To electrolyze water. Electrolysis is a process by which a chemical reaction is carried out by means of the passage of an electric current. The cathode will be recognized by a pink colour from the phenolphthalein indicator due to hydroxide production.
Phenolphthalein turns pink in the presence of base and clear in the presence of acid.
Two molecules of hydrogen are produced for every molecule of oxygen produced. The gases produced at the electrodes can also be collected and tested. A positive test for the presence of hydrogen is a soft pop sound when a burning match is placed in the mouth of the container. The test for the presence of oxygen is to place a smoldering match in the mouth of the container. If the match glows oxygen is present.