Ways to Split Water for Hydrogen and Oxygen
Two hydrogen atoms are covalently bonded to an oxygen molecule in water. When the covalent bonds in the water molecule are broken, the hydrogen atoms combine to produce hydrogen gas, and the oxygen atom combines with a free oxygen atom from another split water molecule to produce oxygen gas. Electrolysis uses an electric current to split molecules of water into hydrogen and oxygen gas.
Electrolysis
Electrolysis requires an electric power source, water and a method of transmitting the current through the water. The overall setup is a battery that's connected to electrodes that are placed into a container of water. Use a 9-volt battery for the power source, although lower-voltage power supplies can be used with sufficient salt dissolved into the water (this increases the rate of electrolysis). The electrodes should be electrically neutral to prevent the electrodes from dissolving into the water.
The "lead" of pencils is actually graphite, a form of carbon. Sharpen both ends of two pencils to expose the graphite at both ends. Push the pencils through a piece of cardstock paper at least one centimeter apart. The cardstock serves to hold the electrodes in place over the water.
Fill a small container with water; dissolve some salt into the water to increase the electrolysis rate. Place the cardstock paper with the electrodes over the container of water so that the bottom of the electrode is submerged and the exposed graphite ends of the pencils are not touching each other.
Attach a wire to the top of each pencil. Make sure the wires are connected to the graphite and not the wood of the pencil. Alligator clips at the end of the wires will make the connection easier. Attach the free ends of the wires to the terminals on the battery. The electrode attached to the anode (negative) terminal will generate bubbles of hydrogen gas, while the electrode attached to the cathode (positive) terminal will generate bubbles of oxygen gas.
Collecting the Gases
The gas can be collected by inverting test tubes filled with water over the electrodes. The rims of the test tubes should be below the water line of the electrolysis chamber. As the gas bubbles rise into the test tubes, the gases will displace the water until they're filled with each gas. When the containers are sufficiently full, cover the openings with a test tube stopper while the test tube rims are still submerged. Remove the test tubes; the one placed over the cathode electrode will be filled with oxygen gas, while the test tube placed over the anode electrode will contain hydrogen.
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