kemptonite1
kemptonite1 t1_iyamgl8 wrote
Reply to comment by thebigger in Eli5: Some ice cream recipes put ice + salt outside the recipient to make it cool faster. But in the winter, salt is put on snow on the street to melt faster. Why one make cool and other melt? by zimobz
Yeah, sand works well for roads for that reason - cars provide plenty of pressure for melting ice through force alone (and chunky sand gives a lot of little pressure points for cars to melt bits of ice under their tires).
But that dodges the real point here - the salt added to an ice cream maker isn’t increasing pressure to melt the ice and make the ice cream freeze faster - the salt is lowering the melting point to encourage the ice to melt faster from conduction, and thus stealing heat at a faster rate from the surrounding objects.
Sand/salt on roads is a slightly different beast. Salt is 100% better at melting ice on roads (because it does the pressure thing sand does AND lowers the freezing point) BUT salt corrodes metal. And corroding the underside of cars isn’t great. So sand is often used (I’m also from the north).
kemptonite1 t1_iyaedhm wrote
Reply to comment by thebigger in Eli5: Some ice cream recipes put ice + salt outside the recipient to make it cool faster. But in the winter, salt is put on snow on the street to melt faster. Why one make cool and other melt? by zimobz
Well, you kind of have a thought going here. Applying pressure to ice does indeed melt it (applied force can break those crystal formations just like heat can) but the effect is pretty minimal. It takes a lot of force to melt ice (like, heavy hammer forces, not scattered salt force). The freezing point is indeed lowered, which is why the ice melts faster. A simple google search confirms this. 🤷♂️If your thought was true then scattering pebbles on ice would be just as effective as ice, which it isn’t.
kemptonite1 t1_iy6w6td wrote
Reply to Eli5: Some ice cream recipes put ice + salt outside the recipient to make it cool faster. But in the winter, salt is put on snow on the street to melt faster. Why one make cool and other melt? by zimobz
It’s been described accurately here, but maybe a way to visualize it could help too. As people have stated, salted ice melts faster than non-salted ice because the freezing point is lowered. Why is quickly melting ice good for making ice cream? Well, in order for ice to melt (salted or non-salted) a lot of heat needs to enter that ice to break the crystal water formations. Like, a LOT of heat. That energy has to come from somewhere, and unlike outside (where the sun can help), your ice cream maker ice has only one option - steal it! So the energy hungry salted ice gobbles up all the heat from every nearby object - and freezes your ice cream solid.
kemptonite1 t1_iybyl18 wrote
Reply to comment by thebigger in Eli5: Some ice cream recipes put ice + salt outside the recipient to make it cool faster. But in the winter, salt is put on snow on the street to melt faster. Why one make cool and other melt? by zimobz
So… I’ve never seen someone use so much space to talk about something they don’t seem to understand very well. I’m not an expert, but I do have a degree in physics and have taken several college level thermodynamics classes.
Your last point in particular is contradictory. And really hits the nail on the head.
Heat transfer (in conduction at least) depends on several things - the area of contact between objects, the coefficient of heat transfer, and the difference of temperature between the objects. (Q/t=kA(T1-T2)/d)
We can’t change the area of contact between the ice and the metal tube containing the cream. We can’t change the coefficient either (not meaningfully). What we CAN change is the difference in temperature-by lowering the freezing point of the ice, we lower the temperature the ice can raise to before it begins melting. A mixture that is partially ice and partially water (such as a mass of melting ice in an ice cream maker) will ALWAYS be at the freezing temperature (0 C for normal ice, or -5ish C for salty ice).
Because the salty ice is colder, heat is transferred more rapidly due to the difference in temperature being greater.
So yes, the freezing point is relevant. Ice cannot have a higher temperature than its freezing point. Therefore salty ice having a lower freezing point means greater temperature difference and therefore greater heat transfer and therefore faster freezing ice cream. Does that make sense?