Electrolysis of Water

The main aim of this experiment is to study the ‘electrolysis of water’.

Let us understand first what is the electrolysis of water. Electrolysis of water is the process of decomposing water into hydrogen and oxygen gas by passing electricity through its aqueous solution (water).

2 H₂O (l) + electrical energy ? 2 H₂ (g) + O₂ (g)

To perform this experiment, we require apparatus and materials such as two beakers, glass rod, spatula, test tubes, Sodium sulfate, Universal indicator solution, Distilled water, two Graphite electrodes, three conducting wires, 3 V battery and a Plug key.

Electrolysis of Water — Procedure

To study the electrolysis of water, we have to follow the following procedure:

To 40 mL water, add a few spatulae full of sodium sulfate and stir well to prepare its concentrated solution.
Add the universal indicator solution dropwise till the colour of the solution becomes dark green.
Fill two test tubes with the dark green solution to their full capacity.
In a beaker, place the conducting wires and fix the two graphite electrodes to the conducting wires.
Fill the beaker with the remaining coloured solution (green-coloured).
Lower the two test tubes filled with the coloured solution over the two graphite electrodes carefully.
Connect a 3 V battery and the plug key to the two electrodes with connecting wires. Keep the key open.
Now, close the key and observe the two electrodes carefully.
Notice the bubbles forming at the two electrodes.
Notice the colour change in the two test tubes.

Conclusion

In this experiment, we conclude that:

In water electrolysis, oxygen gas is liberated at the anode and hydrogen gas is liberated at the cathode. During the process, hydrogen and oxygen gasses are liberated in the ratio of 2:1.

FAQs on Electrolysis of Water

Q.1: How would you show that the gas obtained at the cathode during electrolysis of water is hydrogen and that obtained at the anode is oxygen?

Ans: When we bring the burning splint near the electrode, which makes a pop sound, the gas liberated is hydrogen. When we bring the glowing splint near the electrode and it reignites, then the gas is oxygen. From this, we can prove that the gas liberated at the cathode is hydrogen, and that of the anode is oxygen.

Q.2: What happens during the electrolysis of water?

Ans: During the electrolysis of water, decomposition of water into oxygen gas and hydrogen gas occurs. This happens by passing an electric current through the solution. Hydrogen gas is produced at the cathode, whereas oxygen gas is produced at the anode.

Q.3: Why is electrolysis of water carried out in an acidic medium but not in pure water?

Ans: Pure water cannot conduct electricity. It is almost non-electrolyte. When we add a small amount of acid to water, it dissociates into hydrogen and hydroxide ions and increases the concentration of H+ ions. Therefore, electrolysis is carried out in acidified water but not in pure water.

Q.4: Which type of chemical reaction is the electrolysis of water?

Ans: Electrolysis is an example of a decomposition reaction. It is because water splits into its constituent elements, hydrogen and oxygen, under the effect of an electric current.