Why Does Salt Melt Ice?
# Why Does Salt Melt Ice?
Every winter, trucks spread tonnes of salt on frozen roads, and the ice starts to disappear. It looks like the salt is warming the road. It is not. Salt melts ice through chemistry, not heat, and understanding it explains everything from icy sidewalks to old-fashioned ice cream makers.
Ice is never completely dry
Even well below freezing, the surface of ice carries a microscopically thin film of liquid water. When salt lands on ice, it first dissolves into that film. And here is the key fact: salty water freezes at a lower temperature than pure water. Pure water freezes at 0 degrees Celsius, but once salt is dissolved in it, the mixture can stay liquid well below that.
So the film of water on the ice is now too cold to freeze but perfectly able to stay liquid. It touches the ice, a little more ice melts into it, that new water dissolves more salt, which melts a little more ice, and the process feeds itself until the salt is used up or the ice is gone.
What is happening at the surface
At any moment, the boundary between ice and water is a busy two-way street. Water molecules constantly break away from the ice surface, and molecules from the liquid constantly attach to it. When both rates match, you are exactly at the freezing point and nothing changes. Warm things up and more molecules leave than arrive, so the ice melts.
Dissolved salt does nothing to the rate at which molecules escape the ice. What it changes is the arrivals. Some of the particles in the liquid are now salt ions instead of water molecules. Those ions bump into the ice surface just as often, but they cannot lock into the ice crystal, so the number of water molecules successfully attaching per second drops. With departures outpacing arrivals, the ice melts, and the temperature at which the two rates balance, the new freezing point, is lower. Chemists call this freezing point depression, and it is a colligative property, meaning it depends on how many dissolved particles there are, not on what they are.
Melting ice with salt actually makes things colder
There is a bonus effect that surprises people: dissolving salt in water absorbs heat. You can feel it yourself. Stir salt into a glass of water and the glass gets colder. Combine that with melting, which also pulls in heat, and a salt-and-ice mixture can drop well below 0 degrees Celsius. That is exactly why traditional hand-crank ice cream makers pack the cream container in ice mixed with salt: the salty slush gets colder than plain ice ever could, cold enough to freeze the dessert inside.
Where salt stops working
Salt is not magic, and it has a hard limit. The saltier the water, the lower its freezing point, but water can only dissolve so much salt. A fully saturated salt solution freezes at about 0 degrees Fahrenheit, around minus 18 degrees Celsius. Below that, sprinkling salt on a frozen sidewalk does essentially nothing. Road crews in colder regions often switch to calcium chloride, which works at lower temperatures and is widely used alongside ordinary rock salt.
One last neat consequence of the same physics: when salty ocean water freezes at the poles, the salt ions are left out of the ice crystal. The ice that forms is fresh water. Sea ice, and the glaciers that calve from polar ice shelves, melt into water you could drink.
The takeaway
Salt melts ice because salty water refuses to freeze at 0 degrees Celsius. It dissolves into the ever-present liquid film on ice, tips the molecular balance at the surface toward melting, and even absorbs heat while doing it. It works until roughly minus 18 degrees Celsius, and below that, winter needs a different tool.
Facts checked: October 27, 2026
Sources: Pelton, A., and Margrave, J. Why do we put salt on icy sidewalks in the winter? Scientific American (with the published correction explaining freezing point depression and the heat-absorbing dissolution of salt). HyperPhysics, Georgia State University. Freezing Point Depression in Solutions (colligative property, and the ice cream maker application). UCSB Science Line, University of California Santa Barbara. Why does salt melt ice? (molecular exchange at the ice surface, and why sea ice freezes fresh).