Electrolysis of Water
Electrolysis of water can be easily demonstrated with the help of an experiment. In this experimental set up, a glass beaker fixed with two carbon electrodes is filled with water upto one third of its volume. The positive carbon electrode acts as anode and the negative carbon electrode acts as cathode. Two test tubes are placed on the electrodes as shown in Figure 13. 1.
The electrodes are connected to a battery and current is passed until the test tubes are filled with a particular gas.
If the gas collected is tested using a burning splint we can notice that the gas in cathode side burns with a popping sound when the extinguish splint is brought near the mouth of the test tube. This property is usually shown by hydrogen gas and so it is confirmed that the gas inside the test tube is hydrogen. The burning splint placed near the anode side burns more brightly confirming that it is oxygen gas. This experiment shows that water is made up of hydrogen and oxygen. The ratio of hydrogen and oxygen is 2:1. Hence, for every two volumes of hydrogen collected at the cathode, there is one volume of oxygen collected at the anode.
2H2O ---Electrolysis → 2H2↑ + O2↑
Take some anhydrous copper (II) sulphate powder and place it in a watch glass. Add water drop by drop to the anhydrous copper sulphate. Do you notice any colour change in the powder? You can notice the powder turning blue. It is a test for water.
(i) The reaction between anhydrous copper (II) sulphate and water is used as a test for water.
(ii) The white solid turns blue in the presence of water.