edited by JAMES 0. SCHRECK
fil trcrtes e residues
Univenityaf Northern Colorado
Greeley, CO 80639
Wet Dry Ice Robert Becker Kirkwood High School, Kirkwood, MO 63122
The fact that dry ice tends to sublime rather than melt fascinates just about everyone. It is customary to discuss with studc"ts the significance of phase dmgrams and explain that drv ice is in fact cnpablc of melting-but only at pressures higher than carbon dioxide's triple point pressure of 5.1 atm. That lesson would be more meaningful, however, if it were accompanied by a laboratory exercise in which the students could observe firsthand this rather impressive phenomenon and, perhaps, derive experimental estimates for the triple point pressure! Previous methods for demonstrating this principle have adequately emphasized the hazards involved in dealing with pressurized systems. These methods have generally relied on strong, rigid containers and the monitoring of pressure gauges to safeguard against explosions.' Yet, no safeguards are completely foolproof, especially in the context of a student-performed laboratom exercise. Thus, rather than attempt& . . to safefiuard against explosions, we desiped the folh~wing trchniuue to safceuardfor them! Low-dmsitv ~olvcrhvlcne has been substitited in place of typical materiais sueh as Plexiglas, which might shatter or create sharp edges upon rupturing. Furthermore, the entire process has been downscaled dramatically to the point where a n "explosion" becomes, a t worst, little more than a crisp "pop". Therefore, this technique represents a safe, simple, and inexpensive microscale experiment that students can set up and perform (and re-perform) all in a matter of minutes. j ~ n d r w cI
.I Chom Fh,r 1989 66 547
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Journal of Chemical Education
Pliers
water inside the micro pressure gauge
Figure 1. Set-upfor micro carbon dioxide liquefaction (includingmicro nrers##re nanmel
Drop of
colored woter
\
m
8
m,
0
wox plug
Figure 2. Micro pressure gauge (not to scale).A sample reading for the trapped air column shown above would be 5.5 cm. Procedure: Microscale Technique for Liquefying Carbon Dioxide Fill a pliable, transparent, plastic cup (e.g., "Solo" brand) with tap water to a depth of P 5 cm (about one-half full), and set it aside. Cut the tapered tip off a "Graduated" Beral disposable pipet and slide 8-10 BB-sized pieces of dry ice down the stem and into the hulb. Using a pair of pliers, clamp the o ening of the stem securely shut so that no gas can escape. Finally, holding the pipet by the pliers, lower it into the cup just enough to submerge the bulh (Fig. 1). Observe from the side ofthe cnp the behavior ofthe dry ice.
B .
Observations After about 20 s, the dry ice pieces begin to grow moist, appearing much like regular ice does when it melts. After a few more seconds, the dry ice is completely melted and a small puddle of clear, colorless liquid can be seen a t the bottom of the bulb. At this point, releasing the pressure (by simply loosening the grip on the pliers) turns the sample instantaneously back into a solid-this time in the form of a fluffy, snowlike powder, the same consistency as the dry ice generated by a carbon dioxide fire exting~isher.~ Retightening the grip on the pliers is all that is required to start the cycle over again. Extension: Incorporation of a Micro Pressure Gauge. This procedure can be made more quantitative by the following extension. Prepare a micro pressure gauge, like the ones used for Boyle's law experiments, by taking a 7-cm length micro tube (cut from the stem of a thin stem Beral pipet) and sealing closed one end with a small dab of hot-melt glue. Tie a length of fine thread around the tube just below the glue plug, and trim away any excess glue to allow this end to pass easily through the stem of the mt-off graduated pipet in which the dry ice will be placed. Using a fine-tipped permanent marker, measure from the inside edge of the plug and mark off every centimeter along the length of the tube. Once the tube has been incremented, use a finely drawn-out glass pipet or a pulled-out, thin-stem Beral pipet to place a small drop of dark-colored water inside the open end of the tube, and record its position on the centimeter scale from the inside edge of the drop (Fig. 2). This measurement represents the length of the air column trapped inside the tube between the wax plug and the water drop. This micro-tube pressure gauge can now be lowered by the thread (open end downward) into the stem of the graduated pipetjust aRer the dry ice has been added and just before the opening has been clamped shut. The length of the trapped air column can be monitored, and, by application of Boyles' law, the pressure inside the pipet a t any point in time can easily be derived. Observe that the
pressure quickly increases, then levels offwhile the dry ice melts, and then starts to climb again as the vapor pressure over the liquid carbon dioxide increases with the rising temperature. If the grip on the pliers then is carefully loosened, one can observe this plateau effect in reverse as the dry ice refreezes. Results With such a crudely constructed gauge and with the inevitable cooling of the trapped air column during the exueriment.. anv " readines should be regarded as loose auproximations a t best. Nevertheless, the experimental values derived by this method for carbon dioxide's triple point pressure fall consistently within the range of 5-6 atm, in close agreement with the values cited in most te~tbooks.~,'
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Safety Considerations Direct handling of solid carbon dioxide may cause fmstbite. To avoid this risk, place 10-20 g of crushed dry ice into a n expanded polystyrene cup, lay the cup on its side, and, holding the pipet horizontally, use the cut-off stem to scoop up the pieces and slide them down into the hulb. The bottom of the cup serves as a convenient backstop. It is important to note that the water in this ~rocedureis intended to serve a number of purposes. It keeps the bulb from becoming too cold and rieid on the bottom and keeus condensation from forming on the outside of the bulb, so that observations of the dry ice inside remain unobstructed. Furthermore, should the bulb rupture, the water helps to muffle the sound, although it usually produces a rather lively (though harmless) splash in exchange.6Out of water, incidentally, this sound can be surprisingly loud; depending on the integrity of the hulb, yet it rarely seems to exceed the volume of a typical balloon "pop". If these pops are considered to be too loud, however, one may wish toissue ear plugs to the students. There is no "shrapnel" to speak of; when the bulb does pop, it is actually tearing open suddenly a t its weakest spot. Should the bulb rupture prematurely before the dry ice is completely melted (which neuer occurred during testing), the water would also be there to catch any small pieces of dry ice that might be expelled. Even with these safety precautions, however, the author strongly recommends the use of safety goggles when performing this experiment.
2Thelongerthe plier handles,the moreeffectively thiscan bedone. If the tightest grip still allows gas to leak, it might help to fold the end ofthe stem overbeforeclam~ina. Wider-stemmed Dioets mav be used as well. The wideropening mates it easierto inseriihe dry ice pieces but also makes it more difficultto clamp the stem shut. 3The pipet, with the pressurized carbon dioxide liquid inside, is essentially a microscale fire extinguisher, and, with an accurate aim of the stem, it can be used to extinguish a small candle flame from several centimeters away! (Ofcourse, the carbon dioxide in a real fire extinguisher is typically at room temperature and therefore requires quite a bit more pressure to keep it in the liquid state.) 4Ebbing, D. General Chemistry, 3rd ed.; Houghton-Mifflin: New York . . 1990.
S~cd$rie, D.; Rock. P. General Chemistry, 2nd ed.; Freeman: SanFrancisco, 1987. 6A piece cellophane may be secured over the rim of the cup with a slit made in the center for inserting the graduated pipet bulb. This reduces the splash considerably,
Volume 68 Number 9 September 1991
783
