Ice always carries an incredibly thin film of liquid water on its surface, with water molecules constantly leaving (melting) and rejoining (re-freezing). At 0 °C those two perfectly balance out, so the ice holds steady. Sprinkle salt on it and the salt dissolves into that film and gets right in the way of water molecules trying to lock back on. Re-freezing slows down, melting wins, and the ice turns to slush — even though it is below 0 °C. That is why we salt icy roads. But there is a limit: make it cold enough, around −21 °C, and even salt gives up. Slide the salt and the temperature in the simulator and watch the freezing point drop.
Most people think salt warms the ice to melt it. In fact salt cannot add heat; it lowers the temperature at which water will stay frozen, so ice melts even below 0 °C and the slush actually gets colder.
What's actually happening
Almost everyone pictures salt as somehow heating the ice, but it does the opposite — salting ice actually makes the slush colder than the ice was. The real action is about an invisible truce. The surface of any piece of ice is never perfectly still: water molecules are forever escaping into liquid and others are settling back into the solid. At exactly 0 °C these two flows are evenly matched, which is the whole meaning of the freezing point — it is the temperature where freezing and melting tie.
Salt breaks the tie. When it dissolves into the thin liquid film on the ice, the salt ions crowd the surface and physically obstruct water molecules from finding their place back in the crystal. Freezing gets harder; melting carries on unchecked; so the balance only re-settles at a lower temperature than before. Dissolve enough salt and you can push the freezing point well below zero — which is why a road at −10 °C, which should be a solid sheet, becomes wet and safe once it is gritted. It is not warmer; the water is simply no longer willing to freeze at that temperature. The effect depends only on how many particles you dissolve, and salt is a bargain because every unit breaks into two ions, doing double duty.
The trick has limits and uses beyond winter roads. Pile on more and more salt and the freezing point keeps dropping, but only to a floor — for rock salt, around −21 °C, after which extra salt does nothing and councils switch to calcium chloride, which reaches nearly −29 °C. The same physics runs in reverse in an old-fashioned ice-cream churn: salt mixed into ice forces it to melt, and because melting drinks up heat, the salty ice-bath plunges to perhaps −15 °C — cold enough to freeze the cream in the bucket sitting in it. Lower a freezing point in one place, and you can steal enough cold to freeze something else.
Salt lowers water's freezing point rather than warming it, which is why gritted roads stay wet below zero, down to about −21 °C.
- 1Lay a piece of cotton thread across an ice cube and sprinkle a pinch of salt along the thread.
- 2Wait about a minute. The salt melts a shallow groove, then, with the salt now diluted, that water re-freezes around the thread, locking it in.
- 3Lift the thread and the ice cube comes with it. You just watched the freezing point drop and then recover, all in one go.
Common questions
The effect depends only on how many particles you dissolve, and salt is a bargain because every unit splits into two ions, Na⁺ and Cl⁻, doing double duty.
Yes. Ordinary rock salt has a floor around −21 °C, after which extra salt does nothing, so cold-climate crews switch to calcium chloride, which keeps melting ice down to roughly −29 °C.
The forced melting soaks up heat, dropping the mix to around −15 to −20 °C. This is exactly how hand-cranked ice-cream makers freeze the cream sitting in the salty ice-bath.