Physical Science Blog

Sugar and spice and…salt?

Molasses puts a sticky spin on road salt across the country

December 17, 2010

The popularity of ever-more-exotic sea salts has been making salt-sprinkled caramels and other candies a common sight on holiday shelves. As odd as that combination may seem, sugar and salt will soon be making another unexpected co-appearance around the country: this time, on wintery roads.

One of the surest signs of winter in cold-weather regions of the United States is the buildup of salt in tire treads and on roads (and in your boots, and in your house…). The concept of salt-lining roads is easily explained: when you add salt to water, its freezing point dips below the normal 32 degrees Fahrenheit, so the outdoor temperature has to dip even lower than that for ice to form on the roads.

Some areas have made inventive alterations to basic rock salt: one Iowa town dumped garlic salt on the roads when a spice supplier donated nine tons that were headed to a landfill. And across the states, many civil engineers have long added sugar beet molasses to their road salt to make sure it stays where it’s supposed to—one county in the United Kingdom is just catching on. The stickiness of the molasses keeps the salt on roads longer, saving money by reducing the number of times each town has to salt. The mixture is also supposed to be less corrosive than the plain rock salt that is normally used, and it reduces the environmentally damaging concentration of salt on the road (too much salt makes for briny groundwater, which can end up dehydrating plants and killing aquatic life).

But is sugar-coated salt as efficient as traditional road salt at keeping ice from forming? Though the most common example of freezing point depression is salt in water (there’s a good reason why oceans don’t freeze in the winter), in reality it doesn’t really matter what you add to water to lower its freezing point. Freezing point depression is a colligative property, which means that it depends on the number of particles in a solution but not on their identity. Theoretically, anything that dissolves in water could be used as a de-icer on roads; salt is just used because it’s cheap and easy to find.

Depending on who is doing the assessing, the sugary salt mixture performs either better or worse than traditional salt at melting snow and ice. If all things were equal—if the same amounts of each mixture at the same overall concentrations were applied to roads—there shouldn’t be a difference. But the molasses mixture may scatter less than plain salt, leaving more of it on the road to do its job. The sticky salt may be the way to go for many states.

About the Author

Katie Palmer

Katie Palmer graduated from Williams College with a B.A. in chemistry and a concentration in neuroscience, though her classmates knew her best as “that cellist.” Guided by interests in cognitive science and neuroscience, she has probed rat brains for hints of anxiety traits and attempted to reconcile consciousness with the reality of the nervous system. Katie hopes to follow these topics and more in the process of bringing science to the public. You can follow her on her personal webpage or on Twitter.



Alan Dove says:

Freezing point depression is only part of the story for road salt. Choosing a salt whose dissolution is highly exothermic (such as calcium chloride or calcium hydroxide), adds the heat of solution to the melting effect. That’s why those salts work even below 0 degrees F, while plain sodium chloride doesn’t. The calcium salts are also much kinder to vegetation and waterways. Unfortunately, they’re more expensive, which is why so many states still use NaCl on the roads.

High Point says:

I wonder how difficult it is to remove the corrosive salt/syrup mixture from a vehicle?

Chemistry Road Salt Extended Essay says:

Don’t the properties of the salt affect the effectiveness? Such as how likely and how easily a particle/ion can interrupt the hydrogen bonds between water molecules?

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