Splitting Molecules: Electrolysis

Students are always excited when they get to do something that they perceive as difficult. Splitting a water molecule sounds pretty complicated! In reality this is a pretty simple process. As a teacher who is two years through Open Sci Ed implementation, I am always looking for opportunities for students to do a little more because in some instances the curriculum can get a little discussion and observation heavy. The electrolysis demonstration in 7.1 Lesson 10 was a perfect opportunity for students to complete the set up and observe the process. My challenge was to come up with a simple set up using materials that were easy to obtain and inexpensive.

Material Notes (per set up)

• 8 oz plastic cup - clear works best for easy viewing
• 2 thumb tacks
• Styrofoam - I just used some form a package we received
• Masking tape
• 9V battery
• Distilled or deionized water
• Magnesium sulfate (epsom salt)
• 2 test tubes (150mm) with stoppers
• Test tube tongs
• Matches or lighter
• Wooden splints

The Set-Up
Step One: Using styrofoam from a package we received in the building I made stands that matched the height of a 9V battery. The stand was large enough to hold an 8oz plastic cup.

Step Two: Press two thumb tacks through the bottom of an 8oz plastic cup. The distance between the tacks should be the same as distance between the terminals on the 9V battery.

Step Three: Mix your hydrolysis solution. To make 500ml of solution you will need to dissolve 60 grams of magnesium sulfate (epsom salt) in 500 ml in distilled or deionized water. This will make a 1M solution. If you make a higher molarity solution, the reaction will run faster.

Student Set-Up

Step One: Students fill their cup with tacks in the bottom about ¾ full with the hydrolysis solution.

Step Two: Put the battery into the stand.

Step Three: Fill two test tubes TO THE BRIM with hydrolysis solution. I have students do this over an empty beaker or other container so they can truly fill it all the way.

Step Four: Place your thumb over the top of the tube and lower the tube into the cup on the stand. Once you have the top of the test tube below the water line you can remove your thumb. Position each test tube so that the tip of the tack is inside the one of the tubes.

Step Five: Place the cup on the stand over the battery so that the tacks in the cup are in contact with the battery terminals. When connected correctly you should see bubbles. Since there are 2 hydrogen atoms for every oxygen atom, one tube will fill faster than the other - not really twice as fast since oxygen atoms are larger but there will be a noticeable difference.

What’s Next?
Once students have observed for a while I have them carefully bring their cups to one area where we test the gasses formed. I do this as the demo portion for safety reasons. In the OpenSciEd curriculum they are trying to figure out what gasses are based on observations and the Common Gasses Chart from Lesson 5. Here is what will happen with the gas you have created. In both cases I hold the test tube with tongs while applying the flame.

*Hydrogen is very light so pull the tube from the cup keeping the opening pointed downward. The gas will stay in the top of the tube. You can use a rubber stopper if you choose. I lit a wood splint and put it upward into the tube, the gas will explode making a popping noise. Scares me every time but the kids love it and since each setup was making hydrogen gas we get to do it lots of times.

*Oxygen is a bit heavier than air, so I get better results if I have the wooden splint lit and then pull the tube out of the cup. Again you can use a rubber stopper if you choose. Then place the wooden splint right below the tube and slowly put it in the tube. The oxygen should brighten the flame.