edited by
GEORGE L. GILBERT Denison University Granville. Ohio 43023
Measurement Errors: A Lecture Demonstration Submitted by: Checked by:
Robert J. M u n n University of Maryland College P a r k , Md 20742 Becky J. Hickey University of Washington Seattle, Washington 98195
The ideas of measurement errors, significant digits, precision and accuracy are often intruduced early in an intnrductorv chemistry course. The followiny lecture demonstration can he used to realistically discuss precision, accuracy, averaging, data rejection, and significant digits. The demonstration requires minimal equipment and requires about 5 min t o complete. The equipment required is several identical 8% X 11-in. pieces of paper (preferably with rounded corners), some file cards t o record individual student results and several meter rules (preferably with protective metal ends). Some time hefore the demonstration the metal ends of every rule hut one are removed and 1-2 cm sawed off each rule. The metal ends are then replaced as carefully as possible. In addition, each sheet of paper is folded 2 or 3 times in each direction so that it is difficult to flatten effectively. T h e demonstration proceeds by selecting students and requesting that they measure the length of a piece of paper in centimeters and record the result on a card. By using an appropriate number of measuring instruments a significant quantity of data can he collected in 5 min without losing the attention of the class. The results are collected and transferred to the board for discussion. A typical set of results, obtained in the second class meeting of an introductory course, gave the following results: 29.4 cm, 29.4,29.5 cm, 27.5, 29.1 cm and 29.2 cm. A single meter rule (with a scale running in both directions) was used. The "experimental" outcome can he controlled largely by the way the rule is handed to the student. The first discussion point is devoted to the importance of recording the units of measurement. T h e second point concerns the rejection of data. The audience is always agreeable to rejecting the "obviously wrong" measurement-27.5 cm. This measurement is crossed out, hut not erased, and the discussion turns to how we determine the "best" value for the length of the paper. This discussion involves the ideas of certain and uncertain dieits and the average as the "best" value. In calculating the avrrage it is otien instructi\,t. toaska calc~~lntor owner in the audienct. to do the calc~llntion.'I'hiswill mme often than not yield an average with grossly inflated precision. After a discussion of the relation of significant digits to precision we finally turn to the term accuracy.
Tested Demonstrations is a monthly feature designed to present lecture demonstrations and experiments in a format convenient for classroom use. Readers interested in either submitting or checking demonstrations should contact the column editor. An outline of format requirements was given on page 166 of the March 1976 issue of This Journal.
This segment starts with a unit conversion of the standard paper size in inches to centimeters yielding a length of 27.5 cm. At this point we return to the original data and show that we have eliminated the accurate data and retained the precise data from the original set. Random and systematic errors can he discussed also if appropriate. Finally, the "secret" of the meter stick is revealed, and a discussion of the proper method of using measuring devices of this type is given.
Formation and Dissolution of Precipitates N. S. Nogar and W. A. J a l e n a k Uniuersity of Nebraska Lincoln, 68588 The concept of limited solubility often proves difficult for first year chemistry students, particularly if they have not had extensive previous experience in the laboratory. An explanation of precipitation and dissolution is often more effective if accompanied by a demonstration. In general, however, it is difficult to demonstrate these coocents to a laree " lecture class. In order to circumvent the problem, we have used light scattering to detect the presence of particles suspended in solution. A low power (0.5-mw) He-Ne laser beam is directed through a beaker containing -100 ml distilled water and a stirring bar. The light passes through the heaker without being scattered, and so cannot he seen from the side. A few ml of -1 M sodium chloride solution is then added to the heaker, followed by dronwise ..~ -,~ ~- addition of -1 X lo-:' silver nitrate. As the silver chloride begins to precipitate, light is scattered, and the laser heam traces a hrieht track through the solution. This can he rear of a largeUlecturehall if the lights are seen even from dimmed. Dissolution of orecinitates hv comolex ion formation . . c;rn he dem,,n.;tr,~trdthen h\. the additiun
