Why Does Salt Make Ice Melt?

Even when an ice surface looks solid, a small amount of liquid water remains in dynamic balance with the ice. Salt landing on the surface dissolves in this water and separates into mobile ions. With a lower proportion of water molecules in the solution, it becomes harder for them to arrange into the crystal lattice of pure ice. The solution therefore freezes below the freezing point of pure water. This is freezing-point depression, a colligative property that depends on the number of dissolved particles.

If the surroundings are below 0°C but still above the freezing point of the salt solution at its current concentration, the old balance is disturbed. More ice melts and forms brine until temperature and concentration establish a new equilibrium. Salt does not melt ice by “giving off heat”. Melting absorbs latent heat, so an ice–salt mixture can become colder than the original ice. Traditional ice-cream making uses precisely this effect to create a cooling bath below 0°C around the container.

The effect has limits. Meltwater dilutes the salt, while lower temperatures require more concentrated brine. Below a certain point, ordinary sodium chloride can no longer keep the mixture liquid. Road salt is therefore not effective in every severe freeze, and excessive use can corrode materials and enter soils and waterways. The central mechanism is not that salt “damages” ice; dissolved ions change the equilibrium conditions between liquid water and solid ice.

https://pubs.acs.org/doi/10.1021/acs.jchemed.2c00626
https://pubs.acs.org/doi/10.1021/acs.energyfuels.3c02362