edited hv
tested demonstrations Modified Hydrogen Balloon Explosion Submined by Stephen S. Lawrence Saginaw Valley State University University Center, MI 48710
The Physical Reality of Molecules: They're Dense and They Move Around! Submined by Todd P. Silverstein Willamene University Salem, OR 97301
Checked by
David Franz Lycoming College Williamsport, PA 17701
Checked by
Melvyn M. Mosher Missouri Southern State Colleae Joplin, MO 64801
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The explosion of the hydrogen-oxygen halloon i s one of the most frequently used demonstrations to introduce students to the study of chemistry, stoichiometry, or gas laws. For the first-year, nonscience students, a variation of this demonstration has been used that reinforces the chemical equation describing the interaction of these two gases. When ignited with a match a t a safe distance, the resulting explosion may represent the potential for injury particularly if stoichiometric volumes of the two gases are used. .Extreme caution and acoustical protection are advised during this procedure.
Two 9-in. helium quality balloons are required with the first halloon placed inside the second. This is easily done hv rolline., the aoerture of the outer halloon back to form a douphnut-shaped opming. Once t h ~ 1s s accomplished, the snaw betwtrrn the two balloons 1s filled initinllv with ahout Z ~ of Lwater. More water than this will keep the halloon from floating freely, and less water may cut back on the effectiveness of this demonstration. The exact quantity of water to be used will depend on the amount of hydrogen used to fill the inner halloon and, of course, the size of balloons. The success of this demonstration depends on adequately filling the inner balloon to near capacity. More consistent results are obtained if the balloons are taped into a fixed position on a n iron ring and ignited from directly below, so a s to avoid simply popping only the outer halloon. Following t h e explosion, highly d i s p e r s e d , m i n u t e amounts of water are typically sprayed i n all directions. Students as near as 12 feet and as far away as ahout 20 feet have reported feeling or seeing these water droplets. The formation of water can he evidenced by placing a few sheets of colored typing paper beneath the balloon prior to its ignition. With practice, the location of the water between the balloons can he manipulated, and the resulting fine spray may be aimed in almost any direction. The Socratic Method i s then used to start enereetic discussions of stoichiometry, states of matter, pressure, gas laws, or any related tooics. A result of these discussions alwavs include the fact that the water formed is actually gaseo;s. Admittedly, this demonstration amounts to trickery to make its point. However, for the liberal arts major, visual and concrete evidence (water droplets) imparts a certain personal familiarity that is lacking in the classical explosion. This temporary deception has to be balanced against the increased student enthusiasm and participation and the direct effect these have on student comprehension of the presented material and their performance in the class.
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Upon reading Mariko Suzuki's fine demonstration of molecular motion i n this Journal (Vol. 70, p 8211, I realized t h a t I use one t h a t i s even simpler and perhaps more graphic. This demonstration requires simply a hot plate, a 2-L beaker half-filled with tap water, a bottle of India ink, and a disposable Pasteur pipet with rubber bulb. Allow the beaker of room tunpwaturt: tap water to stand for a minute. then carefulls insert the oioet tilled with ink below the sukace of the water and slowly expel most of the ink into the water. (Tryto avoid expelling any bubbles.) The ink sinks and forms a flat layer a t the bottom of the beaker. The layer i s stable for many minutes (or even hours). Avoid "sloshing" the beaker and carefully place i t on the pre-warmed (high setting) hot plate. An impressive change is seen within 10 s: languid curls of ink solution shoot upward, mixing the entire solution within a minute or SO. This stark, simple demonstration leaves plenty of space for the students themselves to draw conclusions about the behavior of matter. They conclude immediately that the ink solution sinks because i t is more dense than water. Why is i t more dense? I t generally takes a series of hints before the class realizes that if you dissolve 1 g of something (e.g., ink) into 10 mL of water and the volume remains roughly 10 mL, then the density of the solution is now 11 g110 mL (1.1 g/mL) compared to 10 gI10 mL (1.0 g1mL) for pure water. This effect is distinct from that of the different densities of pure liquids, which can be demons t r a t e d using t h e final mixed ink solution by adding methylene chloride (which sinks) and hexane (which floats). The final mixture consists of three clearly demarcated layers, with the black aqueous ink solution separating the two clear organic phases. From the effect of heating on the initially "placid" ink layer, students deduce fairly readily t h a t solutions are made up of molecules that move (and mix) faster when they are heated. Heat, therefore, fosters molecular motion, and temperature, which i s measured with a thermo-meter, is iust a scale that we use to measure heat content. Students also can deduce readily a t this point how and why the mercury in a thermometer rises with added heat: more molecular motion leads to liauid ex~ansion.I t i s iust a short step from here, using a gas-filled piston as a model, to introduce the gas laws (PETP z n , P =l/V) and the basic tenets of the kinetic molecular theory. Volume 72 Number 2 February 1995
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