In chemistry, mixtures are sometimes called homogeneous or heterogeneous. The difference between them is the extent to which, and how uniformly, their different components are mixed together. For example, if you have a bowl of mixed nuts in front of you, you can see clearly that it is made up of different parts, but look at a bottle of white vinegar, and all you see is a colorless liquid.

Most natural substances, and just about anything you can think of, is a mixture, including air, water, soil, orange juice and milk. A mixture is simply a combination of two or more substances that are not chemically united and do not exist in fixed proportions to each other. A mixture can be split into pure compounds or elements. A mixture may have changeable physical properties; for example, a mixture of water and alcohol boils over a range of temperatures.

Homogeneous mixtures, more commonly called solutions, have the same uniform appearance and composition throughout (the prefix "homo" means the same). Solutions consist of particles as small as atoms or molecules; in other words, too small to be visible to the eye. It's impossible to pick out components of a homogeneous mixture. For example, a sugar solution and white vinegar are homogeneous because only colorless liquids can be seen. Homogeneous mixtures only have one phase (state of matter): gas, liquid or solid. This means you will never observe both a gas and a liquid or a liquid and a solid in a homogeneous mixture. Other homogenous mixtures are air, rainwater and vodka.

Heterogenous mixtures are made up of visibly different substances or phases (the prefix "hetero" means different). A suspension is a type of heterogeneous mixture with large particles, visible to the eye. For example, a mixture of sand and water is a suspension because you can see the sand particles in the water. Likewise, salad dressing made of oil and vinegar is a suspension because you can see two liquid layers. Other heterogeneous mixtures are clouds in air, cereal in milk, blood (while blood might look homogeneous at first, on a microscopic level, it's heterogeneous), mixed nuts, pizza and pasta in sauce.

In most cases, it is possible to physically separate components of a heterogeneous mixture, but not a homogeneous mixture. For example, you can remove cereal from milk and pasta from sauce. If you are not sure whether a mixture is homogeneous or heterogeneous, consider its sample size. Some heterogeneous mixtures can appear homogeneous from a distance, such as sand on a beach. If the composition of a mixture appears uniform no matter where you sample it, is is homogeneous; sand on a beach is heterogeneous because when you look at it up close, you can identify different types of particles, such as sand, shells and organic matter.