In the Classroom
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George Gilbert Denison University Granville, OH 43023
A Simple Experiment for Demonstration of Phase Diagram of Carbon Dioxide Submitted by:
Van T. Lieu
California State University, Long Beach, CA 90840
Checked by:
Jeff Gibbons and George Gilbert
Denison University, Granville, OH 43023
The principle of the phase diagram is taught in firstyear chemistry and physical chemistry courses. Although the subject matter is discussed in detail in textbooks and other publications (1–3), no suitable demonstration experiment appears to have been reported in the literature. To meet this need, we have developed a simple experiment that can be used to demonstrate the phase diagram of carbon dioxide. The equipment consists of a small plastic syringe and a pair of tweezers.1 Seal the needle end of a 1-mL tuberculin syringe2 (without the detachable needle) with epoxy resin. Allow to dry overnight. Using a pair of tweezers, fill the syringe one-third full with small pieces of dry ice. Let the syringe stand, upright, for a minute to allow air within the syringe to be displaced by denser carbon dioxide. Insert the plunger and gradually compress the gas–solid mixture to a small volume. As the dry ice sublimes and cools the carbon dioxide gas to or near to the temperature of dry ice, the solid and vapor phases are in equilibrium. This corresponds to point A on the phase diagram of carbon dioxide in Figure 1. As the syringe is gradually warmed by the environment, the temperature and pressure of the gas–solid mixture increase; equilibrium conditions now exist between the two phases. This corresponds to points along Curve ATp. At point Tp, dry ice begins to melt to form liquid carbon dioxide. At this point, solid, liquid and vapor coexist at the unique temperature of –56.6 °C and pressure of 5.1 atm. This is the triple point of carbon dioxide. As the syringe continues to warm and all the dry ice is melted, the point of equilibrium moves up along the curve TpB where only vapor and liquid coexist in equilibrium. At point B, where the temperature is somewhere between –20 and –56.6 °C and pressure somewhere between 20 and 5.1 atm, if you release the pressure on the plunger while still holding it, much of the liquid–gas mixture immediately bursts out in a near-adiabatic expansion, leaving behind in the syringe snowflake-like solid carbon dioxide that completely sublimes within l or 2 minutes. In conclusion, this experiment is simple and easy to perform and can be completed within minutes. For class demonstration, the experiment could be performed on an overhead projector with the use of a larger size plastic syringe.
Figure 1. Phase diagram of carbon dioxide.
Literature Cited 1. Brescia, F.; Arents, J.; Meislich, H.; Turk, A. General Chemistry, 5th ed.; Academic: San Diego, 1988. 2. Petrucci, R. H. Gemeral Chemistry: Principles and Modern Applications, 4th ed.; Macmillan: New York, 1982. 3. Kilkahl, N. K. J. Chem. Educ. 1994, 12, 1052–1055.
1 Rubber gloves and safety goggles should be worn during the experiment and be included as part of the equipment. 2 Can be obtained from most medical and scientific supply vendors.
Vol. 73 No. 9 September 1996 • Journal of Chemical Education
837
