I'm thinking of something similar to the liquid used in the classic dipping birds. Temperature differential in two "bulbs" at each end of a tube should cause vapor to expand and push liquid to the top, shifting the weight and tipping it. The closer to room temperature the boiling point is, the better. Thanks.

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    $\begingroup$ Dichloromethane $\endgroup$
    – MaxW
    Commented May 3, 2019 at 14:56
  • $\begingroup$ Dichloromethane is a carcinogen and liver toxin (if this is for schools). If you want to be green, try "A dunking bird of the second kind". It just uses water and patience. Published in American Journal of Physics: aapt.scitation.org/doi/10.1119/1.1703543 $\endgroup$
    – ACR
    Commented May 3, 2019 at 15:10
  • 1
    $\begingroup$ @M.Farooq It's suspected to be carcenogenic. And not very badly, because with its widespread use, one would know, otherwise. $\endgroup$
    – Karl
    Commented May 3, 2019 at 19:11
  • $\begingroup$ Benzene is present in fuels, and it is a known human carcinogen. Widespread use does not guarantee safety (standards change all the time). If you talk to an old timer, they always say that they used benzene freely in labs. Not anymore. Organic chemists use DCM all the time and work with worse substances. Still they live very long or sometimes quite short. I would avoid volatile halogenated materials in general because they are quite bad for the liver even if they are suspected carcinogens. $\endgroup$
    – ACR
    Commented May 3, 2019 at 21:52
  • $\begingroup$ Dichloromethane is surely toxic. While it can be acceptable for our columns I would not use it for demonstration aimed to general audience nor in a general class activity. $\endgroup$
    – Alchimista
    Commented May 4, 2019 at 6:31

2 Answers 2


Yes, there are lots. Start with Wikipedia's list of refrigerants; while it doesn't call out flammability, any fully-halogenated compounds will be effectively non-flammable. Trichlorofluoromethane has a standard boiling point just above standard room temperature.

You didn't say anything about pressure in your question. If you don't mind pressures higher than one standard atmosphere, and don't mind toxicity, candidates like ammonia, sulfur dioxide, and chlorine come into play. If you don't mind low pressure, you could even use water, although it might freeze as it boils off.

  • $\begingroup$ Don't forget bromine $\endgroup$
    – A.K.
    Commented May 3, 2019 at 19:18
  • $\begingroup$ @A.K. He's the rugged guy in Lord of the Rings who gets shot up with arrows, right? $\endgroup$ Commented May 3, 2019 at 22:59
  • $\begingroup$ I suppose the higher the pressure the faster the whole sequence would progress, so lets says as high a pressure as is reasonable. $\endgroup$ Commented May 5, 2019 at 3:07
  • $\begingroup$ Can you tell us more about your safety constraints (flammability, pressure limits)? HFC-134a (1,1,1,2-Tetrafluoroethane) has a room-temperature vapor pressure between 4 and 5 atmospheres, maybe 25%-60% higher than the pressure in a full two-liter soda bottle. $\endgroup$
    – jeffB
    Commented May 5, 2019 at 14:26
  • $\begingroup$ The higher the pressure the better. A higher pressure differential at as little of a temperature difference would be best. It should be able to hold at least 10 atm. Flammability isn't a huge concern. It will all be inside a sealed system. It's just a bit easier and safer to work with in general. $\endgroup$ Commented May 9, 2019 at 14:22

Most commercial refrigerants would work. There is a caveat, which is that they are usually also ozone depleting and are also greenhouse gasses with high , and the EPA will ream your bunghole if you handlGlobal Warming Potential (GWP)e them improperly. Almost all of these refrigerants are halogenated hydrocarbons (usually HFCs or CFCs).

One of the more modern refeigerants is R-1233zd (1-Chloro-3,3,3-trifluoropropene). R-1233zd is neither an HFC or a CFC, it is an HFO (hydrofluoro-olefin), It is safer to handle and more environmentally friendly than the former categories (zero ozone depletion and low GWP). However, it's also very expensive (~$50/lb), and you'll likely need an EPA 608 license to purchase it (though I may be mistaken here).

Update R-1233zd is actually flammable but only slightly, and it also has a very slow combustion rate of <10cm/s.


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