Re: Salt is often put on icy sidewalks to melt the ice. How does this work?

Great observations! And how cool that you are making ice cream in class!

To your question, let's start with the salt on an icy sidewalk. The process by which salt "melts" ice is called freezing point depression. The principle at work here is that the more "foreign stuff" in a liquid, the harder it is for that liquid to freeze because these foreign ions and particles interrupt the crystalline structure of the liquid as it is trying to freeze.

How? Well, the ice on a typical sidewalk is not perfectly clean (dirt and other particles will be embedded in it), which means it will not be perfectly frozen (unless you live somewhere like McGrath, Alaska where the coldest recorded temperature is -75F). Salt will dissociate ("breakdown" into Na+ and 2 Cl- ions) as soon as it touches water, preventing that water from freezing. That liquid and those ions will then "contaminate" the ice crystals around it and so on, until the ice is both melted and prevented from re-freezing.

On to the ice cream situation. We already know that the salt will melt the ice because of freezing point depression, but then what? The rock salt doesn't make the ICE colder, it makes the liquid water colder as the salt dissolves in it. Salt dissolving in water is considered an "endothermic" reaction (it absorbs heat), making the water/salt solution (brine) colder as it happens, and helping the ice cream ingredients to freeze into something yummy.

Here are a few related answers that might be helpful:
How does adding Salt to water affect its freezing degree and why?
Why does pure ice melt slower in salt water than fresh water?
Why do you put salt in the ice when making ice cream?

I hope this information helps!

Kieran