Activity pubs.acs.org/jchemeduc
Sticky Ions: A Student-Centered Activity Using Magnetic Models to Explore the Dissolving of Ionic Compounds Sheila Ryan* and Deborah G. Herrington Department of Chemistry, Grand Valley State University, Allendale, Michigan 49401, United States S Supporting Information *
ABSTRACT: Understanding what happens at the particulate level when ionic compounds dissolve in water is difficult for many students, yet this understanding is critical in explaining many macroscopic observations. This article describes a studentcentered activity designed to help strengthen students’ conceptual understanding of this process at the particulate level and translate this understanding to the symbolic level. In this activity, students use magnetic models to explore how mono- and polyatomic ions interact with water molecules and with each other. Manipulating the models addresses the common students’ misconceptions (1) that ionic compounds remain as ion pairs and (2) that polyatomic ions dissociate into individual monatomic ions. These models also help students see how water molecules orient around the individual ions. Writing dissociation reaction equations for the modeled dissolving processes helps students better understand the relationship between subscripts and coefficients in writing ionic equations. Post-test and final exam scores on questions related to the particulate and symbolic representations of the dissolving of ionic compounds show improved and retained understanding of this concept. KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Physical Chemistry, Inquiry-Based/Discovery Learning, Hands-On Learning/Manipulatives, Ionic Bonding, Nonmajor Course, Solutions/Solvents
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chemistry), symbolic models (e.g., reaction equations), maps/ diagrams/tables (e.g., the periodic table), and simulations (used to model sophisticated processes such as chemical reactions).8 Studies show particulate level computer animations help students answer conceptual questions about chemical or physical processes.9,10 Additionally, there has been some recent work in the use of tactile or haptic feedback in conjunction with models, indicating that sensory feedback (e.g., the feel of attractive or repulsive interactions) can improve students learning from models.11,12 Hence, this activity uses magnetic models representing ions (monatomic and polyatomic) and water molecules that students manipulate to show what happens to the ions and surrounding water molecules when ionic compounds dissolve in water. They then make connections to the symbolic level by writing equations for the processes they model.
tudies show that students have difficulties understanding the solution process for ionic compounds in water.1−5 Misconceptions include failing to understand the uniform distribution of the solute, instead thinking a new substance is formed,4 equating dissolving with melting,1 and believing that ion pairs stay together as “molecules”.2 Even if students understand that aqueous ionic compounds dissociate into ions, they often have difficulty predicting what ions form based on chemical formulas. Studies have shown, for example, that student equations depicting the dissolving of ionic compounds such as BaBr2 and K2SO4 frequently contain subscript errors4 and that undergraduate students’ drawings of aqueous ionic compounds show a variety of incorrect ideas, most notably not dissociating the ions, breaking the polyatomic ions apart, and misinterpretation of subscripts in formulas (e.g., writing Cl2 in NiCl2 or (OH)2 in Ni(OH)2 as “diatomics”).5 Furthermore, similar errors were found in faculty and graduate students’ drawings.5 These are consistent with the most common student errors observed in our classes. Research suggests that the exploration of macroscopic, particulate, and symbolic representations of chemistry concepts is most effective in guiding students to a conceptual understanding of chemistry topics.6,7 As students cannot see atoms, molecules, and ions, one way to help them explore these processes at the particulate level is through the use of models. Models can take a variety of forms including mathematical models (e.g., P1V1 = P2V2), scale models (e.g., miniature cars), pedagogical analogical models (concrete models to depict unobservable or abstract concepts, e.g., molecular models in © 2014 American Chemical Society and Division of Chemical Education, Inc.
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ACTIVITY DESIGN Models are important products of science and major teaching and learning tools in science education.8 Consequently, the development and use of models is one of the eight Science and Engineering Practices in the new Framework for K−12 Science Education.13 Models are particularly important in teaching abstract concepts, and several key considerations for the development and use of models for teaching have been identified.8 Published: May 2, 2014 860
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to work through a series of questions designed to help them construct an accurate understanding of the dissolving process for ionic compounds.
