Why Does Salt Melt Ice on Roads?
Salt melts ice on roads because saltwater has a lower freezing point than pure water. Salting road ice doesn't change its temperature, but it lowers the freeing point of the ice below the air temperature, so the ice turns back into liquid water.
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How Ice Freezes
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The molecules in liquid water move about fairly freely. As the water freezes, the molecules lose kinetic energy and align themselves more rigidly into crystals, which are solid.
The Effect of Impurities
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Impurities in the water, such as sodium and chlorine ions from dissolved salt, interfere with the water molecules' ability to align into crystals, lowering the water's freezing point. Sugars, alcohol, glycol and other substances have the same effect, to varying degrees.
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The Lower Limit
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How much the freezing point is lowered depends on how much salt is dissolved in the water, up to a point. The amount of table salt (sodium chloride or NaCl) required to drop the freezing point to minus 21.1 degrees Centigrade is the limit of what the water can hold. Any salt added at that point simply doesn't dissolve. So the freezing point cannot be lowered any farther with NaCl alone. This is called the eutectic point.
Calcium Chloride
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Most road agencies use the salt calcium chloride (CaCl) for road clearing, as it has a much lower eutectic point--minus 50 degrees Centigrade--and is a bit more efficient at bringing down the water's freezing point, so less is needed.
Concerns
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Icy roads are a major safety hazard, and snowplowing does not remove ice down to the pavement. The application of salt is a relatively cheap and easy way to remove the lingering ice after snowplowing. The rule of thumb in Minneapolis, for example, is 100 to 150 pounds of salt per lane per mile. However, drivers, homeowners and city officials have long noticed the salt's corrosive effect on auto bodies and curbside improvements and installations such as streetlights. More recently, studies in Minneapolis--which uses about 350,000 tons of salt per year--have shown an increase in the salinity of area lakes, streams and groundwater over the past two decades, which has been attributed to road salt.
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