John A. Ricketts DePauw University Greencastle, Indiana
A Laboratory Exercise Emphasizing Deductive Chemical Reasoning
T o stimulate the student experiments in the beginning, chemistry laboratory should not only demonstrate a chemical principle and illustrate laboratory techniques, but also offer a mental challenge. The desirability of such experiments has been discussed by both Blick' and Y o u ~ g . In ~ this laboratory this is the student's first experiment. Essentially it involves the techniques of qualitative analysis. It differs from the exercise described by Summerbell3 in which an ion foreign to the classical qualitative scheme is given the student after he has familiarized himself with the procedure, and that described by MacWood4in which the behaviors of nine different solutions are studied with respect to each other. The student observes the chemical behavior of the aqueous solutions of five pure nitrate salts toward several common laboratory reagents. From his observations he synthesizes a scheme of analysis that can be used to identify the components of a mixture containing any combination of the five salts. The student then tests his procedure by analyzing an unknown mixture. Outline of the Experiment
Since the most common laboratory blunders in qualitative analysis are incomplete precipitation of a particular ion or insufficient washing of a precipitate, the proper technique of precipitation and filtration is emphasized in a preliminary exercise. Silver chloride is precipitated from silver nitrate solution by the addition of copper(I1) chloride solution. The resulting mixture is filtered, and the precipitate is washed until the washings are free of the copper(I1) ion, the presence of which is detected by the formation of the copper(I1)ammonia complex upon the addition of ammonium hydroxide to the washings. The prime purpose of this experiment is deduction from observation; consequently, the stock solutions of the five nitrate salts are simply labeled I, 11, 111, IV, and V. The salts are silver nitrate, mercury(1) nitrate, cadmium nitrate, aluminum nitrate, and barium nitrate. The stock solutions contain 10 mg of cation per ml of solution. The laboratory reagents are dilute solutions (6M) of sodium hydroxide, hydrochloric acid, sulfuric acid, nitric acid, and ammonium hydroxide. The student performs the following test tube reactions-twentv-five in all. Senarate 3 ml samules of BLICK, D. J., J. CHEM.EDUC., 32, 264 (1953). 'YOUNG, J. A,, J. CHEM.EDUC., 34, 238 (1957). SUMMERBELL, R. H., LESTINA, G.,KING,L. C., AND NEUMANN, H. M., J. CHEM.EDUC., 32, 475 (1955). MACWOOD, G . E., LASSELTEE, E. N., AND BREEM, G., J. CHEM.EDUC.,17,520 (1940).
each of the reagents are added to individual 3 ml portions of each of the five nitrate salts. To those test tubes in which a precipitate appears upon adding hydrochloric acid, excess ammonium hydroxide is added with stirring. If any of these precipitates dissolves, the solution is neutralized with nitric acid. To those test tubes in which a precipitate appears upon adding either ammonium hydroxide or sodium hydroxide, an excess of the precipitating base is added to determine whether or not the precipitate is soluble on excess of the base. To those precipitates that dissolve, nitric acid is added until the precipitate that f i s t reforms redissolves. The student records his observations in his notebook in a five-by-five array. (A typical square is shown here.) From these ohsewations he devises a method of analysis for a mixture that might contain any combination of the five salts. His procedure is approved before he is given an unknown mixture. It is particularly effective when a student has an incorrect procedure to have him try his scheme on a solution-a solution compounded by the student himself--that contains all five of the ions. Reagent
\
HCL
~0iuti.n'
AFNO~
white ppt. no reao801.NHaOH tion r c ~ ~with t .
I1 Hgz(N03.
white ppt. white ppi. turns bisok addn.
I
'
NaOH
NHtOH
H*S01
brown ppt.
ad. execes
brown ppt
black ppt.
black pot.
NHIOH
HNO,
NHqOH
I11 Cd(N032
no reaction
no reaetion
IV AL(NOdx
no reaction
no reao-
v
Ba(N01h
I
no reaction
/
tion
white ppt.
white ppt. white ppt. sol. excess NHdOH
/
white eelatinoua not.
white ~ p t .
I
a d . exees NaOH
no reaotion
A typical compilation d student obrervations.
Conclusion
This particular laboratory exercise has been performed by about 120 students including 80 teachers of science in elementary and junior high schools. Seventy-five per cent of these analyzed their mixtures correctly; each mixture contained at least three of the five ions. Most mistakes in the analyses were in the detection of cadmium and aluminum. This experiment gave the student a sense of accomplishment from the beginning chemistry laboratory periods before having had much Volume 37, Number 6, June 1960
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formal instruction. In addition it seemed to allay the student's fear of the chemistry laboratory. Later when the student has gained enough chemical maturity, the correct scheme is explained by means of chemical equa-
312
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Journol o f Chemical Education
tions. This particular laboratory exercise should find profitable application in the high school laboratory since even those students who had taken high school chemistry enjoyed the experiment.
