How rock salt melts ice, and why less is often more

The question is not, “To salt, or not to salt?” It’s is, “What kind of salt? And in what form?”

Each winter, Philadelphia uses 20,000 to 40,000 tons of rock salt to keep its roads ice-free. In a bad season, like last year, that number can balloon to more than 80,000 tons. But what is actually happening when salt melts ice and snow, and do we need so much?

Blocking ice

When the temperature falls, water molecules slow down and are more likely to meet up. At 32 degrees Fahrenheit, they snap into place within a beautiful, repeating pattern of hexagons: ice.

Salt works because when it dissolves, it impedes this process.

“If you can put an atom in between water molecules, then you’re preventing it from finding its nearest neighbor and forming crystals,” explains Nicolas Alvarez, a chemical engineer at Drexel University.

In the case of rock salt, which is just table salt, or sodium chloride, there’s one atom of sodium and one atom of chlorine to do this disrupting. The way it’s used on most sidewalks, it’s able to lower the freezing point to 25 degrees.

The danger of freeze-thaw

The relatively small reduction in freezing temperature—just 7 degrees—is why in Philadelphia rock salt will often fail to keep water from freezing, and why temperature fluctuations also cause problems for concrete.

Concrete is porous and allows a certain amount of water—and any dissolved salt—inside. If the temperature drops below the lowered freezing point, the water will expand.

“If you walk down the street, you’ll see little pits that have formed on your sidewalk or on your driveways,” says Alvarez. “That’s usually because of the stresses from the ice freezing inside the concrete.”

Another salt, calcium chloride, can avoid those damaging freeze-thaw cycles because it lowers the freezing temperature much more—to minus 25 degrees. Compared to rock salt, it has an extra chlorine atom to squeeze in and destabilize ice.

“When it’s dissolving into the water, [calcium chloride] actually heats up,” says Alvarez. “So it acts the fastest in terms of melting the ice.”

That speed and lower effective temperature, however, come with a price tag that’s about double that of rock salt. So most large-scale operations—including the city of Philadelphia—rely solely on sodium chloride.

A persistent challenge

Decades of heavy use have taken a toll on the environment because salt doesn’t just wash away. Stuart Findlay, an aquatic biologist with the Cary Institute of Ecosystem Studies, says sodium chloride levels remain elevated into the summer months–right when organisms are most vulnerable.

“The amphibians that inhabit lakes, streams, and ponds in the Northeast U.S. are not used to any salt concentration,” he says. “And the young stages of these organisms actually breathe across their skin–that’s how they get oxygen. So any chemical that’s dissolved in the water, they’re very, very sensitive to that.”

One rock salt alternative, calcium magnesium acetate, is sometimes used in special places, such as bridges, because it won’t eat away at metal—or add sodium chloride to waterways. But it’s far more expensive, and can still be harmful to the environment because it depletes oxygen in water.

“Like it or not, sodium chloride is going to be the one that’s used most commonly,” says Findlay. “So the real trick is to minimize how much you can get away with.”

Reducing use

One way for homeowners to cut back is to pay attention to the physical shape of their deicer.

“If it’s a very rocky crystalline structure, like you would get from rock salt,” says Alvarez, “then you can have situations where the bulk of the material is not in contact with the ice.”

Smoother pellets will dissolve more quickly and work faster.

But by far the most efficient delivery of salt over an entire surface is a brine solution. It coats evenly, and won’t be blown away by wind. Brine can also more easily avoid the situation in which too much salt will actually begin to raise the freezing temperature.

In general, adding more deicer will work to further lower the freezing point. But each salt has a limit, known as its eutectic point.

While it may be ugly, the salty residue left on the streets is a brine solution waiting to happen.

“The next time it potentially snows,” says Alvarez, “that is what is going to save you from the black ice.”