The Chemical Composition of Road Salt

The Chemical Composition of Road Salt

What Road Salt Is and How It Works

By Anne Marie Helmenstine, Ph.D.

Updated July 09, 2019

When cold weather arrives, stores stock up on big bags of road salt and you may see it sprinkled on sidewalks and roads to melt ice. But what is road salt and how does it work?

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 it is typically 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 via 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, an extra source of water is not necessary because there is enough liquid water present, either coating the hygroscopic salt pieces or produced by friction from traffic.

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 de-ice 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" de-icer, 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.

Safer Alternatives to Road Salt

All forms of salt pose some environmental dangers, so many communities have searched for alternatives to keep ice off roads. In Wisconsin, cheese brine is used as a de-icer. The brine is a by-product that's normally thrown away, so it's free. Some towns have tried using molasses to reduce the corrosivity of salt. The molasses is mixed with saline solution, so freezing point depression is still active. The Canadian company EcoTraction makes granules from volcanic rock, which help melt ice because the dark color absorbs heat, plus it aids traction by embedding into ice and snow. The town of Ankeny, Iowa, experimented with excess garlic salt they had on hand. Another option, not yet in service, is to use solar power to help melt ice and snow so it wouldn't need to be plowed or chemically removed.