Electrolyte solutions and conductivity

An electrolyte is a substance that produces ions in solution, and electrolytes fall into two categories that behave very differently. A strong electrolyte, such as hydrochloric acid or sodium chloride, dissociates essentially completely into its constituent ions when dissolved, so a solution's ion concentration can be calculated directly from the nominal concentration of dissolved compound. A weak electrolyte, such as acetic acid or ammonia, dissociates only partially, reaching a dynamic equilibrium between the undissociated molecule and its ions, with the equilibrium position (and hence the actual fraction dissociated) governed by the electrolyte's dissociation constant and the solution's concentration. Because a weak electrolyte's dissociation is genuinely an equilibrium, its degree of dissociation changes with concentration in a way a strong electrolyte's does not.

This distinction is directly measurable through a solution's conductivity — its ability to carry electric current, which requires mobile charge carriers (ions) to be present. Molar conductivity, the conductivity normalised per mole of dissolved electrolyte, isolates how efficiently each mole of compound contributes to conduction, independent of concentration. For a strong electrolyte, molar conductivity decreases only mildly as concentration increases, reflecting the fact that essentially all the compound is already dissociated at any concentration — the small decrease that does occur is due to increasing ion-ion electrostatic interactions at higher concentration slowing ion movement, not a change in how much has dissociated. For a weak electrolyte, molar conductivity drops far more sharply as concentration increases, because increasing concentration actually shifts the dissociation equilibrium toward the undissociated form (consistent with Le Chatelier's principle applied to the dissociation equilibrium), reducing the fraction of the compound that is actually ionised and therefore able to carry current. Extrapolating molar conductivity to infinite dilution, where ion-ion interactions vanish and, for weak electrolytes, dissociation becomes essentially complete, gives the limiting molar conductivity — a quantity that, for strong electrolytes, can be measured directly, and that for weak electrolytes must be obtained indirectly (via Kohlrausch's law of independent ion migration) since full dissociation is never actually reached at any finite concentration.