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Build a simple electrolysis device using a 9-volt battery wrapped in oil-based modeling clay, trap the two gases produced, and end with a bang as you test their composition.
Watch your device closely. Right away, you’ll probably notice bubbles forming at the electrodes. Does one electrode seem to emit more bubbles than the other? Is it the positive or the negative electrode?
If you want, you can add your acid-base indicator to the glass of saltwater at this point. Do you notice any change in color?
Allow this setup to stand and collect bubbles for several hours, or even overnight. After several hours, you should be able to see that one of the test tubes has collected a lot more gas in it than the other.
You can check the composition of the gas in the test tubes by doing a “splint test.” Caution: Work with an adult, and use eye protection! You’ll be working with flames.
First, test the gas in the test tube that has more gas in it. It should be the one over the negative electrode. If both tubes are completely filled with gas, make sure you look at the battery to see which tube is over the negative electrode. Put on your goggles. Light the wooden splint on fire (see left photo below). Then, quickly pull the test tube out of the water and hold the flaming splint near the opening (see right photo below). Hold firmly! You might be startled by a loud POP!
Now test the gas in the test tube that filled more slowly. Again, light your wooden splint—but this time, blow out the flame so there’s just a glowing ember at the end of the stick.
Quickly pull the second test tube out of the water and insert the glowing splint into the open end (see left photo below). This time you should see the splint relight (see right photo below). As soon as the splint relights, pull it out of the tube, blow it out, and insert it again. You might be able to get the splint to relight several times.
Electrolysis is chemical decomposition produced by electricity—in this case, the chemical you’re decomposing is water.
The molecular formula for water is H2O, where H stands for the element hydrogen and O stands for the element oxygen. In a glass of water, many of the molecules naturally separate out into hydrogen ions (H+) that are positively charged and hydroxide ions (OH-) that are negatively charged. Your electrolysis device causes reactions that pull apart the water even more.
Since opposite charges attract, the oxygen-containing hydroxide ions migrate toward the positive electrode and the hydrogen ions migrate toward the negative electrode.
Elementally, both oxygen and hydrogen prefer to be diatomic, or two-atom molecules. At the positive electrode, oxygen atoms get pulled from the hydroxide ions and then combine to make oxygen gas (O2) bubbles. Likewise, at the negative terminal, hydrogen ions combine to make hydrogen gas (H2) bubbles. Below is the chemical equation that describes what happens.
2H2O(l) → 2H2(g) + O2(g)
Both oxygen and hydrogen gases are clear and odorless. So how do you know which test tube contains which gas? Here’s a clue: One filled faster than the other. There are twice as many hydrogen atoms available to form a gas, and thus the volume of hydrogen gas that forms should be greater than that of the oxygen gas.
The splint test gives another clue: Hydrogen gas is very flammable—a fact made famous by the Hindenburg zeppelin disaster—and makes an explosive popping sound when lit. Oxygen, on the other hand, is not actually flammable, but it is necessary for combustion, which is why your split relit in oxygen gas.
Epsom salt, also known as magnesium sulfate (MgSO4), is dissolved in the water to help your battery break up the water more efficiently. Epsom salt breaks up into charged particles called ions, and these help carry the electric current through the solution.
Did you add acid-base indictor to your experiment? If you did, you probably noticed color changes happening around your electrodes and inside your test tubes. The production of gas at each terminal changes the pH in the surrounding solution. To find out more about this, check out the related Snack, Indicating Electrolysis.
Simply submerging a “naked” 9-volt, carbon-zinc battery in Epsom salt solution will produce bubbles—and also show the importance of protecting the battery in the modeling clay. The battery will quickly short out and stop working, and the positive battery terminal will rust immediately from the oxygen produce there.
One of the safest and cheapest acid-base indicators is red cabbage juice, which is easy to make on your own: Chop up one small head of red cabbage. Heat a saucepan of water until boiling, then shut off the heat and add the cabbage. Let it steep for 10 minutes, until the liquid turns a deep purple. Strain it and discard the solids. Your cabbage juice indicator is now ready to use in this and many other experiments. It will turn pink in an acidic environment, purple in a neutral environment, and blue-green or even yellow in an alkaline environment.
Break water into hydrogen and oxygen with a simple device.
Make a conductivity meter and let your electrolytes shine.
Construct a simple battery that can power a light.
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Attribution: Exploratorium Teacher Institute