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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.

Across disciplines and across the state, from Nobel Prize winning scientist David Lee to notable science education advocate Bill Nye, researchers and scientists have been called on to respond to these questions. For more than seven years, kids - and a few adults - have been submitting their queries to find out the answer to life's everyday questions.

Previous Week's Question Published: 9 September, 2004 Next Week's Question
Dry ice isn't always dry
Question
When is dry ice a fluid?

Question
When most people hear the words, carbon dioxide, they think of a colorless, odorless gas. This is understandable because that is the form in which people normally encounter this material under the conditions on earth. However, there are conditions where carbon dioxide is quite different in appearance and behavior. For example, at low temperatures carbon dioxide can be transformed into a solid. This solid is referred to as "dry ice" because at normal pressure it turns directly into a gas rather than a liquid when it absorbs heat. That is, dry ice sublimes (at a temperature of -78.5°C) so is often used as a coolant. If a mixture of carbon dioxide gas and solid is subjected to a pressure of slightly more than 5 atmospheres at low temperatures (-56°C), it can be transformed into a liquid. When the pressure is released the liquid turns back into solid and gas. Under the appropriate conditions this process can be repeated many times, as I often do as a demonstration in my General Chemistry class at SUNY Cortland.

In addition, carbon dioxide can be converted into another phase called a supercritical fluid (SCF) at temperatures above 30°C and pressures above 73 atmospheres. In this form carbon dioxide has several special properties. In some ways it resembles a liquid while in others it behaves like a gas. It can dissolve other materials with the efficiency of a liquid but flows as easily as a gas. These properties have led to the use of supercritical carbon dioxide for the extraction of many substances from foods, such as caffeine from coffee or nicotine from tobacco as well as others. The use of supercritical carbon dioxide in this way eliminates potentially harmful residue from the food, as occurred with the use of previous solvents. In addition, it promotes more efficient removal of the material of interest (because SCFs can penetrate the food more completely). Finally, its use results in easier recovery of the material extracted because carbon dioxide can be converted back into a gas just by changing the pressure, leaving the extracted material behind.

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