January 17, 2014

The Nitty-Gritty on Road Salt

Ah, winter. The season when all cars turn white, regardless of what colour they had when they left the factory.

Could it be chromatic sympathy with the hoary landscape? Ach no, it's more to do with the reality of driving on icy roads. As soon as the first snow falls, out come the trucks piled high with road salt. The spreader gets activated, and the roads are covered with a sprinkling of coarse rock salt, sometimes mixed with sand for added grit, sometimes with more unexpected ingredients like beet juice.

Wait: beet juice?

Okay, hold your Vitamix. Before you start experimenting with spreading veggie juices on your driveway, let's step back a bit. Let's go back to rock salt and ask how that helps snowy or icy roads in the first place.

Ice and packed snow make roads very slippery, so public works engineers are keen to keep that from accumulating on road surfaces. Rock salt helps because it lowers the freezing point of water. Pure water freezes at 0C or 32F. [You could argue about just how pure rainwater is (or its frozen equivalent, snow), but compared to the amounts of salt added on the road, you can forget about any impurities in the water that falls from the sky].

On the diagram below, pure water is way on the left, with zero salt (NaCl) added, and pure salt is way on the right. For water without salt, the freezing point is at T=0C. As you add salt into the water, you move towards the right in the phase diagram, and the freezing point goes down, following the red line.

This phase diagrams tells you a few things. First of all, it takes quite a bit of salt to depress the freezing point: 10% salt will only win you a few degrees. That's why it takes trucksful of salt to get the job done.

Secondly, salting the road can only take you so far: to be precise, –21.1 degrees centigrade at the utmost, in ideal lab conditions. In practice salting roads is only effective for temperatures down to –5C or so. So in a Deep Freeze, like that experienced in the US in early 2014, when temperatures reached –40C and even lower, putting any amount of salt on the road will not help at all.

And then there's the aftermath: whatever doesn't get spread over car bodies where it goes to work making rust, or flows away with the meltwater, can be seen as a white residue on the road, once the snow and ice is gone. Rains wash it away to follow the meltwater, and ends up in the soil next to the road and in streams and waterways.

That's a problem: salt does terrible things to soil. In fact, in the old days salting the fields belonging to criminals and conquered peoples was a popular and effective method of punishment, because nothing would grow on those fields for years. And freshwater streams are, if anything, even more sensitive to salinisation. So putting too much salt on the road is not a good thing.

This is where beet juice comes in. As in, sugar beets.

In fact, the beet juice used for roads is a by-product of the sugar industry. The phase diagram for a sugar solution is very similar to that of a salt solution: as you add sugar to water, the freezing temperature is lowered (to –9.5C for the case of sucrose).

But here's the really cool thing: when you add both salt and sugar to water, they reinforce each others tendency to lower the freezing point. It can be effective down to –18C (or 0F). So adding beet juice to a salty brine and spraying that on the road, preferrably just before snowfall, is super effective at keeping ice from forming on the road, and can reduce salt use by a third. Added bonus: beet juice reduces the corrosive tendency of salt brine.

(Do roads sprayed with beet juice turn a nice fuchsia colour? --Sorry, no: sugar beets are off white inside. Too bad, the melting and drying patterns of red beet juice could produce some nice psychedelic road art).

For your own driveway, how about this experiment: After removing as much snow as you can, try spreading a layer of pulp leftover from your juicing, especially if you use a lot of fruit. The sugars left in the pulp may help lower the freezing point. At the least, the pulp will provide extra traction, and it's not nearly as abrasive as sand if you track it indoors. And after the snow is gone, you can just sweep it onto your lawn where it will benefit the grass rather than killing it. I'm not a juicer; if you decide to try this, please let me know how it worked out.

To minimise rusting and corrosion on your car, try to get to a car wash before the temperatures rise: while it's cold, the chemical reactions are slowed way down. Give the attendant a nice tip and ask them to put extra care into rinsing the underside of your car. And hope it doesn't start to snow again too soon.



You may also like:
1. How to Save Water (and Work) When Washing Your Car
2. How to scrape ice off your windows (Novel uses for a credit card)


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