The electrolysis of water is a good demonstration, but it makes an even better student activity. We have found several demonstrations that we have turned into "labettes".
A "labette" is a chemical activity that we can all share. The activity is often integrated into a lesson rather than a separate lesson in itself. In this case, we are learning that electricity can cause chemical change. We sharpen two pencils at both ends, tape them to a 9-volt battery, add a battery clip from Radio Shack and tape the wires to the top ends of the pencils. This apparatus falls apart pretty easily, but it is easy to make and repair. Since the students can make the apparatus, the whole thing is more accessible.
We place the ends of the pencil into a solution that contains water, universal indicator, and a pinch of the salt, sodium sulfate. The sodium sulfate makes the solution a better conductor without changing the pH of the solution. The universal indicator changes color with acid or base.
The electrolysis of water is really fairly complicated. One pencil is negative (-) because the battery pushes electrons into the graphite. When water touches this negative pencil, the water gains electrons. This is called reduction and the pencil is called the cathode.
The reaction is:
2H2O)l) + 2e- → H2(g) + 2OH-(aq).
The hydroxide ions formed make the solution basic and the universal indicator turns purple.
The other pencil is positive (+) because the battery pulls electrons out of the graphite. When water touches this positively-charged pencil, it loses electrons. This is called oxidation and the pencil is called the anode.
The reaction is:
2H2O(l) → O2(g) + 4H+(aq) + 4e-.
The hydrogen ions formed make the solution acidic and the universal indicator turns red. This is a very colorful activity that adds to the interest. As you move the pencils around, the solution becomes very pretty. When you remove the pencils and mix the solution, it returns to its original green color (usually) because the H+ ions and OH- ions neutralize each other.
There are also bubbles of oxygen gas (O2) and hydrogen gas (H2) that form, but they are not always easy to see. We continue this lesson by doing the electrolysis of KI(aq) in a solution that also contains the acid-base indicator, phenolphthalein. We can repeat this with a solution of CuCl2.
I did not invent this activity. I learned some of it from Irwin Tallesnick in Canada. I learned other parts from other teachers with whom I have worked. It took me awhile to finally get around to making my first pencil apparatus, but the student joy and understanding that comes from it makes it a worthwhile effort. Here is my Handout [Word] | [Acrobat] and Answers [Word] | [Acrobat].