How Does Calcium Chloride Melt Ice?

Colligative Properties

Water is a solvent, meaning that it is a liquid capable of dissolving solids into solution. More specifically, water is a polar solvent, best at dissolving salts and other charged molecules. When a solvent, polar or otherwise, dissolves a significant enough amount of solids, the increase of molecules contained within the solution starts to have an impact on that solvent's physical properties. These affected properties are known collectively as the "colligative properties" of the solvent. Colligative properties hinge only on the total number of individual particles. Atomic and molecular size have no bearing on the observed effect.

For water, a well-known colligative property is a decrease in freezing point temperature. As such, in sub-freezing temperatures, people will throw salt (especially calcium chloride) onto the ground around entrancing to prevent or remove ice. The salt is dissolving into calcium and chloride ions in the water, allowing the latter to remain a liquid at lower and lower temperatures.