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Mixtures and Compounds

Mixtures are heterogeneous forms of matter. Mixtures are composed of variable proportions of molecules and atoms.

Compounds are homogeneous forms of matter. Their constituent elements (atoms and/or ions) are always present in fixed proportions (1:1 depicted here).

Examples of mixtures:

Examples of compounds:

Properties of Mixtures

Properties of Compounds

Separating the Components of a Mixture

Most laboratory work in biology requires the use of techniques to separate the components of mixtures. This is done by exploiting some property that distinguishes the components, such as their relative


Dialysis is the separation of small solute molecules or ions (e.g., glucose, Na+, Cl-) from macromolecules (e.g., starch) by virtue of their differing rates of diffusion through a differentially permeable membrane.

An example:

Cellophane is perforated with tiny pores that permit ions and small molecules to pass through but exclude molecules with molecular weights greater than about 12,000. If we fill a piece of cellophane tubing with a mixture of starch and sugar and place it in pure water, the sugar molecules (red dots) will diffuse out into the water until equilibrium is reached; that is, until their concentration is equal on both sides of the membrane. Because of their large size, all the starch (blue disks) will be retained within the tubing.


Chromatography is the term used for several techniques for separating the components of a mixture. Follow the links below for examples.

Link to a description of paper chromatography, where the molecules are separated by size and solubility
Link to a description of exclusion chromatography, where the molecules in a mixture are separated by size.
Link to a description of affinity chromatography, where molecules are separated on the basis of their attraction to material in the chromatographic column.


Electrophoresis uses a direct electric current to separate the components of a mixture by the differing electrical charge.
Link to a description of how the proteins in blood serum are separated by electrophoresis.

Pure Substances

Some of the pure substances isolated from mixtures cannot be further broken down. Oxygen (O2) is an example. It is one of the elements; the fundamental building blocks of matter.
Link to discussion of elements.
Most pure substances are compounds. Table salt, sodium chloride (NaCl), is an example; water (H2O) is another. If we pass an electrical current through molten NaCl, two new substances will be formed:

In this operation, a compound has been decomposed into its constitutive elements.

Note the differences between separating the components of a mixture and those of a compound.

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21 February 2011