PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 348

11.1.2 Electrolytes and ions

Electrolytes are chemical substances which are present in melts or solutions partially or fully
in the form of ions. The positively charged ion is called the cation, the negatively charged ion
is the anion. We will also use the term electrolyte for electrolyte solutions and melts.

Note: Water is the most common solvent of electrolytes. Some polar compounds, e.g.

methanol, formaldehyde, acetonitrile, or ammonia can be also used as solvents.

Astrong electrolyteis a substance that is present in a melt or solvent solely in the form of
its ions. Typical strong electrolytes are aqueous solutions of most salts, strong inorganic acids
(nitric, hydrochloric,... ), hydroxides of alkaline metals and alkaline earth (sodium hydroxide,
calcium hydroxide,... ).

Example

An example of a strong electrolyte is a sodium chloride melt in which complete dissociation to

ions occurs according to the reaction

NaCl→Na++ Cl−,

where Na+is a sodium cation and Cl−is a chloride anion. Sodium chloride dissolved in water

dissociates according to the same reaction.

Aweak electrolyteis a substance that is present in a melt or solvent both in the form of its
ions and in the form of undissociated molecules. Typical weak electrolytes are water, aqueous
solutions of weak inorganic acids (carbonic acid, boric acid,... ), most aqueous solutions of
organic acids (acetic acid, oxalic acid,... ), and solutions of weak hydroxides (ammonium
hydroxide,... ).

Example

An example of a weak electrolyte is the solution of acetic acid in water. During the reaction

CH 3 COOH=CH 3 COO−+H+

chemical equilibrium is established between the undissociated acid and its ions.