Question

write activity to show electrolysis of water as an example of decomposition reaction

Answer 1

Here is an activity to show the electrolysis of water as an example of decomposition reaction-

First the acidified water used as electrolyte breaks down into  H+ and OH- ions .As anode OH- goes and loses electron. 2OH- = H20 +1/2O2 + 2electron- this reaction takes place. At anode $O_{2}$  is evolved and these electron moves to the cathode through the wire. At cathode H+ ion is attracted and it gains electron to form H2 (2H+ +2 election = H2)
So we can see that gas evolved at cathode was double than the gas evolved at anode. That's the process of the decomposition of the water.
Hope this answer helps...

Answer 2

lectrolysis of water is the decomposition of water into hydrogen and oxygen ions.In electrolysis two electrodes are submerged in an electrolyte and connected to battery.Further water is added to this and gets dissipated into hydrogen and oxygen gas(ions combine with free electrons to form gas).Hydrogen is evolved at Cathode and Oxygen is evolved at Anode.

Since a single compound(H2O) dissipates in 2 elements (H2 and O2) this is a decomposition reaction.