When salt and ice are mixed, some interesting science occurs. It’s common practice to use salt to melt ice on roads and walkways, but if you compare the melting of ice cubes in fresh water and salt water, you’ll find that the saline water actually melts more slowly and the temperature drops. Why is this happening? Ice is made cold by salt. How cold is it?
Salt Lowers the Temperature of Ice Water
Salt can cause the temperature of ice to drop from freezing or 0 °C to as low as -21 °C when added to water (which is technically ice water). That’s a huge difference! When the temperature drops, why does it do so? For water molecules to melt, energy (heat) must be absorbed from the environment to overcome hydrogen bonding.
If salt is added to ice melting, it alters how readily water can refreeze back into ice, which is an endothermic process. Ice melts in pure water, cools the surroundings and water, and some of the absorbed energy is released again when the water returns to ice. At 0 °C, ice melts and freezes at the same rate, so you don’t see ice melting.
By lowering the freezing point of water, salt lowers its freezing point. Ions from salt interfere with water molecules aligning to crystallize into ice, among other things.
Because salted ice isn’t pure water anymore, and because the freezing point is lower, it can’t refreeze as readily. More ice melts, absorbing more heat, lowering the temperature even further.
If you don’t have a freezer and want to make ice cream, this is great news. The drop in temperature will give you a frozen treat in a matter of minutes if you place the ingredients in a bag and place it in a bucket of salted ice!