Analogical teaching models, such as the models we use in our activity, are always simplified and enhanced to address the target concept. In our case, we are particularly interested in helping students understand (i) oppositely charged ions separate when dissolved in water, (ii) these ions are surrounded by water molecules that interact with the ions a certain way due to the polarity of the water molecule, and (iii) that polyatomic ions stay together as a unit when compounds containing these ions dissolve in water. Thus, the models used in the Sticky Ions activity are made of magnets that have imbedded polarity such that students find, for example, cations attract anions and the oxygen end of water molecules and repel other cations and the hydrogen ends of water molecules. These ionic and ion-dipole attractions can be broken relatively easily; however, polyatomic ions and water cannot be broken apart as they are attached to plexiglass (Figure 1). These models are modified from those previously described by Davies14 in this Journal.
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ACTIVITY DETAILS This activity is appropriate for use in a high school chemistry class or for introductory level or nonmajor college chemistry courses. It takes about 60−75 min to complete and can be used either in a lab or lecture setting. The activity is designed to address the following learning objectives. Students will be able to (1) Predict what individual species (ions) form when an ionic compound dissolves in water. (2) Describe how the polarity of water molecules influences its interaction with dissolved ions. (3) Make connections to macroscopic observations by explaining the differences in conductivity for solutions of polar covalent compounds versus ionic compounds. Accordingly, prerequisite knowledge includes formation of ions and an introduction to polyatomic ions and covalent and ionic bonding. In our course, prior to conducting this activity, we have also covered bond and molecular polarity for covalent compounds, drawing Lewis structures for ions (including polyatomic ions) and covalent compounds, and intermolecular forces. However, it is possible to do this activity before covering these latter topics if the instructor starts the activity with a brief discussion of the polar nature of water. The activity begins with several questions that require students to retrieve and review this prior knowledge.15,16 A quick survey of students’ answers and drawings allows the instructor to assess adequate mastery of prior concepts, which groups may need additional assistance, and which student misconceptions are most prevalent prior to the activity so that she or he can look closely at these after the activity to see if they still remain or have been largely remediated. Students then work with the magnetic models to answer a series of questions designed to help them construct the understanding that when ionic compounds dissolve, the ions separate and that water interacts with and surrounds the ions through ion-dipole interactions. Students manipulate the models and draw pictures as well as write equations to record their observations. The model manipulations are separated into two parts (A and B). In Part A, students consider just monatomic ions, drawing the interactions between water molecules and monatomic anions and cations (Figure 2) and modeling the solvation process for various ionic compounds consisting of just monatomic ions (Figure 3). Seeing and feeling the interaction between the ions and water molecules can help students understand why (some) ionic compounds dissolveattractions between the partial charges on water
Figure 1. Reading clockwise from the top left: a simple salt such as NaCl; a cation solvated by water; a single polyatomic ion with one central atom and four peripheral atoms such as SO42− or PO43−; a single polyatomic ion with one central atom and three peripheral atoms such as NO3− or CO32−.
These models are purposefully simplified to focus on key difficult aspects of the target concept. Thusly, the Sticky Ions activity is not intended to be the sole means of teaching about aqueous solutions of ionic compounds but rather as one activity that addresses some common student difficulties with this topic. The limitations of the magnetic models used in Sticky Ions include (i) the relative size of the ions is not to scale; (ii) the ionic attraction can be broken while the covalent bonds in the water molecules and polyatomic ions cannot, thus suggesting incorrectly that ionic bonds are weaker than covalent bonds; (iii) the charges on the monatomic ions are given as + and − with no relative size (e.g., + is used for 1+ or 2+ ions); and (iv) in an actual solution there would be many more water molecules than ions. Additionally, for simplicity’s sake, the activity has students examine only a single formula unit of each ionic compound when modeling dissolving, rather than using a more accurate, but more unwieldy, modeled compound consisting of multiple formula units. Suggestions for addressing these limitations with students are provided in the Activity Details section as well as in the teacher guide, which is available in the online Supporting Information. It is also important to provide opportunities for discussion and negotiation of meaning when using models to aide students in constructing the desired knowledge.8 Such negotiation also models for students what communities of scientists do. In the Sticky Ions activity, students work in groups, using the models
Figure 2. Example of student drawing for modeled interaction between water molecules and individual monatomic anions and cations. 861
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in part B, how each polyatomic ion behaves as a discrete chemical species. The polyatomic ion models also help address the misconception that when dissolved the polyatomic ions split apart as the magnets are glued onto plexiglass; thus, the students observe that the polyatomic ion must stay as a unit. After completing parts A and B, students answer several analysis and discussion questions designed to reinforce the key ideas from the activity (e.g., polyatomic ions do not split up and water interacts with ions through ion-dipole attractive forces), help them extend their thinking to related ideas (e.g., why do not all ionic compounds dissolve in water), and address some of the limitations of the model. A class discussion of these questions following the activity allows the instructor to ensure that students recognized key attributes of the models and modeled interactions (e.g., that covalent bonds are much stronger than ion-dipole interactions as you can easily separate the water molecules from the ions but cannot break the polyatomic ions apart) as well as limitations (e.g., that ionic bonds are not really much weaker than covalent bonds). At this time, the instructor can also use multiple polyatomic and monatomic ions to construct a model of several formula units of a compound to reinforce that ionic compounds are really composed of arrays of ions as opposed to single neutral units. Finally, as a follow-up, students can make connections between their investigations of the solution process at the particulate level and macroscopic phenomena by examining the effect of ions in aqueous solution on conductivity. The final assessment questions provided can be used as a follow-up assignment to assess individual student comprehension. Complete student and teacher guides for this activity can be found in the online Supporting Information.
