tested demonstrcltions How Vacuum Cleaners Pick Up Paper: Demonstrating the Relationship between Air Pressure and Vacuums
Acknowledgment The author would like to thank John Terry and Jamin Warnke for their input and interest.
Submined by
Thomas L. Neils Wisconsin Lutheran College 8830 W. Bluemound Road Milwaukee, W153226
Checked by
The study of air pressure is often used as an introduction to the study of gases in general chemistry courses. Several effective demonstrations have been developed to show that air does exert a force on all objects and that pressure differences can cause nonrigid containers to collapse. These demonstrations often involve the use of a vacuum pump to reduce the pressure inside a container. Because of this type of demonstration, many students mistakenly helieve that the vacuum is sucking in the walls of the container, and they completely miss the point of the example. They think that the vacuum pulls the walls in, rather than that the higher pressure on the outside crushes the walls. This misunderstanding can be clarified easily by the following demonstration that involves a vacuum cleaner with a hose, several pieces of paper, and a sheet of silicone rubber. First, the students are shown that a vacuum cleaner will pick up a small piece of paper. When asked what happened, most students will say that the vacuum cleaner sucked up the paper. Next, a second piece of paper, smaller in size than the circumference of the end of the vacuum cleaner hose, is placed on the sheet of rubber. The end of the vacuum cleaner hose is placed over the paper and pressed firmly and evenly into the rubber. The vacuum cleaner is turned on for a short time and then turned off. The hose must be kept firmly pressed into the rubber until the vacuum cleaner has stopped completely. When the hose is lifted, the paper is still there! In-Rushing Air Carries the Paper This experiment shows that the vacuum does not pull the paper up, as most students believe. The actual process, in which air rushes in to equalize the pressure and carries the paper along with it, can then be explained. The explanation may include the point that rug attachments for vacuum cleaners must have "beater bars" to lift heavy dirt particles into the air so that they will he carried into the cleaner by the in-rushing air. This demonstration can be adapted for use with an overhead projector. Cut a small hole, with a diameter smaller than that of the vacuum hose, in the rubber sheet and place it on the projector. The piece of paper can be set in this hole and students will he able to see that it did not get pulled into the vacuum cleaner by the suction.
GEORGE L. GILBERT Denison University Granville.OH 43023
Combustion of Hydrocarbons: A Stoichiometry Demonstration Submitted by M. Dale Alexander New Mexico State University Las Cruces, NM 88003 Checked by Wayne C. Wolsey Macalester Colleae St. Paul, MN 55105 One of the chemical concepts that many students find uninteresting, if not boring, is that of stoichiometry The simple stoichiometry demonstration described here is one that students find to be not only interesting hut also relevant. Materials Bunsen burner: H-baseburner equipped with bottom sliding air shutter (Cenea,Cat. No. 11003-03)is preferred, but burners with side vents may be employed. .small propane torch buranc hnckpnrker's stove \Gnrc~aGaz or MSR Rnpldfire Srove or Scnpto Hutnne Fuel canister 2 1 or,rqu~ppedulth #4 lighter inlet
.
Procedure Operation of the Bunsen burner is discussed in terms of the equation for the combustion reaction when natural gas is employed.
CH, + 20, + CO, + 2H20 The ratio of the volumes of oxygen to methane is seen to be 21. Considering the composition of air (115 oxygen by volume), the relative volumes of air and natural gas are, therefore, 10:l (air to methane). The students are then shown what happens to the Bunsen burner flame when the mixture is first made too rich and then too lean with the air shutter. The burner is then adjusted to give the best mixture. Now the equation for the combustion of propane is discussed C,H,+ 5 0 % - 3C02+ 4H,O and the students are shown that ratio of the volume of air to that of propane (25:l) is considerably greater than is the 'A small propane torch equipped with a needle valve can be used for the propane source. If the burner head of the torch is unscrewed fromthe shaft, the shaft can be connected to the Bunsen burner with rubber tubino. To orovide a butane source. the same ~rocedurecan be Jseo erncoyrig a smal b~laneoac6pacner's stove If a Scrlpto B~taneF.e canster is useo. wncn s not eqJppea w l h a neeole va.ve a secono person 1s neeoed lo pLsn in the n pple valve
Volume 70 Number 4 April 1993
327
case for methane. If the bumer, which is perfectly adjusted for methane. is fueled with nrooane instead.' the students is too &h, an observaimmediately note that the && er tion consistent with the volume stoichiometry of the reaction. Even when the air shutter is opened fully to give the maximum air supply, the flame is still luminous, which indicates that the combustion is still incomplete. The procedure is repeated except that butane is employed.' In this case the ratio of the volume of air to that of butane for complete combustion is even greater (32.5:l). 2C4Hlo+ 130, + 8C02+ 10HzO The Bunsen flame using butane, when the burner is adjusted for natural gas, is noticeably more luminous than when propane is used. Even when the maximum amount of air is admitted, the flame is still very luminous, and if the air shutter is completely closed, the flame is very sooty Variations If compressed air is available, the students can be shown that if the air supply is sufficient, both propane and butane can undergo complete combustion. Employing a T-shaped or Y-shaped connecting tube, air from the compressed air source and the fuel gas (propane or butane) can be premixed before introduction into the Bunsen burner, and by controlling the amount of air, the flame can be adjusted to give complete combustion. Precautions Keep the propane and butane canisters well away from the Bunsen burner flame.
