The general method used in the electrolysis of a solution is illustrated in Fig. 31. The vessel D contains the electrolyte. Two plates or rods, A and B, made of suitable material, are connected with the wires from a battery (or dynamo) and dipped into the electrolyte, as shown in the figure. These plates or rods are called electrodes. The electrode connected with the zinc plate of the battery is the negative electrode or cathode, while that connected with the carbon plate is the positive electrode or anode.
Theory of electrolytic dissociation. The facts which have just been described in connection with solutions, together with many others, have led chemists to adopt a theory of solutions called the theory of electrolytic dissociation. The main assumptions in this theory are the following.
1. Formation of ions. Many compounds when dissolved in water undergo an important change. A portion of their molecules fall apart, or dissociate, into two or more parts, called ions. Thus sodium nitrate (NaNO3) dissociates into the ions Na and NO3; sodium chloride, into the ions Na and Cl. These ions are free to move about in the solution independently of each other like independent molecules, and for this reason were given the name ion, which signifies a wanderer.
2. The electrical charge of ions. Each ion carries a heavy electrical charge, and in this respect differs from an atom or molecule. It is evident that the sodium in the form of an ion must differ in some important way from ordinary sodium, for sodium ions, formed from sodium nitrate, give no visible evidence of their presence in water, whereas metallic sodium at once decomposes the water. The electrical charge, therefore, greatly modifies the usual chemical properties of the element.
3. The positive charges equal the negative charges. The ions formed by the dissociation of any molecule are of two kinds. One kind is charged with positive electricity and the other with negative electricity; moreover the sum of all the positive charges is always equal to the sum of all the negative charges. The solution as a whole is therefore electrically neutral. If we represent dissociation by the usual chemical equations, with the electrical charges indicated by + and - signs following the symbols, the dissociation of sodium chloride molecules is represented thus:
NaCl --> Na+, Cl-.
The positive charge on each sodium ion exactly equals the negative charge on each chlorine ion. Sodium sulphate dissociates, as shown in the equation
Na2SO4 --> 2Na+, SO4-.
Here the positive charge on the two sodium ions equals the double negative charge on the SO4 ion.
4. Not all compounds dissociate. Only those compounds dissociate whose solutions form electrolytes. Thus salt dissociates when dissolved in water, the resulting solution being an electrolyte. Sugar, on the other hand, does not dissociate and its solution is not a conductor of the electric current.