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An electrolyte is any chemical substance that dissolves in water to produce ions, or charged atoms, which thereby make the solution a good conductor of electricity. A strong electrolyte is generally completely ionized in water and gives equal numbers of cations, or positively-charged ions, and anions, or negatively charged ions. A weak electrolyte, on the other hand, will ionize only slightly in water and produce few ions, so their solutions are poor conductors of electricity.
Pure water itself is not a good conductor of electricity, and distilled or deionized water, from which all ions have been removed, won't readily conduct an electrical current. If a strong electrolyte like table salt, NaCl, is added, however, the salt will dissolve to give Na+ ions and Cl- ions. The Na+ are capable of accepting electrons from the negative pole of the power source while the Cl- will transport electrons to the positive pole, resulting in a net flow of electricity through the solution. The more salt that's added, the more conducting the solution will be, up to the point of saturation.
In a weak electrolyte, this dissociation into ions occurs only to a small extent, typically much less than 10%. The vast bulk of the weak electrolyte remains in its original unionized form in solution. In such cases, there are insufficient ions present to carry an electric current.
The electrolytic strength of a substance can readily be determined by measuring the electrical conductance of a solution of the substance of known concentration. Tables are available listing the Qsp or ion constant of many substances, which is a measure of their degree of ionization.
Telling a weak electrolyte apart from a strong one just by inspecting its chemical formula is not so straightforward. A weak electrolyte will usually be composed of covalent bonds, or chemical bonds in which electrons are shared by two atoms. A strong electrolyte will have at least one ionic bond, in which electrons from one atom are transferred to another to produce two ions which are then held together by electrostatic forces. Most chemical bonds have both some ionic and covalent character, however, and so some knowledge of chemistry is required to make a reasonable estimate of a compound's electrolytic strength.
In general, though, most organic acids and their salts, and the salts of organic bases, are weak electrolytes. A substance with low water solubility may also be classified as a weak electrolyte. Technically, however, solubility is not the same as electrolytic strength.
In addition to weak and strong electrolytes, non-electrolytes are substances that don't ionize to any appreciable extent in aqueous solution, and their solutions don't conduct electricity at all. Most organic substances, unless they contain an acid or base functionality, are non-conductors and therefore non-electrolytes. Sugar and alcohol, for example, are organic compounds with no acid or base functionality and will therefore not produce ions in solution.
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