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On September 17th, 1998 the Ithaca Journal ran its first "Ask A Scientist!" article in which Professor Neil Ashcroft , who was then the director of CCMR, answered the question "What is Jupiter made of?" Since then, we have received over 1,000 questions from students and adults from all over the world. Select questions are answered weekly and published in the Ithaca Journal and on our web site. "Ask A Scientist!" reaches more than 21,000 Central New York residents through the Ithaca Journal and countless others around the world throught the "Ask a Scientist!" web site.

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Previous Week's Question Published: 23 October, 2003 Next Week's Question
Balloon's pressure drops when place in liquid nitrogen
Question
If you put a balloon of oxygen into liquid nitrogen what would happen to the balloon?

Question
Basically the pressure decreases. This is exactly as we would expect based on all sorts of observations. For example we know that the pressure in a car's tire falls when it gets cold outside. So we expect that if we start with a pressure of an atmosphere of oxygen in our test chamber (say we use a metal chamber and not a balloon), then I would expect that the pressure in the chamber would fall by roughly a factor of four (being the ratio of 308K [Kelvin] or 35 Celsius - a hot summer day - to 77K the boiling point of liquid nitrogen).

So now we are down to 0.25 atmosphere. But oxygen isn't an ideal - gas and can liquefy. At one atmosphere pressure, the temperature at which it liquefies is about 90K.

As the temperature decreases, the pressure at which a gas liquefies becomes lower. The relationship between temperature (in Kelvin) and pressure isn't linear, so you have to look up the pressure for a particular temperature in a table. It turns out that at a quarter of an atmosphere, the temperature at which liquid oxygen forms is 78.77 K, or a little higher than that of liquid nitrogen at one atmosphere pressure.

So now what? Well, the answer is that some of the oxygen will liquefy, occupying a much smaller volume, and the pressure will fall some more till it gets down to the pressure at 77K (roughly 1/5 of an atmosphere). Then the liquefaction will stop.

So if you pressurized a balloon to an atmosphere pressure (the pressure inside and the pressure outside are equal) then the pressure inside will drop to 1/5 of an atmosphere (which is the pressure above the liquid oxygen at 77K), the balloon will wrinkle up and liquid oxygen will collect in a pool at the bottom of the balloon. If you started with a high pressure of oxygen (blew up the balloon even more) then you'd have the same pressure inside but more oxygen liquid at the bottom of the balloon (and the balloon would wind up being more or less the same size in either case). The pressure difference (between the 1/5 atmosphere inside and the 1 atmosphere outside) is then supported by the balloon's walls because the balloon has "stiffened" as the rubber freezes up.

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