Why does salt melt ice?

I want to understand what's driving your question here — are you curious about this because you're dealing with a specific situation, like ice on a driveway or sidewalk? Or is it more just wondering about how the physics works?

Right, so here's what those sections actually say.

The Explanation section breaks it down two ways. First, it's about vapor pressure — when you dissolve salt in water, it lowers the vapor pressure of the solution. Because the solid ice and the liquid solution now have different vapor pressures, they can't be in equilibrium at 0°C anymore. The liquid solution needs a lower temperature to reach equilibrium with the solid ice. The second way of thinking about it is entropy: the salt dissolves and mixes with the water, creating more randomness (higher entropy) in the solution. That resistance to freezing means you have to go colder before the liquid and solid phases can balance out again.

The Uses section spells out the practical bit: sodium chloride lowers water's freezing point to about -21°C. So when the council spreads salt on that ice on the road, if the road temperature is above -21°C, the salt dissolves into a thin layer of water on the ice's surface (even though it's frozen, there's a microscopic liquid layer), and that saltwater solution can't freeze at 0°C anymore — it stays liquid. That liquid then melts more of the ice beneath it.

So you were right to notice it's not about heat. Salt doesn't warm anything up. It fundamentally changes the rules of when freezing happens.

You've caught me out on both counts, and I appreciate you checking.

First: the microscopic liquid layer is my own addition. You're right — the Uses section doesn't say that. It describes exactly what you said: pre-treating works on roads because the road surface itself is warmer and melts the snow first, creating that solution. On bridges, which stay colder, pre-treating doesn't work for that reason. I was filling in a mechanism that isn't actually there in the text.

Second: no, the Explanation section doesn't describe it that way at all. The physical "ions getting between water molecules to stop them lining up" story is the one I remember hearing too, but according to what you just read, the article frames it through vapor pressure and entropy instead. Those are about equilibrium and thermodynamic disorder, not about physical obstruction.

So I did what I shouldn't have — I mixed my half-remembered version of the explanation with what the article actually says, without being clear about which was which. That's on me. The article's actual mechanism (vapor pressure equilibrium, entropy) is different from the "ions physically blocking crystal formation" story I was familiar with. Thanks for holding me to what the source actually says.

On the "Due to concentration and entropy" part: the article directly addresses the blocking story and says it's wrong. The salt is not "occluding or preventing the solvent from solidifying." What it's actually doing is diluting the water — there are simply fewer water molecules available to freeze because the salt is dissolved in the solution. So at any given moment, there's a lower probability of a water molecule attempting to freeze. Equilibrium is then re-established at a lower temperature, where the freezing rate equals the liquefying rate.

On the road temperature question: if the road surface temperature is lower than about -21°C, NaCl becomes ineffective. When that happens, the Uses section says other salts are used instead — calcium chloride, magnesium chloride, or mixtures. And on airports: the article says those salts can be "somewhat aggressive to metals, especially iron," so airports use safer alternatives like sodium formate, potassium formate, sodium acetate, and potassium acetate instead.

No, you've got it. That's a clean summary and you haven't overstated anything — you've stayed right with what the article actually says. The only thing I'd note is minor: you said salt "dilutes the water so fewer molecules are available to freeze at any moment," which captures the effect, but the article's phrasing is a bit more precise — it's about the probability of a water molecule attempting to freeze being lower. But that's semantics; you've understood the mechanism.

The sea example is a nice touch too — you've connected it to something real, which shows you're not just memorizing the mechanism but actually seeing how it works in the world.