Electrolysis of Water
Decompose water into its elements by electrolysis.
Materials
· Goggles
· One small cup
· Two 1-mL beral pipettes
· Baking soda solution
· Scissors
· Power
· Two pin
･ Tape
· 2 alligator clip wires
Procedure
1. Cut each beral pipette stem 2 cm below the bulb. Make sure they are even. If the pipette has
graduation markings, use these as a guide.
2. Into each beral pipette bulb, push the pin into the side of the bulb, about 2/3 of the way down the bulb,
toward the stem. Make sure you create the hole in the smooth part of the bulb, not in the seam. Into
each hole you’ve created, gently push a piece of graphite.
3. Cut the top of your cup down to about 4 cm high.
5. Pour baking soda solution to fill your small cup about 1/2 full.
6. Fill your pipettes completely with baking soda solution.
a. To fill the pipettes completely, when approximately half-full of solution, turn upside down to squeeze
the rest of the air out before filling with water. You may have to do this several times to completely fill the
pipettes.
b. Make sure not to squeeze the area of the bulb with the graphite, so you do not break the graphite.
7. Place the pipettes stems down in the remaining solution in the small cup, insert the stems into the
straws so they are upright. Make sure the pipette stem holes are submerged in the solution. See Fig. 1-8.
8. Clip the alligator clips to the graphite electrodes, carefully, so you do not break the graphite.
9. Clip the alligator clips to the battery.
10. Observe the system as you allow it to run. Keep in mind the following questions:
a. Which electrode on the battery is each connected to which electrode on the electrolysis apparatus?
b. What is happening at each electrode?
c. Why is it important that the pipettes are both resting in the baking soda solution?
11. When you finish running the system, mark the level of the liquid in each pipette.
a. You can pull the pipettes out of the cup to do this, just be careful not to squeeze them.
b. Make a second mark on each pipette where the bulb stops curving at and becomes straight.
c. Measure the distance between each mark on each pipette and record your results.
12. So that we can do a class flame test, pull out the + pipette, remove the graphite electrode and
squeeze the collected gas into an inverted test tube marked + that will be at the back of the room.
Repeat for the – pipette.
Either observe your teacher flame test the gaseous products of electrolysis or watch a video of the
electrolysis of water, e.g.: www.youtube.com/watch?v=OTEX38bQ-2w
Fig. 1–8. Equipment set-up for electrolysis of water.
Questions
Table 1–3 shows some physical properties of pure hydrogen (H 2), oxygen (O2), nitrogen (N2), water vapor
(H2O) and air. Use this information as you answer the questions that follow.
1. What did you observe as the system ran?
2. If each wire went from one terminal of the battery to one electrode, what completed the circuit?
3. What was the gas generated at the electrode connected to the positive (+) pole of the battery? Give
your evidence.
4. What was the gas generated at the electrode connected to the negative (–) pole of the battery? Give
your evidence.
5. How much gas formed at each electrode? How do the volumes of the gases compare? How might you
explain any differences? How does this provide evidence for or against the chemical formula for water?
6. As you observed the splint tests on the gases, was there evidence that water re-formed? If so, under
what conditions was it formed?
7. Which gas formed by the electrolysis of water is denser? What is your evidence?
8. How do the properties of water differ from its individual elements? Is the electrolysis of water a
chemical or physical change? What is your evidence?
Table 1-3. Splint test for gases
Gas
Splint test
Hydrogen (H2)
When a burning splint (piece of wood) is introduced to a sample of pure hydrogen
gas, it will burn with a popping sound.
Oxygen (O2)
When a smoldering splint is introduced to a sample of pure oxygen gas, the splint will
reignite.
Nitrogen (N2)
When a burning splint is introduced to a sample of pure nitrogen gas, it will be
extinguished.
Water Vapor (H2O)
When a burning splint is introduced to a sample of pure water vapor, it will be
extinguished.
Air
When a burning splint is introduced to air, it will continue to burn.
~ 79% Nitrogen, (N2)
~ 21% Oxygen, (O2)
Electrolysis
How did electrolysis split water into hydrogen and oxygen? At the negative terminal of the battery,
electrons are produced. Those electrons flow through the wire to the electrode in solution (see Fig. 1-9).
At the negative (-) electrode, electrons are transferred to the water and picked up by hydrogen atoms in
water. This allows hydrogen to split from the rest of the water molecule and form H 2 molecules. At the
other electrode, the positive (+) electrode, electrons are donated by the oxygen in the water molecule,
which allows oxygen to split from the water and form O2 molecules. Electrons flow from this electrode
back to the positive terminal of the battery.
Fig. 1-9. Mechanism of electrolysis of water.
The chemical reaction that occurred at the negative electrode was:
2 H20 + 2e- H2 + 2OHThe chemical reaction that occurred at the positive electrode was:
2 H20 02 + 4H+ + 4eWhen water is heated and boils, a physical change takes place as liquid water evaporates into gaseous
water vapor. Boiling is a physical change, because even though the water changes phase from liquid to
gas, the liquid and gas are both made of the water molecule. The water compound is not broken apart.
In the activity, we could observe the physical change of phases, from liquid water to gaseous hydrogen
and oxygen. Electrolysis is a type of chemical change, however, because hydrogen and oxygen are
chemically different substances from water. Putting hydrogen and oxygen back together by combustion,
or igniting them with a flame, is also a chemical change. Decomposing water is relatively difficult. It can be
accomplished through electrolysis and by some other chemical reaction. Heating alone cannot
decompose water. In fact, water is not broken apart even by the heat of a volcanic eruption!