In our previous episode, we saw our valiant heroes attempt to make ice cream from a secret recipe, using only south pole snow (at -28°F) to freeze it into a delicious soft-serve consistency. It took approximately 12 minutes, considerably longer than it took in the classroom using regular ice and rock salt. What caused this unexpected (at least for me!) result? Watch and learn, little one - watch and learn...
In the classroom, the salt melted the ice into water at about +28°F, which was cold enough to transfer the heat out of the cream mix. Liquid is a far better conductor of heat than air, so when I tried it with snow, which is primarily air trapped in the ice crystals, my hypothesis is that the air merely insulated the cream mixture and prevented it from freezing quickly.
So: if I could somehow create a supercold liquid from the snow, then the ice cream would freeze quickly. But if I just added water, the water would freeze into ice crystals, so I'd lose my liquid. I need a liquid that will remain liquid, even at super cold temperatures!
Antifreeze is readily available here, in the forms of ethylene glycol and propylene glycol. Neither of which is very good for human consumption. Alcohol would do the trick, and be less toxic, so I borrowed some isopropyl alcohol from the Med Clinic (it's the same stuff they sterilize your arm with, before giving you an injection). Alcohol stays liquid at very cold temperatures, and is often used in thermometers (the ones with red liquid inside). But we had to be careful to wipe off the inner bag well, because isopropyl alcohol still isn't great for you, and it would taste bad.
I gathered a group of assistants and we took our experiment back outside. The isopropyl alcohol turned the snow into a super-cold slush, so cold in fact that it was probably dangerous to handle with bare hands. It worked magnificently!
This experiment, unlike the salt and ice method, would be tough to replicate in the classroom. You need ice or snow that is very very cold to start with, probably colder than you can get it in your freezer. For those of you who live in places where it gets really cold in winter, you might be able to try it.
Dry ice would be a bad idea, if you sealed the outer ziplock bag. Why do you think that would be bad? I was going to write, "I'll leave that as an exercise for the reader to figure out" but that would open me up to liability from people who read that as a license to go try it - so, I'll just tell you! What happens to dry ice as it warms up? It turns from solid straight to gas. The bag would expand quickly, and eventually pop, scattering dry ice chunks and isopropyl alcohol all over the place, and probably poke out your eye. No amount of ice cream can make up for a poked out eye!
Your chemistry teacher might be able to do it using dry ice in a beaker instead of inside a sealed bag... can't hurt to ask!