When the Roads Freeze Over
This winter of 2008-2009 has been a rather cold one so far compared to winters of past years, and those people who reside in the northern parts of the world know that the winter months bring snow to the ground. Snow is basically frozen water, and it could be fun due to its fluffy consistency, but we all know that snow on roads make it dangerous for drivers of motor vehicles. How do people keep the roads clear of slippery precipitation during the winter months?
Highway Rest Area Amidst a Frozen Tundra
The solution to keeping the roads clear of snow is usually spreading road salt on the ground. This road salt is basically the same thing as table salt (sodium chloride, or NaCl for those who prefer chemical symbols). This is because the heat capacity of salt lowers the freezing temperature of water by a finite amount. The normal freezing point of water is 0 degrees Celsius. With the freezing temperature lowered, the snow will then melt because it is no longer below the freezing point of water. For example, if it is -5 degrees Celsius outdoors, and the road salt lowers the freezing temperature of water from 0 degrees to -10 degrees Celcius, then the snow on the roads will melt because the -5 degree outdoor temperature is warmer than the -10 degrees required for water to freeze with road salt on it.
Now, when the temperature outdoors is extremely cold, there is a problem in using road salt to keep the roads clean. Road salt, with its own heat capacity property, can only lower the freezing point of water by a certain amount. What happens when the ambient temperature outdoors falls below this adjusted freezing point of water? Road salt is no longer effective in keeping roads safe, and as a result the roads freeze over and become dangerously slippery (especially if the road salt did its job and melted the snow during the day, but overnight the temperature drops sharply and causes the melted snow to re-freeze into ice). In situations like this, drivers need to realize that they need to slow down and drive extra-carefully. Road-cleaning crews can use materials like sand to coat the roads and make them less slippery, but that is not a fool-proof way of making frozen roads less dangerous.
Now isn’t this freaking awesome?