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Chemical Eye on a Cold Snap
by Preston MacDougall


January 24, 2005

"A river of artic air coming down from Canada" is how many weather reports have described the recent cold snap refreshing a huge swath of the country, from Minneapolis to Miami. My nose told me that Murfreesboro, Tennessee, was right in the middle of it.

jpg Preston MacDougall

Last Monday morning in Grand Forks, North Dakota, the mercury set a record for that date when it went down to 37 below, on the Fahrenheit scale. The "river" characterization accurately implied that there was also a good amount of wind-chill to dress for.

Coincidentally, 40 below is the same frigid temperature on both the Fahrenheit scale and the Celsius scale, which they use across the border from Grand Forks in Winnipeg, Manitoba. Folks in North Dakota and Manitoba will agree that 40 below is definitely cold, but scientists will tell you that it is not absolutely cold.

Temperatures on the Fahrenheit and Celsius scales are defined relative to fixed points. For instance, on the Celsius scale, proposed in 1742 by a Swede named Anders Celsius, the freezing and boiling points of pure water, under normal atmospheric pressure, define O. and 100. degrees, respectively. All other temperatures are relative to these.

Water figures prominently in the definition of the Fahrenheit scale as well, but not pure water. If you've ever made ice cream the old-fashioned way - churning a mixture of rock salt and crushed ice that surrounds a can of fresh cream, sugar and other goodies - you know that the saltwater slurry gets darn cold. The ice cream mixture will slowly harden, or "freeze", but the salt and ice appear to melt into one another. Using the same
thermodynamic principle, highways can be kept ice-free in the winter, but only to a point.

The point was discovered in 1724 by Daniel Fahrenheit, a German physicist, who wondered what concentration of saltwater would yield the lowest possible freezing point. It takes a lot of salt to reach the optimum mixture, but once you're there, you're as low as you can go. Any more, or any less salt, and the mixture will freeze at a temperature that is above zero on the Fahrenheit scale.

As folks in Grand Forks know, you can go lower. A century after Fahrenheit and Celsius weighed-in with their scales, a Scot named William Thomson became very interested in temperature's role in the practical applications of thermodynamics. In 1864, from carefully measured properties of relatively warm gases, he predicted that temperature on the Celsius scale could never go below 273.7 degrees below zero, ever. He called this temperature "absolute zero", and the temperature scale that begins and ends there is named after his subsequently acquired title of Baron Kelvin of Largs.

Under normal atmospheric pressure, air starts to liquefy at 190 degrees below zero on the Celsius scale. So there were some major experimental challenges to surpass before scientists would know whether Lord Kelvin was right. Air is mostly made up of the gases nitrogen and oxygen, in about a 4 to 1 ratio. Liquid nitrogen boils at 77 degrees on the Kelvin scale (which is a really cool 320 degrees below zero on the Fahrenheit scale), and is often used by dermatologists to remove pre-cancerous skin lesions.

Last year, during National Chemistry Week in mid-October, students in the MTSU Chemistry Club delighted local elementary school students with another use of liquid nitrogen - making ice cream! (I guess the tediousness of churning salt and ice for thirty minutes or more just doesn't appeal to youngsters today.) Using the same ingredients of cream, sugar and goodies, but just mixing in a couple liters of liquid nitrogen, the frozen treat was ready to eat in a few minutes. No salt, no ice, no bucket or crank (unless you count the seasoned chemistry professor who misses the thermodynamic beauty of the old-fashioned method).

There are other gases with even lower boiling points, and in the game of boiling point limbo, helium wins. The liquefaction of helium was a major breakthrough in experimental physics, ushering in the new field of
"low-temperature physics", where all sorts of weird and wonderful phenomena occur, such as superconductivity.

The Dutchman, H. Kamerlingh Onnes, achieved this feat in 1908, when he observed liquid helium for the first time at the near-bottom temperature of 452 degrees below zero Fahrenheit, or a measly 4.2 degrees on the Kelvin scale. Since the planet Jupiter is composed primarily of the elements hydrogen and helium, you could say that he observed "drops of Jupiter" long before Train recorded their chart-busting song.

Eventually, physicists were able to wring the last few degrees out of matter. And, as Kelvin predicted, there are no below zero temperatures on his scale. It is now possible to get within millionths of a degree above
absolute zero, and it turns out that he was only off by half a degree!

That's impressive. It seems that Lord Kelvin had his temperature scales down cold.

Preston MacDougall is a chemistry professor at Middle Tennessee State University. His "Chemical Eye" commentaries are featured in the Arts and Public Affairs portion of the Nashville/Murfreesboro NPR station WMOT (


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