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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Imagine water as Lego bricks.
When the bricks are clean, they snap together nicely and you get a big block of Lego. This is plain water freezing to make an ice cube.
Now imagine playing with Lego at the beach. The sand gets everywhere, between the individual Legos. This makes it really hard to make a nice big block of Lego. You have to clean all the sand away.
This is kind of what is happening with the salt in your water. It is getting between the water to prevent them from stacking up nicely to make an ice cube.
To make an ice cube from salt water, you have to do more work and make things colder to get the water clean in places to stack together nicely. Therefore, salt water freezes at a lower temperature than plain water.
Now that you have salt water, you have something that freezes colder than plain water, you can use it to freeze thing that normally don’t freeze with ice.
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