|MadSci Network: Chemistry|
Well, it's pretty easy to do. Trouble is, it is dangerous. It should never be done without proper safety gear and knowledgable adult supervision (that means a science teacher, not your crazy neighbor or wierd Uncle Joe). Having said that, we'll continue.
First of all, you've already done the hard part; that is, splitting the water molecule. Doing this took energy which is stored in the bonds of the new hydrogen and oxygen molecules. It takes only the addition of a little more energy (called activation energy) to release this again. It is a lot like a match. You've got to strike it (add energy) to allow it to react (burn).
The safest way to demonstrate the recombining of hydrogen and oxygen is called the "hydrogen pop" test. In this, you use a SMALL upside-down test tube to collect hydrogen gas then hold a lit match to the mouth. You will hear a POP (well...it's more like the yip of a small dog). The hydrogen combines with the relatively small amount of oxygen in the air (DON'T do this with the oxygen you made!!!!) When you look at the test tube after, you will see condensation on the glass inside. Congratulations, you've just created water!
By the way, did you know that there are other ways of recombining hydrogen and oxygen to make water where ELECTRICITY, not heat, is produced. This is called a fuel cell and if you want to learn more, write back anytime!!
Try the links in the MadSci Library for more information on Chemistry.