C
-
1
'
A
R. J. FRIESEN, editor univenity of Waterloo Waterloo, Ontario, Canada
Solid State Labs: The Bubble Raft
P. D. McCormick Thorold Secondagv School Ormond Street Thorold, Ontario The lack of explicit methods for interpreting the patterns observed in soap bubble rafts has, for the most part, precluded the use of this fascinating experiment as a two-dimensional dynamic model of a crystalline solid. The following activities provide considerable insight into the orderly packing of spherical objects and, by inference, into the properties of metallic crystals. The Equipment A ripple tank with either bottom or top illumination serves admirably as a bubble raft container, and a fine glass U-tube connected to a low pressure gas supply, such as a balloon, a gas tap or any gas metering device that you can control, can he manipulated to produce l-mm bubbles in a raft form (Fig. 1). Solutions of dishwashing detergent to which glycerin is added produce stable bubble rafts. These can be swept with a glass rod into clusters, and close packing areas become readily apparent.
Figure 1 . Ripple tank for bubble raft production
Try These Experiments 1) A group of bubbles that constitutes a "crystal" has rows of bubbles that form concentric equilateral triangles. This can he demonstrated by cutting cardboard triangles of the proper size, and superimposing them on the projected image. 2) Neighboring cj s t a l s have their rows oriented in different directions and this can be demonstrated using two or more triangles, whereupon their orientations are seen to be different. The less organized regions between neighhorinr crvstals are then eood renresentations of cwstal houn&ari& 3) The effect of "im~uritv"atoms on the cwstal stmct& of the raft can be demonstrated by inGoducing a larger or smaller bubble, and examining the orientation using the triangles. 4) The effect of compressive or tensile forces on a solid can he illustrated by placing two plastic or wooden sticks parallel to one another at each end of the raft, and alternately pressing and stretching the raft. Ohyeme the vacancies, grain boundaries and dislocations under compression and tension. This illustrates what happens when you bend a metal bar and i t springs back. Permanent deformation can he illustrated by increasing the tension or compression. 5) Dislocations in crystals are defined by a change in direction of grain patterns, by movement within the crystal, and by changes in the structure of the crystal itself. If a parallelogram ABCD is used to define a crystal orientation, and is placed over an area containing a dislocation, i t does not close exactly (Fig. 2), and the resulting vector AE is called the Burgers vector and defines the dislocation. A parallelogram around a perfect crystal, or a crystal
Figure 2. represented
by
and illustrating the
paralleiogram
urgers
tor.
vet.
containing a vacancy only, will close perfectly and the Burgers vector will be zero. 6) Dislocations can he introduced by bursting one or more bubbles with a hot wire, and the ways in which they move can he studied under conditions of tension and compression. The interaction of two or more dislocations can be noted, and the effect of vacancies on dislocations can be studied. 7) Any other ideas?
The synthesis of the hexafluorochlorine ion is reported in an article in Inorganic Chemistry, 12, (7). 1580 (1973). The CIFs+ cation was prepared in the form of its PtFssalt. It is a canary yellow solid and a powerful oxidizer reacting explosively with organic materials or water. The cation is octahedral. Would you expect this?
Ks, Experiment: The Solubility Product for Barium Hydroxide John P. Reynolds l'Amoreaux Collegiate Institute 2501 Bridletowne Circle Agincourt, Ontario M I W 1 W7 The soluhility product experiment most often used in high school is that involving silver acetate. There are no others mentioned in most high school texts. I have found that an experiment, using barium hydroxide, Ba(OHh, yields results just as valid; two completely separate approaches are used, and the cost involved is very much less. The concepts involved in the treatment of solubility product are much more clearly impressed on the student. Volume 52, Number 8. August 1975 / 521
Method A: Direct Determination of Solubility at Room Temperature 10 g) and Weigh a sample of B ~ ( O H ) Z . ~ H(approx. ~O place it in a 100-ml volumetric flask making it up to volume with distilled water. Shake the flask vigorously and filter the contents, being careful to wash the flask with the filtrate only. This allows all undissolved material to he recovered. Dry the residue in the open air for two days, and this will leave the student with a residue quite accurately described by the formula Ba(OH)2.Hz0.' Weigh this and save the filtrate. Make appropriate calculations in order t o determine solubility in g/lW ml. I t is important to point out the change in formula of the hydrate. From solubility and the equation for the dissociation of Ba(OH)2, a value for K., can be obtained.
takes a different quantity of the barium hydroxide hydrate (say, from 1 g to 10 g) and carries out the experiment according to Methods A & B, some will have a precipitate; others will not. Comparing the ion products with observation and perhaps graphing the ion products versus sample size, you can clearly demonstrate this concept. The Effect of Temperature on Krp and Solubility The solubility, and thus K,, for barium hydroxide, varies considerably with temperature. Using data from the 51st edition of the "CRC Handbook", we note the following K,, in mld water (15°C) is 225 X 10W while K,, in hot water (78°C) is 1.08 X 10'
Method 6: Determination of [OH-] by Titration Take a 10-ml aliquot of the saturated solution (i.e., the filtrate) prepared in A. Titrate this with approximately 0.1 M nitric acid. The nitric acid can be standardized against a known concentration of sodium hydroxide, as a demonstration, if the students have not previously encountered titration techniques. This time using the hydroxide ion concentration found in the titration, and referring to the equation, a value for K,, can he found. The relationship between [OH-] and [Ba2+]is important here. Sample data from the class are as follows
If the experiment is carried out at a different temperature, between room temperature and 35"C, a titration of aliquots of the warm filtrate of a saturated solution would allow one to calculate K,,. Use a water bath to prepare a saturated solution at an elevated temperature, and record the temperature at the moment of filtering. Any precipitate forming as the filtrate cools will redissolve on titration. Sample data T = 32-C K,, = 2.56 X lo-' T = 28'C K,, = 1.08 x lo-'
Will a Precipitate Form?
T = 2-X Kag = 4.03 X 10' This section has proven to be temperamental, and at one time will produce good results while at others will produce quite poor results. I suspect that this could be due to production of barium carbonate in the saturated solution. Imagination may serve to bring in other ideas, and cut down on the sources of error. For example, in Ba(0H)z.XH20, X could be determined for the "dried" material. A discussion of error sources can impress on the student the significance of his results. I'm sure this will be of use in teaching many of the concepts found in the Solubility Product unit, and I would appreciate any comments teachers might have.
The condition for the formation of a precipitate, [BaZ+][OH-12 > K,,, can be demonstrated. If each group
p. 165.
Method A
Method B
2.00 1.02 214 200
4.28 1.25 3.00 200
X
10F2
X X
lo-'
X
lo-'
largest Ratio -= 20 smallest
X
lo-'
X X
lo-" 10.'
X
largest Ratio -= 3 4 smallest
This experiment can be extended depending on the time, class, etc.
522 / Jourml of Chemical Education
'Article by William Benton, Encyclopedia Britannieo, Vol. 3,
