Methane tower or methane bubbles
Requirements:
- Bucket (5 liters)
- 2 liters water
- 20 milliliters detergent (dishwashing liquid works good)
- Gas separator (for example the top of a camping gas burner or a shower head)
- Methane gas
What makes this experiment interesting is that methane gas has a smaller density than regular air. It’s 0.668 kg/m3 for methane versus 1.205 kg/m3 for air. This means that methane gas will rise up in the air under normal circumstances. We take the bucket and fill it with 2 liters of water plus the soap. Now we want to start making bubbles in the soapy water, the major difference with normal bubbles and these bubbles is that they’re filled with methane gas rather than air. Because of the smaller density for methane gas the bubbles won’t spill over the edge of the bucket but they’ll form a nice column which can grow meters high.
The fun begins when you have a proper methane tower. Make sure the smoke detectors are off!
With some fellow students we’re setting up a workshop “Translating spectacular experiments to the classroom situation” and the methane tower is a part of this. It looks really easy to build when you see the YouTube movies, but when you try it yourself you’ll find out it’s much harder than it looks. It took us a couple of hours to come up with the right ratio. Although methane gas has a smaller density than air, soap does not! So having a lot of soap in your tower will make your tower fall over to the side.
If you want your pupils to experiment with this themselves you could use a 250 ml beaker. Fill it with 100 ml water and 2 ml soap. A pasteur pipette works great as gas separator. You do might want to give it a try yourself before you let your students take off. 
You could use this experiment when you’re talking about density.