What Is a Mixture in Chemistry?

A graphic illustrating a chemical mixture

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A mixture is what you get when you combine two substances in such a way that no chemical reaction occurs between the components, and you can separate them again. In a mixture, each component maintains its own chemical identity. Typically mechanical blending combines components of a mixture, although other processes may produce a mixture (e.g., diffusion, osmosis).

Technically, the term "mixture" is misused when a recipe calls for you to mix, for example, flour and eggs. A chemical reaction does occur between those cooking ingredients. You can't undo it. However, mixing dry ingredients, such as flour, salt, and sugar, does produce an actual mixture.

Even though the components of a mixture are unchanged, a mixture may have different physical properties than either of its components. For example, if you combine alcohol and water, the mixture has a different melting point and boiling point than either component.

Examples of Mixtures

  • Sand and water
  • Salt and water
  • Sugar and salt
  • Ethanol in water
  • Air
  • Soda
  • Salt and pepper
  • Solutions, colloids, suspensions

Examples That Are Not Mixtures

  • Baking soda and vinegar
  • Borax and glue to make slime
  • Combining hydrochloric acid (HCl) and sodium hydroxide (NaOH)

Classification of Mixtures

Mixtures may be categorized as either homogeneous or heterogeneous.

A homogeneous mixture has a uniform composition that doesn't readily separate. Every part of a homogeneous mixture has the same properties. In a homogeneous mixture, there is typically a solute and a solvent, and the resulting substance consists of a single phase. Examples of homogeneous mixtures include air and saline solution. A homogeneous mixture may contain any number of components. While a saline solution is simply salt (the solute) dissolved in water (the solvent), air contains many gases. The solutes in air include oxygen, carbon dioxide, and water vapor. The solvent in the air is nitrogen. Typically, the particle size of the solute in a homogeneous mixture is petite.

A heterogeneous mixture, in contrast, does not exhibit uniform properties. It's often possible to see the particles in the mixture and separate them from each other. Examples of heterogeneous mixtures include a wet sponge, sand, gravel, trail mix, and chalk suspended in water.

To some extent, whether a mixture is classified as homogeneous or heterogeneous is a matter of scale. For example, mist may appear to be homogeneous when viewed on a large scale, yet if magnified, the concentration of water won't be uniform from one area to another. Similarly, some mixtures that appear heterogeneous at a normal scale become more homogeneous on a large scale. Sand is heterogeneous if you examine it in the palm of your hand, yet seems homogeneous if you view an entire beach. Nearly any mixture, viewed on a molecular scale, is heterogeneous. Math is applied to determine whether a mixture is homogeneous or heterogeneous. If no statistical variation between properties is observed, a mixture should be treated as being homogeneous.