Dissolving Salts in Water: Students' Particulate Explanations of

Article Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Dissolving Salts in Water: Students’ Particulate Explanations of Temperature Changes Timothy N. Abell and Stacey Lowery Bretz* Department of Chemistry and Biochemistry, Miami University, Oxford, Ohio 45056, United States ABSTRACT: This study investigates how students account for a macroscopic temperature change during the dissolution of ionic salts through particulate level explanations. Semi-structured interviews were conducted with general chemistry, physical chemistry, and biophysical chemistry students. During the interviews, students conducted handson tasks that included the touching of beakers containing exothermic or endothermic dissolution processes. Data analysis resulted in categorizing students into groups based on their ideas about bond breaking, bond making, and energy changes. Students’ particulate understandings of the dissolving process did not appear to impact their explanations of the energy changes they observed. Only two students (one from general chemistry and one from biophysical chemistry) correctly described both the dissolving process and the macroscopic energy changes. No students invoked the concepts of potential energy, lattice energy, or enthalpy of hydration to explain their observations. KEYWORDS: First-Year Undergraduate, General, Chemical Education Research, Misconceptions, Discrepant Events, Aqueous Solution Chemistry, Ionic Bonding, Solutions, Solvents, Thermodynamics, Water, Water Chemistry FEATURE: Chemical Education Research

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INTRODUCTION Many misconceptions have been characterized in chemistry, especially related to dissolution.1−12 Several studies have found that students believe a reaction occurs between the solute and solvent.2−4,8,12 Smith and Nakhleh found that students think chemical bonds form between dissolved ions and water, but it is unclear if students think of this as a new compound being formed.4 Ebenezer and Erickson reported that students believe new compounds are formed through a chemical reaction when either sugar or sodium chloride is dissolved in water.8 Naah and Sanger found similar results and speculated that their interview protocol, which included a demonstration that neither solid lithium chloride nor liquid water alone conduct electricity but the combination of the two does, may have convinced students that a new compound was formed.2 Two other common misconceptions include the idea that ionic compounds dissolve to form neutral atoms or molecular pairs and ignoring the interactions between water and the ions, i.e., hydration spheres. Kelly and Jones found that none of the general chemistry students in their study included hydration spheres in their particulate representations of solutions and half of the students drew aqueous sodium chloride as molecular pairs.6 Naah and Sanger found that when writing balanced equations for the dissolution of ionic solids in water, some first semester general chemistry students believe that the compound breaks apart into neutral atoms in solution.2 While students’ ideas about dissolving ionic and molecular solids in water has often been investigated, only a few of these © XXXX American Chemical Society and Division of Chemical Education, Inc.

studies examined students’ thinking about connections between the macroscopic domain and particulate representations.4,9,11 In a study conducted by Teichert et al., students made predictions about the conductivity of various solutions and explained their predictions by generating particulate representations.9 After carrying out the conductivity tests, students were allowed to make changes to their particulate representations based on their observations. Smith and Nakhleh asked students to make predictions about and provide particulate level explanations for what they believed would happen when various solutes were placed in either water or cooking oil.4 The students then observed the demonstrations and explained any differences between their predictions and their observations. Ebenezer and Fraser, in their interviews with chemical engineering students, not only used visual macroscopic observations but also asked the students to make tactile observations.11 Students held beakers as salts dissolved, allowing them to feel whether the beaker grew hot or cold. The students then explained why the temperature change occurred during the dissolution process. Ebenezer’s and Fraser’s analysis focused on where the energy was transferred from or to during the process, but they did not examine the students’ ideas about how the dissolution process occurs. While many students were inconsistent with their explanations of the energy changes, the authors attributed this finding to the Received: November 6, 2017 Revised: January 30, 2018

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DOI: 10.1021/acs.jchemed.7b00845 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Article

domains provide the rudiments of a language with which chemists communicate. Even though some have argued that the three domains ought not be considered distinct from one another because the symbolic domain cannot be separated from the macroscopic and particulate domains of knowledge,21 experts can nonetheless easily transition among these three domains but novices struggle to make these connections.22 The research protocol described below was intentionally designed to purposefully investigate students’ connections among these three domains in the context of energy changes during solution formation.

fact that the students were interviewed prior to instruction on the topic. In addition to misconceptions about solution formation, students also have fundamentally incorrect ideas related to chemical bonding and bond energy as multiple studies have reported that students believe bonds store energy and that energy is released when bonds are broken.11,13−15 Interestingly, two of these studies found that students think bond breaking can be either an exothermic or an endothermic process.13,15 Boo reported that some students view bond breaking as a two-step process. First, energy must be put in to begin breaking the bond and then energy is released once the bond is broken.13 Boo also found that students think an input of energy is required to create, or “build,” new bonds. The research reported below sits at the nexus of these previously reported studies regarding students’ ideas about dissolution and their thinking about chemical bonds and energy. The purpose of this study was to investigate how students generated particulate level explanations in order to explain macroscopic energy changes from their tactile observations during the dissolution process. We also wanted to characterize students’ thinking regarding the dissolution process and what impact these ideas have on students’ explanations of the temperature change.

