JOURNAL OF CHEMICAL EDUCATION
A CHEMISTRY IDENTIFICATION QUESTION
WILL S. DeLOACH Mississippi State College for Women, Columbus, Mississippi
GIVENseven substames studied this semester, identified by Numbers 1, 2, 3, 4, 5, 6, and 7. Nos. 1, 2, 5, and 61lreelements. Nos. 1, 3, and 7 are gases ordinarily. Nos. 2, 5, and 6 are solids ordinarily. Many of them have oxidizing or reducing properties but 1 is a strong oxidant, 5 is a strong reductsnt, and 4 is s strong oxidant when hot. All seven are produced on an important scale commercially. When 2 is burned in air: 7 is farmed. No. 3 dissolves in water to form a basic solution. Nos. 4 and 7 form acidic solutions in water. No. 7 is used in the industrial preparation of 4. All of the gases have odors, and 3 and 7 are colorless. No. 5 reacts vigorously with water, forming a basic solution. No. 6 will bum in air to form a compound that is a good dehydrating agent. The four elements arranged in order of increasing number of valence electrons are: 5, 6, 2, 1. Nos. 5 and 6 must he stared out of contact with air. Nos. 3 and 4 contain one element in common, and 4 and 7 contain two elements in common. Nos. 1 and 7 have industrial use as bleaches. No. 2 has two orystdline forms. Nos. 3 and 4 in water solution are common laboratory reagents. No. 5 has s. silvery luster. No. 6 has two a.ll.llotmpic forms, one of which is much more reactive than the other. The reactive form of 6 is quite poisonous. Nos. 3 and 7 have same use in elmtrio refrigerators. Identify the seven substances by writing their names or formulae in the spaces below.
"Scrambled clues" questions of this sort have been used by the author to add variety to general and organic chemistry tests and examinations, to catch the interest of the better students, and to require more than mere recall of memorized facts and theories. Such questions are not tedious t o prepare, can touch on much material, and can be scored rapidly. I t is necessary to take care to keep them from being too easy--so that everyone answers them-or ro difficult as to be impossible with the knowledge the students are expected to have. The student's difficultiesin analyzing the problem can of course be increased by having a, larger number of unknowns or by making the clues less obvious. It seems to be a good idea to show relationships between the unknowns as far as possible, but not to require the solution of one to depend completely on another. Time is an important factor since many students are unable to make much pxogress on this sort of problem if they feel rushed. One such question was given to a class in general chemistry on the final examination at the end of each
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semester, and the table gives the correlation coefficients between the score on this item, in each case, and the total score on the remainder of the examination. Question
Correlation
Number qf
coefficient"
persm
" 7 , . 1-, is the correlation coefficient between item 1 and the total score on the test excluding item 1.
Although these data are based on a rather small number of cases the coefficients4.70 and 0.68-indicate that this type of question is probably substantially valid. The correlation coefficient between the scores on the
two questions is also given for those persons, 21 in number, who took both examinations. This figure0.61-gives some measure of the reliability of this type of question, there being a considerable time lapse and a difference in content between the two questions. The answers to the question given at the beginning are: (1) CIS, (2) S, (3) NH,, (4) HSO4, (5) Na, (6) P, and (7) SOz. The scores actually obtained when this question was given to a class of 22 students ranged from zero (2 persons) to the maximum possihle (also 2 persons). ACKNOWLEDGMENT
The author wishes to thank Dr. Julian C. Stanley, Jr., of the Psychology Department, George Peabody College, for his help in preparing this paper.
