Why Does Salt Make Ice Melt?

Salt does not warm a frozen road. It changes the temperature at which liquid water and solid ice can exist together.

Even below the usual freezing point, an ice surface contains a very thin layer in which some water molecules move between solid and liquid states. When salt lands on that surface, it dissolves in the available liquid water and separates into charged particles.

Pure water freezes when its molecules can organize into an orderly crystal structure. Dissolved particles make that arrangement less favorable. As a result, salty water must be cooled to a lower temperature before it freezes. If the air and road are cold but still warmer than the new freezing point of the salty solution, some ice melts and produces brine.

The effectiveness depends on temperature and concentration. Ordinary sodium chloride works well around the freezing point but becomes much less useful in severe cold. Calcium chloride and magnesium chloride can function at lower temperatures and may release heat while dissolving, although they are often more expensive.

Road crews sometimes spread liquid brine before a storm. This can prevent snow and ice from bonding firmly to the pavement, making later removal easier. Salt also has costs. It can corrode vehicles and bridges, damage plants, alter soil, and enter lakes and groundwater.

The same principle appears in an old fashioned ice cream maker. Salt is placed around the ice surrounding the container, lowering the temperature of the melting mixture enough to freeze the cream inside.

Salt does not defeat cold. It creates a liquid solution whose freezing point is lower than that of ordinary water, forcing ice to melt until a new balance is reached.