370. Electrolysis is the name given to the process of decomposing chemical compounds by passing the electric current through them. The compound decomposed is the electrolyte. [Fig. 112] shows a tumbler of liquid (electrolyte) through which the current is to pass in the direction of the arrow. Two carbon plates, A and C, are in the liquid, and are joined to the source of electricity. The current enters at A (anode) and leaves at C (cathode).
EXPERIMENT 147. To study the electrolysis of water.
Apparatus. The two simple cells ([§ 275]) joined in series ([§ 364]), although two Daniell or two dry cells will be better. A tumbler of water containing a few drops of sulphuric acid to make the water a conductor. Two pieces of sheet copper will serve as the electrodes. The galvanoscope may also be put into the circuit as in [Fig. 113].
371. Directions. (A) Allow the current to pass, and note (1) whether gas is set free at both electrodes, A and C, and (2) at which the quantity of gas is the greater. If very little gas is produced use more cells.
(B) Remove A and C from the liquid, to remove the gas, then watch the action of the needle of G V as the water is again decomposed.
372. Composition of Water. The two gases liberated in [Exp. 147] were hydrogen (H) and oxygen (O). The chemical formula for water is H2O, which means that it is composed of two parts, by volume, of H and one part of O. With proper apparatus these gases may be collected, tested, and the amounts measured.
373. Electromotive Force of Polarization. We know that H and O have a strong chemical attraction, or affinity, for each other. In order, then, for the current to decompose water, this attraction between the gases must be overcome; and as soon as the current ceases, these gases try to rush together again to form water. This sets up an electromotive force of almost 1.5 volts; in fact, a current is produced if the H and O be allowed to form water again (See Storage Cells). To decompose water the current must have an E. M. F. of over 1.5 volts to overcome this E. M. F. of polarization. It was seen in the study of simple cells that the current became rapidly weaker as hydrogen was deposited upon the copper plate, on account of this opposing electromotive force.