# The dry ice "witch's brew" **URL:** \ **Category:** Factual Questions\ **Created:** [September 15, 2021, 2:06pm UTC](https://boards.straightdope.com/t/the-dry-ice-witchs-brew/950698 "2021-09-15T14:06:54Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [September 15, 2021, 2:06pm UTC](https://boards.straightdope.com/t/the-dry-ice-witchs-brew/950698/1 "2021-09-15T14:06:54Z")
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? ---
**Author:** ![AlsoNamedBort](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alsonamedbort/32/3213_2.png) [@AlsoNamedBort](https://boards.straightdope.com/u/AlsoNamedBort)\ **Post date:** [September 15, 2021, 2:44pm UTC](https://boards.straightdope.com/t/the-dry-ice-witchs-brew/950698/2 "2021-09-15T14:44:22Z")
> [@CookingWithGas](#): > > 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. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [September 15, 2021, 3:01pm UTC](https://boards.straightdope.com/t/the-dry-ice-witchs-brew/950698/3 "2021-09-15T15:01:13Z")
I believe the fog is just water condensing out in the very cold air. > **[Here Is the Reason Why Dry Ice Makes Fog](https://www.thoughtco.com/why-dry-ice-makes-fog-606404)** > > Dry ice is frozen carbon dioxide and is used for smoke and fog special effects. Learn what dry ice is and how it is used to make fog. ---
**Author:** ![filmore](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@filmore](https://boards.straightdope.com/u/filmore)\ **Post date:** [September 15, 2021, 3:12pm UTC](https://boards.straightdope.com/t/the-dry-ice-witchs-brew/950698/4 "2021-09-15T15:12:52Z")
> [@CookingWithGas](#): > > I always wanted to do that but never had any dry ice. 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: [https://penguindryice.com/where-to-buy-dry-ice/](https://penguindryice.com/where-to-buy-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. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [September 15, 2021, 3:47pm UTC](https://boards.straightdope.com/t/the-dry-ice-witchs-brew/950698/5 "2021-09-15T15:47:55Z")
> [@filmore](#): > > You may be able to find dry ice sold in your local grocery store. 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.) ---
**Author:** ![crowmanyclouds](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crowmanyclouds/32/19884_2.png) [@crowmanyclouds](https://boards.straightdope.com/u/crowmanyclouds)\ **Post date:** [September 15, 2021, 3:51pm UTC](https://boards.straightdope.com/t/the-dry-ice-witchs-brew/950698/6 "2021-09-15T15:51:21Z")
> [@filmore](#): > > 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. > **[Leidenfrost effect](https://en.wikipedia.org/wiki/Leidenfrost_effect)** > > The Leidenfrost effect is a physical phenomenon in which a liquid, close to a solid surface of another body that is significantly hotter than the liquid's boiling point, produces an insulating vapor layer that keeps the liquid from boiling rapidly. Because of this repulsive force, a droplet hovers over the surface, rather than making physical contact with it. The effect is named after the German doctor Johann Gottlob Leidenfrost, who described it in A Tract About Some Qualities of Common Water. ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [September 15, 2021, 3:51pm UTC](https://boards.straightdope.com/t/the-dry-ice-witchs-brew/950698/7 "2021-09-15T15:51:52Z")
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. > [@filmore](#): > > 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. The one with the molten metal is the [Leidenfrost Effect](https://en.wikipedia.org/wiki/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. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [September 15, 2021, 3:52pm UTC](https://boards.straightdope.com/t/the-dry-ice-witchs-brew/950698/8 "2021-09-15T15:52:28Z")
> [@filmore](#): > > 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. 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.