What Is Road Salt?

Chemical Composition of Road Salt and How It Works

Most road salt is rock salt or halite from salt mines.
Most road salt is rock salt or halite from salt mines. Winslow Productions, Getty Images

Question: What is road salt and how does it work?

Answer: Road salt is halite, which is the natural mined mineral form of table salt or sodium chloride (NaCl). While table salt has been purified, rock salt contains mineral impurities, so road salt typically is brownish or gray in color. Machines mine the salt, which is crushed and packaged for delivery. Additives may be mixed with the road salt to prevent caking and ease delivery using gritting machines.

Examples of additives include sodium hexacyanoferrate(II) and sugar.

How Road Salt Works

Road salt works by lowering the freezing point of water -- a process termed freezing point depression. In a nutshell, the salt breaks into its component ions in a small amount of liquid water. The added particles make it more difficult for the water to freeze into ice, lowering the freezing point of the water. So, for road salt to work, there needs to be a tiny bit of liquid water. This is part of the reason why road salt is not effective in extremely cold weather when water would freeze too easily. Usually, there is enough liquid water present, either coating the hygroscopic salt pieces or produced by friction from traffic, that an extra source of water is not necessary.

When cold weather is forecast, it is common to pre-treat roads with brine, which is a solution of salt and water. This helps prevent ice from forming and reduces the amount of road salt needed to de-ice the surface later.

Once ice starts to form, road salt is applied in gravel or pea-sized chunks. Road salt may be mixed with dry or damp sand to aid the process, too.

Other Chemicals Used as De-icers

While rock salt is the most affordable and commonly used chemical to deice roads, sand also may be used. Other chemicals are also available.

Most of these other chemicals are more commonly used for sidewalks or driveways. Each chemical, including road salt, has pros and cons. One of the biggest advantages of rock salt is that it is readily available and inexpensive. However, it does not work under extremely cold conditions and it does pose significant environmental risks. The primary concern is that the sodium and chlorine get into the ground and water and raise the salinity. Also, because rock salt is impure, other undesirable compounds present as contaminants are released into the ecosystem. Examples of contaminants include lead, cadmium, chromium, iron, aluminum, manganese, and phosphorus. There is no "perfect" deicer, so the goal is to use the best chemical for the situation and to use the lowest effective quantity.

Note that sodium chloride, potassium chloride, magnesium chloride, and calcium chloride are all chemically "salts", so any of them could be correctly termed "road salt". The chemicals listed as corrosive may damage concrete, vehicles, and other structures.

Deicer Chemicals
ProductLowest Effective
Temperature (°F)
CorrosiveAquatic
Toxicity
Environmental
Factors
rock salt (NaCl)20yesmediumtree damage
potassium chloride (KCl)12yeshighK fertilizer
magnesium chloride (MgCl2)5yeshighadds Mg to soil
calcium chloride (CaCl2)-25extremelymediumadds Ca to soil
calcium magnesium acetate (C8H12CaMgO8)0noindirectlowers aquatic O2
potassium acetate (CH3CO2K)-15noindirectlowers aquatic O2
urea (CH4N2O)15noindirectN fertilizer
sand--noindirectsediments

 

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Helmenstine, Anne Marie, Ph.D. "What Is Road Salt?" ThoughtCo, Mar. 6, 2017, thoughtco.com/chemical-composition-of-road-salt-609168. Helmenstine, Anne Marie, Ph.D. (2017, March 6). What Is Road Salt? Retrieved from https://www.thoughtco.com/chemical-composition-of-road-salt-609168 Helmenstine, Anne Marie, Ph.D. "What Is Road Salt?" ThoughtCo. https://www.thoughtco.com/chemical-composition-of-road-salt-609168 (accessed October 23, 2017).