Solid and liquid substances which are able to conduct the electric current can be roughly divided into two categories. These are metallic conductors and electrolytic conductors. In metallic conductors or electronic conductors, the electricity is carried by the mobile electrons. When an electric potential is applied to metallic conductors, the electrons are forced to stream in one direction while the positive ions remain stationary. Thus, the flow of electricity is not accompanied by any appreciable movement of matter.
Conducting materials of the second type are known as electrolytic conductors or electrolytes. Electrolytes are distinguished from metallic conductors by the fact that the current is carried by ions and not by electrons. As we know that ions consist of atoms or groups of atoms which have lost or gained electrons, thus acquiring positive or negative charges, respectively. The application of an electrical potential causes these charged particles of matter to move, the positive ions move towards the cathode and the negative ions move towards the anode. It follows, therefore, that passage of an electric current through an electrolyte is always accompanied by transfer of matter. This transfer is manifested by changes of concentration, and also by visible separation of material at the points where the electric current enters and leaves the electrolyte. Although many molten salts and hydroxides are electrolytic conductors, the treatment here will be restricted to electrolytes consisting of a salt, acid or base dissolved in a suitable solvent, such as water.