How to Remove an Oxygen Molecule From Water

Water contains two hydrogen atoms and one oxygen atom in each molecule. In electrolysis, an electric current passed through the water causes the molecules of water to decompose into hydrogen gas and oxygen gas. Hydrogen gas bubbles around the anode (negative) terminal, and oxygen gas bubbles around the cathode (positive) terminal. A flame test using a glowing wood ember on the collected gases will result in the hydrogen gas reacting quickly and violently with the oxygen gas, causing the ember to glow brightly and intensely.

Things You'll Need

• Two pencils
• Pencil sharpener
• Two wires (at least 15 cm long)
• Noteccard
• 9-volt battery
• Tall drinking glass
• Water

Instructions

1. Remove the metal cap containing the eraser from two pencils.

2. Sharpen the pencils at both ends so the graphite ("lead") core is exposed.

3. Push the two pencils halfway through a note card. The pencils should be placed at least one centimeter apart.

4. Fill a tall drinking glass with water.

5. Place the note card with pencils over the top of the glass and ensure the bottom tips of the pencils are submerged but not touching the bottom of the glass.

6. Attach a wire to the exposed graphite rod (not the wood shell) at the top of each pencil. Connect one wire to the cathode (-) battery terminal of the battery and the other wire to the anode (+) battery terminal.

7. Observe the bubbles forming at the graphite tips in the water. The bubbles being produced at the tip connected to the cathode battery terminal are oxygen gas. The anode tip is producing bubbles of hydrogen gas.

Tips & Warnings

• Dissolving salt in the water will increase the rate of water decomposition.

• The apparatus may be modified to collect the gases being produced by inverting water-filled containers into the water and angling the electrodes to bubble into separate containers.

• Hydrogen gas is highly combustible and will react violently in the presence of heat and oxygen.

• Pure oxygen is highly reactive with many compounds.

References

• Electrolysis: Obtaining hydrogen from water: The Basis for a Solar-Hydrogen Economy
• Electrolysis of Salt Water
• Electrolysis of Water Using an Electric Current