When we speak of ice, we say that it’s frozen solid. That isn’t true. Ice is actually always in the process of melting and refreezing. When ice stays frozen, the process of melting and freezing are in balance and happen at the same rate (equilibrium). In order to melt the snow and ice on our roads and sidewalks, salt is used.
Why does salt melt frozen water?
When salt is spread over snow, the salt dissolves into the melted water. The salt then interferes with the refreezing process. The snow is no longer freezing and melting at the same rate. More water remains melted. The salt is a freezing point depressant. Ordinarily, the freezing point of water is 32 degrees Fahrenheit (F). With the addition of salt, the freezing point can be lowered to 15 F. The more particles of salt are in solution, the lower the freezing point. When the temperature is lower than 15 F, sand is used to increase traction on roads because the salted water will freeze into ice.
What kind of salt can be used?
Many different compounds could be used to melt snow and ice. The main requirement is that they form a solution when dissolved in water. Therefore, you need to look at what the compound will do to the environment. Most salts are corrosive and can be harmful.
Sodium chloride (table salt) isn’t the only choice for deicing roads and sidewalks. In fact, calcium chloride lowers the freezing point of water to -20 F, but the road becomes slippery at 0 F. Several different salts are commonly used: ammonium sulfate, calcium chloride, calcium magnesium acetate, magnesium chloride, potassium acetate, potassium chloride, and sodium chloride. Each salt has pros and cons. The pros include speed of melting, use as a fertilizer, and effective temperature ranges. The cons involve damage to concrete, corrosive, and can make the surfaces slippery.
In 2008, Seattle, Washington used sand instead of salt on their roads. The local paper stated that the reason was to avoid adding salt to Puget Sound. Unfortunately, this resulted in only 4-wheel drive vehicles and cars with chains to travel the city’s streets. Balancing the needs of the environment and the public safety is challenging.
Why can’t you keep adding salt and lower the temperature further?
The salt is only in the liquid water. As the water refreezes into ice, it leaves the salt molecules in solution. The salt then becomes more concentrated in the remaining water. Eventually, the salt cannot remain in solution and begins to crystallize back into salt.