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RESEARCH QUESTIONS Two research questions were the focus of this inquiry: 1. What particulate explanations do students generate regarding the macroscopic manifestation of energy during dissolution? 2. What do students think happens when an ionic salt is put into water? 3. How do students’ ideas about the dissolution process affect their explanations of temperature changes during solution formation?

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PARTICIPANTS The students in this study were enrolled in one of three different courses at a university in southwest Ohio. Second-semester general chemistry (n = 19 GC) students were sampled from multiple lecture sections with different faculty and were interviewed after instruction and course assessment about the dissolution process. The textbook used in this course addressed both the enthalpy and entropy changes that occur during solution formation and included particulate representations of ion− dipole interactions.21 The GC students also completed laboratory experiments in which they calculated the change in enthalpy during dissolution of ionic compounds and explored the colligative property of boiling point based on the number of dissolved solute particles. Students from two upper-division courses, physical chemistry (n = 7 PC) and biophysical chemistry (n = 6 BPC), were also interviewed. These courses focus on the thermodynamics of gas phase and biological systems, respectively. All the GC students were science (nonchemistry) majors, and the PC and BPC students were all junior or senior chemistry, biochemistry, or chemical engineering majors. The students were 18 females, 14 males, and mostly Caucasian/white (n = 22) with 10 students who were African American/black, Asian, or Hispanic. This sample was representative of both the university and the courses in which these students were enrolled. All 32 students were informed of their rights as human subjects and consented to participate in the interview. Each student was compensated with a nominal gift card.

THEORETICAL FRAMEWORKS

Novak’s Theory of Meaningful Learning

Novak’s theory of Meaningful Learning was used to guide the development of this research. This theory states that in order for students to learn meaningfully, they must have relevant prior knowledge, the new material must be meaningful and relatable to existing prior knowledge, and the students must choose to incorporate the meaningful material into their existing knowledge.16,17 If all three of these conditions are not met, then the new material might be incorporated with improper connections or memorized without any connections to prior knowledge. These missing or incorrect connections that occur when students construct their knowledge are what lead to misconceptions.18 This research was designed to explore the nature of the connections that students make between bonding and energy changes in the context of explaining why solution formation is exothermic in some cases and endothermic in other instances. Johnstone’s Triangle

In order to investigate connections between students’ thinking about their tactile experiences and their mental particulate models of matter, we designed an interview protocol informed by what are colloquially known as “Johnstone’s Domains.” Alex Johnstone has argued that one of the challenges with learning chemistry is that knowledge exists across three representational domains: the symbolic, particulate, and macroscopic domains.19,20 These three

Figure 1. Phases of the interview showing the order in which the tasks were presented to students. B



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METHODS

Semi-structured interviews were conducted by the first author in both fall 2016 and spring 2017. The interviews were both audio- and video-recorded. The participants were supplied with a LiveScribe pen and paper so that any student-generated representations could be collected.22 The interview consisted of five phases, and each interview lasted approximately 1 h. Phase 1 of the interview explored the prior knowledge of the students. The students were asked to explain what they knew about dissolving a compound to make a solution, how and why precipitates form, what it means for a reaction or process to be spontaneous, and what they knew about enthalpy and entropy. As with any semi-structured interview, students were asked probing questions to follow up on their responses. In phases 2−5 of the interview (Figure 1), students were asked to make macroscopic observations during several repeated cycles of Predict−Observe−Explain tasks.23 Students were asked to predict what they thought would occur before performing the task, and then, after observing the task, they were asked to explain what they had observed. In phases 2, 3, and 4 of the interview, students were given solid ionic salts and vials of water. During phase 2, students observed the exothermic dissolution of magnesium chloride and felt with their hands the release of energy as the solution formed. In phase 3, students observed that silver chloride was insoluble in water. The third salt provided to the students was silver nitrate in phase 4. As with phase 2, the students observed the dissolution of the solid, but this time felt the vial grow cold because the solution formation was endothermic. In phase 5 of the interview, the students were asked to combine the two aqueous solutions that they had created in phases 2 and 4 so they would react to form a precipitate, i.e., silver chloride. This manuscript reports findings from phases 2 and 4 of the interviews, that is, the parts of the interview where students were asked to explain why one solid dissolved and they felt energy being released yet the other solid dissolved and they felt the solution grow colder. In both of these phases, students were asked to explain the roles of enthalpy and entropy in what they had observed. The interview protocol included questions about the energy changes involved with bond breaking, bond formation, and the formation of solvent−solute interactions. Each interview was transcribed verbatim, and the transcripts were annotated with any drawings or representations generated by students during the interviews. The transcripts were managed using a qualitative software program, NVivo 11.24 The data were both inductively and deductively coded for students’ ideas about the dissolving process and the energy changes associated with it. The codes were then grouped into categories through constant comparative analysis.25

