I'm looking for a cheap and safe way to demonstrate how releasing pressurised gas from a container can take the container from room temperature to freezing (cold enough for ice to form on the container itself) in a classroom environment. I basically have two ideas:
- Diving flask. This is relatively safe because there is just air inside, so there are no dangerous substances involved. The price of renting one is also not too high, and I do have a licence, so there won't be any bureaucratic problems involving the excuse "but I'm not going to be using it for diving". However, it is heavy, it needs to be rented and transported, and I don't actually know that it's going to become cold enough. I never tried releasing that much that fast. Releasing air directly from the flask is also quite noisy.
- Compressed air for electronic cleaning. This is easier to get a hold of, and easier to transport, although the price is about the same. In this case I know that it will become cold enough. However, it's usually not really air inside it, which will mandate a well ventilated environment. It's also smaller, so it's more difficult to get away with me demonstrating it alone as opposed to everyone getting their own to try it out (or, at least in groups), which again makes me worried about ventilation (and also costs)
Are there any other obvious instances of compressed gas that would work that I've missed? Have you done this activity (either as teacher or as student), and want to share what worked for you?
