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Two-liter soda bottles are popular at summer picnics, birthday parties, and other celebrations. When they’re full, soda bottles are rigid enough to stack, but when they’re empty, the thin plastic makes soda bottles easy to crush and recycle. In fact, two-liter bottles seem to lose that sturdiness as soon as you open the top. A rigid bottle magically becomes soft and squishy just from taking off the cap. So why are unopened soda bottles so rigid when an opened bottle feels so much softer?

The answer has surprisingly little to do with the soda itself. In fact, thin-walled plastic bottles feel strong because of a single ingredient added during the manufacturing process. After filling the bottle with soda or another beverage, manufacturers add a gas to the top of the bottle––and that gas is nitrogen.

At their factories, beverage manufacturers use a process known as inertion to fill the tops of two-liter bottles with nitrogen gas. The nitrogen maintains a higher pressure inside the bottle than the surrounding air pressure. That pressure ensures that the thin plastic walls won’t collapse, which means that grocery stores and customers can stack two-liter bottles.

Of course, once the cap has been opened, releasing the pressure, the plastic once again feels thin and flexible.

Nitrogen is an ideal gas to use for maintaining high pressure. Because of its molecular size, nitrogen does not leak through plastic or rubber as quickly as smaller molecules like oxygen. And just as nitrogen makes two-liter bottles strong, it can also help maintain the pressure in your car tires. Inerting tires with nitrogen provides many of the same benefits as using the gas for beverage storage. Nitrogen doesn’t leak through the rubber of a tire as quickly as compressed air, so tires filled with nitrogen stay full longer.

Maintaining the correct tire pressure is even more important than maintaining pressure in a two-liter bottle, because tire pressure saves money on gas and makes driving safer. So, the next time you see a stack of two-liter bottles, remember that nitrogen can also increase pressure in your tires.

Post Author: Genevieve Carlton Ph.D.

Ph.D. - Research Historian from Northwestern University. A writer and researcher she has published pieces for Ranker, Stacker and Atlas Obscura. She has published a nonfiction history book with the University of Chicago Press and a number of scholarly articles with top journals.

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