Air is a mixture. So is seawater, granite, and a bag of trail mix. A mixture is a combination of substances that are not chemically bonded. Each substance retains its own identity. The proportions can vary. The components can usually be separated by physical means, like filtration, distillation, or evaporation.

Mixtures are either homogeneous or heterogeneous. A homogeneous mixture, also called a solution, has the same composition throughout. Saltwater and air are homogeneous. A heterogeneous mixture has visibly different parts. Sand and iron filings, oil and water, and chocolate chip cookie dough are heterogeneous. The distinction depends on the scale of observation. Milk looks homogeneous but is actually a colloid, with fat droplets dispersed in water.

Mixtures are everywhere in chemistry and daily life. Alloys are mixtures of metals. Fuels are mixtures of hydrocarbons. Foods are complex mixtures of proteins, fats, carbohydrates, and water. Pharmaceuticals are mixtures of active ingredients and excipients. Separating mixtures is a major part of chemical engineering. Distillation, extraction, chromatography, and filtration are all separation techniques.

The key difference between a mixture and a compound is that a compound has a fixed composition and new properties. A mixture has variable composition and retains the properties of its components. Saltwater can be diluted or concentrated. Water is always H2O.