Titration O b S e ~ a t i o n ~ Initiator(s)
wt. ratios acid color
a) Phenol Red
base color
orange
red
pink
orange
pink
yellow
4:l
blue
green
5:1
bluelgreen blue
1:l
yellow
b) Methyl Red Hydrochloride /Phenolphthalein c) Methyl Red Hydrochloride
5:l
d) Methyl Red Hydrochloride IMethylene Blue e) Brilliant Green ImCreso Purple f) Brornophenol Blue /Alizarin Red
violet
When the rainbow is mentioned, the students then are asked for the colors of the rainbow. As each color is mentioned, the appropriate test tube is filled by squirting the NaOH solution into it. The rack markings indicate to which test tube the NaOH solution should be added as the color is mentioned. If the solid indicators are added carefully to beakers so the material is along the inside edge, the beakers will appear empty, allowing for the use of a n overhead projector for this demonstration. When discussing the chemical principles involved in the demonstration, it is noted that the second solution used is a NaOH solution five rimes theconrentration of the HCI in the initial solution. The additionofthe NaOH solution converts the test tube solutions from acidic to basic solutions. resulting in the observed wlor changes.
Critical Point Phase Separation in Binary Liquid Mixtures Submined by
L. H. Bowen and M. L. Benevides
Nonh ~ i r o l i aStale ~ n v e r s k Ralelgh, NC 27695
(a)hexane and nitmbenzene, and
(b)triethylamine and water.
Phenolphthalein is added to the latter mixture. makinrr the aqueous layer pink. In order to avoid odors (t&thylaI mine, in particular, is noxious and toxic), the test tubes are stoppered and covered with a plastic bag sealed tightly over the mouth of the tube, which prevents any vapors from escaping. At room temperature 0 for mixing of a 50-50 mixture, and thus phase separation occurs.
-
Check4 by Daniel T. Haworth Marqueue University Milwaukee, W153238
-
-
The critical point for a one-component system, the tem~ e r a t u r eand oressure where liauid and eas densities become equal, is not suitable for demonstration purposes because it ~enerallvoccurs at hieh oressure and often high temperature as well. In addition the two phases are commonlv colorless. A discussion of critical ~ o i n Dhenomena t suchas the disappearance of the meniscus is lkndered by lack of demonstrable examples. However, for m a w twocomponent systems, critieai behavior c& be obtained in the liquid phase. Both upper and lower critical temperatures occur in the system water-niwtine a t 210 OC and 61 'C, respectively (under pressure) (1).More wmmonly, twocomponent systems exhibit either upper or lower critical temperatures. The upper critical temperature T,, is the highest temperature a t which separation into two phases is the lowest occurs. The lower critical temperature TI,
-
-
temperature at which phase separation occurs. Thus, in nicotine-water, the two-phase region occurs only between 61 OC and 210 'C. The phase diagrams for an example of eachof these types ofbehavior are giveninAtkins'physica1 chemistry text (1). These phase diagrams suggest a simple demonstration. Because we have been unable to fmd such a demonstration mentioned in this Journal or in related sources, we now give our version of critical phase separation. It is simple, colorful, and surprising to students. The effects observed can be explained at a freshman level, but also they lend themselves to a more sophisticated discussion of non-ideal terms in the thermodynamics of mixing. Hexane-nitrobenof zene and water-triethylamine have T,, of 293 K and TI, 292 K, respectively, both slightly below the usual room temperature ( I ) . For the demonstration, two large test tubes are filled (in a hood) about 213 with approximately a 50-50 volume mixture of
-
A
-
Literature Cited 1. Atkhs, P W.Physkal Chemktq, 4th ed., Freeman:NervYmk, 19W,pp 193-195.
Volume 70 Number 9 September 1993
775