Figure 3. Example of student drawing and chemical equation for modeled dissociation of lithium and sulfide ions that occurs when it dissolves in water. For the sketch of this process, students were told to omit the waters to make it easier to observe the identity and number of ions present upon dissociation.
molecules and the charged ionsand better visualize those interactions, that is, which end of the water molecules will be attracted to which ions. Students start by modeling a compound formed from +1 and −1 ions and then move to compounds formed from +2 and −1 ions and +1 and −2 ions. Magnets with a “+” sign on them can represent any monatomic positively charged ion (+1, +2, or +3), and the same is true for the magnet representing the negative monatomic ion. This and the relative sizes of the ions are limitations of these models that should be explicitly discussed with students either at the beginning or end of the modeling activity. In Part B, students use the models and answer questions similar to Part A, but with compounds that include polyatomic ions (Figures 4 and 5). For simplicity purposes, in both Parts A
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EVALUATION The activity was used in three large sections (67−70 students) of a nonmajor college chemistry course. For two sections, each group of four students worked with a full set of magnetic models; for the third section, the students still completed the activity in groups, but for each step requiring models, the instructor demonstrated with the models projected on a screen. All students took a pre- and post-test to assess their understanding of what happens when ionic compounds dissolve in water and their ability to write symbolic representations (balanced ionic equations) and draw particulate level representations. Additionally, the final exam, given around three weeks later, included several matched questions. As shown in Table 1, students generally showed large gains from preactivity to postactivity and relatively good retention on the final exam. Interestingly, completing the activity with student manipulation of the models or viewing the instructor manipulate the models appeared to be equally effective,
Figure 4. Example student drawing of modeled interaction between water molecules and polyatomic anions.
Figure 5. Example of student drawing and chemical equation for dissolving of calcium phosphate. Similar to the dissolving illustrated in Figure 3, waters have been omitted here.
and B, students use just one formula unit to demonstrate the solution process. This helps students see which ions form and,
Table 1. Student Results for Preliminary, Post-Activity, and Final Exam Assessments % Correct Multiple Choice Question (Question 1)
% Drawing Waters Around Ions (Question 2)
Balanced Equation (Question 3)
Drawing Ionic Compound Dissolving (Question 4)
Section
Description
n
Pre
Post
Pre
Post
Final
Pre
Post
Final
Pre
Post
Final
F11 04 F11 02 W12 03
hands-on hands-on demo
47 45 44
32% 27% 25%
87% 100% 93%
40% 53% 50%
91% 87% 82%
91% 87% 86%
26% 9% 18%
83% 89% 82%
68% 64% 64%
30% 27% 27%
94% 100% 93%
81% 87% 86%
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Understanding of Chemical Change. J. Res. Sci. Teach. 2004, 41, 317− 337. (7) Bunce, D. M.; Gabel, D. Differential Effects on the Achievement of Males and Females of Teaching the Particulate Nature of Chemistry. J. Res. Sci. Teach. 2002, 39, 911−927. (8) Harrison, A. G.; Treagust, D. F. Modelling in Science Lessons: Are there Better Ways to Learn with Models? Sch. Sci. Math. 1998, 98 (8), 420−429. (9) Appling, J. R.; Peake, L. C. Instructional Technology and Molecular Visualization. J. Sci. Educ. Technol. 2004, 13, 361−365. (10) Kelly, R. M.; Phelps, A. J.; Sanger, M. J. The Effects of a Computer Animation on Students’ Conceptual Understanding of a Can-Crushing Demonstration at the Macroscopic, Microscopic, and Symbolic Levels. Chem. Educ. 2004, 9, 184−189. (11) Bivall, P.; Ainsworth, S.; Tibell, L. A. E. Do Haptic Representations Help Complex Molecular Learning? Sci. Educ. 2011, 95, 700−719. (12) Clark, D.; Jorde, D. Helping Students Revise Disruptive Experientially Supported Ideas about Thermodynamics: Computer Visualizations and Tactile Models. J. Res. Sci. Teach. 