Why does salt melt ice?
Because salt lowers water’s freezing point. Dissolved in the thin film of water on ice, it makes a salty liquid that can’t freeze at 32°F, so the ice melts. Plain rock salt works down to about 15°F.
Quick Answer
Salt melts ice by lowering the freezing point of water, a process called freezing-point depression. Ice and the thin layer of liquid water on its surface are always trading water molecules back and forth. When salt dissolves in that liquid, it breaks into sodium and chloride ions that take the place of some water molecules, so fewer water molecules rejoin the ice and freezing slows while melting continues. A 10 percent salt solution freezes at about 20°F and a 20 percent solution at about 2°F. The effect has limits: many road agencies consider plain salt impractical at around 15°F or colder because it works too slowly, and the coldest a salt-and-water mix can stay liquid is about −6°F (−21°C). At lower temperatures, crews turn to other chemicals, such as magnesium chloride and calcium chloride, which work in colder conditions, and they often pretreat roads with salty brine before a storm.
By the Numbers
Key Facts
- The science
- Dissolved salt lowers the freezing point of water (freezing-point depression)
- Rock salt’s limit
- Works well down to about 15°F and is slow below that
- Strongest brine
- A salt-and-water mix can stay liquid down to about −6°F (−21°C)
- Colder options
- Magnesium chloride and calcium chloride work at lower temperatures
How salt changes the freezing point
| Salt in the water | Freezes at about |
|---|---|
| None (pure water) | 32°F (0°C) |
| 10% | 20°F (−6°C) |
| 20% | 2°F (−16°C) |
| The most salt can do | −6°F (−21°C) |
Sources: University of Minnesota LTAP and the University of Illinois Physics Van. Real roads rarely reach these ideal mixes.
Ice is busy, even when it looks still
At the surface of ice, water molecules are constantly leaving the solid and joining the liquid, while others rejoin the ice. At 32°F, the two balance out. Salt changes the balance. Its ions take up space in the liquid, so fewer water molecules rejoin the ice each second. Freezing slows, melting keeps going, and the ice shrinks.
Why there are limits
The more salt in the water, the lower the freezing point, but only to a point. Plain rock salt becomes slow and impractical at about 15°F. Magnesium chloride and calcium chloride can keep working in colder weather. Salt also needs a little liquid water to start the process, which is why crews often spray a salty brine on pavement before a storm.
Why salty ice gets colder
Melting ice absorbs heat from its surroundings. When salt makes the ice melt faster, it draws even more heat out of the mix, so a salt-and-ice mixture can drop well below 32°F. Cooks have long used that trick to chill ice cream.
Myth vs. Fact
MythSalt melts ice by giving off heat.
FactIt doesn’t. Salt lowers the freezing point, and the melting draws heat from the surroundings, which is why a salt-and-ice mix gets colder.
MythSalt works at any temperature.
FactPlain rock salt becomes slow and impractical at about 15°F and below, so crews use other chemicals or pretreat with brine.
The Bottom Line
Salt melts ice because dissolved salt lowers water’s freezing point, so ice melts at temperatures where plain water would stay frozen. Ordinary salt works well down to about 15°F.
Sources & Further Reading
- Scientific American — Why do we put salt on icy sidewalks in the winter?
- Scientific American — Why Does Salting Roads Make Them Safer?
- University of Minnesota LTAP — Winter Salt Guide: What Is Salt and How Does It Work?
- Physics Van, University of Illinois — Freezing Saltwater
- Lunar and Planetary Institute — All About Ice: The Melting Point
- General Chemistry Online, Frostburg State University — Why does salt melt ice?