PHASE EQUILIBRIA IN THE CHEMICAL SYSTEMS H2O AND CO2: USEFUL DEMONSTRATIONS TO ENHANCE AN INTRODUCTORY GEOLOGY LECTURE CLASS

Demonstrations involving phases in the chemical systems H₂O and CO₂ are used to help motivate introductory geology students in a non-science geology class and enhance the lecture on the hydrologic cycle. In demonstration 1, a vacuum pump is used to evacuate the atmosphere in a sealed 1000 ml glass flask containing liquid H₂O. (For safety, a 3/4" plexiglass shield protects the audience.) As the atmosphere is drawn out through the sidearm of the flask the liquid H₂O begins to boil. The H₂O phase diagram shows that water can boil at room temperature if the pressure is lowered. Most of the students are amazed. At the same time and at 1 atm, a beaker of water heated on a hot plate is boiling at 100°C. We compare the two utilizing the H₂O phase diagram. A discussion ensues on such topics as liquid H₂O on Mars and other planetary bodies. The H₂O phase diagram also shows the negative slope of the univariant line between the solid and liquid phase. Now we can explain the phenomenon of ice-skating and certain aspects of glacial movement. In demonstration 2, a beaker of H₂O ice is compared to a beaker of CO₂ ice. The instructor asks " Why isn't this CO₂ ice melting? Is it defective?" Here we compare the phase diagrams for the chemical systems H₂O and CO_2. The phase diagram for CO₂ shows that at room temperature and 1 atm the dry ice sublimates and does not melt. In demonstration 3, Magdenburg Hemispheres are used to show the effect of atmospheric pressure. Here the atmosphere is evacuated between the Magdenburg Hemispheres. After the valve is closed, the instructor asks for the assistance of a student to pull the Magdenburg Hemispheres apart. The student is unable to separate them. The force needed to separate these hemispheres is approximately 400 lbs. However, they easily separate as the instructor opens the valve. These demonstrations involve students, excite students, motivate students and enhance the lecture on the hydrologic cycle.