Bring Science Home An electrolyzing science project from Science Buddies
Key concepts Water Chemistry Electricity Molecules Introduction Background If making water from its elements is so dangerous, what about the reverse reaction? Splitting water into its two components is much easier to do and is called water electrolysis. Making hydrogen or oxygen this way seems simple. But as you probably suspected, this reverse reaction needs an energy input, which is why it is also called an endothermic reaction. In water electrolysis, the source of energy used for the reaction to occur is electricity. An easy way to generate electricity is with a battery. Because pure water is not good at conducting electricity, however, electrolysis requires the addition of an electrolyte, such as salt or acid. The electrolyte dissolves in water and separates into ions (electrically charged particles) that move through the solutions and are able to conduct electricity this way. To add electricity to the solution, you also need two electrical conductors that make contact with the water. These are called electrodes and are mostly metals or other conductive materials. When electrical current is applied to the electrodes, the ions (electrically charged atoms) in the electrolyte—including the positively charged protons (H+) and negatively charged hydroxyl ions (OH-) that result from the self-ionization of water—start moving toward the electrode with the opposite charge where either hydrogen or oxygen is produced. You can see this for yourself in this activity and even capture both gases! Materials
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Observations and results When you looked closely, you might have observed that on the graphite electrode connected to the negative pole of the battery more gas was formed than on the other side. Collecting the two gases with the jumbo straws probably demonstrated this even better. After 10 minutes the water level at the negative pole should have been about half that of the positive side, meaning that you collected about double the amount of hydrogen gas compared with oxygen. The difference is due to the fact that one water molecule has two hydrogen atoms to one oxygen atom, as explained above. This means it takes two water molecules (2 H2O) to make one oxygen molecule (O2). At the same time, however, two molecules of water (2 H2O) can make two molecules of hydrogen (2 H2). Whereas hydrogen and oxygen are formed at the electrodes, the leftover reaction products from water are protons (H+ on the side of oxygen) and hydroxyl ions (OH- on the side of hydrogen). You can visualize this by putting a pH strip into the solutions in the jumbo straws above each electrode. The solution in the straw that was put on top of the negative battery pole electrode should show a basic pH (7 or greater) whereas the other one should be acidic (pH less than 7). Other electrode reactions can happen, too, if there are ions in the solution that compete with the hydrogen or oxygen production. You might have observed once you added salt (sodium chloride) to your electrolyte, it started smelling like at a swimming pool. Instead of oxygen, chlorine is produced at the positive battery pole, which is also used for disinfection of swimming pool water. If you used metal thumbtacks as electrodes instead of graphite pins, the metal (usually steel or brass) will dissolve, or corrode, at the positive battery pole and instead of (or in addition to) gas production, you should see that the metal thumbtack turns reddish brown. This demonstrates that electrolysis is not only a technique for splitting water into its components but can also drive other reactions that otherwise would not occur spontaneously. Cleanup More to explore This activity brought to you in partnership with Science Buddies Discover world-changing science. Explore our digital archive back to 1845, including articles by more than 150 Nobel Prize winners. Subscribe Now! |