THE GAS MENAGERIE
During break, astronomy students explore several demonstrations of gas principles. Left, Todd Whitmore prepares a device that illustrates gas pressure. To his left, Darlene Allen makes a cloud in a bottle. Across from her, Tanisha Stewart has been experimenting with applications of a device that pressurizes soda. To the right, Andre' Allen has been using a home-made thermometer, but all are transfixed with the anticipation of the outcome of Mr. Whitmore's demonstration.
While expensive apparatus are needed to do a careful exploration, if an exotic and sophisticated apparatus is introduced without sufficient context rooted in the familiar, students tend to gain little from demonstrations, believing that special conditions are provided to create the phenomenon, rather than to reveal a fundamental principle. "The Gas Menagerie" is a collection of simple, hands-on investigations that will help cultivate an appreciation of gas behavior with culturally familiar items.
The Pressure Popper
This simple demonstration was shown to me by Mel Joesten, Professor Emeritus of Chemistry and Education at Vanderbilt University. He founded a volunteer society of graduate that go into schools and have the students do simple investigations with inexpensive apparatus. This is one of their favorites. All that is needed is:
35 mm film canister : Free from photo developers Seltzer Tablet : 3.39/box 36 Water : None The canister is filled with just a little water, and less than half a piece of a standard seltzer tablet is dropped in. The cap is placed on, and one waits a few seconds. In less than about half a minute, enough carbon dioxide is produced from the seltzer to increase the gas and the pressure sufficiently to blow the cap off to several feet above.
Todd Whitmore, above, is startled when his "Pressure Popper" goes off a few seconds after arming it.
Students might be advised to hold the container upright on the table, as it often turns over and spills when blows, as is apparent in the picture above. In addition to illustrating the basic nature of gas pressure, it illustrates an important principle in industrial safety. When a container of liquid boils, such as those surrounded by a fire, the additional gas along with its increased temperature causes a dangerous increase in internal pressure. A "Boiling Liquid Expanding Vapor Explosion" or "BLEVE" (rhymes with "heavy") will result if the container ruptures. The author is aware of two cases of BLEVE's badly injuring students when someone attempted to take some liquid nitrogen, which boils at room temperature, in a plastic soda bottle and screwed the cap on.
As an additional exploration of the "Pressure Popper", we had a class of physics for education majors attempts to measure the pressure at which the eruption occurs with a PASCO Absolute Pressure Sensor (CI-6532A) with a PASCO 750 Interface. A ¼ " hole was drilled in the top of the film canister for one of the additional spouts that comes with the sensor. Silicone aquarium sealer was used very successfully.
Tameka Shamery, above, awaits a cue to drop in some seltzer when Lindsey Couler, foreground, begins data acquisition. Roger Klee, middle, is ready to put on the cap. Penni Wallace, right, is hopeful that her change in the configuration of the experiment will result in a successful burst of the cap this time.
As a good Computer-Based-Laboratory should be, the students had to make several attempts to get a successful run. They varied the amount of water, seltzer, and the way is was clamped (the clamp distorted the bottle so the cap did not fit well and just came loose). Below is their data taken with PASCO Data Studio, with the successful run appearing clearly as tremendous build up of pressure which is suddenly released.
Several attempts to measure the pressure at which the cap is blown off by a group of students. The successful run is shown in green, as the pressure quickly rises to 143 kPa and immediately drops to ambient value after the cap is blown. Attempt for which the cap just came loose are evident by lower levels of pressure with fluctuation. Boyle's Law A simple way to illustrate that the volume of a gas decreases with pressure is with a sealed polyethylene soda bottle. The harder the bottle is squeezed, the smaller the volume becomes. Even higher pressures may be explored beyond average strength with the help of an inexpensive and clever device.
The "Fizz Keeper" ® pump cap is device that pressurizes leftover soda to keep it from going flat. It consists of a piston that screws into the top of a standard 355 mL or 2.0 L soda bottle. It can be used to create a very high pressure environment inside the bottle. WARNING: These experiments should only be done with a PLASTIC bottle. A pressurized glass bottle would be a tremendous hazard if dropped.
Fizz Keeper ® $3.25 355 mL plastic soda bottle Refuse Small Marshmallows $1.19
For this startling and deceptive demonstration, students are shown a soda bottle about 1/3 full of "after-dinner mints". The bottle is shaken, and the "after-dinner mints" rattle a little bit. A student is asked to unscrew the pump cap. There is loud hiss, and suddenly the "dinner mints" expand to the full volume of the container, revealing their true identity as small marshmallows!
On the left is a photo of small marshmallows pressurized by a pump cap. On the right is the same marshmallows with the pressure released to atmospheric pressure.
The operating principle behind the pump cap may be directly studied well with a small 355 mL soda. When the soda is opened, bubbles erupt and float to the surface. The reason that sodas "go flat" is because they eventually loose enough carbon dioxide through fizzing that the tart taste, due to the dissolved CO2, wanes. When a pump cap is used on a fresh 355 mL bottle, in just a few pumps the volume of bubbles adhering to the bottom are seen to shrink dramatically. Their buoyancy and rate of growth is correspondingly decreased, so they just stay on the bottle and few new bubbles appear.
