Balloons and the Teaching of Chemistry F. Dewhurst School of Life Sciences, Leicester Polytechnic, P.O. Box 143, Leicester LEI 9BH, U.K. F. I?. Dewhurst School of Sciences, Nene College, St. Georges Avenue, Northampton NN2 6JB, U.K.
Balloons are a remarkably useful tool for the chemical educationalist. Balloons not only can provide cheap, safe, and easy-to-use alternatives to more complex and costly equipment, but also they can he used as effective teaching aids to demonstrate principles of structural chemistry. At the beginning of the nineteenth century it was common for gases to be collected into bladders for use in chemical and physiological experiments. I t is modern practice to handle lahoratory gases in high pressure cylinders equipped with reducing valves and pressure gauges. Unfortunately, it is very costly to equip a class with enough cylinders, etc. for each student or even each student pair. There are also safety hazards arising from the use of high pressure cylinders by less advanced students unless they are supervised very closely. This close supervision is not possible for a single instructor in charge of a large group. It is worth noting that stafflstudent ratios are determined more by economic considerations than by safety considerations. In most cases lahoratory practices, rather than staff ratios, must be modified to overcome safety problems. Balloons a s Stirring Devices Kleebergl has descrihed the use of air-filled, rubber, toy balloons as a simple medical lahoratory stirring device. The air is released in a slow stream to stir a fluid in field laboratories, in places where electricity is not available or in areas prone to electricity supply failures. The use of a glass tap (preferably two-way) and a curved glass tube drawn into a capillary a t the end to deliver the air and regulate its flow was described. A cycle pump or gas cylinder was used to fill the balloon which gave an air stream for well over 2 hr (up to 6 hr with a large balloon). We have found that Kleeherg's idea has considerahle potential for application in the teaching lahoratory. The method can be used to stir reaction mixtures in synthetic experiments and is also useful for stirring titrations, particularly microtitrations using microburettes or "agla" syringes. Small magnetic stirrers are quite cheap, hut it still can be a serious drain on funds to equip a large group of students, and, also, many laboratories, particularly older or less-advanced lahoratories, lack sufficient power points. The use of a nitrogen cylinder to fill the halloons enables an inert atmosphere to be provided for oxidation sensitive systems etc. Plastic T or Y pieces with rubber tubing and screw or pinch clips can provide a cheap and adequate substitute for glass taps. With younger, less-experienced, or clumsy students, glass capillaries can he replaced advantageously by less fragile, flexible, plastic capillaries drawn out from heated polyethylene tubing etc. Balloons a s Gas Reservoirs A further development of Kleeberg's idea which we have found to have considerable potential in the educational laboratory is the use of balloons to provide students with gas samples for carrying out simple experiments. The use of balloons filled from a cylinder is particularly attractive with junior science classes who can safely carry out experiments with 44 I Journal of Chemical Education
common gases such as nitrogen, oxygen, and carhon dioxide. Inflammable gases such as hydrogen, acetylene, ethylene, or ethane may also he used safely since the quantities involved are small and the nature of the container (a balloon) precludes sufficient pressure buildina- up- to cause a dangerous explosion. . Experiments involving passing gases into solutions or mixing and igniting of gases can also be carried out without explosion risks from "sucking back" or "strike back." Toxic gases such as chlorine, sulfur dioxide, carbon monoxide, or hydrogen sulfide may be dispensed safely into balloons for use in the fume cupboard. Most fume cupboards cannot cope with the volume of toxic gas which could be released from even a small cylinder or si~ h o nif a valve sticks. and serious accidents have occurred because of this. It is also possible to dilute toxic gases hy introducine- them carefullv into a balloon alreadv ~artiallvfilled .. with nitrogen or air. Recentlv, Cowlina2has described the use of balloons as safe and convenient hydrogen containers for small scale catalytic hydrogenations. Now that most synthetic intermediates are fully characterized spectroscopi~allythere is little need to measure the volume of hydrogen taken up during the reaction. Cowling used this technique for postgraduate research in chemistry, but the method is even more useful for undergraduate teaching purposes. Balloons and Biological Chemistry The use of balloons as gas reservoirs has many. applications .. in the biological field. ~ & o o n sare an obvious means of collecting.samples of expired air for respiratory studies. They provide a convenient reservoir for stirring tissue homogenates and cultures, for providing aerohic or anaerobic incubation conditions, and for studies in the area of nitrogen fixation. In the sphere of biochemistry they are a particularly convenient source of carbon monoxide for spectroscopic and other studies on hemoglobin, cytochromes, and cytochrome-linked enzyme systems. They also provide a convenient source of carhon dioxide for anesthetizing and immobilizing insects. Balloons and Molecular Shapes Jones and Bentley3 and Jones4 have fully discussed the use of modelling balloons sold in toy shops to illustrate the shapes of simple covalent molecules in terms of the mutual repulsion of electron pairs in the valence shell of an atom. The balloons are elongated in shape and may be twisted in the middle after inflation and intertwined with each other at the twisted necks. If two balloons are twisted toeether in this wav the four lobes are directed toward the corners of a regular tetrahedron. The balloons take up the shape s~ontaneouslvbecause of mechanical repulsion, and the.anaiogy to the behavior of electron pairs is convincing. Balloons may he tied together at the necks or tips, and if four different colored inflated balloons are 'Kleeberg, J., Med. Lob. Tech., 28,185, (1911). 2Cowling,A. P., Chem. Brit.,15,433, (1919). 3Jones, H. R. and Bentley, R. B., Proe. Chem. Soc., 439, (1961). 4Janes, H. R.,Educ., Chem. 2,25, (1965).
ioined in this wav the" aeain take ur, a tetrahedral configu;ation. A fifth uiinflatedballoon may he tied by its tip into this tetrahedral assemblv and inflated "in situ" to represent itself can he demonstrated by removing the outgoing group with the aid of a pin. This demonstration is particularly striking as the models are ahout two feet high. Jones has discussed in detail how coordination numbers of 3 (BFa), 5 (PF5),6 (SFs), and 7 (IFT),how douhle-honded
structures such as thionyl chloride, how cis- trans- isomerism ~ in octahedral complexes in alkenes, and how S Nsubstitution can he illustrated using balloons. Rather surprisingly, he did not mention their use in demonstrating optical isomerism. The authors have not exhaustively reviewed the use of these versatile and inexpensive objects in chemical education but hope that they have drawn the attention of educationalists to a useful tool whose potential is far from being completely exploited.
Volume 58, Number 1, January 198 1 / 45
