Don't Worry, It's a Dry Cold

When people first move to Edmonton, they quickly begin hearing tales of how cold our winters are. These tales are met with the exasperated shout of disbelief: "It gets HOW cold?!" To which native prairie dwellers, such as myself, reply: "Don't worry, it's a dry cold." So what? Ever wonder why it feels way colder at -10 in Guelph than it does at -20 in Edmonton? It has to do with humidity (or the lack of) and a branch of chemistry referred to as "thermochemistry".

Our story begins with James Joule (1818-1920) and the first law of thermodynamics. Joule, being a brewer by trade, also shows us that chemists are rarely far from ethanol. 

First Law of Thermodynamics: energy can neither be created nor destroyed, only converted from one form to another. Heat is a form of energy. It can be transferred across the boundary between a system and its surroundings. Temperature is the measure of heat. Also important to know it that the direction of heat transfer is always from the thing that has the heat to the thing that doesn't. So leaving your front door open in the winter will not let any cold air in. It is physically impossible; however, you can let a whole lot of heat out. Another important term to know is heat capacity: the amount of heat required to change the temperature of a system by 1 degree. 

It is currently a balmy +1 in Edmonton right now.  The North Saskatchewan river isn't even frozen over, but I can tell you with certainty, I will not be jumping in for a swim. Even though the temperature of that river is actually warmer than the air surrounding it, it still feels a heck of a lot colder. This is because heat transfer is more efficient between a liquid and a solid than it is between a solid and gas. Heat is lost, at a molecular level, by collisions between the warmer body (you) and the colder body (the river or the air). Because of the fact that a liquid is more dense than a gas (especially a gas at cold temperature), there are more opportunities for molecules to collide, meaning more opportunities for heat transfer. The other important point is that the heat capacity of water is really big. Meaning that it takes a lot of heat to warm the water just one degree. If there is a lot of water in the air, it can condense on you, giving you that "wet" feeling. This will lead to more opportunity for heat transfer, making you feel much colder at -10, than if you are living in a place like Edmonton, where it is so dry that your skin begins to feel like an exoskeleton that you are much too big for. (I recommend Moisturel as a moisturiser for anyone looking to combat dry skin.) 

This heat capacity of water isn't all bad though. Ask people in Vancouver. See, because water takes so long to heat, it also takes a really long time to cool. Meaning that in the winter, places near water, like Vancouver, don't get that cold. Of course, one big snow storm and the whole city shuts down because they only own one snow plow, and have no idea how to function in weather that gives the rest of Canada the monicker "The Great White North". You can also use this heat capacity to your advantage when cooking dinner. Want to thaw your frozen meat faster? Stick it in room temperature water. 

So Edmontonians, when that mercury dips, and your skin and hair desiccate to a point beyond all recognition, be thankful for it. After all, it's a dry cold, so it really isn't that bad. Just bundle up.  

References:

Petrucci, R. H.; Harwood, W. S.; Herring, F. G. General Chemistry 2002 Prentice Hall Inc., Upper Saddle River, NJ.

Balchin, J. Quantum Leaps: 100 Scientists Who Changed the World 2010 Arcturus Publishin Limited, London.

Laidler, K. J.; Meiser, J. H.; Sanctuary, B. C. Physical Chemistry 4th ed. 2003 Houghton Mifflin Company, Boston, MA.

Winter Tires: Don't Tread the Snow

Well winter has arrived in Edmonton. It is currently -17, with a windchill that makes it feel like -25 C. Over 15 cm of snow has fallen in 72 hours. The roads have become a delightful mix of ice and snow, making driving difficult. And it is not just here in Edmonton that citizens have been hit with a mound of snow and freezing temperatures. Calgarians are currently praying for their next chinook. So how can chemistry help you survive winter? With the science of winter tires! Why are winter tires mandatory in Quebec? Why are some Albertans lobbying for the same law in this province? Are winter tires that important? Well, anyone I have asked have all stated that they love winter tires and are shocked at the difference it has made. The difference all comes down to glass transition temperature (Tg). 

Take a look around your home. I am sure that you can find numerous examples of different types of plastics. Some are rubbery, some are hard, some are fiberous. These characteristics are going to determine how different polymers (plastics are a type of polymer) are going to be used. Now think of a plastic bucket. The kind that you may have used as a kid to build sandcastles. That thing was indestructible during the summer, but leave it outside in Edmonton right now and drop it, that same bucket would shatter into a million pieces. What we are observing is a change in "state" of the polymer. Now this might sound odd, considering it is still solid, and the states of matter are solid, liquid, and gas. So how can we be seeing a change in state? Enter the glass transition.

Polymers can have two solid states: they can be glassy; these are hard plastics, like cellphone cases and water bottles; or they can be rubbery; these are flexible plastics, like rubber balls, or tires. The glass transition temperature (Tg) is the temperature at which a polymer switches between the glassy state and the rubbery state. If a polymer is used BELOW its glass transition temperature, it will be glassy or hard. If a polymer is used ABOVE its glass transition temperature, it will be rubbery or flexible. The polycarbonate water bottle on my desk is an example of a plastic that I am using BELOW its glass transition temperature, while the flexible silicone spatula I used to make my breakfast is an example of a plastic I am using ABOVE its glass transition temperature. Going back to the plastic bucket example: in the summer, the bucket is above its Tg, so there is some flexibility to it and therefore, doesn't break easily. In the winter time, the bucket is below its Tg, making it glassy, and more fragile, so it breaks. 

At cold temperatures, rubber tires are also going to go through this change. Rubber tires were such a great advancement (thank you John Boyd Dunlop) in the tire because these air-filled rubber tires absorbed shock, had more contact area with the road surface, and consequently, gave more traction. The more a tire interacts with the road, the more traction a vehicle has. In the snowy, icy winter, we need all the traction we can get. To get a nice, flexible tire that has lots of contact with the road, it needs to be used above its Tg. However, in Canada, our winters are going to push that. Our -40 C winter days are going to bring a regular tire down to, if not crossing, its Tg. This will make it more rigid, and therefore, it will have less contact with the road surface, which will decrease the traction, precisely at a time when drivers want MORE traction. Also, the treads on the tire will become less flexible, allowing for snow to build up in them, further reducing traction.

Winter tires are made of a type of rubber that has a much lower Tg than summer tires or all season tires. This means that even as the mercury drops, the tire will not approach the Tg, and will stay flexible, resulting in more road contact, less snow build up, and MORE TRACTION. More traction means less sliding, smaller stopping distances, and safer driving. Enjoy safer winter driving thanks to the chemistry of polymers and the glass transition temperature. Get yourself some winter tires.    

For further winter survival reading check out a previous entry: Careful of the Icy Patch

Want more on winter survival through chemistry? Be sure to leave your questions and comments.