A Bulletin Board Demonstration on Humidity Submined by Kurt Smith and Sally ~ o l o m o n ' Drexel University Philadelphia. PA 19104 Checked by J. H. Dreisbach University of Scranton Scranton, PA 18411 Postine Yive" demonstrations is one verveffedive wav to uscarhehistry bulletln board. ~demonst;ationofthe&lor chanees of cobalt chloride with relative humidity is described below. Procedure
An 18.5-em circle of coarse filter paper (Whatman #2 Qualitative) is thomughly soaked in a very concentrated solution of cobalffI1) chloride hexahydrate prepared by adding 10 g CoC12. 6H20 to 25 mL water. The filter paper is allowed to dry, then attached to a n explanation of the reactions involved in thecolor changes ( s e e . ~ r i n c i ~ l eThe s~. filter paper and explanation me posted on a bulletin board where students note that the paper turns reddish- violet on wet humid days and blue on dry days.
Principles For students who have been introduced to complex ions, you can explain that Co(I1) complex ions with six ligands are pink to violet and those with four ligands are blue? I n the well-known equilibrium below, produced by dissolving cobalt(I1) chloride hexahydrate in water, the blue color of the tetrachlorocobaltate(II) complex ion appears upon adding concentrated HC1. Addition of water regenerates the reddish color of the hexaaquacobalt(I1) ion.
reddish-violet
blue
Students can apply LeChfitelier's principle to conclude that removal of water (as on dry days) leads to increased concentration of the blue complex ion and that adding water produces more of the reddish-violet ion. For students who have not yet encountered complex ions, you can remark that the paper is coated with a substance, called cobalt\Il,chloride hexahydrate, whch turns reddish bvnbsorbina wateron humid days and blue by losing water on dry day;. A chart can be posted on which younger students are assigned to record the date, the humidity, and the color. 'Author to whom correspondence should be directed. 'Conon F. A. and Wilkinson G. Advanced inorganic Chemistm Wiley, NewYork, 1980;Chapter 21, pp 77C-772.
Volume 68 Number 12 December 1991
1039
