Final answer:
Compounds consist of two or more types of atoms bonded together with fixed composition and unique properties, while mixtures are physical combinations of substances with variable compositions.
Step-by-step explanation:
Compounds and mixtures differ significantly in their composition. Compounds have a fixed composition and exhibit distinct chemical and physical properties that are different from the elements that compose them. For example, sodium chloride or table salt is made from sodium, a soft shiny metal, and chlorine, a green gas, but salt itself is a white crystalline solid with neither of the original elements' properties.
Mixtures, in contrast, are physical combinations of two or more substances that retain their individual properties. Mixtures can be heterogeneous, where the composition may vary from point to point, such as in mixed nuts or vegetable soup, or homogeneous, where the composition is uniform, like in salt dissolved in water.
Pure substances, which include both elements and compounds, have a constant composition throughout. Essentially, while mixtures can have varied compositions, compounds and elements as pure substances do not. This fundamental distinction is important in the study of chemistry and the classification of matter.