How Cold Does Ice Get With Salt?

Adding Salt to Ice and Freezing Point Depression

Adding salt to ice doesn't just melt it. It also makes it colder because of freezing point depression.
Adding salt to ice doesn't just melt it. It also makes it colder because of freezing point depression. Dave King / Getty Images

Some interesting science occurs when you mix salt and ice. Salt is used to help melt ice and prevent it from re-freezing on roads and walkways, yet if you compare the melting of ice cubes in fresh water and salt water, you'll find ice actually melts more slowly in the saline and the temperature gets colder. How can this be? How cold does salt make ice?

Salt Lowers the Temperature of Ice Water

When you add salt to ice (which always has an outer film of water, so it's technically ice water), the temperature can drop from freezing or 0 °C to as low as -21 °C. That's a big difference! Why does the temperature get lower? When ice melts, energy (heat) must be absorbed from the environment to overcome the hydrogen bonding holding the water molecules together.

Melting ice is an endothermic process whether there is salt involved or not, but when you add the salt you alter how readily water can refreeze back into ice. In pure water, ice melts, cools the surroundings and water, and some of the energy that is absorbed is released again as the water returns to ice. At 0 °C ice melts and freezes at the same rate, so you don't see ice melting at this temperature.

Salt lowers the freezing point of water via freezing point depression. Among other processes, the ions from the salt get in the way of water molecules aligning to crystallize into ice. When salted ice melts, the water can't refreeze as readily because the saline isn't pure water anymore and because the freezing point is colder. As more ice melts, more heat is absorbed, bringing the temperature down even lower. This is great news if you want to make ice cream and don't have a freezer. If you put the ingredients in a bag and place the bag in a bucket of salted ice, the drop in temperature will give you a frozen treat in next to no time!