Magnesium chloride dissolves into three ions -- one magnesium cation and two chloride anions. On the flip side, adding a tiny amount of insoluble particulates can actually help water freeze at a higher temperature. While there is a bit of freezing point depression, it's localized near the particles.
The particles act as nucleation sites that allow for ice formation. This is the premise behind the formation of snowflakes in clouds and how ski resorts make snow when it's slightly warming than freezing.
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Develop and improve products. List of Partners vendors. Share Flipboard Email. Anne Marie Helmenstine, Ph. Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels. 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. The solid salt simply can't get into the structure of the frozen water to start the dissolving process.
The actual reason that the application of salt causes ice to melt is that a solution of water and dissolved salt has a lower freezing point than pure water.
When added to ice, salt first dissolves in the film of liquid water that is always present on the surface, thereby lowering its freezing point below the ices temperature. Ice in contact with salty water therefore melts, creating more liquid water, which dissolves more salt, thereby causing more ice to melt, and so on. The higher the concentration of dissolved salt, the lower its overall freezing point. There is a limit, however, to the amount of salt that can be dissolved in water.
Water containing a maximum amount of dissolved salt has a freezing point of about zero degrees Fahrenheit. Therefore, the application of salt will not melt the ice on a sidewalk if the temperature is below zero degrees F. To understand why water containing dissolved salt has a lower freezing point than pure water, consider that when ice and water are in contact there is a dynamic exchange at the interface of the two phase states.
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.
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