It sounds like this“Salt makes the ice water cold because the salt is colder than the ice.”
Thinking salt itself is cold
Why it feels right
It is a natural guess. When you add salt to ice, things get colder, and it is natural to ask what caused it: the new ingredient, so the salt must be the cold thing. Salt also comes from a cupboard that may feel cool, and we have all seen frost on a salty road.
Why it’s wrong
The salt starts at room temperature. It makes the mixture colder because it lets ice melt below 0 degrees, and melting takes heat from the water. The cold comes from the melting, not from the salt.
A worked example
Put a thermometer in a pinch of salt and it reads room temperature, about 20 degrees. Sprinkle it on warm water and the water doesn't get colder. The salt isn't a source of cold at all.
What happens in ice water: ice and liquid sit at 0 degrees, with as much freezing as melting. Add salt, and the freezing slows down but the melting carries on. Ice now melts without being refrozen, and melting takes 334 joules per gram from the surroundings. That heat comes from the water, the glass and the salt itself, so they all cool.
The mixture goes on cooling until its temperature reaches the new, lower freezing point, at which melting and freezing balance again. The cold isn't added; it is made by taking heat away. This is also why an ice and salt mix, left in a warm room, gradually warms again as the heat flows back in.
How to catch yourself
- You say salt 'is cold' or 'brings the cold'.
- You expect salt to cool plain warm water.
- You can't say where the heat goes when the mixture gets colder.
What to hold instead
Salt starts at room temperature. The mixture gets colder because salt lets ice melt below 0 degrees, and melting soaks up heat from the water and the glass until the temperature reaches the new freezing point.
Where you’ll meet it
- Making ice cream in a bag of ice and salt.
- Cooling a bottle quickly in a bucket of salted ice.
- Wondering why road salt works only while there is ice to melt.
Questions people ask
- Why does adding salt to ice make it colder?
- Salt lets the ice melt at a lower temperature, and the melting takes heat from the water and the glass. The temperature falls until it reaches the new freezing point of the salty water.
- Does salt on its own make water colder?
- No. Dissolving table salt in water changes its temperature only by a fraction of a degree. The big drop happens only when there is ice to melt.