Course Outline

General Chemistry

03/24/07

A sample of matter is pure if has has a single identity throughout the sample (i.e. it has only one substance in it). A pure substance is either an element or a compound. 24 karat gold is an example of a pure element, and distilled water is an example of a pure compound.

A mixture is a sample of matter that has more than one substance physically blended so that each retains its own identity. A mixture can be composed of elements, compounds, or both. For example, mercury amalgam dental fillings are a mixture of elements, sea water is a mixture of compounds, and air is a mixture of both.

Along with its identity, each substance in a mixture keeps its own characteristics. Many of the properties of a mixture ends up being a blend of the properties of the components of that mixture. If we add salt to water, we get a mixture that looks like water but tastes like salt. This distinguishes a mixture from a compound (which also contains more than one type of substance, in a way). The properties of a compound are vastly different from the properties of the elements it 'contains.' If we react sodium, a silvery-gray metal, with chlorine, a poisonous greenish gas, we get table salt, a white, crystalline solid.

Some experiments performed in the General Chemistry laboratory require mixtures to be separated and purified. Examples of separation/purification techniques include crystallization, distillation, and chromatography, all of which take advantage of the differences in the properties of the components of a mixture.

Mixtures can be further classified according to how well the components are blended. If a mixture is perfectly uniform, it is a homogeneous mixture, also known as a solution. Air is an example of a homogeneous mixture. Heterogeneous mixtures are ones that are less completely blended. When we examine a heterogeneous mixture, we see the different components as lumps, layers, and/or cloudiness. Concrete is an example of a heterogeneous mixture.

For more information about mixtures see the Chemistry Web Book.

Concept Check: Of the objects you can see around you, which are pure substances and which are mixtures?

Answer: There may not be any pure substances near you. Most common objects are mixtures since the natural tendency is for matter to mix. It takes energy to separate substances and keep them pure. The metals and plastics used to make computers are mixtures. Wood and paper used to make your books and pencils are mixtures. Even a pencil 'lead' is a mixture of graphite and clay binder. Metals and gemstones used in high quality jewellery may be pure, though the colors of some gems, like amethyst, are due to impurities. Copper used in wiring is fairly pure, but copper coins are mixtures. Even bright, shiny aluminum foil that keeps your snacks fresh and tasty is coated with an invisible oxide layer.

© Copyright 2006, Kelley Whitley, ChemProfessor. All rights reserved.

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