Why Does Ice Melt When You Sprinkle Salt on It?

Sprinkling salt on ice makes it melt, which is why rock salt is used to make walkways and roadways safer after a snowstorm. The reason it works is because it raises the melting point of ice. At a molecular level, the rate of freezing is lowered below the (constant) rate of melting, because the spacing between the water molecules is increased.

Rates of Freezing and Melting

At zero degress Celsius, the rate at which water molecules freeze balances the rate at which water molecules melt, meaning the ice is in equilibrium. Increasing the temperature by adding heat makes the water molecules bounce around faster, too fast to stay ice, and the ice melts. If the temperature is reduced, the water molecules do not have enough thermal (kinetic) energy to overcome their attractive forces, and they turn into ice.

The Ice Lattice

Introducing a foreign contaminant at zero degrees Celsius can also change the balance between melting and freezing. For instance, a contaminant such as salt makes water molecules pack less neatly into the crystalline pattern of ice molecules. Also, there is greater distance between the water molecules over which the attractive forces act. Therefore, the rate of water solidification goes down, while the rate of melting stays the same, so the rate of melting is now greater than the rate of freezing.

A Lower Limit

The above describes what happens at zero degrees Celsius, and it is implied that it happens at lower temperatures, as well. But once it reaches -21.1 Celsius, or -6 Fahrenheit, the salt freezes too, though this occurs in laboratory conditions only. The lowest melting point on roads is typically about -9.5 degrees Celsius, or 15 degrees Fahrenheit. If it is colder sand is more appropriate.

Other Contaminants

What about using other contaminants, such as sugar? Will the freezing point depression still occur? Yes. For every mole of contaminant dissolved in a kilogram of water, the freezing point is depressed by roughly 1.7 to 1.9 degrees Celsius. Rock salt is used because it is inexpensive.

Reducing Water Concentration

There is a variation in the ability to depress the melting point of water. The more ions a salt breaks up into, the more effective it is in slowing the rate of freezing. Calcium chloride lowers the melting point of water much more than salt because it breaks up into three ions instead of two.

References

• General Chemistry Online!