An element is a pure substance that cannot be broken down into simpler substances by chemical means. A compound is a pure substance that is made up of two or more elements chemically combined in fixed proportions. A homogeneous mixture is a mixture in which the components are evenly distributed throughout the mixture. A heterogeneous mixture is a mixture in which the components are not evenly distributed throughout the mixture.

Milk is a heterogeneous mixture because it contains a variety of components, including water, proteins, fats, carbohydrates, and minerals. These components are not evenly distributed throughout the milk, as the fat tends to rise to the top.