[majikandy]

Always heard about this but never tried it. Sounds like fun. Great description, very clear and much better than just saying it lowers the temperature! Nice writing.

[timrice]

Author here, thank you!

It's been bouncing around in the back of my brain for a long time.

I couldn't find any clear and concise explanations about what really happens when salt is added to ice, so I did some research and wrote it out myself :D

[tylerhou]

Here is another way that I thought of it.

1. To make ice cream, we need to cool milk/cream below the freezing point of water (because milk/cream contains water).

2. To cool things, liquids have good thermal conductivity properties, so we would prefer to use a liquid.

3. We need some substance which is still a liquid at slightly below freezing.

4. It happens that salted water has this property and is relatively cheap.

[majikandy]

Remember for point 4 it needs something in that salted water that can keep it at the lower temperature.

[tylerhou]

Also, we would like a phase transition slightly below freezing so that it's easier to keep a stable temperature.

[ljf]

Thanks for this - I was watching a video of ice cream making with my son the other day, and the guy making the ice-cream said how it lowered the temp, and I totally didn't believe it was correct and started to explain my theory before realising I had no idea. Great to see it laid out so clearly!

[majikandy]

More importantly, how was the ice cream you made? Apparently liquid nitrogen ice cream makes smaller crystals or something and tastes better? That could be the sequel…

[lesuorac]

> Apparently liquid nitrogen ice cream makes smaller crystals or something and tastes better?

It's about how fast you freeze the ice cream so the crystals don't grow.

--

You can get N2 from a local gas supplier but a lot of grocery stores stock CO2 (Dry Ice) that can also be used for ice cream.

CO2 has the disadvantage of if you get it in the ice cream it makes it carbonated but the smoke looks like a witches cauldron so it looks cool imo.

[lapetitejort]

My favorite use of dry ice is making Boo Bubbles:

1. Create a mixture of soap, water, and glycerin. This will make bubbles harder to pop.

2. Get a sealable container, poke a hole in the lid, the glue a tube in the hole.

3. Fill the container with water, then drop in pieces of dry ice. The CO2 should escape through the tube.

4. Stick the other end of the tube into the soap mixture.

5. Large bubbles will form with a cloudy gas of CO2.

6. Use towels to carry the bubbles, throw in the air, and combine with other bubbles.

I do this every Halloween. Kids get a kick out of it. A lucky few will stick around and learn about CO2, tensile strength, etc.

[simondotau]

Surely there must be at least some flavours which could be compatible with carbonated ice cream. Cola ice cream, perhaps?

[iamkroot]

Delicious, of course!

And yes, the other commenter is correct. LNO2 works so well because it freezes the ice cream so fast that the crystals don't have time to grow very large, which produces a nice and smooth texture in the final product.

[fuzzfactor]

When you do the math or work it at the bench, the difference is quite remarkable in the number of kilos of ice needed (per kilo of ice cream) when starting with 0C ice versus -20C ice.

Hint: start with rock salt at -20C also.

Edit: and the prepared cream premixed and chilled to 0C.

[AngryData]

You can quickly experience it yourself by holding an ice cube in each hand but pouring salt on one of them. While both cubes contain approximately the same deficit of energy, the salted one with the lower equilibrium temperature will pull heat out of your hand much faster and feel much colder. I guess you could always put it in two cups too.

[DIARRHEA_xd]

Almost. For the salted cube, remember to pour the salt on your hand and then put the ice cube on top.

[AdamH12113]

This is indeed a great explanation, and all the better for being both clear and concise.

[wombatpm]

If you play around with methonal and ice, you can get even lower temperatures