Why Is Salt Put on Sidewalks to Melt the Ice?

Equilibrium

• When water reaches 32 degrees Fahrenheit (or 0 degrees Celsius), it freezes and forms ice. Water's freezing point means that it has reached equilibrium, where no additional heat can be released to continue melting. Adding salt disrupts this equilibrium. As the salt reacts with the ice, it releases heat, which slows down the water from freezing and forming more ice. In turn, this quickens the process of melting.

Molecular Differences

• Heating up or cooling down water changes how fast or slow the molecules in water move. In its liquid state, the molecules in water move faster than they do when water has been transformed into its solid (ice) state. As the salt hits the ice, the molecular structure of both the ice and salt begin bonding with each other.

Solubility

• Solubility--or rate of dissolving--is also a factor for the effectiveness of salt in melting ice. When salt is dropped into water, it begins dissolving. But, there is a limit to the amount of salt that can be dissolved in water. Once salt has reached its point of solubility, even if you add more salt, it will not dissolve in the water. The solubility for salt is much higher when the temperature of water is also higher.

Chain Reaction

• As soon as the salt hits the ice, a chain reaction begins. The chemical reaction between both the ice and salt gives off heat, which starts to melt the ice. Once ice begins melting into water, salt can begin dissolving in that water. This gives off additional heat, which promotes more melting.

Efficiency Experiment

• Rock salt tends to be the most common, because it is a fairly inexpensive treatment for melting ice quickly. According to KRCG's Kelly Gunderson, who ran a series of experiments in 2007, a calcium chloride mix melted ice the quickest. Even more efficient, she found, was calcium chloride mixed with rock salt.