Why Does Salt Melt Ice but Stop Working in Extreme Cold?

Spreading salt on an icy road does not “burn” the ice by releasing heat. Even below 0°C, an extremely thin layer of liquid water normally remains on an ice surface. When sodium chloride dissolves in that layer, it forms brine. Water molecules then have greater difficulty arranging themselves into an ordered ice crystal, so the liquid and solid phases reach equilibrium at a lower temperature. This is freezing-point depression.

If the surroundings are colder than the freezing point of pure water but warmer than the freezing point of that brine, some ice melts and allows more salt to dissolve. The more evenly salt is distributed and the more surface water is available, the faster it generally acts. This also helps explain why pre-mixed brine can contact a road more quickly than dry salt.

Salt is not effective without limit. As the temperature falls, less liquid water is available, while dissolution and diffusion slow down. Below the lowest freezing temperature of the sodium chloride–water system, even brine at the most favourable concentration cannot remain liquid. NOAA gives that minimum as about −21.1°C. In real road operations, ordinary salt becomes inefficient well before that point because time, moisture, traffic and pavement conditions also matter.

The accurate claim, then, is not that salt prevents ice from ever forming. Salt shifts the freezing conditions to a lower temperature. The same principle is used in making ice cream: salt lets an ice–water mixture cool below 0°C; it does not warm it.

https://www.noaa.gov/
https://salinity.oceansciences.org/