Research: Science and Education edited by
Chemical Education Research
Diane M. Bunce The Catholic University of America Washington, D.C. 20064
Students’ Understanding of Solution Chemistry Concepts Tacettin Pınarbas¸ı* and Nurtac¸ Canpolat Department of Science and Mathematics Education, K. K. Education Faculty, Atatürk University, 25240, Erzurum, Turkey; *
[email protected] Content knowledge of general chemistry is generally tested with mathematically-based questions. It is often assumed that success in solving mathematical problems indicates mastery of a chemical concept, and it is accepted that problem-solving ability indicates an understanding of chemical concepts. Unfortunately, students often solve problems successfully using memorized algorithms. Little connection between solving an algorithmically-based problem and understanding the chemical concept behind that problem was found by various researchers (1–6). These studies showed that many students who are successful at solving mathematical problems do not understand the chemical concepts behind their memorized algorithmic solutions. According to Janiuk (7) the present trends in education emphasize the student, as opposed to the instructor, as the main object of the teaching process. The instructor’s task is to augment the student’s cognitive ability. To succeed the instructor must possess knowledge about the course content and the nature of the learning process. The goal of recent studies (8) has been to determine the knowledge the students begin with and its effect on learning, so that the process of learning can be optimized. This includes choosing the best ways of reasoning so that students will progress from their initial knowledge to the knowledge delineated by the educational goals (7). Thus science educators have become increasingly aware of the importance of students’ preconceptions of science concepts so that the teaching of science can be improved. Chemistry has been regarded as a difficult subject by chemistry teachers, researchers, and educators (9). The reasons for this vary from the abstract nature of many chemical concepts to the difficulty of the nomenclature and symbology used in chemistry. Students often develop misconceptions about scientific concepts, for example, chemical change (8, 10), the particulate nature of matter (11, 12), chemical equilibrium (13), and solubility and solution (14–25). These misconceptions have serious effects on subsequent learning (8). Therefore, it is important to discover students’ preconceptions in order to plan future teaching activities. This study examined students’ understanding of some concepts in solution chemistry. Method This study investigated some concepts relating to solution chemistry such as unsaturated, saturated, and supersaturated solutions; physical properties of a solution; and gas solubility. A diagnostic test concerning these concepts was developed specifically for this study. 1328
The test consisted of four multiple-choice questions (see the Appendix). Question 1 tested the students’ understanding of unsaturated, saturated, and supersaturated solution concepts and required students to apply these concepts to a microscopic representation. Question 2 tested students’ understanding of the concept of vapor pressure lowering and required students to predict the reason for it. Question 3 measured students’ understanding of how the solubility of a gas in water changes when an inert gas is added to the system. Finally, Question 4 tested the students’ understanding of the relationship between vapor pressure and boiling point and required students to make a comparison between solutions and pure solvents in terms of their vapor pressures and temperatures at their boiling point. All questions were pilot tested and the required modifications were made prior to the administration of the test. The content validity of the test questions was assessed by one chemistry professor and two research assistants. These questions were administered to 107 undergraduates enrolled in General Chemistry-II in 2001 at Atatürk University in Turkey. A traditional problem-solving oriented lecture approach was followed in the course. The students were taught by a separate lecturer, and as outside researchers, the authors did not interfere in the teaching process. The test was conducted under normal class conditions without previous warning. The students were told that the results of the test would be used for research purposes and would be kept confidential. To find out possible trends or typical misconceptions among the entire sample, all the students were asked to write an explanation for their multiple-choice answer. In addition seven students participated in a 15-minute informal interview in which the interviewer took brief notes to ascertain the trends in students’ understanding of the concepts. At the end of the course, the students in the study took a written exam in which some questions required the students to state the definitions of the concepts included in the diagnostic questions to determine whether they knew the definitions of the concepts. To further probe students’ conceptual understanding, nine students were chosen at random to be formally interviewed. The interviews were scheduled and took place in an empty room. Each interview took approximately 30 minutes and students’ permission to audiotape was obtained. Later, the interviews were transcribed and analyzed. Students’ responses to the diagnostic test were analyzed, percentages of the answers were calculated, and wrong answers with the highest percentage were considered and discussed. Students’ responses given in the formal interviews were studied and misunderstandings were identified.
