EXPERIMENT 5.1

PURPOSE:

To perform a decomposition reaction through the use of electrolysis to separate water into hydrogen and oxygen gas.

MATERIALS:

• Water
• 9 volt battery (A new one works best)
• Two test tubes (You can purchase these at a hobby store. If you cannot get them, use the tubes that florists put on the stems of cut flowers.)
• Beaker or glass (It must be deep enough so that when it is nearly full of water, the battery can stand vertically in the glass and still be fully submerged in the water.)
• Epsom salts (You can get these at any drugstore or large supermarket.)
• Tablespoon
• Eye protection such as goggles or safety glasses

BACKGROUND:

In Experiment 2.3, you observed the electrolysis of water by attaching wires to a battery and placing those wires in a solution of water and baking soda. In this experiment, you will use a slightly different method of water electrolysis to perform a decomposition reaction and to collect information about the chemical composition of water.

QUESTION:

What substances are produced when water decomposes? What ratio of products do you expect to create through the decomposition reaction?

HYPOTHESIS:

Write what you predict to collect in the test tubes when water decomposes. Include the approximate amounts you think you'll collect relative to each other. In other words, do you predict you will collect more hydrogen gas or oxygen gas, or will they be the same amount?

PROCEDURE:

1. Fill the beaker with water.
2. Add 3 tablespoons of Epsom salts and stir so that they dissolve. Don't worry if there are some undissolved Epsom salts at the bottom of the glass.
3. Stand the battery vertically at the bottom of the glass. You should immediately see bubbles forming on each terminal.
4. Allow the battery to sit like this for 10 minutes. The reason you must do this is that there is air trapped in the battery, and the air escapes in the form of large bubbles. If you watch your battery for a few moments and see some large bubbles coming from the center of the terminals or from the edges of the battery, those are air bubbles that will mess up the results of the experiment.
5. After 10 minutes, take your test tubes and fill them completely with the solution in the beaker. The best way to do this is to fully immerse the test tubes, tilting them to let all the air out. In the end, there should be no air bubbles in the tubes. If you cannot fill the test tubes from the beaker, make another mixture of water and Epsom salts in a bowl and fill the test tubes from the bowl.
6. Take the two tubes and, while keeping their openings fully immersed in the solution, hold them upside down. You should still see no air bubbles in the tubes. The solution will not pour out as long as you keep the tops of the tubes submerged in the solution at all times.
7. As simultaneously as possible, place one tube over each terminal of the battery, once again making sure that the openings of the tubes are always submerged in the solution. Your experiment should look something like this:

8. With a little effort, you can make the test tubes balance on the battery so that you do not have to hold them there. If you have trouble doing that, you can tape the test tubes together by two knives across the glass, sandwiching the test tubes between the knives.
9. Once you have gotten the test tubes to stand on the battery on their own, watch what's happening. The gases forming at each terminal travel up the test tube until they reach the top. At that point, they fill the top of the test tube, pushing away the water that was there. As time goes on, then, the gases produced at the terminals will be collected at the top of the test tubes. Draw your apparatus and record your initial observations in your data table.
10. Let the experiment sit for a while. Go back periodically and check, and you will see that the water level in the test tubes is decreasing. This is happening because as the gases are produced, they exert pressure on the water in the tubes. This pushes down on the water in the tubes, forcing water out of the tubes and into the glass. Add any observations to your data table.
11. The solution will probably turn a nasty color after a while, because substances in the battery will eventually leak into the solution. That's okay, though. It looks ugly, but it does not affect the results of the experiment. Be sure not to get any of the solution in your mouth! The chemicals coming from the battery can be toxic at high concentrations.
12. When one of the test tubes is filled halfway with gas, draw what the experiment looks like in your data table. Be sure to note which terminal is the positive one and which is the negative one. If it is not marked on the battery, the larger terminal is negative, and the smaller terminal is positive.
13. Clean up: Throw the battery away, pour the contents of the glass into the sink, rinse the glass thoroughly, and flush all the liquid down the drain with plenty of water. Put everything away.

DATA TABLE:

INTERACTIVE SIMULATION:

Watch this video demonstration of water electrolysis to better understand the decomposition reaction.

CONCLUSION:

Based on your observations, answer the following questions:

1. Were you able to collect gases in the test tubes on the positive and negative terminals of the battery? How much gas was in each test tube relative to each other?
2. Knowing the composition of water (H₂O), can you tell which terminal produced oxygen gas and which terminal produced hydrogen gas?
3. Write a conclusion paragraph answering these questions and making connections to the text.

SUBMIT YOUR RESULTS: