When you add a salt to ice with a little bit of water, the salt lowers the freezing point of the water, keeping it from re-freezing as easily and helping to melt the rest of the ice.
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If you live in an area with a cold and icy winter, you have probably
experienced salt on sidewalks and roads, used to melt the ice and snow
and keep it from refreezing. Salt is also used to make homemade ice
cream. In both cases, the salt works by lowering the melting or
freezing point of water. The effect is termed freezing point depression'.
How Freezing Point Depression Works
When you add salt to water, you introduce dissolved foreign particles
into the water. The freezing point of water becomes lower as more
particles are added until the point where the salt stops dissolving.
For a solution of table salt (sodium chloride, NaCl) in water, this
temperature is -21°C (-6°F) under controlled lab conditions. In the
real world, on a real sidewalk, sodium chloride can melt ice only down
to about -9°C (15°F).
Colligative Properties
Freezing point depression is a
colligative property of water. A colligative property is one which
depends on the number of particles in a substance. All liquid solvents
with dissolved particles (solutes) demonstrate colligative properties.
Other colligative properties include boiling point elevation, vapor
pressure lowering, and osmotic pressure.
More Particles Mean More Melting Power
Sodium chloride
isn't the only salt used for de-icing, nor is it necessarily the best
choice. Sodium chloride dissolves into two types of particles: one
sodium ion and one chloride ion per sodium chloride 'molecule'. A
compound that yields more ions into a water solution would lower the
freezing point of water more than salt. For example, calcium chloride
(CaCl2)
dissolves into three ions (one of calcium and two of chloride) and
lowers the freezing point of water more than sodium chloride. Here are
some other de-icing compounds:
Chemicals Used to Melt Ice
| Name | Formula | Lowest Practical Temp | Pros | Cons | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Ammonium sulfate | (NH4)2SO4 | -7°C (20°F) | Fertilizer | Damages concrete | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Calcium chloride | CaCl2 | -29°C (-20°F) | Melts ice faster than sodium chloride | Attracts moisture, surfaces slippery below -18°C (0°F) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Calcium magnesium acetate (CMA) | Calcium carbonate CaCO3, magnesium carbonate MgCO3, and acetic acid CH3COOH | -9°C (15°F) | Safest for concrete & vegetation | Works better to prevent re-icing than as ice remover | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Magnesium chloride | MgCl2 | -15°C (5°F) | Melts ice faster than sodium chloride | Attracts moisture | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Potassium acetate | CH3COOK | -9°C (15°F) | Biodegradable | Corrosive | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Potassium chloride | KCl | -7°C (20°F) | Fertilizer | Damages concrete | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Sodium chloride (rock salt, halite) | NaCl | -9°C (15°F) | Keeps sidewalks dry | Corrosive, damages concrete & vegetation | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Urea | NH2CONH2 | -7°C (20°F) | Fertilizer | Agricultural grade is corrosive |
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