If you live somewhere that gets snow and ice in the winter, you've likely enjoyed a “snow day" off from school when snow and ice made the roads too dangerous for driving.
As your city works to clear snow and ice from the roads, you may have noticed trucks spilling salt or sand on the road. That's no accident! It's all part of the plan.
In warm water, molecules move quickly and freely, making liquids fluid and able to adapt to any shape of container you pour them into. As water freezes, though, molecules begin to move more slowly. Eventually the molecules begin sticking together to form ice, a process we call freezing.
When you put salt on ice, liquid water molecules that have not yet frozen on the surface of the ice dissolve a bit of the salt, creating a sort of saltwater solution called “brine." The brine prevents the liquid water molecules from freezing.
As more water molecules begin to speed up and break free from the ice, they turn into liquid and dissolve even more of the salt.
Why does this happen? Salt lowers the freezing point of water. Ice forms when water is cooled to 32° F (0° C). When you add salt to water, the freezing point drops.
If you add a 10 percent salt solution, the freezing point drops to 20° F. A 20 percent salt solution freezes at 2° F.
If you have ever sprinkled salt on your icy front porch steps, you already know you can watch this process in action. When you sprinkle salt on ice, you will notice that it begins to dissolve immediately around each grain of salt.
This melting continues to spread, until you are left with salt grains on the porch steps. With the ice gone, the salt also provides traction, making it safer for visitors to come to your front door.
Unfortunately, if temperatures dip below approximately 15° F, salt may not get the job done. Salt needs water to begin the melting process.
If all the water is frozen solid, it cannot get into the ice structure to start the melting. When this happens, workers often turn to sand instead.
Many people believe that sand and salt are used for the same purpose — to melt the ice. But they're wrong!
While workers use salt to melt ice, they rely on sand for improved traction. Sand crystals increase friction and help prevent vehicle tires from slipping on slick roads, making travel easier for drivers.
Sand can also help prevent new ice from forming on roads. Imagine each grain of sand as a tiny little ball. In each scoop of sand, there are millions of balls constantly in motion. Each time a car drives over them or the wind blows, the grains move around.
The movement of the sand grains makes it difficult for water molecules to stick together and form ice. Although sand may not entirely prevent ice from forming, its constant motion makes it difficult for ice to build up as quickly as it can on untreated roads.