Anyone who lives in a place that experiences winter, a list that has grown significantly thanks to the Polar Vortex of 2014, has seen salt sprinkled all over the highways to try and combat ice formation. This nearly magical ritual happens regularly in many places, from the front walk to the on ramp. It sounds silly at first, almost like an old superstition, but salt does in fact melt ice up to a certain temperature. The how and why of this ultimate life hack is actually pretty easy, once you take a much closer look at what’s happening at the molecular level.
How Ice Actually Works
When you look at ice you see a solid. That’s what we were taught ice was in school, and it’s how many people think of ice when they’re adults. Ice’s identity is a little more fluid though, all puns intended. You see, parts of ice are solid. The water freezes into a strong, crystalline structure that is quite strong, and which expands when it freezes. However, while the bulk of any given chunk of ice is solid, the outermost layer is in a state of flux from water to ice and back again. This is actually the reason ice is slippery, but more on that here.
This is where salt comes into things. Ice forms at roughly 32 degrees Fahrenheit, but even at that point there’s still some transition as water turns to ice, and the ice flows back up into the water. There’s a lot of motion happening among the top layers of molecules, and that excitement causes a lot of disruption around the edges. By introducing salt into that equation, it throws a monkey wrench into the whole operation.
What the Salt Does
Salt dissolves in water. This means that the upper layers of ice become salt water, and the salt content makes it more difficult for the water to take the crystalline structure of ice. The higher the content of salt is, the colder it has to be in order to freeze water. A 10% solution of ice water won’t freeze until it hits 20 degrees Fahrenheit, and a 20% salt solution won’t freeze until it hits 2 degrees. Because the salt crystals infect the ice, it helps break down the solid and start turning it back into a liquid.
Salt does have limitations to what it can and can’t do when it comes to ice control, though. If it’s colder than 2 degrees, for instance, no amount of salt will be able to penetrate the block and turn it back into liquid. When that happens it’s actually a smarter idea to spread sand, which is gritty and will help create traction for drivers who have to traverse the roads. Some people may use radiant heating beneath a road or sidewalk to keep it warm, and some enterprising homeowners might use a hydrophobic coating so that water is pushed off of a surface and ice never gets a chance to form. Seriously, watch a video of that here; it’s mesmerizing.
“Why do They Use Salt to Melt Ice on the Road in Winter?” by Anonymous at How Stuff Works
“Why Does Salt Melt Ice?” by Anonymous at Knows Why
“Why Does Salt Melt Ice?” by Anne Marie Helmenstine at About