1. Electrolysis of Water An Example of Chemical Decomposition by
Joseph A. Castellano, Ph.D.
TOPS of Santa Clara Valley

2. A Decomposition Reaction
A simple experiment is used to show how low voltage electricity decomposes the water molecule into hydrogen and oxygen atoms.

3. Create the Electrodes
Cut the eraser off two number 2 pencils and cut the pencils
in half to give two pieces about 10 cm long. Sharpen both
ends of each pencil.
The graphite in each pencil will become the “electrodes.”

4. Form the Electrode Assembly
Using two small binder clips (2 cm wide) and one large
paper clip (5 cm long), connect the pencils together:

5. Prepare the Electrolyte
Mass 20 grams of pure sodium chloride * or sodium
sulfate and transfer it to a 500 ml Erlenmeyer flask.
Add 200 ml of water to the flask and stir to dissolve
all of the crystalline solid to produce a clear 10% solution.
The salt solution acts as an electrolyte.
Pour the 10% solution into a 250 ml beaker.
* Make sure the salt does not contain calcium silicate,
which tends to make the solution cloudy.

6. Complete the Experimental Set-up
Place the “electrode” assembly on top of the beaker
containing the electrolyte solution; make sure the
tips of the pencils are well immersed in the liquid.
Attach an alligator clip with a red wire to the tip of
the pencil on the right and connect the red wire to
the positive (+) terminal side of a 9-volt battery.
The right side pencil with then be the anode.
Attach an alligator clip with a black wire to the tip
of the pencil on the left and connect the black wire
to the negative (-) terminal side of a 9-volt battery.
The left side pencil with then be the cathode.

7. The Water Electrolysis Set-Up (10% Na2SO4 or NaCl solution)
Cathode
(-)
Anode
(+)
9 volt
Battery
Electrolyte
(10% Na2SO4 or NaCl solution)

8. The Water Electrolysis Experiment
Notice more gas bubbles form at the
cathode (left) than at the anode (right).

9. Cathode & Anode Reactions
The reaction at the cathode is REDUCTION – a gain
of electrons:
2 H2O + 2 e H2 + 2 OH-
The reaction at the anode is OXIDATION – a loss of
electrons:
2 H2O - 2 e O2 + 4 H+
Combining the two reactions, we obtain:
4 H2O H2 + O2 + 2 H2O or
2 H2O H2 + O2

10. Electrolysis of Water SummaryH O O H H H O O2
2 H2
2 H2O
Two molecules of water decompose into one molecule of diatomic oxygen and two molecules of diatomic hydrogen.

11. This presentation was produced as a public service to help middle school and high school science teachers develop experiments and demonstrations that can be used in their classrooms.
More science experiments and demonstrations are available on the TOPS of Santa Clara Valley web-site:
TOPS (Teaching Opportunities for Partners in Science) is a program aimed at stimulating greater interest in science by middle school students.