Fizz Keeper $3.25 2L bottle of soda $ 0.99 Raisins $1.67
A study recently appeared in The Physics Teacher (I am unable to find the exact reference at this time) on putting raisins in soda. The raisins acquire bubbles and eventually rise to the surface. Upon breaching the surface, the bubbles burst, and the raisin sinks to the bottom again. A handful of raisins will create a cycle of rising and sinking. Caution: put them in only one at a time, or you will be cleaning up a big mess when the soda suddenly erupts! As shown in the photo below, the raisin rising/sinking cycle is stopped when the bottle is pressurized.
Left: Raisins in a bottle of soda rise and sink as bubbles form on them and burst upon breaching the surface. Right: The process is immediately halted when the bottle is pressurized.
While the concept is similar to the well known Cartesian diver toy, the demonstration is of life-or-death importance to divers who ascend too quickly. The reduction of pressure can cause the formation of dangerous bubbles in the blood stream, causing DCS (DeCompression Sickness), also called "the bends".
Perhaps the earliest thermometer was created by Galileo utilizing the fact that the pressure and volume of a gas increase when temperature is increased. A simple version is a flask of liquid and air which is sealed except for a tube open to the environment. The level of liquid indicates the temperature of the air inside the flask. One disadvantage is that the absolute reading of the device depends upon atmospheric pressure, which of course varies. However, for situations of relatively constant temperature, this effect can be used to indicate atmospheric pressure. Common in early American homes, a weatherglass was sort of like a sealed glass "teapot", which hung on a hook. A higher level of liquid in the spout indicates imminence of storms as the gas inside expands and pushes out the liquid from the spout. Weatherglasses often had a "coaster" as the spout would overflow in times of very low pressure.
To make an inexpensive Galilean thermometer with common materials, here is all that is needed.
A tall glass bottle with a wide mouth (Looza Fruit Juice) $2.99 A tall soda straw. $1.39/box of 50 Plastic ruler (optional) can be photocopied on transparency Colored fluid dyed water will do Silicone sealant $2.99/tube Liquid crystal terrarium thermometer (available at pet supply, optional) $3.19
A brand of fruit juice was found with a suitably shaped jar, with a wide mouth and somewhat cylindrical shape that allows the liquid level to be clearly observed.. The paper label was removed by soaking it in water, and the glass was scrubbed clean of glue with vinegar. A ¼ " hole is drilled through the cap of the bottle, and then the straw is inserted through it and sealed with silicone. A plastic ruler may be glued along the straw. Additionally a liquid crystal terrarium thermometer may be adhered to the interior of the bottle. A small quantity of colored water is added to the bottom of the jar, or colored oil such as olive or motor oil to avoid fogging of the interior. After the lid (with the straw and ruler inserted into the jar) is screwed on, the experimenter may wish to have some default height to the fluid level, by blowing a little extra air into the vessel. By just placing hands upon the jar to heat the air inside, the level of liquid begins rising within a few seconds, and the level was observed to rise three inches within one minute.
Adiabatic Processes The pump cap is sold with terrarium thermometers by Arbor Scientific with the following activity as the suggestion. One places the thermometer inside a 2 L bottle and pressurizes it with the pump cap. The pressure can be quickly observed to rise several degrees Fahrenheit from adiabatic heating. What we found most interesting was allowing the system to cool back to room temperature, and then to release the pressure. The temperature is observed to fall several degrees below room temperature, directly exemplifying refrigeration!
One natural process in which adiabatic cooling is extremely important is the formation of clouds. The pressure of rising air is reduced as altitude is gained, and it is cooled as it rises. When the temperature is sufficiently low, condensation prevails around particles and a cloud is born. Cloud formation is typically demonstated by compressing air and releasing it. While devices that do so are one company's best selling item , all that is needed is:
Cloud in a Bottle
3L bottle with cap Refuse Water Free Match $0.10/ box
A little water is placed in the bottom of the bottle, and a lit match is dropped inside as a source of suspended particles for the water vapor to condense around. When the bottle is compressed with the cap on, the temperature inside is raised, increasing the rate of evaporation of the water inside until a new equilibrium is reached, but with a greater proportion of water vapor. After the bottle is compressed for several seconds, when it is released, the air inside immediately clouds as the temperature decreases below the new raised dewpoint. If the bottle is squeezed again, the cloud disappears, but reappears upon release. During a drought, one student took particular interest in this demonstration!
A 3 L bottle contains a little water and a lit match is dropped inside. The cap is screwed on, and the bottle is compressed. When released, a cloud forms in the interior. Acknowledgements
Many thanks are due Peiter Vissher, Stan Jones, and Gene Byrd of the University of Alabama Department of Physics and Astronomy for their assistance in developing this collection of demonstrations. Also Craig Bohren of Pennsylvania State University Department of Meteorology, Charles Knight of the National Center for Atmospheric Research, and Melvin Joesten of Vanderbilt University Department of Chemistry and Peabody School of Education were of critical assistance for the development and explanation of these demonstrations.