This winter has been filled with so much rain and snow. I started wondering what the difference is between the salt they place on icy roads and the salt you use to make homemade ice cream?
Is it the same kind of salt? What does the salt on the road do? Does it help with friction or melt the ice?
I thought the salt mixed with ice, to make ice cream, makes the freeze temperature lower?
I’m confused…
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Its the same concept in both cases, it lowers the freezing point
You’re going to be more familiar with this from the other end. Boiling water is 100C right? If you have a pot of boiling water on your stove on low you get some vapor coming off. If you crank the stove up to high does the temperature of the water increase? Nope, just makes water boil off faster. All new energy added goes into the phase change (liquid-gas) and not into increasing the temperature
Same concept for freezing. A water-ice mix exists at the freezing point of the mix so a bag of ice water is pretty close to 0C. Add some salt and you lower the freezing point but the mix still needs to hang out at the freezing point so now your bag of ice-salt-water is -5C.
For a road, salt means that instead of needing the temperature to be above 0C to melt the ice on the road, it now just needs to be above -5C to melt the ice and not let it refreeze. The sun can add just enough energy to the mix to cause the mainly ice to transition to salty water with some ice still at -5C but the road is now clear
For ice cream, you need to get the temperature down below 0C to get good freezing of the sugary cream. By filling the outside of the maker with ice and salt and water you end up with a -5C cooler outside of the inner spinning liner which lets you freeze the mixture up a lot better than a 0C liner would.
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