2004, 41 (1), 1− 23. (13) National Research Council. A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas; The National Academies Press: Washington, DC, 2012. (14) Davies, W. G. Magnetic Models of Ions and Water Molecules for Overhead Projection. J. Chem. Educ. 1991, 68, 245−246. (15) An alternative for students who do not have adequate experience with predicting the formulas of ionic compounds is to have students play Go Fish for an Ion to generate a list of possible ionic compounds. Instructions, score sheets, and a complete set of cards including point values are available for purchase from Flinn Scientific. (16) The Sticky Ions activity and other inquiry-based activities designed by instructors in the TI Program at GVSU may be accessed at http://www.gvsu.edu/targetinquiry/tidocuments-home.htm (accessed Apr 2014). A password is required to obtain materials, but registration is free. The site provides free instructor and student guides, facilitation notes, student misconceptions addressed by each laboratory, and help with setup and assessment questions.
indicating that this activity can be used for large classes where making a large number of models would be prohibitive. Examples of the assessment questions used as well as a more detailed discussion of the results can be found in the online Supporting Information.
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CONCLUSIONS Dissolving of ionic compounds is a difficult concept for students to grasp. Using the Sticky Ions activity to allow students to explore dissolving of ionic compounds at the particulate level using magnetic models appears to help them better understand this process with respect to the separation of ions in solution, polyatomic ions as discrete chemical units, and the interaction of water molecules with ions in solution. Students that participated in the activity, both using the models themselves and observing them as a demonstration, made substantial gains in their understanding of how ionic compounds dissolve.
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ASSOCIATED CONTENT
S Supporting Information *
Student and teacher guides for the activity; a detailed discussion of the pre- and postassessment results, including sample questions. This material is available via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This activity was designed as part of the Target Inquiry (TI) Program at Grand Valley State University (GVSU). TI is funded by the National Science Foundation (DRL-0553215), the Camille and Henry Dreyfus Foundation, and GVSU. Any opinions, findings, conclusions, or recommendations expressed in these materials are those of the authors and do not necessarily reflect the views of the National Science Foundation. Thanks to TI instructors and teachers for piloting this activity and providing feedback and to CHM 102 students at GVSU.
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REFERENCES
(1) Abraham, M. R.; Williamson, V. M.; Westbrook, S. L. A CrossAge Study of the Understanding of Five Chemistry Concepts. J. Res. Sci. Teach. 1994, 31, 147−165. (2) Devetak, I.; Vogrinc, J.; Glažar, S. A. Assessing 16-Year-Old Students’ Understanding of Aqueous Solution at Submicroscopic Level. Res. Sci. Educ. 2009, 39, 157−179. (3) Ebenezer, J. V.; Erickson, G. L. Chemistry Students’ Conceptions of Solubility: A Phenomenography. Sci. Educ. 1996, 80, 181−201. (4) Naah, B. M., & Sanger, M. J. Student Misconceptions in Writing Balanced Equations for Dissolving Ionic Compounds in Water. Chem. Educ. Res. Prac. 2012; retrieved from http://dx.doi.org/10.1039/ C2RP00015F (accessed Apr 2014). (5) Smith, K. J.; Metz, P. A. Evaluating Student Understanding of Solution Chemistry through Microscopic Representations. J. Chem. Educ. 1996, 73, 233−235. (6) Ardac, D.; Akaygun, S. Effectiveness of Multimedia-Based Instruction that Emphasizes Molecular Representations on Students’ 863
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