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Why does salt make ice water colder?

Salt on ice makes it colder, not warmer. Guess first, then see why melting takes heat, why salt lowers the freezing point, and where the limit is.

The steps5 · 26 min
  1. 1Melting takes heat: 334 joules for every gram5 min
  2. 2Dissolved salt lowers the freezing point6 min
  3. 3The mixture cools until it reaches the new freezing point6 min
  4. 4There is a limit: about -21 degrees5 min
  5. 5It is the particle count, not the kind of stuff5 min
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What it takes apart

Sound like you?

The ideas people most often get wrong on the way, in their own words. Each step starts from one of them.

“Ice melts by itself once it is warm enough, and melting takes no energy of its own.”Taken apart in step 1Thinking melting is free once it is warm enough“A mix of ice and water is always at 0 degrees, whatever you add to it.”Taken apart in step 2Thinking ice water cannot go below 0 degrees“Salt makes the ice water cold because the salt is colder than the ice.”Taken apart in step 3Thinking salt itself is cold“The more salt you add, the colder the ice water gets, with no limit.”Taken apart in step 4Thinking more salt always makes it colder“Salt melts ice through a special reaction between salt and ice, unlike other things you might dissolve.”Taken apart in step 5Thinking salt has a special chemical effect on ice
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The short answerRather read it than work it out?Show it (it gives the steps away)

Turning ice into water at 0 degrees takes 334 joules per gram, with no change in temperature. 100 g of ice takes 33.4 kJ, drawn from the water, glass and air around it. So melting ice always cools its surroundings, and ice water stays at 0 degrees while ice remains.

Dissolved particles get in the way of water molecules rejoining the ice, so water with salt in it freezes below 0 degrees. The drop is about i x 1.86 x molality: 10 g of salt in 100 g of water, with i = 2 for the two ions, lowers it by about 6.4 degrees. Zero is the freezing point of pure water only.

With salt in the water, ice still melts but can't refreeze as fast, so melting wins. Each gram that melts takes 334 joules from the mixture, so it cools. It keeps cooling until its temperature reaches the new, lower freezing point and melting and freezing balance. The cold comes from the melting, not from the salt, which starts at room temperature.

The freezing point falls only while salt keeps dissolving. For table salt it bottoms out at about -21 degrees, at about 23% salt by mass, 30 g per 100 g of water. Extra salt beyond that sits at the bottom with no further effect, which is why ice baths and road salt have a floor.

Any dissolved substance lowers the freezing point, in proportion to how many particles it makes. Table salt splits into two ions, and a gram holds many units, so it makes about twelve times as many particles per gram as sugar. That is why salt, not sugar, goes on roads, and why calcium chloride, with three ions, works at even lower temperatures.

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