Separate water by electrolysis

Separate Water by Electrolysis

Objective:  Use electricity to separate water into hydrogen and oxygen gas. Separating molecules with electricity is called electrolysis.

2 test tubes*
Plastic container with lid
9V battery
2 metal thumbtacks
Baking soda
Deep bowl or bucket
Permanent marker

ProcedureDownload the Word document.

  1. Push the two thumbtacks into the plastic container through the bottom at the same distance apart as the terminals on the battery. Use the marker to label one side positive (+) and one side negative (-).
  2. Cut two holes into the plastic lid just large enough to fit test tubes through.
  3. Prop the test tubes over the thumbtacks with the points reaching into the test tubes. Attach the lid to hold the test tubes in place.
  4. Fill the deep bowl or bucket with enough water to completely submerge the electrolysis assembly.
  5. Holding the assembly under water, fill the test tubes and plastic container with water until there is no air in the test tubes whatsoever. Air at the top of the plastic container is not a problem.
  6. The terminals of the 9V battery are labeled with a plus and minus sign. Hold the battery against the thumbtacks with the positive terminal lining up with your positive tack, and the negative terminal lining up with your negative tack. Don't try the balancing act in the picture for now; just hold it with your hands.
  7. Do you see anything happening at the points of the thumbtacks?
  8. Electricity cannot travel through the water from one battery terminal to the other. We need to add an electrolyte to carry the charge through the water. Dissolve as much sodium bicarbonate** as you can into your deep bowl of water. It's okay if some extra sodium bicarbonate drops to the bottom.
  9. Repeat steps 5-7.

*If you don't have test tubes, look around your house for anything clear that you can catch gas in and see it happening.
**Read the ingredients on the baking soda box.


  1. The bazillion water molecules in your test tubes each have the chemical formula H2O. That means they each have two hydrogen atoms attached to one oxygen atom. Electrolysis pulls the water molecules apart and draws the hydrogen to one electrode and the oxygen to the other electrode. Once they are separated, which element will you have more of? Can you tell by the quantity which electrode is collecting hydrogen and which electrode is collecting oxygen?
  2. Metals are very reactive with oxygen. What is happening to the thumbtack on the oxygen side? Flip the battery so the terminals are mis-matched with your positive and negative labels. Now which side is generating oxygen? Do your test tubes still contain pure gases after switching electrodes?

Prove it!
If you start over and collect pure gases, you can see their behavior with fire. This requires matches, or a wooden splint that is on fire. Wooden coffee stirrers work well.

  1. Hydrogen is explosive, and with this tiny amount, you can achieve a small "POP" by inserting a lit match into the gas.
  2. Oxygen is flammable, but not quite explosive. With this tiny amount, you can re-light a match that has lost its flame by inserting the glowing match into the oxygen.
  3. If you have a hard time catching the gases, browse the many YouTube videos and watch other people do it.