Salt Questions · Science, chemistry & environment
Why does salt prevent ice from forming at 32 degrees?
Short answer
To freeze, water molecules must line up into an orderly ice crystal. Dissolved sodium and chloride ions get in the way, so the water has to be colder before freezing can happen. The more salt is dissolved, the lower the freezing point, down to about −21 °C (−6 °F) for the saltiest brine.
Freezing point of salt water by concentration
Bar length shows degrees below zero; approximate values.
A colligative property
Freezing-point depression depends on how many particles are dissolved, not what they are. Each unit of salt splits into two ions, so salt is about twice as effective per molecule as sugar.
How much salt, how much colder
Approximate freezing points of salt water by concentration:
- 5% salt: about −3 °C
- 10%: about −6.6 °C
- 15%: about −11 °C
- 20%: about −16.5 °C
- 23.3%: −21.1 °C (the lowest possible)
An equilibrium view
At 0 °C, ice and water exchange molecules equally. Dissolved salt lowers the rate at which water molecules join the ice, while ice keeps melting at the same rate, so the ice melts until the temperature drops enough to rebalance.
Related on The Salt Almanac
More science, chemistry & environment questions
- What happens when you mix salt and vinegar chemically?
- Why does iron rust faster in salt water?
- What is the pH level of a salt water solution?
- How do you separate salt from water through evaporation?
- What is the crystalline structure of sodium chloride?
- Why is road salt bad for the environment and plants?
Cite this page
For school projects and reports. Teachers often prefer the original sources too: see the list below.
The Salt Almanac. (2026, November 8). Why does salt prevent ice from forming at 32 degrees. https://saltalmanac.com/answers/why-does-salt-prevent-ice-from-forming-at-32-degrees
“Why does salt prevent ice from forming at 32 degrees.” The Salt Almanac, 8 Nov. 2026, saltalmanac.com/answers/why-does-salt-prevent-ice-from-forming-at-32-degrees. Accessed [today's date].
“Why does salt prevent ice from forming at 32 degrees.” The Salt Almanac. Last modified November 8, 2026. https://saltalmanac.com/answers/why-does-salt-prevent-ice-from-forming-at-32-degrees.
Original sources
- Wikipedia. Freezing-point depression. https://en.wikipedia.org/wiki/Freezing-point_depression
- Wikipedia. Sodium chloride (data page). https://en.wikipedia.org/wiki/Sodium_chloride_(data_page)