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The perfect example demonstrating this is the electrolysis of water.
If we take some water in a beaker and add sulphuric acid to it then it becomes acidulated water and a great conductor of electricity.
Then we pass electric current through them and we see that one test tube has more space than the other.
This proves that water has 2 parts of hydrogen and 1 part of oxygen.
 Oxygen is collected at anode and Hydrogen at Cathode.
                        2H₂O ------------------->  2H₂ + O₂

In fig. Gas A is hydrogen and gas B is oxygen.
1.Take a plastic mug.Drill two holes at its base and fit rubber stoppers in these holes.insert carbon electrodes
2.Connect these rods to a 9 Volt battery
3.Fill mug with water such that the electrodes are immersed.Add few water of dilute sulphuric acid in water
4.Take two test tubes filled with water and invert them over two carbon electrodes 
5.Switch on the current and leave the apparatus undisturbed for some time
6.You will see the formation of bubbles at both the electrodes .These bubbles displace water in test tubes 
7.The gas collected in the test tube attached to a cathode is twice in volume than the gas collected an anode 
8.Once the test tubes are filled with respective gasses, remove them carefully
9.The match stick or candle burns with a pop sound in the test tube a cathode, indicating presence of hydrogen