: How is salt a solid state and water freezes at 0 °C but when you put salt on roads at -10 it stays in liquid state?

Question

So it’s -10 °C where I am today and there are loads of puddles on the pavements and roads because they have been salted (I’m aware that the ground temperature might be higher than -10). But I can’t wrap my mind around the fact that above 0 °C salt can be solid but when it’s diluted into water it lowers the freezing temperature.
But to what temperature? Is there a point where salt is no longer effective on roads and pavements?
Does it depend on the amount of salt mixed with water?
When the sea freezes is it only the H2O that solidifies? What about the salt?
Please, this has kept me up at night!

In: Chemistry

Answer 1

“freezing” is basically the point where the molecules in the substance stop moving around and instead form a solid (often crystalline) structure. This happens because individual molecules have weak forces of attraction towards each other, that’s a bit like magnetic attraction. When molecules have a lot of energy (ie, they’re hot), they’re far too excited to be interested in obeying those forces of attraction and they whizz around all over the place. To solidify them you have to remove energy so they’re moving slow enough that the forces of intermolecular attraction can take hold.

What salt does is it dissolves in water, and when salt dissolves in water, and when salt dissolves in water, the water molecules get much more interested in the salt molecules than other water molecules. Their attraction gets occupied by the salt so you basically get a bunch of individual salt ions each surrounded by a crowd of water molecule groupies. This prevents the salt ions attracting to each other and solidifying, and it reduces the feelings of attraction the water molecules feel towards each other, so they’re less likely to start doing that attraction thing they need to do to solidify.

Water that’s fully saturated with salt (a point where every single water molecule has a salt ion to be interested in, and any more salt you add won’t dissolve) freezes at -21, at which point the water molecules separate from the salt and the salt goes back to being a separate solid. If it’s colder than -21, applying salt to roads won’t melt the ice.