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Here is a very simple way to demonstrate that water contains hydrogen and oxygen. In order to show electrolysis, you'll need a 9 volt battery, some bits of wire, tape, a small saucer, some salt, and two small pencils sharpened at each end.

You can use this demonstration in lower grades just to illustrate that oxygen and hydrogen are the components of water. But we've also included enough information to make it useful for high school chemistry students.

However you make use of the demo, it only takes seconds to set up, requires no special equipment, and it works every time! The first time we saw this demonstrated we were amazed at how simple it is to separate water into its components ... no complicated tubing, glassware, and electrical power supply needed!
You can use a clear glass dish, and show the process on an overhead projector.

Here's the setup. That's all there is!

Add a sprinkle or two of salt to the water ... not much is needed. Each pencil becomes an electrode, and the moment you hook up the battery, bubbles will begin appearing at the tip of each pencil: oxygen at the positive electrode, and hydrogen at the negative one.

You can try collecting the gases with inverted small test tubes, and use a burning splint to test for hydrogen (the tube will pop) and a glowing splint to test for oxygen (the splint will burst into flame).

Below on this page we've included some terms describing electrolysis, and an explanation of the process that is suitable for high school chemistry students.

When you add salt to the water, the salt ions (which are highly polar) help pull the water molecules apart into ions too. Each part of the water molecule has a charge. The OH- ion is negative, and the H+ ion is positive.

This solution in water forms an electrolyte, allowing current to flow when a voltage is applied. The H+ ions, called cations, move toward the cathode (negative electrode), and the OH- ions, called anions, move toward the anode (positive electrode).

If you add some universal indicator solution to the salt water, you will be able to see a colour change corresponding to acid near the cathode (H+ ions in water) and base near the anode (OH- ions in water).

At the anode, water is oxidized:
2H2O  —>  O2  +  4H+  +  4e-
At the cathode, water is reduced:
4H2O  +  4e-   —>   2H2  +  4OH-
Note that there is a net balance of electrons in the water.

Bubbles of oxygen gas (O2) form at the anode, and bubbles of hydrogen gas (H2) form at the cathode.

The bubbles are easily seen. Twice as much hydrogen gas is produced as oxygen gas.

The net reaction:   2H2O   —>   2H2  +  O2

Content, Graphics, & Design by Bill Willis 2002
Wunderland Website Design


Updated: 2016/06/30

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