Why Does Salt Melt Ice?

In the winter, you may have seen people pouring salt over ice on their driveway, sidewalks, and roads. A couple minutes, perhaps hours later, the ice is melted into a puddle of water or sludge. So, how does the salt work?

First, you have to understand what happens when ice and water come in contact. Either the surface molecules on the ice escape into the water, also known as melting, or water molecules are captured on the ice, also known freezing. The warmer the temperature, the faster the molecules move, so ice molecules escape into water. Cold temperature makes the water molecules slow, which allows the ice to capture the water molecules.  


The process of the ice molecules capturing water molecules can easily be disrupted. If you add salt to ice and water, the salt molecules get mixed in and prevents the water molecules from connecting to the ice. Foreign molecules don’t prevent melting, so the rate of melting is faster than freezing. Any substance can be used to disturb the balance between ice and water, for example, sugar and alcohol. Both can melt ice. Salt is used because it is inexpensive and you can acquire it in almost any store.


To refreeze water with added salt, you have to cool the mixture to below the usual freezing temperature. More salt means a lower temperature needed to refreeze. The salt water also needs a crystal of ice to start the freezing. Otherwise, the water with added salt remains the same even if it’s colder. 

There is actually a certain temperature where it’s so cold, the salt molecules start separating from the water, so the salt doesn’t work in every situation.  Around -21 degrees celsius, or -5.8 fahrenheit is just about right.  Of course, we don’t need to worry about in Wisconsin.

For detailed information and a cool animation that shows how the salt, water, and ice molecules work, go to: http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-melts-ice.shtml