insights Pressure Determinations Using Open-end Manometers George Desmarais 817 Central Avenue Dover, NH 03820
edited by
STAFF
stances "pushing" on the line. (The Hg vapor pressure is negligible.) On the right side where the arrow points, the pressure is due only to the atmosphere since this is all that is "pushing" on that line. Since the levels of the Hg columns are stationary, it must be true that where the lines are drawn, pga8 + p ~ g
From my classroom experience, I have found that many students do not understand what pressure changes have occurred to modify the levels of the mercury columns in an open-end manometer. Some of them subtract the difference in the levels when they should add; others add the difference when they should subtract. I developed the following method to increase their understanding of the level changes and, in turn, their ability to quantify these changes correctly. I indicate that a short line should be drawn across the top of the lower Hg level and that a coincident line should be drawn across the other part of the U-tube. Because the pressure in fluids a t a given level is the same throughout that level, the process is then one of determining the pressure a t the two lines by simply focussing on what is "pushing" on the lines. For example, consider the following: In the accompanying diagram, the curved lines represent the levels of the Hg columns. On the left side of the U-tube where the arrow points, the pressure is due to the gas and the Hg column because these are the only sub-
therefore, pms =
-p
~ g
For the case in which the Hg column is lower on the left side of the U-tube, similar reasoning yields pw =
+p ~ g
Caution -~~ Due to the toxic nature of mercury, as both a liquid and vapor, proper handling and storage procedures must be followed. ~
The Quest for Mistakes Jordi Carvajal Extensio Sagrada Familia Sardenya 341, Barcelona 08025 Spain
Scientific and technical papers and magazines cover news of a wide range of topics: chemistry, physics, biology, medicine, and many others. However, many of the articles are only surnmanes for different agencies, written by nonspecialized journalists. ORen mistakes occur. In addition, t;anslations often lead to errors in product names. One of the most frequent mistakes is the translation of the EuAs a result glish billion (109)intothe Spanish billion (10'~). of this error, you can often read that "the Earth began 5,000,000,000,000years ago"! I use this situation to foster my students' abilities to analyze critically and to compare scientific news sources. In practice, every group of three or four students are assigned a specific paper or magazine. These students read the scientific news for two weeks looking for possible mistakes or inaccuracies. Finally, we spend a whole session dealing with their discoveries, talking about the most interesting aspects and creating new scientific interests.
498
Journal of Chemical Education
