Earth Science - Other

How Salt Melts Ice and Snow



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When snow falls from the sky, it is so beautiful, until someone slips on the ice. At that point, people will start to place salt on their pathways to melt the snow. It is a curious thing how salt does its job to help keep peoplesafe from slips and falls. Salt does its job by lowering the melting or freezing point of water. This is called “freezing point depression.”

Ice will form when the temperature reaches the freezing point of 32 degrees Fahrenheit, or zero degrees Celsius. When salt is added to water, dissolved foreign objects are introduced to the water. The freezing point then lowers as more salt is added it continues to drop until the point where the salt will stop dissolving. A ten percent salt solution lowers the freezing point to about 20 degrees Fahrenheit or six degrees below zero Celsius. A 20 percent solution freezes at two degrees Fahrenheit or sixteen degrees below zero Celsius. Regular table salt, or sodium chloride, NaCl, when used will lower the freezing temperature to 15 degrees Fahrenheit or nine degrees below zero Celsius.

If observed, the dissolving process can be witnessed. When the salt is dropped on ice, the ice around the salt will immediately melt. The melting area will spread from that point.

Other factors can also alter the reaction time, such as the ice, the pavement and the air.

The freezing point is considered a colligative property of water. This property depends on the number of particles in an object. Solutes or liquid solvents with dissolved particles show colligative properties. Other such properties are boiling oint elevation, vapor pressure lowering and osmotic pressure.

There are many other items that can be used to de-ice besides salt, which is a corrosive and can damage concrete and vegetation. Ammonium sulfate can be used down to 20 degrees Fahrenheit. Its advantage is that it is a fertilizer but it does damage concrete.

Calcium Chloride can be used to temperatures of 20 degrees below zero Fahrenheit. It melts ice faster than salt but it attracts moisture and below zero degrees Fahrenheit it can make pathways slippery.

Another substance that can be used instead of salt is calcium magnesium acetate. This is better to prevent re-icing than as a remover but it is the safest chemical for concrete and lawns.

Others include magnesium chloride, potassium acetate, potassium chloride, sodium, chloride also known as rock salt, and urea. Each of these has plusses and minuses. Check to see what would work the best in the situation in which it would be used. 

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