and unreactive (a desirable attribute when used in column chromatography). Because the preparation of y-Also3is tedious, aluminum hydroxide was considered as an alternative reagent: AI(OH),(s)+ 3HCKaq) +AlC13(aq)+ 3H20 Al(OH)3(s)+ NaOH(aq)-+ NaAl(OH)4(aq) Ademonstration of ampboterism based on the properties ofAlIOH,,that is suitable for large lecture hall demonstrations is a&,mplisbed readily hyadding a small m o u n t of alum1(rouahlv 30-50 g~to800 mL ofwater in a 1-Lbeaker and stirring iagnetic>lly. Addition of a small amount of concentrated sulfuric acid produces the clear acidic solution needed for the demonstration. In front of the students small increments of a 6 M NaOH solution can be added. The alum solution will become turbid as AI(OHb begins to precipitate; then suddenly the solution will clear. A13+ (aq)+ 3NaOH +Al(OH)3(s)+ 3Na'(aq)
A few drops of universal indicator (or red cabbage .juice) can be usedduring the titration with great effect to aid in the vlewlne and to illustrate the extreme changes in pH.' The reverse tltration (base to acid) is likewise iilustrailve. A solution of alum (either acidic or basic) can be stored indefinitely for quick use in a demonstration. Literature Cited 1MacZura, G.:Goodboy, K P.;Koenig,J.J. InEneyelopPdm ofchrmim1 Toehnolagy,3 d ed..: Bushey, G. J.; Klinaberg, A, van Nes, L., Eds: Wiley: New Yark, 1978; pp
Fast Molecular Motion Kerro Knox Princeton University Princeton, NJ 08544 Checked by:
George Wollaston Clarion Unviersity Clarion, PA 16214 The expression for the root-mean-square velocity of the molecules in the ideal gas is derived or given in many freshman chemistry textbooks.
where R is the gas constant; Tis the absolute temperature; and M is the molecular weight (in kg for R in joules!). Substituting numbers into the equation leads to some astonishingly high speeds; even the sluggish Brz at 25 'C goes 216 m/s on the average, or in units more familiar to American students, 483 rilph. A demonstration is described here that shows that molecules do indeed move verv fast. and over long distances in a short time when nothing is in the way. hisr rapid motion is contrasted with the more usual rate of dfision at 1
'KAI(so,)~.~~H~o is easily prepared by reaction of aluminum cans (or foil)in KOH and then neutralizing with sulfuricacid (2). 2Thevisual effectof precipitation and dissolution can be amplified by the use of a lighted stage (3). 574
Journal of Chemical Education
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Ampule for holding liquid bromine The Setup Caution: Open bromine containers should always be manipulated in a hood with rubber gloves. The demonstration tubes are safe in the classroom because they are closed. Later they are opened in a hood. The rubber stoppers may be affected by the reactive element
glass tubes that are about 1-m long are halfTwo 64.covered with taped white paper. (The standard 4-ft length glass tubes are satisfactory.) They can be closed at both ends bv No. 13 rubber stoppers. (It helps if the ends of the tubes are flared a little td accommodak the stoppers better, but that is not absolutely necessary.) One stopper .. has a stopcock running through it. Two or three small glass ampules that are partly fdled with liquid bromine are placed a t one end of each tube, in a horizontal position. (See below for making these containers). The stoppers are then inserted. A piece of iron, which is held by a small alnico magnet on the outside, is placed inside the other end of each. IA short. thick steel bolt is adequate.) The tubes are stoppered a t this end-ne with the stopcock. Rubber bands are put around the tubes just under the magnets to keep them from sliding, with the iron pieces, down the tubes in the next step. The tubes are then raised to a vertical position with the ampules a t the bottom. Then thev are clamped in ring stands with the inm pieces held at (he top by the magnets. The stopcock is connected to a water aspirator. All this can be donebefore class. The glass ampules are made from 6-mm glass tubing by blowing a small bulb a t one end-not too thick and not too thin. The other end is then pulled out and necked down, as close to the bulbasconveni&t.'l'hen it iscut om. The result is shown in the firure: It should lie flat on top ofthe No. 13 rubber stopper without extending over the 'dges. The ampules are filled by putting them into a fdter flask with about 5 mm of liquid bromine in the bottom. The flask is stoppered and evacuated with an aspirator. When the vacuum is released in the flask, the bulbs partially fill with the liquid. The bulbs are emptied into a petrie dish. (Sliehtlv roueh handline is recommended to break the we& bkbs now when it much less inconvenient.) The open ends of the bulbs are then sealed in a flame. I t is a good idea to test the filled and sealed ampules at this staec bv outtine them back into the cmotv . " filter flask and then pulling a vacuum on them. Again any weak ones will break.
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The Demonstration The demonstration is begun by pumping out one of the tubes, then closing the stopcock. The magnet on the atmospheric pressure tube is removed. The iron piece drops, breaking one or more of the bulbs. (Havim more than one bulb is ;imply for security. However, if no-bulb is broken, the piece of iron can be simply picked up and dropped again). Some brown vapor is seen against the white paperbackground a t the bo&m of the t;be. Then the mignet on the evacuated tube is pulled. As fast as the eye can see, the tube is instantly filed with brown vapor, thereby demonstrating very rapid molecular motion over a long distance without collisions. For the rest of the lecture, the atmospheric pressure tube will slowly develop a distribution of brown vapor as the bromine diffises into the air above it.
