How do I keep hydrogen or oxygen in a test tube when pulling them out of water? I have a Styrofoam cup with two metal thumbtacks pushed though the bottom, and about 100 ml of distilled water mixed with 6 grams of sodium hydroxide in the cup. The cup is sitting on a 9 volt battery which separates the hydrogen and oxygen in the cup through the thumbtacks.

I put a test tube over the thumbtack that is producing oxygen and one over the thumbtack producing hydrogen. I'm trying to capture the hydrogen and oxygen in the test tubes and light them on fire. My problem is that when I pull out the test tube, the little water left in the tube pulls out all the gas when it falls out. How do I keep that from happening?

Here's a video of what I'm trying to do.

  • $\begingroup$ Close the tubes with a rubber cap before pull them out of the water. $\endgroup$
    – RFG
    Commented Nov 21, 2013 at 18:32
  • 1
    $\begingroup$ I assume you're always keeping the hydrogen tube facing downwards since hydrogen is much less dense than air. The oxygen tube has to face upwards outside of water since it is slightly denser than air, or it will spill out. A residual column of water inside the tube might cause your gasses to disperse because as the water leaves, atmospheric air enters suddenly and can push the pure gasses out. It should be fine if you wait for the tubes to be completely filled with gas, or if you slowly let air inside the tubes. You can also use your thumb to cap the tubes when necessary. $\endgroup$ Commented Nov 21, 2013 at 21:35
  • $\begingroup$ In a high school experiment we took plastic pipettes, cut the tubes off so it was just the bulb, and marked them at regular intervals. We set up the electrolysis with 2 wires, so oxygen was being made at one and hydrogen at the other. We filled the bulb with the electrolysis solution and placed it over one wire to collect 1 gas, then carefully moved it to the other wire. As the gas was collected, it displaced the liquid. We then carefully placed the bulb with the gas on exposed wires from a kitchen lighter and ignited it to blow it across the lab. $\endgroup$
    – user137
    Commented Jul 27, 2014 at 5:29

1 Answer 1


The cheapest and simplest modification to the procedure in the video is to use plastic test tubes with threaded caps. When a tube is full of gas, tighten the cap just short of making a seal. Then squeeze the tube to force out any water at the bottom and tighten the cap.

This will work best if you use a cap of the type having a plastic cone on the interior of the cap, rather than just a flat cap. One down side to this method is that there will be a slight negative pressure in the tube with respect to the ambient air, so when you open it a small amount of air will be sucked in. And remember that when you do remove the caps, hydrogen is much less dense than air and will want to rise, so keep the tube inverted until you do whatever you plan to do with it. Likewise, oxygen is slightly less dense than air, so keep it pointing upwards when uncapped.

Remember that you are working with a pretty strongly caustic solution here; wear gloves and goggles.


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