Journal of Chemical Education • Vol. 80 No. 11 November 2003 • JChemEd.chem.wisc.edu
Research: Science and Education Table 1. Students’ Misconceptions Identified through Students’ Written Responses, Informal Interviews, and Formal Interviews Misconceptions
Written Responses
Informal Interview
Formal Interview
√
√
√
√
√
A solution containing undissolved solute is a supersaturated solution. Undissolved solute is a component of solution. Because of the attractive forces between solute particles and solvent particles, the vapor pressure of a solution is less than that of pure solvent.
√
√
√
The amount of gas dissolved in a solvent is proportional to total pressure of gases above the solution.
√
√
√
Boiling liquids at atmospheric pressure have different vapor pressures.
√
√
√
Results and Discussion In the interviews, the analysis of the students’ responses relating to the definitions of the concepts and the results of the written exam suggested that the majority of the students correctly stated the definitions of the concepts. The analysis of the students’ written responses showed that many students tended to leave the explanation section of the questions blank or repeated some of the statements from the questions rather than giving reasoning for their answers. However, some major misconceptions were identified through the analysis of combinations of the students’ written responses, the informal interviews, and the formal interviews. These findings are summarized in Table 1. These misconceptions are discussed in detail in the following sections.
Question 1 Students’ responses by percentages are presented in Table 2. A majority of the students, 77.5%, considered choice b as correct, stating that solutions A, B, C represent saturated, supersaturated, and unsaturated solutions, respectively. Only 16.8% of the students answered the question correctly. These results indicate that while students are able to state successfully the definition of unsaturated, saturated, and supersaturated solutions, three-in-four students could not use this information to choose the correct representations of the solutions. From the results, it could be said that many students have difficulty in making the distinction especially between a saturated solution and a supersaturated solution. That is, the students may regard a solution as supersaturated if dissolved and undissolved solute are in equilibrium (as shown on the diagram labeled B in the Appendix). The findings obtained from the interviews support this view (R and I stand for researcher and interviewee, respectively.): Solution B is a supersaturated solution because it contains some excess sugar. A solution containing undissolved solute (indicating the undissolved sugar) means a supersaturated solution. (I1)
To identify the sources of the students’ misunderstanding, they were asked to explain their answers during interviews. The following excerpt summarizes the most common source of the misunderstanding about supersaturated solution.
R: Could you tell me why did you classify this one (pointing out the saturated solution on the diagram) as supersaturated solution? I: Because there is more solute in supersaturated solution than that in saturated one and this solution has excess solute (pointing out the undissolved solute), so it is supersaturated. R: Do you think that the solution consists of undissolved solute and the liquid phase above it? I: Right. R: If we remove undissolved solute by filtering, what would you call the rest of it? I: Saturated solution. (I4)
This dialogue indicates that the students have a weak understanding of solution. Students believed incorrectly that the undissolved solute is a component of the solution. Another example showing the students’ understanding of supersaturated solution is given in the following excerpt: R: If you were asked to prepare a supersaturated sugar– water solution, what would you do? I: I would add sugar little-by-little into water by stirring until it does not dissolve any more and sufficient undissolved sugar appears at the bottom. (I4)
This dialogue suggests that students do not know how a supersaturated solution is prepared. As a result, it could be said that students’ misunderstanding of saturated and supersaturated solutions might come from the lack of knowledge and understanding of preparation of saturated and supersaturated solutions.
Table 2. Students’ Responses to Test Questions Question
Response (%) a
b
c
d
e
1
1.1
77.5
16.8
4.6
0
2
54.2
14.1
2.8
17.7
11.2
3
12.1
2.8
0
85.1
4
1.8
27.1
66.3
----
0 4.8
NOTE: Correct answers are in bold type.
