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Having a Gas with Water

Science Snack
Having a Gas with Water
Use electricity to break water into its elemental components.
Having a Gas with Water
Use electricity to break water into its elemental components.

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.

Video Demonstration
Having a Gas with Water | Science Snacks
Tools and Materials
  • 9-volt battery—the inexpensive, carbon-zinc kind, often labeled "super heavy duty": You can use old batteries (though they may take a bit longer to work), but do not use lithium, alkaline, or rechargeable batteries
  • Two rubber bands
  • Two stainless-steel screws, each at least 1.5 inches (40 mm) long
  • One block of oil-based modeling clay, also known as polymer clay (brand names include Sculpey, Fimo, Plasticine, and Plastilina) Caution: Salt-dough clays such Play-Doh will not work and might cause a short circuit
  • Clear glass or plastic cup or beaker
  • Lukewarm water
  • Epsom salt; Caution: Do not substitute table salt (NaCl) for this activity, since it can produce chlorine gas
  • Teaspoon
  • Safety goggles
  • Two laboratory-grade glass test tubes (thicker-walled “combustion” tubes are recommended)
  • Lighter or matches
  • Wooden splint or wooden coffee stirrer
  • Optional: acid-base indicator such as phenol red, phenolphthalein, bromothymol blue, or cabbage juice indicator (see the Teaching Tips section below for instructions on how to make your own)
    Note: Make sure you know which colors indicate acid and/or base when using your chosen indicator, and that you’re aware of any safety issues involved in its use
Assembly
  1. Wrap one rubber band around the 9-volt battery so that it lies across both of the battery terminals. Wrap the other rubber band around the battery so that it crosses the first rubber band perpendicularly, passing between the two battery terminals.
  2. Slide the two stainless-steel screws beneath the rubber band so that each screw is held to the top of each battery terminal (see photo below). The heads of the screws should point in the same direction, and the screws should not touch each other.
  3. Since your battery will be submerged in saltwater, you’ll need to protect it by surrounding it with modeling clay. Press small pieces of clay between and around the electrodes (see left photo below). As you work, note which terminal is positive and which is negative by carving a mark into the surrounding clay, and be sure to keep the tips of the screws exposed (click to enlarge right photo below). Continue pressing and smoothing the clay until your device is sealed all around. Be sure your terminal marks are recognizable and the tips of the screws are clear. This is your electrolysis device, and the screws are your electrodes—they’ll be conducting electricity during the experiment.
  4. Fill your glass about three-quarters full with lukewarm water.
  5. Put on your safety goggles.
  6. Pour one teaspoon (5 mL) of Epsom salt into the water. Stir your solution to help dissolve the salt.
  7. From your glass of saltwater, carefully fill each test-tube to the brim. Put them aside, but prop them up vertically so they don’t spill.
  8. Carefully lower your electrolysis device into the solution with the electrodes (screws) pointing upwards. The ends of the screws should be under the surface of the saltwater. If they’re not, just add a little more water.
  9. Once your electrolysis device is submerged in the saltwater, you should begin to see tiny bubbles emerging from the electrodes. To capture these bubbles, cover the mouth of one solution-filled test tube with a thumb or finger, pressing firmly to make a water-tight seal. Then, carefully invert the test tube and submerge its mouth into the glass of saltwater, making sure you don’t lose any solution. Your thumb or finger will get wet. Finally, remove your thumb or finger from under the inverted and submerged test tube. The solution should stay in the tube.
  10. Keeping the mouth of the test tube under the surface of the water, carefully move the opening of the inverted test tube over one of the bubbling electrodes. Make sure that the gas bubbles from only one electrode are able to rise up and collect inside the tube. The electrode should be able to support the test tube in a vertical position while it collects gas.
  11. Repeat Steps 9 and 10 with the other test tube, positioning it over the other bubbling electrode (click to enlarge the photos below).
To Do and Notice

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.

What's Going On?

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 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.

$$2H_{2}O(l) \rightarrow 2H_{2}(g) + O_{2}(g)$$

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.

Going Further

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.

Teaching Tips

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.