the composition is the same throughout, while in a heterogeneous mixture, the composition varies.
For example, let's consider saltwater. Saltwater is a mixture because it contains two substances: salt (sodium chloride) and water. The salt and water can be separated by evaporation, as the salt will remain as a solid while the water evaporates. In contrast, pure salt (a pure substance) will always have the same composition and properties, regardless of where it is obtained from.