A student asked me this today. I understand that salt doesn’t actually melt ice, but lowers its melting temp, but how do I explain this to a child?
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This is one of those things that is only ever explained sorta right.
First, the part everyone misses: the ice you’re using to freeze the ice cream is well below freezing.
So far so good – you need it to be below freezing to freeze something, right?
Problem is it starts to melt. And when it does you’ve got cold ice swimming in warmer liquid water, which is now exactly at freezing — too warm to freeze the ice cream.
Toss some salt in and the melting water can now be liquid below the normal freezing point. That means that really cold ice is surrounded by nice cold water that is cold enough to freeze ice cream.
None of this would work if the ice wasn’t already colder than freezing.
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