Chemical Education Today
Reports from Other Journals
A View of the Science Education Research Literature: Student Understanding of Chemical Change by William R. Robinson
In their paper, “Students’ understanding of chemical reactions” which appeared in the International Journal of Science Education (1998, 20, 303–316), Maija Ahtee and Irma Varjola cite studies indicating that many students of a variety of ages and nationalities are unable to distinguish between physical change and chemical change. Students often explain phenomena based on intuitions rather than on science concepts learned in school; they do not use atoms and molecules in explanations unless prompted; they are unable to differentiate among concepts such as compound, mixture, solution, and atom; they do not identify formation of a new substance as a characteristic of a chemical reaction; and they prefer to use concrete attributes rather than attributes with an atomic or molecular basis when discussing change or reactions. Ahtee and Varjola then report their study of Finnish students with results that fit the same pattern. Students in Finland study chemistry and physics in comprehensive schools (grades 7–8, ages 13–16) and in senior secondary schools (grades 10–12, ages 17–19). In grade 7, two 45-minute classes per week for one semester are dedicated to chemistry. Two classes per week are dedicated to physics during the next term. In grade 8, chemistry is taught twice a week for the entire year. Physics is taught twice a week in grade 9. In the senior secondary schools a year of chemistry followed by a year of environmental chemistry is required. In this study, chemistry students responded to the following statements in their own words: 1. In a chemical reaction, a substance transforms into a different substance. Explain what that means. 2. What kind of things do you think will indicate that a chemical reaction has taken place?
The questions were administered to students in grades 7 and 8 about one month after the discussion on chemical reactions in their classes, to secondary school students at the end of their first year of chemistry, and to students at the University of Helsinki in a first-year general chemistry course for those who want more that just a basic course in chemistry.
The responses were sorted into the following five groups using the criteria in parentheses: A. No understanding (irrelevant response, repeats the definition given, serious error) B. Some knowledge (example given without explanation, reaction can be erroneous) C. Understanding but the response included errors (responses that include at least one of the responses in E, but still contain errors) D. Partial understanding (responses that contain one of the responses in E) E. Sound understanding (responses that include: reorganization of atoms, breaking and reforming bonds, and changes in chemical or physical properties).
As might be expected, the level of understanding regarding the nature of a chemical reaction (question 1) increased with increasing exposure to the topic; although only a quarter of the senior secondary students had a partial or sound understanding of the concepts. Table 1 records the distribution of responses. One of the more common mistakes in answering question 1 was the inappropriate use of terms such as substance, compound, element, atom, and molecule. Students treated objects of the macroscopic world (substance, element) as if they were objects of the microscopic world (atom, molecule). The authors argue that confusion between the real (macroscopic) and the theoretical (microscopic) frameworks is reinforced as students use the same word for element and the corresponding atom. Example statements from secondary students are: “Substances form bonding” and “Substances change outer electrons between them.” The second question in the study was aimed at determining whether or not students understood that a physical change is not a sufficient condition to indicate a chemical reaction. Only four grade 8 students, one senior secondary student, and eleven university students mentioned changes in chemical properties as a condition of a chemical reaction—
Table 1. Distribution (%) of Answers to Question 1 and Number of Participants by Grade Level Evaluation a
Grade 7 (n b = 159)
Grade 8 (n = 105)
Senior Secondar y (n = 178)
University (n = 137)
A
31
30
20
9
B
46
50
35
15
C
9
14
21
17
D
14
7
19
45
E
0
0
6
14
a
Refer to text for definitions of the evaluations.
b
Number of participants at each level.
JChemEd.chem.wisc.edu • Vol. 76 No. 3 March 1999 • Journal of Chemical Education
297
Chemical Education Today
Reports from Other Journals Table 2. Visible Characteristics of a Chemical Change and Three Different Physical Changes Chemical or Physical Change
Change in Form
Change in Color
Heat Needed
Change in State
Something Disappears
Burning wood
×
×
×
×
×
Dissolving salt in water
×
×
—
×
×
Melting ice
×
—
×
×
—
Heating iron to glowing
—
×
×
—
—
55 to 90 percent of each group associated a change in physical properties with a chemical reaction. It is not difficult to understand why students confuse physical and chemical changes. Table 2, taken from Ahtee and Varjola’s paper, lists characteristics of one chemical change and three physical changes that can be demonstrated readily in an introductory course. The table indicates that what instructors see as examples of chemical and physical changes are not readily distinguished by novices. Both the chemical and the physical changes have observable properties in common, and neither has a unique property. This is what students see when these changes are demonstrated, and it is not possible for them to distinguish between the chemical change
and the set of physical changes on the basis of these visible behaviors alone. The results of this paper add to an increasing body of evidence that speaks to the need for chemistry teachers to be extremely careful and consistent in not mixing terms that refer to macroscopic and microscopic entities and in not allowing their students to be sloppy with these terms. It also supports the idea that demonstrations can mislead unless they are carefully explored and discussed. William R. Robinson is in the Department of Chemistry, Purdue University, West Lafayette, IN 47907;
[email protected].