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Question 2
Question 3
The students’ responses for Question 2 are summarized in Table 2. Slightly more than half of the students, 54.2%, chose a, stating that attractive forces in solution caused the vapor pressure lowering. Almost 18% of the students chose d, believing that both attractive and repulsive forces changed the vapor pressure of the solution. Only 14.1% of the students answered the question correctly. The low percentage of the correct answer indicates that only a small number of the students understood the chemical idea behind this process. It also indicates that many of the respondents hold misunderstandings about the nature of the process. It is known that the vapor pressure of a solution is less than that of pure solvent because of the presence of the dissolved solute particles. Many textbooks emphasize that vapor pressure lowering depends only on the number of solute particles present in solution without regard for the nature of the particles. A high percentage of the students, however, attempted to explain this process in terms of interactions between solute and solvent particles. These students believed that the vapor pressure of solutions is less than that of pure solvent, because attractive forces between solute and solvent molecules prevent solvent molecules from escaping out of solution. Excerpts from the interviews confirm this rationale:
Percentages of students’ answers for Question 3 are presented in Table 2. This question had the lowest percentage of correct answers. Although only 12.1% of the students answered this question correctly, the majority, 85.1%, chose e. A gas dissolved in a liquid will be in equilibrium with the same gas in the vapor phase. This means that the solubility of the gas in the liquid will vary with the partial pressure of the gas in the vapor. As can be seen from Table 2, however, most students chose e thinking that the amount of gas dissolved in a solvent is proportional to total pressure of gaseous mixture above the solution. The following excerpt supports this way of thinking:
R: How could you compare the vapor pressures of 0.01 M KCl solution to that of 0.01 M NaCl solution at the same temperature? I: Both must be the same. R: Why do you think so? I: Their concentrations are the same. R: But they are different solutions? I: Yes, but it does not depend on the nature of solute. ... R: How do you compare the vapor pressure of a solution and a pure solvent? I: Vapor pressure of solution will be lower. R: OK. You think that vapor pressure of solution is less than that of pure solvent. Why do you think so? I: I think it is because of the fact that solute particles attract solvent molecules, preventing them escaping from solution and reduce vapor pressure. (I5)
From the above dialogue, it appears that the respondent is aware that the vapor pressure of solution is independent of the nature of the solute but depends on the number of the solute particles in the solution. However, it is also apparent from the dialogue that the respondent was unable to use this piece of information in the final part of the dialogue. This misconception can be attributed to overgeneralization of intermolecular interactions used in the explanation of some chemistry concepts such as viscosity, surface tension, solubility, and so forth.
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R: When you open a bottle or can containing a carbonated beverage, the excess CO2 leaves the solution in the form of bubbles. Could you explain why? I: The pressure above the beverage is reduced and this reduces the equilibrium concentration of CO2 in solution. R: OK, what do you think about the final CO2 pressure in equilibrium with CO2 in solution? I: Atmospheric pressure, that is one atm. (I7)
As seen in the dialogue, the respondent did not differentiate the partial pressure of CO2 and the atmospheric pressure. Whenever pressure and temperature effects on solubility were mentioned, students generally recall the fact that the solubility of gases often decreases with increasing temperature and increases with increasing pressure without differentiating the total or partial pressure. Our findings suggest that students are not able to recognize the importance of the partial pressure of a gas in equilibrium with the dissolved gas. Students’ misunderstanding may arise from this notion.
