Deicing the Driveway: What the Different Products Do

There are four types of chloride salts that can be purchased for keeping ice off your driveway and sidewalks. Urea and a new pet-safe product are also available. What are the differences and benefits of each?

Putting salt on your paved walkways in the winter can help keep ice from forming, thus keeping them safe for walkers. The reason this works is because salts ionize, come apart chemically, in water. This makes it harder for the water molecules to join together to form ice. However, as the temperature drops lower and lower, there is a point at which each of the salts will no longer be able to keep the water molecules apart and ice will form.

If you put salt on existing ice or snow it will melt them, but very slowly. This is because it has to get into solution with the liquid at the surface of the ice or snow. Then it does what was described above- prevents further freezing. For a slightly more technical, but very good explanation of this, see General Chemistry Online.

Sodium Chloride is also known as Rock Salt, or Halite. It is also the same as table salt, but in larger chunks. This salt is the least efficient at melting ice, but is the cheapest to buy. It only works until the temperature drops slightly below 20^o F. It is also very harmful to the environment. It releases the greatest number of chloride ions (very reactive) when it dissolves. Chloride corrodes metal, damages low quality concrete, and pollutes water. It can kill the vegetation beside the walkways if enough of it is used, and even "poison" the soil. Rock Salt is generally used by road crews to salt roads and is also commonly used in home ice cream makers.

The next most efficient is Potassium Choloide. You may also find this on your table as sea salt. Potassium Chloride does not irritate the skin and will not kill vegetation. But its temperature range is only slightly better than Sodium Chloride, working above 15^o F. It is sometimes combined with other chemicals to lower its effective temperature. However, it may not be available in all markets. It is toxic in surprisingly low doses ¹; bad news for pets.

The familiar round white pellets seen so often on winter sidewalks are usually Calcium Chloride. It will irritate skin when wet. However, it is effective at temperatures below 0^o F.

Magnesium Chloride has recently joined the winter options for those who need to melt ice and snow. It is fast and efficient down to -13^o F. It is also the most expensive of the salts. It is less toxic to plants, does not leave white residue on boots and carpets, and releases less chloride. With long-term use it can cause corrosion to aluminum and steel, and has been shown to damage electrical wiring. ²

All of these salts can irritate the paws of dogs or cats, and can be toxic if ingested. Although two of the salts are commonly used on foods, if a child or pet eats too much of any of them they can become ill.

Urea (Carbonyl diamide) is also used for deicing in some situations, but although cheap, it may be the worst choice of all. Although it does not irritate animal's paws it releases nitrates in water. This promotes the growth of algae and releases toxic ammonia. It corrodes metal and is only effective to 20^o F. It is essentially fertilizer given a different use.

There is a product on the market which is completely different. It is currently only manufactured by one company under the name Safe Paw. They describe the product as "a patented dual-effect compound, made up of crystalline amide core infused with special glycols." ³ It is supposed to work down to 2^o F, be non-toxic and non-irritating. It is quite expensive, but is apparently very safe.

For more on this topic see National Geographic Green Guide

1. "Can salt substitute kill you?", The Straight Dope

2. "Magnesium Chloride as a De-Icing Agent", USDA

3. Safe Paw