It sounds like this“The more salt you add, the colder the ice water gets, with no limit.”
Thinking more salt always makes it colder
Why it feels right
If a little salt lowers the freezing point by a few degrees, a lot of salt should lower it by a lot. It is a plain straight-line guess, and for small amounts it is nearly right, so it seems there is no reason for it to stop.
Why it’s wrong
The freezing point falls only until the water is saturated with salt. For table salt that happens at about 23% salt by mass, and the lowest temperature is about -21 degrees. Beyond that, extra salt just sits undissolved at the bottom.
A worked example
Salt lowers the freezing point in proportion to how many particles dissolve. But only so much will dissolve. At 0 degrees, about 35 g of table salt dissolves in 100 g of water, and the proportion barely changes with temperature.
The lowest freezing point you can reach with salt and water is about -21.1 degrees, at 23.3% salt by mass. This point is called the eutectic. Below it, the mixture doesn't just stay liquid longer: it freezes into a mix of ice and salt crystals.
As a quick check: 23% salt by mass means 23 g of salt for every 77 g of water, which is about 30 g of salt for 100 g of water. Add more than that and the extra doesn't dissolve, so it has no more effect on the temperature. This is why ice cream makers get down to about -18 degrees, not -50, and why salting a road works well only down to about -10, and poorly below it.
How to catch yourself
- You expect that you could reach any temperature with enough salt.
- You think 'pile on salt' is always the fix for a stubborn ice patch.
- You assume the effect keeps its straight-line rule at every amount.
What to hold instead
Salt lowers the freezing point only until the water is saturated. For table salt the limit is about -21 degrees, at about 23% salt by mass. More salt than that just sits undissolved, so there is a floor to how cold salt and ice can get.
Where you’ll meet it
- Choosing how much salt to put in an ice bath.
- Understanding why road salt fails on very cold nights.
- Reading why the Dead Sea can stay liquid below 0 but still freezes in a hard winter.
Questions people ask
- How cold can salt and ice get?
- About -21 degrees Celsius for table salt, reached at about 23% salt by mass. That is the lowest freezing point of a salt and water mix; adding more salt doesn't lower it further.
- Why doesn't road salt work in very cold weather?
- Salt melts ice only when the temperature is above the freezing point of the brine it makes. At around -10 degrees, a thin layer of road salt cannot dissolve enough to keep up, so other chemicals such as calcium chloride, which works to about -30, are used instead.