DOUGLAS D. SMITH Guilford High Schwl Rockford, Illinois 61 111
The Case of the Unlabeled Bottles
concentrations in the last group range from 0.05 M to 0.2
M). Zuelke' described a lab that was designed to relate ionic chemistry with descriptive chemistry without using detailed equilibrium knowledge. The following excerpt from the article explains the basic procedure. In the exercise. a student receives seven (or eight) solutione in numbered dropper bottles. He is given a lisiof important s ~ e c i e sin the bottles (along with their concentrations), hut is not told which species .& in which bottles; a given set of solutions is usuallv arranged to contain ions from only one or two qualitative analysis groups. The purpose of theexperiment is to identifv the contents of each bottle, using the contentsof theothe;bottlesas theonly reagents.This a nccomplished by adding five drops of a second solution. Because of the possible rxistence of slow reactions, the student is also a(lvised to reverse the above procedure, and make observations durine" the droowise addition of the first solution to the second. He may also mix more than two solutions a t a time in brder to obtain confirmatorv evidence. Aside from the general directions, the student is furnished with the followine list of chemical s ~ e c i e sresent. For example: HzO (this could be an only constituent), 3 M NH3,3M H30+, Bi3+, Ag+, Cd2+, Hg2+, I-, Nos-, Sod2-, C1- (the
La Chatelier's Principle Demonstrated with a Rubber Band' 1) Secure a strong, pure rubber band of about 75 mm X 6 mm. 2) Hang the rubber band from a support at the top of a ring stand. 3) Stretch the band by hanging a 5M1000-g mass from it. Adjust the support so that the bottom of the mass is barely above the base of the ring stand. 4) Quickly andgently heat the length of the band with aBunsen burner. The mass should begin to move upward. As cooling occurs the rubber band should again elongate.
.
5 ) 1Jsine the - ~enuntion: ~ -~~
~
T h e bottles' contents are Bottle 1 Bottle 3 Bottle 5 Bottle 7
H30+, INHs CdZ+,Sod2Hz0
Bottle 2 Hg2+,NosBattle 4 Ag+, NosBottle 6 Bi3+,CI-
Note: This set may be a little tough as (1) the solubility of mercuri( sulfate is such that a reaction will not he noted when the ions art milred and 2) HCl must be added to stabilize when Bi3+is used. There are many possibilities that could be developed. In order give the least confident students a chance for success the follow in^ set has been used. The students should have access to a solubilit) chart andlor the solubility rules, and be told that AgOH may not bt listed but mav show up as AgzOwhich is black. The studentsmay alsc miss the reaction with the &id and carbonate if they are not careful Instructions are for 40 ml of solution. Bottle 1 1.29 g silver nitrate Bottle 2 water Bottle 3 2/3ml of Bottle 4 1.76 copper(I1) concentrated HCl bromide Bottle 5 0.85 g sodium Bottle 6 2.15 g barium carbonate hydroxide
'Zuehlke, Richard W., J. CHEM. EDUC., 43,601, (1966). warmer-r (b) the releasing of tension on a rubber band should allow the band to became shorter and cooler. 6) Give each student a rubber band and have him suddenly streteh it and touch the band to the upper lip. Have him note its temperature. 7) Let the band remain stretched for ahout 30 secondsso it can m l to room temperature. 8) Quickly let the band contract and touch it to the upper Lip. Mwt students should feel a temperature change. If you wish a discussion of the phenomena from a thermodynamics view point, this example is used in a bwk by Jay A. Young,2"Chemistry, Reflections orAnother Teacher."
~
Energy + long ruhher hand =short ruhber hand, note theendorhermic and exothermic relationships. If we are correct, then the lo) suddrn stretching of a hand should make the band
Messy Writing = Poorer Grades Educational Research: Neat handwriters may receive better grades than messy handwriters for similar content in essay answers. Messy writers (granted the messiness does not include creative spelling or imaginative punctuation) often receive lower grades on essays which are supposed to be evaluated on content than students of equal ability who can also produce neater, clearer handwriting. T h e implications for chemistry tides and other items that high schwl teachers may find of immediate use in their classes. Comments and suggestions for Ideas are encouraged and may be sent directly to the column editor.
Backwill, F..I., The School Science Reuiew, 58,71 (1976). Young, .lay A., "Chemistry, Rmectims of Anorher Teacher," Silver Rurdett, Morrirrown, New Jersey. 1965, Chapter 5. 1 2
teachers may he that when grading essay portions of tests or reports, the messy handwriters will receive less credit than they deserve and the neat handwriters more. Huck and Bounds' found that "(a) graders who are neat writers are biased by the neatness of the essay, and (b) graders who are messy writers are not influenced by their factor of 'essay neatness'." Conclusions from the study may be that (1)messy handwriting graders could be proud of their objectivity, (2) many teachers should maybe use objective tests rather than essay tests, or (3) when using essay questions, techniques will need to be utilized to assure fair grading.
'
Huck, S.W., and Bounds, William G., American Educational Research Journal, IX, 279, (1972). Volume 54, Number 11. November 1977 1 701
