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Spraying salt on roads is a safety measure. Credit: stoatphoto / Shutterstock.com
Brrr … it's cold outside! Children flock to the television in the hope of hearing that there will be a snowy day; the bread and milk aisles of the grocery stores are empty because of an imminent snowstorm; and utility trucks spray salt or salt water on the roads.
We all know why the first two arrive. The kids are excited about a day off filled with chocolate and snowmen. Adults supply themselves with basic necessities. But what's new with these trucks?
They strive to protect slippery road users by spraying rock salt or salt water solution to prevent the formation of ice. This salt is very similar to the salt you have on your table: it's the same sodium chloride, NaCl. Some proprietary blends contain other salts, such as potbadium chloride (KCl) and magnesium chloride (MgCl), but they are not as commonly used.
Road salt is not as pure as what you use in your food. it has a brownish gray color, mainly due to mineral contamination. Subjecting the environment to this salt by runoff can have unintended consequences, including negative effects on plants, aquatic animals and wetlands.
But it is an economical and effective way to protect the ice roads thanks to a simple scientific principle: the lowering of the freezing point of the solutions. The freezing point of pure water, the temperature at which it becomes ice, is 32 degrees Fahrenheit. So, if there is snow, melted snow or freezing rain and the soil is above 6 ° C (32 ° F), solid ice will form in the streets and sidewalks.
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The salt prevents the water molecules from solidifying into ice crystals at a temperature of 6 ° C (32 ° F). It remains rather slimy at this temperature before moving towards 15 ° C. Credit: Julie Pollock, CC BY-ND
However, if the water is mixed with salt, the freezing temperature of the solution is below 32 ° F. The salt prevents the water molecules from forming solid ice crystals. The degree of depression of the freezing point depends on the salinity of the solution.
It is important to note that the salt must be in a solution with liquid water for this principle to be obeyed. This is why many cities spray a salt solution before the ice forms.
The salt spilled on the ice depends on the sun or the friction of the car tires that cross it to melt the ice in a slush that can mix with salt and not refreeze. Solid salt pretreatment depends on the warmer surface of the road to melt snow or freezing rain so that it can mix well with salt. This is also why pretreatment bridges – which are colder than other roads – usually does not work, and why you see signs "frozen bridge before road".
These salt solutions reduce the freezing temperature of the water to about 15 ° F. Unfortunately, for people facing freezing temperatures, salt treatment will not remove ice on their roads.
Another strategy used at these low temperatures is to put sand on the ice. Sand does not change the melting temperature, it just provides a rough surface to your tires to prevent slipping.
The science of freezing point depression can be applied to any solution, and many research groups have focused on the development of alternatives with fewer negative consequences for the environment. They include additives such as molbades and beet juice. So maybe you can expect to clean not only the white salt of the bottom of your jeans after a winter walk, but also the pink salt.
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