water
9-volt battery
2 paper clips
another container filled with water; Unbend the paper clips and connect one to each terminal of the battery.
Place the other ends, not touching, into a container of water. That's it!
You'll get bubbles off both wires. The one with more bubbles is giving off pure hydrogen. The other bubbles are impure oxygen. You can test which gas is hydrogen by lighting a match or lighter over the container. The hydrogen bubbles will burn; the oxygen bubbles will not burn.
Collect the hydrogen gas by inverting a water-filled tube or jar over the wire producing the hydrogen gas. The reason you want water in the container is so you can collect hydrogen without obtaining air. Air contains 20% oxygen, which you want to keep out of the container to keep it from becoming dangerously flammable. For the same reason, don't collect the gas coming off both wires into the same container, since the mixture could burn explosively upon ignition. If you wish, you can collect the oxygen in the same way as the hydrogen, but be aware this gas is not very pure.
Cap or seal the container before inverting it, to avoid exposure to air. Disconnect the battery.