The Use of Fuses in Chemical Demonstrations Submined by
John J. Fortrnan and Jeffrey Schreier Wright State University Daylon. OH 45435
Checked by Gordon A. Parker University of Toledo Toledo, OH 43606
It is oRen convenient or entertaining to use fuses to set off such pyrotechnic demonstrations as colored hydrogen balloon explosions ( I ) potassium chloratdsugar torches (2) .nitrocellulose/ black powderlpowdered metal sparkling 'flower-pots" (31, or thermite reactions Waterproof wick (black powder fuse) can be purchased a t most hobby shops or gun dealers. It bums hot enough for the delayed ignition of many pyrotechnic demonstrations. Torches and Sparkler Pots The wick can be used for the igniting the potassium chloratelsugar torches in place of using concentrated sulfuric acid on suear (2). or for nitrocellulose soarkler flasks instead of u;ng b&zoyl peroxide and anaiine (3).In either case. the fuse is simnlv inserted into the solids that have been mixed according& the appropriate instructions (2,3). The Thermite Reaction I n similar fashion the fuse can be used in place of either magnesium ribbon (6)or potassium permanganate plus glycerin (4,7-10) to ignite the thermite reaction. Thermite starting mixture is placed in a small indentation made in 328
Journal of Chemical Education
the coarse thennite in the crucible. A smaller depression is made in the fine startine mixture and filled with a small amount of a 50:50mixture of sugar and potassium chloratr ( 4 . 6 . 11I. The fuse is inserted into this mixture to a depth of about one inch. Due to a recent accident (12, 13) & i s important to offer a warning to those who choose to perform the thermite demonstration.
.CAUTION: Do not allow the molten metal to drop into water. Use a sand trap instead. Alcohol Explosions A waterproof fuse also can be used in place of a spark generated by a Tesla coil jumping between two nails in a polyethylene bottle to ignite a n air/alcohol vapor mixture (14, 15).A hole slightly larger than the size of the fuse is made in the side of a 250-mL to 1-L polyethylene bottle with a nail.
.CAUTION:It ir important that the hortlr he flexible polyethylene as rlrar piastre hnttlrs may shatter.
A little alcohol is added. the flask corked and shaken. most of the excess liquid aicohol poured out, and the flask recorked. In order to avoid getting the end of the wick wet, only then is a short piece of fuse inserted about 2 in. throueh the nail hole. Lieht the fuse and when i t oroduces spark; on the inside of tGe bottle the cork will be shot into the air. The fuse should be pointed away from anyone as should the cork. If it doesn't explode, wait until after there is no red glow before approaching the bottle because the explosion still may occur as long as there is any spark. If the vapor pressure of the alcohol is too great or the hole is too big the escaping vapor will keep the sparks blowing to the outside and the gases mag not detonate. The vapor pressure can be reduced by adding a little water to the iiquid alcohol in the bottle. For various reasons ibmition with the wick is onlv successful about 80% of the time. Alcohol may either bum in the presence of air or an alcohol vapor oxygen (air) mixture may detonate. Whereas, the spark from a Tesla coil always will detonate the gaseous mixture if i t is within the volume oercent exolosive limits (16). sometimes the hurning fuse may cause only burning of the alcohol instead of initiatine the chain reaction of an exolosion. After each use the bottle will not fire again due to lack of oxygen until it is flushed out by filling it with water. Most of the water must then be poured out before reuse. This flushing may be used to instruct students about the need for the exhaust stroke in an internal combustion eneine. The same bottle can be used only about five times before the hole has been melted too laree and the fuse will bum away inside the bottle with sm&e but no explosion. Old holes may be covered with strapping tape to allow the maximum use of the bottle before i t is thrown away. A third wav to imite the exnlosion of the alcohol cannon that is mar; cons%tently su&essful than the waterproof wick, but cheaper than a Tesla coil, is to use a six-volt lantern battery and an electrical rocket igniter1 that can be purchased from hobby shops. The igniters and the waterproof wick usually are found with the model rocket supplies. Still another method is to use a piezoelectric starter button for home gas grills (17). Hydrogen Balloon Explosions The fireworks fuse can be used for initiation of hvdrozen , ., balloon explosions in interesting ways. However, the fuse cannot be extinmished ensilvonce it is lit. so it is best used on balloons whirre falling uhburned hot portions will not
'Estes moae rocket lgnllers 112301. Estes Ina~stnes.1295 H St Penrose, CO 81240 Cost IS abo~t52 50 for a package of s x
