Science Puzzle
Why Do We Put Salt on Icy Roads?
On frosty nights, gritters spread rock salt on the roads. By morning the salted ice has turned to slush, even though the air is still below freezing.
The salt isn't warm and it doesn't give off any heat. If anything, the slushy mix it makes ends up colder than the ice was. So how can cold salt melt ice?
The Answer
Salt lowers the temperature at which water freezes. Salty water stays liquid well below 0 °C, so at minus 3 °C the ice melts into it.
Ice is never completely still. At its surface, water molecules are always breaking free of the ice and others are always joining it. At 0 °C the two happen at the same rate, which is why 0 °C is the melting point of pure water. There's also always a very thin film of liquid water on the ice.
Salt dissolves in that film. Its particles get in the way of water molecules trying to lock back into the ice, while molecules keep breaking free just as fast as before. Melting now beats freezing and the ice turns to slush. A strong salt solution doesn't freeze until about minus 21 °C, which is also why the salty sea freezes far less readily than a freshwater pond.
Breaking up ice takes energy, and here it comes from the ice and water themselves, so the slush gets colder as it forms. People use the same trick to make ice cream in a bag of ice and salt. Rock salt starts to struggle when it's colder than about minus 10 °C, because the melting slows to a crawl.
The principle: Freezing point depression. Dissolving something in water makes it harder for the water to freeze, lowering its freezing point below 0 °C.