Why Do They Put Salt on Icy Roads?
This is sometimes the subject of bad jokes. Occasionally some smart aleck will say, “To melt the ice!” Of course, that’s the reason.
Well, in recent years scientists have found that other man-made chemicals will do the same job. But for years, men and women have put salt on icy roads, on slick sidewalks and on the concrete steps leading up to their houses. Salt seems to be a perfect choice for melting the sometimes treacherous ice.
One of the most interesting things about ice and salt is the unique activity that takes place when the two come in contact. According to research, in laboratories and in classroom experiments, when we put salt on a chunk of ice the interior temperature will actually drop quite a bit!
This might suggest that putting salt on ice is the worst thing to do. But if we look into the structure of the ice, we will see that salt works to reduce the ice chunk. Ice is composed of crystals that hold to each other in a network. The molecules of ice move very slowly, which is why ice is different from water. Any liquid water that is present (with salt in the mix) will take longer to re-freeze. This is key to good highway maintenance. If the salt is present before snow and ice, the water will not freeze so quickly.
If we put salt (sodium chloride) and water (oxygen and hydrogen) together the molecules of the two compounds tend to bind to each other. Freezing occurs in any material as heat is removed. More heat must be removed from the water/salt combination to make it freeze, quite a bit more than water alone. We have effectively lowered the temperature at which the water (with salt) will freeze. It stays in the liquid state longer.
Salt will also have an interesting effect on water that is heated. The salt and water mixture won’t boil until the temperature is higher than the boiling point of water alone.
We could use a lot of other things to melt ice because scientists know that anything that will dissolve in water will change the freezing and boiling points. In fact, when the temperature gets too low, salt won’t do the trick anymore. Some man-made chemicals will work at much lower temperatures. Simple combinations such as sodium acetate will work to melt the ice.
During the winter months in northern climates, highway crews will use ice to lower the freezing point of water. But they may also put sand or cinders down to increase traction. Salt has two jobs in the winter – keeping water from freezing and providing some traction too. Salt crystals used by highway crews are like small rocks or pebbles that give tires more traction. Salt is very cheap and can be used in large quantities without great cost. It works very well too!