Salt doesn't melt ice – here's how it actually makes winter streets safe

February 1, 2019 by Julie Pollock, The Conversation
Spraying salt onto roads is a safety measure. Credit: stoatphoto/Shutterstock.com

Brrr … it's cold out there! Children are flocking to the television in hopes of hearing there will be a snow day; the bread and milk aisles at grocery stores are empty because of an impending snow storm; and utility trucks are out spraying salt or salt water on the roads.

We all know why the first two happen – kids are excited for a day off of school filled with hot chocolate and snowmen. Adults are stocking up on necessities. But what's up with those trucks?

They're working to protect drivers from slippery conditions by spraying or a solution of salt water to prevent . This salt is very similar to the salt you have on your dinner table – it's the same sodium chloride, NaCl. There are some proprietary mixtures that contain other salts – such as potassium chloride (KCl) and magnesium chloride (MgCl) – but they're not as commonly used.

Road salt isn't as pure as what you use on your food; it has a brownish gray color, mostly due to mineral contamination. Subjecting the environment to this salt via runoff can have some including negative effects on plants, aquatic animals and wetlands.

But it's a cheap and effective way to protect roads from ice due to a simple scientific principle: freezing point depression of solutions. The freezing point of pure water, the temperature at which it becomes ice, is 32 degrees Fahrenheit. So if there's snow, sleet or freezing rain and the ground is 32 F or colder, solid ice will form on streets and sidewalks.

Salt prevents the water molecules from solidifying into ice crystals at 32 F, instead staying slushy at that temperature, before eventually freezing around 15 F. Credit: Julie Pollock, CC BY-ND

If the water is mixed with salt, though, the freezing temperature of the is lower than 32 F. The salt impedes the ability of the water molecules to form solid ice crystals. The degree of freezing point depression depends on how salty the solution is.

It's important to note that the salt must be in a solution with liquid in order for this principle to be obeyed. That's why many cities spray a salt solution before any ice forms.

Salt that's dumped on top of ice relies on the sun or the friction of car tires driving over it to initially melt the ice to a slush that can mix with the salt and then won't refreeze. Pre-treating with solid salt relies on the warmer road surface to initially melt any snow or freezing rain so that it can properly mix with the salt. This is also why pre-treatment of bridges – which are colder than other roads – does not typically work, and why you see "bridge freezes before " signs.

These salt solutions decrease the freezing temperature of water to around 15 F. So, unfortunately for folks facing truly frigid temps, treating with salt won't get rid of ice on their roads.

An alternative strategy used at these lower temperatures is putting sand on the ice. Sand doesn't change the , it just provides a rough surface for your tires to prevent slipping and sliding.

The science of freezing point depression can be applied to any solution, and many research groups have focused on developing alternatives with fewer negative environmental consequences. They include additives such as molasses and beet juice. So maybe you can look forward to cleaning not just white salt off the bottom of your jeans after a winter walk, but pink as well.

Explore further: Toward roads that de-ice themselves

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6 comments

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Eikka
4 / 5 (1) Feb 01, 2019
Salt doesn't melt ice


Yes it does, as long as there's a film of liquid solution already on it. Ice dissolves into a salt-water solution that is colder than the freezing point of pure water..

Dump three large spoonfuls of salt into a glass of water, stir until (most) dissolves, and put the glass in the freezer. Take it out after an hour or two, and then drop an ice cube in it. Thermometer says the water is below 32 F, yet the cube melts away.

This is why the salt is sprayed on the roads as a solution rather than dry salt. When it's properly cold, the dry salt will simply get kicked up and blown away, while the wet salt slurry does melt the ice and turns it into a salt-sorbet, which is then churned up by the car wheels.
oakironworker
2 / 5 (1) Feb 01, 2019
why am I seeing an article that covers 8th-grade curriculum
Tagbert
4 / 5 (1) Feb 02, 2019
interesting

An alternative strategy used at these lower temperatures is putting sand on the ice. Sand doesn't change the melting temperature, it just provides a rough surface for your tires to prevent slipping and sliding.


Sawdust and wood chips also work this way and have the added benefit that they stay on the surface of the ice when it melts and then refreezes.
carbon_unit
not rated yet Feb 03, 2019
What irritates me is the rampant over-use of salt on parking lots and sidewalks, probably driven by liability concerns.) Then the crap gets drug into buildings and vehicles and damages concrete. In the fortuitous case of power snow onto frozen ground, a leaf blower works very well to blow walks and drives clean right down to the concrete leaving no residue. Pity all the yard companies which do snow clearance in the winter don't break out their leaf blowers when conditions allow.
greengnome
Feb 04, 2019
This comment has been removed by a moderator.
carbon_unit
not rated yet Feb 04, 2019
Above should be "powder snow" not "power snow"... Froze my toes off clearing that "fortuitous" snow off Friday with temps in the teens after some serious sub zero weather. Today it is pushing 60...

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