By Juliet Myfanwy Johnson | Updated Mar 24, 2022
The freezing point of pure water is 0 °C (32 °F). At this temperature, the kinetic energy of water molecules is low enough that they can arrange into a solid lattice, forming ice. The process is balanced: water is simultaneously melting and freezing at the same rate.
When salt (NaCl) is introduced into the system, it dissociates into sodium and chloride ions. These ions disrupt the ability of water molecules to align into the crystalline structure of ice, effectively lowering the temperature at which the solid phase can form. This phenomenon, known as freezing point depression, means that ice continues to melt while the rate of new ice formation slows.
Because the freezing point is depressed, the water remains in a liquid state even at temperatures that would normally freeze it. As a result, the melting process dominates, and the ice that does exist is less stable, leading to a net reduction in ice thickness.
Any solute—such as alcohol, sugar, or other chemicals—can disturb the equilibrium of water molecules in the same way. Salt is the most common choice for de‑icing roads due to its low cost, high availability, and effectiveness in reducing the freezing point of water by several degrees.
Road crews spread salt on icy surfaces to create a brine layer that remains liquid at sub‑freezing temperatures. This not only melts existing ice but also prevents new ice from forming until temperatures rise above the depressed freezing point.
For more detailed scientific explanations, consult peer‑reviewed studies on colligative properties and phase transitions in aqueous solutions.
