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Q & A: Electrolysis of Water

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Most recent answer: 10/22/2007
Q:
I have heard of an experiment about splitting water into H and O that Iíd like to show my chemistry class but canít find anywhere that shows the setup. Two wires are connected to a 6-volt battery, then run through a beaker of water, and finally each into a separate test tube. One test fills with H and the other with O. You light a match, hold it to the base of the H test tube and the match goes out, then to the base of the O test tube and the match relights. Is this not a safe experiment? Is that why I canít find it anywhere?
- Valerie Cheshier
Solomon Schechter, Dallas, TX
A:
You're right that you can split up water into hydrogen and oxygen through electrolysis, which means by using an electrical current. I suggest checking out this website for more detail
Ignore the remark on the site about using AC current. We've found that the process goes a lot faster if you use maybe 25 V rather than just 6 V.

The main danger one of us noticed doing this in an elementary classroom is that some kids love to come up and grab the apparatus, so make sure that no higher voltages are around. As you feared, there is also danger of a hydrogen explosion if you make a big hydrogen bubble and it then comes in contact with air and a spark. I worry a little whether the hydrogen gas might sometimes mix enough with air as you open the test tube to make an explosion when hold a match to it, rather than just extinguishing the flame, as pure hydrogen would. You could well be right that thatís why people donít do this experiment so much. One safe compromise might be to just generate enough gas to see the two bubbles (and maybe see that one bubble is twice the size of the other) and to skip the ignition part. Of course that does miss showing directly that the two gases are different.

(published on 10/22/2007)

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