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No. 127571347Printed in the USACOVER PHOTOGRAPHY BY MIKE CIESIELSKIQ: My parents just bought new tires fortheir car, and the salesman told them theyshould inflate them with nitrogen gasinstead of regular air. Is paying extra fornitrogen gas in car tires worth it?A: This is a very intriguing question. My initialreaction is to assume that salespeople arejust looking for one more way to squeeze abuck out of gullible customers. After all, air is78% nitrogen. Why would inflating your cartires with pure nitrogen make that big of a difference?But this question is particularly trickyfor me because it pertains to chemistry, whichI have taught for the past 22 years. And cars,though I have driven for the past 29 years, Iknow next to nothing about. I do, however,know where to go for advice about cars: thefamous Magliozzi Brothers, Tom and Ray,a.k.a. “Click and Clack, the Tappet Brothers”!(Their NPR radio show “Car Talk” is aQ uestion F rom the C lassroomBy Bob Beckerweekend favorite.)Their Car Talk website(http://www.cartalk.com) revealsthat they have addressed the nitrogenquestion quite a few times, mostrecently in April 2005. Their advice:For the average driver, filling tires withnitrogen gas is not worth the addedcost. Still, for a chemist, the “nitrogenin tires” question has an intriguing mixof silly and plausible claims swirling about it.Race car drivers do it. Why? Air containsabout 78% nitrogen, 20% oxygen andsmall quantities of other gases, including,most importantly, water vapor. On humiddays, air contains considerably more waterCORELvapor than it does on dry days. What's more,because of the extensive hydrogen bonding,water vapor deviates quite dramatically fromideal gas behavior. Thus, as it heats up, anair-filled tire tends to show much more inconsistencyin pressure changes than does a tirefilled with pure nitrogen gas. In car racing,contact between the tire and the track iseverything, and a fraction of a psi (poundsper square inch) in the tire can make the differencebetween winning, losing, or spinningout of control. NASCAR mechanics figuredout years ago that the pressure in a nitrogenfilledtire was far more consistent and reliablethan the pressure in air-filled tires. Does thatnecessarily mean that the average Joe orJane on the street should swap out the air intheir tires for nitrogen gas? Is it worth spendingthe extra money? Again, probably not.A large number of web sites disagree.But, consider this: These sites tend to be theones selling nitrogen. As such, the informationthey offer should be scrutinized and not merelyaccepted as factual.One argument isthat nitrogen is far lessreactive than oxygenand thus does lessdamage to the insideof the tire. This all maybe true, but it has beenmy experience that tiretreads wear off longbefore any kind ofdeterioration from the inside could make a difference.Maybe nitrogen causes less oxidationof the wheel rims? But again, is this really evenan issue?They also contend that nitrogen-filledtires dissipate heat more quickly and thus runat a lower temperature. Unless this effectcan be attributed to the water vapor, itmakes very little sense, since nitrogen andoxygen have very similar thermal conductivities(0.02583 and 0.02658 W/mKrespectively) and specific heat capacities(29.1 and 29.4 J/molK, respectively).2 ChemMatters, FEBRUARY 2006http://chemistry.org/education/chemmatters.html