The amounts of substances in a mixture can vary. Students should understand that mixtures can change in appearance but this does not mean that any substances have changed in their chemical composition. Mixtures can be combinations of elements or compounds. Most substances found in nature are mixtures. A pure element or a pure compound is rarely found.
Air is a mixture of gases, milk is a mixture of solids and liquids, alloys are mixtures of solids.
Mixtures that do not appear to be distributed the same throughout are said to be heterogeneous, and those that are the same throughout are called homogeneous. Solutions are common types of homogeneous mixtures. Sugar and water form a solution when mixed. The sugar becomes evenly distributed throughout the solution, so that one portion is not sweeter than another. If more sugar is added to the solution, the entire solution becomes sweeter and we say that it is more concentrated.