: 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?


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

Simple put, the sodium ( Na)-and chloride (Cl) diffuse through the water and prevent the water molecules to organize close together to make ice. It works up until -9 to -15 depending on what type of salt you use… sodium, magnesium ect.

Salt water has a freezing point below 0˚C. When you put salt on the ice, and then grind it with car tires, a little water is formed from the friction. This water dissolves some salt, and then the salt water wets the ice. At the boundary, the ice melts and makes more water which dissolves more salt which allows the salt water to spread over more ice, … .

The freezing point depends on the amount of salt in the water, from -3˚ for ocean water to -9.4˚ for water that’s saturated with table salt. You can get a little lower with specialized salts containing magnesium or potassium.

Salt is sodium chloride, NaCl.

When it hits water, those molecules split in half into Na+ and Cl- ions that float around in the water.

When you try to then freeze the water, something interesting happens.

The water molecules want to form up into an orderly hexagonal crystal and make a solid, but there’s a problem: wayward chlorine and sodium ions are haphazardly stuck in the middle and don’t want to leave.

To force them out, you have to suck even more energy out of the system and cool the water further. The more salt you have, the harder it is to freeze the water.

The road salt eventually stops working when you get too cold, even absolute brine water will freeze in intensely cold conditions.

And yes, when sea ice forms most of the salt is expelled.

“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.

> But to what temperature?

If your using plain old rock salt it works up to -21.1C (-6F)

> Is there a point where salt is no longer effective on roads and pavements?

Yes. As you get closer to the temperature above the effectiveness of the salt becomes less.

> Does it depend on the amount of salt mixed with water?

It can (so “yes”?). No one cares about salt being on the road (within reason), but they’re very concerned about ice and snow. So if you have excess salt it’s not a problem. In order to get the maximum possible effect (i.e. to get down to the -6F I mentioned before) you need the mass of salt to be ~37% of the mass of the ice/water. If you have less then the lowest temperature you can have no ice at will be lower.

> When the sea freezes is it only the H2O that solidifies?

For the most part yes, it’s just the water that solidifies. Roads are a little different because there isn’t a big pool of water for the ‘extra’ salt to go into. So if you use salt on the road and it then ends up too cold for the salt to work you end up with a mixture of close to pure ice, and a bunch of ‘wet’ solid salt.