Question 4 The percentages of answers for Question 4 are given in Table 2. The results show that a significant number, 66.3%, of the students chose answer d and slightly more than onequarter of the students, 27.1%, chose the correct answer, b, indicating that most of the students memorize definitions and use them without understanding. Although the students were able to correctly define the boiling point of a substance, they failed to apply this knowledge to real cases. Students who chose d believed that boiling liquids at atmospheric pressure have different vapor pressures. The following excerpt from one student’s response may help to illuminate the source of this misunderstanding: R: Imagine that we have one beaker of pure water and another beaker of salt solution boiling in the front of us. How do you compare the vapor pressures of these boiling liquids? Are they the same or different? I: Different. We have seen that a nonvolatile solute lowers the vapor pressure of the solvent. So, the vapor
Journal of Chemical Education • Vol. 80 No. 11 November 2003 • JChemEd.chem.wisc.edu
Research: Science and Education pressure of the salt solution must be less than that of pure water. (I8)
This dialogue suggests that the interviewee knows that at constant temperature, a solution has a lower vapor pressure than the solvent, but the interviewee misinterpreted this information and incorrectly applied this piece of information to the boiling points of solutions and solvents. Anecdotal evidence also supports the notion that students misunderstand that different liquids have different vapor pressures at their boiling points, such as ethanol and water. Conclusions and Implications for Teaching This study provides insights into the students’ understanding of some concepts in solution chemistry. The high percentages of students’ incorrect answers for the questions indicates that a great proportion of the students were unable to apply their chemical knowledge to real-life situations. This study has identified a number of misconceptions of solutions generated by the students, such as: (i) many students viewed a solution containing undissolved solute as a supersaturated solution; (ii) nearly half of the students thought that attractive forces between solute and solvent molecules in a solution caused vapor pressure lowering; (iii) a significant number of the students held the view that the amount of gas dissolved in a solvent is proportional to total pressure of gaseous mixture above the solution; and (iv) a large number of the students believed that different liquids have different vapor pressures at their normal boiling points. On the whole, the results were not encouraging and show that the students did not learn the concepts well. The reasons for this may be the result of instructional approaches, textbook inadequacies, and the availability and use of the laboratory (26). Moreover, students’ ideas may or may not be influenced in unanticipated ways (8), especially in teaching abstract scientific concepts by using traditional teaching strategies. Science education researchers indicate that many novice learners in chemistry are able to apply algorithms without significant conceptual understanding (27). Therefore, the students can solve numerical problems, but fail to answer conceptual questions. What essentially distinguishes conceptual learners from algorithmic learners is that the former are more advanced and less dualistic in their thinking, more experienced in problem solving, more situational in their knowledge orientation, and more verbal in their reasoning (28). Gil-Perez and Carrascosa (29) stated that one of the most important outcomes of research on science misconceptions has been a better understanding of science-learning difficulties and the awareness of the necessity for profound changes in the teaching process to improve meaningful learning. The findings in this article on students’ conceptions of solutions may contribute to our understanding of some of the difficulties that students experience in their chemistry classes. Also we suggest that the test questions developed can be easily used to ascertain students’ understanding of solution concepts in a learning group. Thus, different instructional strategies might be developed according to the type
