# Why does salt “melt” ice but freeze ice cream?

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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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Salt changes how ice melts, making it melt faster, by making it easier to pull heat out of the thing that the ice is touching. If that thing the ice is touching is a metal container of ice cream mix, then pulling the heat out of it makes it freeze solid. Mixing it during this process introduces air to the frozen mixture, making it super good.

So what happens is that salt lowers the point where water freezes.

So when you’re making your ice cream usually you take like ice made from tap water and you put the ice cream mix in like a mixer or something above the ice. The ice has a lot of holes and gaps as it doesn’t totally fill the space evenly meaning there is a lot of surface area of the mixer not being touched by the ice.

So heat only transfers from the warmer object to the colder object but that doesn’t work very well when there is low contact surface to actually transfer that heat. So you melt the ice with the salt, the water still has the same temperature as the ice but now fills in all of the gaps from the ice which helps to make the heat transfer more conductive and therefore faster

You don’t put the salt in the ice cream, you put it on the ice. Ice melts by absorbing heat. The salt makes it absorb heat faster by altering the melting temperature thereby cooling the ice cream down faster.

Salt requires energy to dissolve and it is taken from the thermal energy of the water and ice mixture and it gets colder. When the meeting point drops some of the ice will melt. Turing ice to liquid water requires energy and reduces the temperature

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The salt is not put in the ice cream but in the ice and water mixture around it. The mixture with the salt will not freeze because its melting temperature is lower. There is no extra salt in the ice cream so its melting temperature has not dropped.

When you add salt to a roadway its temperature will drop. There is a minimum temperature that is useful for a road that is above the minimum freezing temperature of the nine

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.