Observation of phase changes with corresponding pressure changes.
A two ml. plastic microcentrifuge vial and a small shop vise are used together to melt dry ice.
Wear safety glasses for this demo. The vial can explode, or shoot out of the vice, from the pressure of liquid carbon dioxide. Set up a camera with a close shot of an empty vial before putting in a loaded vial.
Crush a pellet of dry ice to make pieces that fit into the vial. Place a couple of pieces in the vial, and snap the lid closed.
Immediately place the vial horizontally in the jaws of the vice, and lightly snug the vice closed. The dry ice will begin to melt after about fifteen seconds.
Once the dry ice is seen to be melting, slowly open the vice jaws so that the vial cap can push out and release the pressure inside. A hissing sound and the liquid carbon dioxide changing into fluffy dry ice snow means success. The vial falling out of the vice jaws and exploding or shooting across the room means a different kind of success.
Practice this demo before using it in class.
Current practice is to remove the cap from the centrifuge vial, and seal between the vial and vise jaw with a silicone rubber stopper. This allows for a controlled and reliable release of the pressure after liquification is shown. Allows for the question about where in the PV diagram the system is going, and consequential solidification of the carbon dixoide liquid.
Confusion about how much the stopper is compressing the contents was expressed when this was shown on video. What is the pressure change? Can we instrument up the silicone stopper. P and T would both be great to have, but not necessary.
The phase diagram tells us how we get to the triple point of carbon dioxide. Starting at the bottom left, the cold low pressure dry ice is in a warm environment, so heat is entering the system. Dry ice sublimes and the pressure rises. The temperature of the dry ice rises, so does the pressure, which allows the dry ice to exist as a solid at higher temperature. Energy from outside the system goes into rising pressure and temperature, which leads up the solid/gas boundary to the triple point. Now the heat energy from outside goes to melting the dry ice.
