Why do people put salt on the roads?
You can easily see that salty water freezes at a different temperature than fresh water. If you live somewhere where it gets cold in the winter, you probably already know that you put salt on the sidewalk to melt the ice so people can walk on it.
Comparing salty water to fresh water
If you don’t, though, you can test this out by filling two cups with regular tap water and putting two spoonsful of salt in one of them. Stir, and put both cups in the freezer. How long does it take each of them to freeze? Does it make a difference if there is even more salt in the water?
Why does this work?
The reason water takes longer to freeze when it is salty is that the salt molecules get in the way of the water molecules when they try to form regular ice crystals. The water molecules can’t form their fixed hexagon pattern until they find a way around the salt crystals.
Bibliography and further reading about salt: