Why Does Salt Melt Ice So Fast?

Salt can keep roads from becoming too icy.

When the weather report calls for icy weather, city officials often put salt on main roads. Salt lowers the freezing point of water so that ice forms at lower temperatures. If ice is already present, adding salt to it raises its melting point so that it melts more quickly. Salt affects how water freezes by altering the speed at which water molecules move.

Difference Between Water and Ice

Ice is water in a solid form. According to the "Lansing State Journal," the difference between water and ice is that the molecules move much more slowly in ice than in water. Adding heat to the frozen water causes the molecules to move more quickly, turning the ice back into a liquid form.

Salt and Water's Freezing Point

Adding salt to water while in a liquid state lowers the freezing point--the temperature at which water freezes. The "Lansing State Journal" says that this occurs because salt sticks to water molecules as it dissolves in the water. To freeze the water, cold air must first separate the salt and water molecules. This process requires colder temperatures than the process of freezing regular water.

Adding Saltwater to Ice

If water has already frozen into ice, pouring saltwater on top may help it melt. According to Frostburg State University, putting liquid water in contact with ice causes some of the ice to melt while some of the water freezes. If plain water is used, the ice melts more slowly than the water freezes and there is no observable change to the ice. Since saltwater freezes at a lower temperature than regular water, this changes the balance of melting to freezing in favor of melting, so that the ice melts faster than the saltwater freezes.

For this reason, putting salt on top of ice can help it melt if it is raining.

Antifreeze and Salt

According to "Lansing State Journal," antifreeze works in the same way as the practice of adding salt to water. Adding antifreeze to a car engine lowers the freezing point of fluids inside the engine. This stops the fluids from freezing on cold mornings so that the engine will run properly.

Other Substances Have Similar Effects

"Lansing State Journal" says that any substance that dissolves in water has the same effect on the water as salt. Sugar and potassium chloride are examples of other substances that dissolve in water and could be used instead of table or rock salt.

References

• "Lansing State Journal": Why Does Salt Melt Ice?
• Frostburg State University: Why Does Salt Melt Ice?