and the source of the preconceptions to minimize the likelihood of misconceptions occurring and to facilitate students’ conceptual understanding. This research did not aim to identify teaching strategies to induce conceptual change, and therefore, no teaching approaches were used to address the observed misconceptions that have a great influence on subsequent learning. Indeed, further work is necessary to develop appropriate teaching– learning approaches that will assist teachers and students to overcome any of the misconceptions students bring into classroom. We suggest that the findings reported here can be utilized in research that develops teaching strategies to overcome such misconceptions. Literature Cited 1. Nakhleh, M. B.; Mitchell, R. C. J. Chem. Educ. 1993, 70, 190–192. 2. Smith, K. J.; Metz, P. A. J. Chem. Educ. 1996, 73, 233–235. 3. Nurrenbern, S.; Pickering, M. J. Chem. Educ. 1987, 64, 508. 4. Pickering, M. J. Chem. Educ. 1990, 67, 254–255. 5. Sawrey, B. J. J. Chem. Educ. 1990, 67, 253–254. 6. Nakhleh, M. B. J. Chem. Educ. 1993, 70, 52–55. 7. Janiuk, R. M. J. Chem. Educ. 1993, 70, 828–829. 8. Ayas, A.; Demirbas, A. J. Chem. Educ. 1997, 74, 518–521. 9. Ben-Zvi, R.; Eylon, B.; Silberstein, J. J. Chem. Educ. 1982, 47, 64–66. 10. Abraham, M. R.; Williamson, V. M.; Westbrook, S. L. J. Res. Sci. Teach. 1994, 31, 147–165. 11. Hibbard, K. M.; Novak, J. D. Sci. Educ. 1975, 59, 559–570. 12. Novick, S.; Nussbaum, J. Sci. Educ. 1978, 62, 273–281. 13. Gussarsky, E.; Gorodetsky, M. J. Res. Sci. Teach. 1988, 25, 319–333. 14. Haidar, A. H.; Abraham, M. R. J. Res. Sci Teach. 1991, 28, 919–938. 15. Lee, O.; Eichinger, D. C.; Anderson C. W.; Berkheimer G. D.; Blakeslee T. D. J. Res. Sci. Teach. 1993, 30, 249–270. 16. Stavy, R. J. Res. Sci. Teach. 1990, 27, 247–266. 17. Fellows, N. J. J. Res. Sci. Teach. 1994, 31, 985–1001. 18. Prieto, T.; Blanco, A.; Rodriguez, A. Int. J. Sci. Educ. 1989, 11, 451–463. 19. Blanco, A.; Prieto, T. Int. J. Sci. Educ. 1997, 19, 303–315. 20. Ebenezer, J. V.; Gaskel, P. J. Sci. Educ. 1995, 79, 1–17. 21. Ebenezer, J. V.; Erickson, G. L. Sci. Educ. 1996, 80, 181–201. 22. Ebenezer, J. V. J. Sci. Educ. Tech. 2001, 10, 73–92. 23. Slone, M.; Bokhurst, F. D. Int. J. Sci. Educ. 1992, 14, 221– 235. 24. Johnson, P. Int. J. Sci. Educ. 1998, 20, 393–412. 25. Kokkotas, P.; Vlachos, I. Int. J. Sci. Educ. 1998, 20, 291–303. 26. Abraham, M. R.; Grzybowski, E. B.; Renner, J. W.; Marek, E. A. J. Res. Sci. Teach. 1992, 29, 105–120. 27. Nakhleh, M. B. J. Chem. Educ. 1992, 69, 191–196. 28. Pushkin, D. B. J. Chem. Educ. 1998, 75, 809–810. 29. Gil-Perez, D.; Carrascosa, J. Sci. Educ. 1990, 79, 77–93.
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Appendix Question 1 There are different concentrations of sugar solutions in beakers A, B, and C. Which of the following options correctly defines the solutions as unsaturated, saturated, and supersaturated? (Increased concentrations are illustrated by the density of the dots in the diagrams. The dots represent the dissolved sugar molecules. The undissolved sugar in beaker B is shown as a darkened area.)
Question 3
PX = 2 atm
I
A
B
A unsaturated saturated supersaturated* unsaturated saturated
(a) (b) (c) (d) (e)
C
B saturated supersaturated saturated supersaturated unsaturated
C supersaturated unsaturated unsaturated saturated supersaturated
PX = 2 atm PN2 = 1 atm
II
PX = 2 atm PN2 = 2 atm
III
At 25 ⬚C the closed containers (I, II, and III) contain solutions of X gas in water as illustrated above. In each container, the partial pressures of X gas are the same (2 atm). At constant T and V, if an inert (N2) gas is injected into the containers II and III where the N2 partial pressures are 1 atm and 2 atm, respectively, how do the equilibrium concentrations of dissolved X gas change in the containers? (The dots in the containers represent X gas molecules dissolved in water.) (a) I = II = III* (d) III > I = II
(b) I > II > III (e) III > II > I
(c) I = III > II
Question 2 At a particular constant temperature, the vapor pressure of water above a dilute sugar–water solution is less than that of pure water. Decide which of the following statement(s) correctly explain(s) this notion.
Question 4 Imagine that a beaker of pure water and another beaker of a saturated aqueous salt solution are both boiling at atmospheric pressure. Which of the following statements do you think is correct for this case?
I. Sugar molecules prevent water molecules from escaping out of solution owing to the attractive forces between the sugar molecules and water molecules.
(a) Both will have the same vapor pressure and the same temperature.
II. In solution, the number of water molecules per unit volume is reduced because of the presence of sugar molecules.
(b) Both will have the same vapor pressure but different boiling temperatures.*
III. Sugar and water molecules repel each other.
(a) I
(b) II*
(c) I and II
(d) I and III
(e) III
(c) Both will have the same temperature but different vapor pressures. (d) Both will have different vapor pressures and different temperatures.
*Note: The correct answer for each question is indicated by an asterisk.
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Journal of Chemical Education • Vol. 80 No. 11 November 2003 • JChemEd.chem.wisc.edu
