I'm working on developing physics lab activities that my students can do at home during the covid quarantines. For an activity involving waves and thermodynamics, I came up with an experiment at home in which I blow over the top of a bottle (beer bottle or 2-liter soda bottle), measure the frequency, and then spray a can of "dust-off" (difluoroethane) into the bottle and measure the frequency again. The frequency gets lower because difluoroethane is much more dense than air.

Environmentally, it looks like difluoroethane is fairly benign. It's used as an alternative to substances that are worse for our planet's atmosphere. People can huff it, and if you do, it's dangerous. It's flammable, so you want to stay away from flames and sparks. All of those things seem like they're not huge problems for my purposes.

But the label also says, "Never spray into an enclosed space, such as a trash can or paper shredder." What's up with this? That's exactly what I need to do for this experiment. Why is this dangerous?

Would there be any safety or environmental advantage to using 2,3,3,3-tetrafluoropropene ("Opteon" or R-1234yf)? It doesn't seem as readily available in small packages.


Following up a comment by @MaxW: Interesting, thanks. I found a video of the liquid exploding when it's poured into water. Also a demonstration that it's flammable in gas form if you collect it in an open-topped container. Burning produces toxic $\ce{HF}$ and $\ce{COF2}$. So I'm hypothesizing (tell me if I'm wrong) that it's safe for my application if you (1) stay away from any source of ignition, (2) do it in a well ventilated area, and (3) in case of ignition, get away and let the fumes dissipate thoroughly. I'd be interested in answers as to whether this seems sufficient.

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    $\begingroup$ Making an educated guess, I'd say that the danger is explosive limits in a container with an open top would be the worrisome problem. Explosive limits, vol% in air: 3.7-18 $\endgroup$ – MaxW May 11 '20 at 20:14
  • $\begingroup$ That 1,1,-difluoroethane (physically?) explodes in contact with water is rather surprising. Unfortunately the explanation given in the video is rather unsatisfying. Have you considering using other gasses and volatile liquids (saturating the air with vapour), like the butane in the video, or diethyl ether, or acetone? As you say, flammability isn't a huge issue for you, with the proper precautions. I do understand sourcing the material is a major issue, but I can't think of clear winner from a hardware store (e.g. dichloromethane would probably work really well, but it's more toxic, etc..). $\endgroup$ – Nicolau Saker Neto May 12 '20 at 2:21
  • $\begingroup$ I don't think the potential explosion of liquid difluoroethane is a risk if you are just working with the gas. And not using vessels filled with water. $\endgroup$ – matt_black May 12 '20 at 13:50
  • $\begingroup$ @NicolauSakerNeto: I'd be happy to find an alternative material, but it has to be something cheap that we can buy and mail to students. The technique involves blowing over the top of the bottle, so I don't want to use acetone, etc. -- it has to be an inert gas. $\endgroup$ – Ben Crowell May 14 '20 at 0:25

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