The dry ice "witch's brew"

I got a package yesterday packed in dry ice. Even though it was not Halloween I had to take a brick of it and put it in a bucket of water to make a foggy “witch’s brew” cauldron. I always wanted to do that but never had any dry ice.

What is the fog? Is it just the dry ice sublimating? Or is there a chemical reaction with the water creating some new gas?

Why doesn’t the dry ice just freeze the water?

It does. After the bubbling and fog die down., you’ll probably find a chunk of dry ice surrounded by frozen water. At the same time the dry ice is warming up the water is cooling down, but it’s not instant. So how much ice you get depends on the temperature of the water and the amount of dry ice you have.

I believe the fog is just water condensing out in the very cold air.

You may be able to find dry ice sold in your local grocery store. And it’s relatively cheap. I see these Penguin coolers in stores around here: Where to buy dry ice - Penguin Brand Dry Ice®

As for not freezing the water, I would guess one complication is the gas pushing the water away from the block of ice. The gas may be cold, but it’s going to act as a sort of insulating layer between the water and the dry ice. I believe a similar effect allows people to quickly swipe their finger through a molten stream of metal. The moisture on their finger turns to steam and keeps the molten metal from actually touching their skin.

It is (or used to be, at least) sold at Ingles. In blocks around a half inch thick ard around the width and lenght of a sheet of notebook paper. I played around with one once. (With mine, atmospheric moisture would form a thin coating of water-ice over any exposed area very quickly–I could actually pick up small chunks safely because I sad touchng H2O, not CO2.)

It absolutely does freeze the water. If you’re using it at a party, you eventually have to go knock the water ice off the dry ice to get it to bubble and fog better. And it slightly carbonates the water it’s in as well.

The one with the molten metal is the Leidenfrost Effect. Basically the water on your skin will create an insulating steam later between your skin and the metal itself. It’s probably in reverse with dry ice; the layer of CO2 that’s generated is protecting the dry ice from your relatively hot finger.

I don’t think that’s it. The energy in the water is the main source for sublimating the dry ice. If you leave it just exposed to air it produces a lot less “smoke” and lasts much longer. But as long as it’s not freezing to the surface of the dry ice the water cooled down by it is going to sink until the point where all the water has cooled down to 4 C.