A fun experiment to conduct when discussing phase diagrams is the melting of solid carbon dioxide (dry ice). To perform this experiment, place small pieces of dry ice (carbon dioxide) in a plastic pipette, seal with a pair of pliers, and position the bulb of the sealed pipette in a beaker of water. Then sit back and watch what happens! Some students of mine recently performed this experiment and filmed it with our high speed camera. Check out the video below.
“Wait!” I can hear some protest. “I thought you were talking about the MELTING of dry ice! All I saw in that video was an explosion!” Well, there is a lot going on in this experiment, and part of what is going on IS the melting of dry ice. Below I will describe how I relate this simple, yet fascinating experiment to the phase diagram of carbon dioxide.
Carbon dioxide is a gas a room temperature and atmospheric pressure. That carbon dioxide exists in the gaseous phase under these conditions can be predicted by the intersection of this temperature and pressure on the phase diagram of carbon dioxide (Figure 1, red circle).
Figure 1: Phase diagram of CO2. Red circle: carbon dioxide at 20oC and 1 atm pressure is a gas. Red arrow: Solid CO2 must be at or below -78.5oC at 1 atm pressure; solid CO2 will go directly from a solid to a gas (sublime) as it warms at this pressure.
Inspection of the phase diagram also allows us to predict several things about solid carbon dioxide (dry ice). First, dry ice must be at or below -78.5 oC at 1 atmosphere pressure. Also, dry ice placed on a warm surface will not melt, because carbon dioxide cannot be liquefied unless subjected to pressures greater than 5.1 atm (Figure 1). Rather, a piece of dry ice will go directly from the solid to the gaseous phase (it will sublime) when it warms at 1 atmosphere pressure (Figure 1, red arrow).
When dry ice is sealed in a plastic pipette, the dry ice warms and sublimes. Consequently, the pressure inside the pipette increases. On the phase diagram, we can envision this process as the dry ice “travelling” along the curved black line from the point at (-78.5oC, 1 atm) to the point at (-56.6oC, 5.1 atm). At this second point (-56.6oC, 5.1 atm), the dry ice begins to melt and all three phases (solid, liquid and gas) are present at once. After reaching this point (called the triple point), the solid CO2 melts. After this, only liquid and gas CO2 are present in the pipette. The liquid warms and boils, allowing the pressure in the pipette to continue to build until…Well, I think you saw what happened in the video above!
A full analysis of this experiment, including some interesting observations of what happens to the CO2 that remains inside the pipette AFTER the explosion is below.
To learn more about this experiment, see the following article in the Journal of Chemical Education, written by Bob Becker: http://pubs.acs.org/doi/pdf/10.1021/ed068p782. A version of this published by Flinn Scientific can be found at: https://www.flinnsci.com/media/621541/91639.pdf.
Acknowledgements: Thanks to Alyssa Molina, Beth Irving, Justina Petersen and Jordan Gillette for capturing film of the dry ice pipette explosion on the high speed camera.