Cold, Rain and Snow (but Mostly Snow and Ice) January 10, 2010Posted by calvinus in Physical Chemistry.
Tags: Physical Chemistry, The Fall, Weather of Malcontents
This is the spring without end
This is the summer of malcontent
This is the winter of your mind
M. E. Smith, 1992
Actually Mark, this is the Winter of Malcontents. In case you had failed to notice, the Angles are unhappy with the recent weather that has afflicted Albion’s Plain.
All this snow and ice is all a bit of a frightful inconvenience, quite frankly. I can’t get my car out of the drive for all of these roughians parking in the middle of the road, just abandoning their cars willy-nilly.
Get over it. It’s cold, it’s slippy. Dress accordingly. Act sensibly.
Councils are running out of salt to spread on the road and to a certain extent, I can understand this. The weather has been a freakish, I mean how often in the last few years has Bromley (for example) had to thole this much snow. And so, in times of hardship, it is right that councils are rationing their supplies of salt. I have heard the odd (and it is odd) criticism of councils that won’t go out and salt the roads and pavements when it gets too cold. “They should have been out…saying that the salt won’t work at -6°C is just an excuse.” Err…actually…it is standard physical chemistry, my friend.
The water molecules in ice crystals are held together nice and tightly in a rigid structure that looks not unlike a honeycomb. Voilà:
In a liquid, the water molecules are pinging around moving all over the place, they are not as tightly bound to each other as in an ice crystal. As the temperature drops, the movement of the water molecules slows down and this is what allows ice to form this regular structure. If you add something like salt into this mix, the atoms in salt disrupt the regular arrangement of water molecules above. When this disruption happens, the water molecules find it easier to move and break free, thus melting the ice that had formed. If you want a nice animation of what happens when water freezes (at a molecular level) clicky here.
However, this only works until the temperature gets cold enough again to slow down the water molecules enough to allow them to be stuck back into the ice crystal. What temperature this happens at depends on the nature of the salt (such as what size the atoms are, the atoms present, etc.) and how much you use. Sodium chloride and calcium chloride are commonly used salts for icy roads. The Finns, rather typically given their climate, have been looking for alternative salts and thought about using potassium formate (although a lot of ants might be needed to keep Finnish roads gritted with this!).
Standard table salt can be effective at temperatures as low as -21°C if you use enough of it. Unfortunately this is at such high quantities of salt that I suspect we will all start complaining about how much our cars are rusting…
 Hellstén PP, Salminen JM, Jørgensen KS, & Nystén TH (2005). Use of potassium formate in road winter deicing can reduce groundwater deterioration. Environmental science & technology, 39 (13), 5095-100 PMID: 16053115