Electrolysis
Electrolysis is the use of electricity to separate elements or compounds. Here the objective is to separate water into hydrogen gas and oxygen gas. This project can become dangerous if you decide to collect the gases. You won’t get pure oxygen unless your battery has platinum electrodes, but you can collect a sample of hydrogen gas. Hydrogen burns easily in air (Hindenburg) and burns vigorously with a clear flame when mixed with oxygen (space shuttle main engines).
Add several tablespoons of salt [edit: baking soda is a safer choice…read the comments] to the water and stir it until it disolves. This step provides free ions, making a solution which conducts electricity.Place the battery in the solution. You should see tiny bubbles on both of the electrodes. The one that produces bubbles faster is producing hydrogen. The battery will only last a few minutes in the solution.
Optionally, you can attach wires or graphite rods to the battery terminals and leave the battery out of the water. Using graphite rods will speed up the process considerably.
