Ever been curious about the science behind putting salt on the road?
In the winter in countries with climates such as that of Canada, frequent snow and cold weather can lead to very icy road conditions. Icy roads are extremely dangerous, and cause a large number of car accidents every year. Salt is used to reduce the amount of ice on roads, with the hope of preventing accidents.
When the salt is added to the road, it lowers the freezing point of the water, because the solution of water and salt has a lower freezing point than pure water. When salt is initially added, it dissolves in the film of liquid water that is always present on the surface of ice, lowering its freezing point below the temperature of the ice. Ice that is in contact with the now salty water melts, which created more liquid water, melting more ice, and so on and so forth. The higher the concentration of salt in the solution, the lower the freezing point. However, there is a maximum amount of salt that can be dissolved in water. Water that contains the maximum amount of dissolved ice has a freezing point of zero degrees Fahrenheit. If the temperature is lower than this, adding salt will not melt the ice on a road. There is another way in which the large pieces of calcium chloride (which is cheaper than sodium chloride) work to break down the ice. When a large vehicle drives over the bits of ice, the weight of the vehicle pushes them down and backwards. The physical force cracks and shatters the ice. Friction can also melt the ice and snow.
There is however, some environmental controversy to the use of salt on roads . When the snow melts, the water now containing ice enters the water supply. This can contaminate the ground water. Once salts are present in ground water, it can take decades for them to decay. When the salinity of groundwater is increased, it has major affects on the health and lifespan of many organisms, namely algae and fish. It also affects mammals such as moose and deer and birds when they drink this water in a variety of ways. It can also cause damage to automobiles, and significant damage to concrete.
Last winter, the city of Seattle refused to use salt on their roads, and instead opted for alternative methods, which were not nearly as effective. In the end, the city decided that the increased number of collisions that resulted is not worth it, and will be switching back to using salt this coming winter. There are many alternative methods that Seattle,among other cities, tried as instead of salt. Sand was sprinkled on the snow but did not melt it, and also caused dust. Road officials attempted to pack down snow. Various road de-icers have also been tried, but the vast majority of the time these contain harmful chemicals as well.
The debate over whether or not to use salt is a difficult one. It is very clear that the use of salt has many negative impacts, but it is also clear that it is by far the most effective road de-icer. It is important that when road salt is used it is used properly and effectively. If the proper amount is used, there are better result and a lower cost. It is also important to know the temperature threshold of the product being used. Overall, the use of salt on icy roads is a scientific process that has had a large impact on society.