By: HowStuffWorks. Your city and local government likely has several de-icing trucks that spread road salt on the highways, streets and sidewalks to melt the ice. First, road salt is simply halite — rock salt — which is table salt in its natural form. The difference is table salt goes through a lengthy purification process, while rock salt does not.
And because rock salt still has impurities, it's brown or gray in color. Road salt works by lowering the freezing point of water via a process called freezing point depression. The freezing point of the water is lowered once the salt is added, so it the salt makes it more difficult for water to freeze. A percent salt solution freezes at 20 degrees Fahrenheit -6 Celsius , and a percent solution freezes at 2 degrees Fahrenheit Celsius. The key is, there has to be at least a tiny bit of water on the road for freezing point depression to work.
That's why you often see trucks pre-treat roads with a brine solution a mixture of salt and water when ice and snow is forecast. If the roads are dry and the DOT simply puts down road salt, it likely won't make much of a difference.
But pre-treating with a brine solution can help ice from ever forming, and will help reduce the amount of road salt trucks will need to spread to de-ice later. Rock salt is one of the most widely used road de-icers, but it's not without critics. For one, rock salt does have its limits. If the temperature of the roadway is lower than about 15 degrees F - 9 C , the salt won't have any effect on the ice. Because of thermal vibrations in the ice, a large number of molecules per second become detached from its surface and enter into the water.
During the same period of time, a large number of water molecules attach themselves to the surface of the ice and become part of the solid phase. At higher temperatures, the former rate is faster than the latter and the ice melts. At lower temperatures the reverse is true. At the freezing point the two rates are equal. If salt is dissolved in the water, the rate of detachment of the ice molecules is unaffected but the rate at which water molecules attach to the ice surface is decreased, mainly because the concentration of water molecules in the liquid molecules per cubic centimeter is lower.
Hence, the melting point is lower. John Margrave, a chemistry professor at Rice University, explains. All icy surfaces in fact contain small puddles of water. Because salt is soluble in water, salt applied to such surfaces dissolves. Liquid water has what is known as a high dielectric constant, which allows the ions in the salt positively charged sodium and negatively charged chlorine to separate. These ions, in turn, react with water molecules and hydratethat is, form hydrated ions charged ions joined to water molecules.
This process gives off heat, because hydrates are more stable than the individual ions. That energy then melts microscopic parts of the ice surface. Thus a substantial amount of salt spread over a large surface can actually thaw the ice.
In addition, if you drive over the ice in your automobile, the pressure helps force the salt into the ice and more of this hydration occurs. The rock salt applied to icy roads in the winter is the same substance that comes out of your salt shaker. The only difference is the size. The most common types of ice melts on the market today are chloride based. When picking a deicer, take all factors into consideration and pick the product best for you and your family. Topics: Home Safety , Weather. Privacy Policy.
Find An Agent. Sodium chloride , most commonly known as rock salt, is the most frequently used type of salt, probably because of how inexpensive it is. Calcium chloride is probably the most effective salt-based product, because it works up to degrees. Be careful when using calcium chloride. Potassium chloride has been deemed as the safer chloride-based option, working in temperatures as low as 12 degrees. Sodium acetate works well in colder temperatures, as low as zero degrees.
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