Mixture Definition and Examples in Science

What a mixture is (and is not)

A mixture contains two or more substances which retain their chemical identity.
A mixture contains two or more substances which retain their chemical identity. antonioiacobelli, Getty Images

In chemistry, a mixture forms when two or more substances are combined such that each substance retains its own chemical identity. Chemical bonds between the components are neither broken nor formed. Note that even though the chemical properties of the components haven't changed, a mixture may exhibit new physical properties, like boiling point and melting point. For example, mixing together water and alcohol produces a mixture that has a higher boiling point and lower melting point than alcohol (lower boiling point and higher boiling point than water).

Examples of Mixtures

  • Flour and sugar may be combined to form a mixture.
  • Sugar and water form a mixture.
  • Marbles and salt may be combined to form a mixture.
  • Smoke is a mixture of solid particles and gases.

Types of Mixtures

Two broad categories of mixtures are heterogeneous and homogeneous mixtures. Heterogeneous mixtures are not uniform throughout the composition (e.g. gravel), while homogeneous mixtures have the same phase and composition, no matter where you sample them (e.g., air). The distinction between heterogeneous and homogeneous mixtures is a matter of magnification or scale. For example, even air can appear to be heterogenous if your sample only contains a few molecules, while a bag of mixed vegetables may appear homogeneous if your sample is an entire truckload full of them. Also note, even if a sample consists of a single element, it may form a heterogeneous mixture. One example would be a mixture of pencil lead and diamonds (both carbon).

Another example could be a mixture of gold powder and nuggets.

Besides being classified as heterogeneous or homogeneous, mixtures may also be described according to the particle size of the components:

Solution - A chemical solution contains very small particle sizes (less than 1 nanometer in diameter).

A solution is physically stable and the components cannot be separated by decanting or centrifuging the sample. Examples of solutions include air (gas), dissolved oxygen in water (liquid), and mercury in gold amalgam (solid), opal (solid), and gelatin (solid).

Colloid - A colloidal solution appears homogeneous to the naked eye, but particles are apparent under microscope magnification. Particle sizes range from 1 nanometer to 1 micrometer. Like solutions, colloids are physically stable. They exhibit the Tyndall effect. Colloid components can't be separated using decantation, but may be isolated by centrifugation. Examples of colloids include hair spray (gas), smoke (gas), whipped cream (liquid foam), blood (liquid), 

Suspension - Particles in a suspension are often large enough that the mixture appears heterogeneous. Stabilizing agents are required to keep the particles from separating. Like colloids, suspensions exhibit the Tyndall effect. Suspensions may be separated using either decantation or centrifugation. Examples of suspensions include dust in air (solid in gas), vinaigrette (liquid in liquid), mud (solid in liquid), sand (solids blended together), and granite (blended solids).

Examples That Are NOT Mixtures

Just because you mix two chemicals together, don't expect you'll always get a mixture! If a chemical reaction occurs, the identity of a reactant changes. This is not a mixture. Combining vinegar and baking soda results in a reaction to produce carbon dioxide and water. So, you don't have a mixture. Combining an acid and a base also does not produce a mixture.