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RESULTS AND DISCUSSION

The transcripts were analyzed to understand how students explained, at a particulate level, the macroscopic temperature changes they observed during the dissolution of first magnesium chloride (exothermic) and then silver nitrate (endothermic). From this analysis, four distinct patterns in the students’ explanations were identified (Table 1). The first group consists of students (n = 9) who thought that breaking bonds within the solute and within the solvent were exothermic processes, while making bonds and forming interactions between the water and solute were endothermic processes. The second group of students (n = 8) correctly described bond breaking as requiring an input of energy and that bond making resulted in the release of energy. Students in group 3 (n = 9) explained that bond breaking can be either an exothermic process or an endothermic process, and these students did not express any ideas related to bond making. The final group of students (n = 6) described idiosyncratic ideas to explain the temperature changes that did not fit well into any of the three other groups. Within the first three groups, multiple themes in students’ thinking were detected. Group 1. Bond Breaking Is Exothermic; Bond Making Is Endothermic

Nine students (n = 5 GC, n = 4 PC/BPC) described bond breaking as exothermic and bond making as endothermic in order to explain both of the temperature changes that they observed. Ally, a General Chemistry student, explained why the dissolution of magnesium chloride was exothermic: “To me I wanna say that breaking the bond releases heat and forming the bond like absorbs heat. Umm and overall this has a stronger release of heat when you’re breaking the bonds.” Ally’s mention of a “stronger release of heat” was prompted when her hands held the beaker that became warmer as the solute dissolved. Despite the prevalence of this previously reported misconception, students in this group held differing ideas about which bonds were broken and formed during the process of dissolving. Analysis of this group’s ideas resulted in two common themes. Some students attributed the dissolving of the solid to the breaking of intermolecular forces between water and the ions, while other students thought a reaction occurred that caused the covalent bonds in water to break. Theme 1a. Water Causes Dissolving. Even though these students (n = 3 GC, n = 3 PC/BPC) did not understand the energy changes involved in bond breaking and bond formation, they did describe that there are partial charges on water and full charges on the ions. Most of these students understood that these charges resulted in interactions between water molecules

Table 1. Distribution of Students’ Particulate-Level Explanations of Observed Temperature Changes during Dissolution Group 1

2

3

4

Explanations Bond breaking is exothermic, bond making is endothermic 1a. Water causes dissolving 1b. Salt reacts with water Bond breaking is endothermic; bond making is exothermic 2a. Water causes dissolving 2b. Salt reacts with water 2c. Water plays an unknown role Bond breaking is exothermic or endothermic 3a. Bond breaking is endothermic or exothermic 3b. Bond breaking is endothermic then exothermic Idiosyncratic C

GC (n)

PC/BPC (n)

Total (n)

5 3 2 6 2 3 1 4 2 2 4

4 3 1 2 1 0 1 5 3 2 2

9 6 3 8 3 3 2 9 5 4 6 DOI: 10.1021/acs.jchemed.7b00845 J. Chem. Educ. XXXX, XXX, XXX−XXX


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A: “Because I’m just balancing charges.” Ally, and all other students in this theme, believed that the salts reacted with water to form an acid and either a metal oxide or a metal hydroxide. These students treated the dissolving process as if it were a double displacement reaction (Box 1).

and ions in the solution, as drawn by Oliver (PC) in Figure 2 when he explained the dissolution of magnesium chloride:

Box 1. The two double displacement reactions that students typically wrote to explain what happened when magnesium chloride was added to water MgCl2(s) + H 2O(l) → 2HCl(aq) + MgO(aq)

MgCl2(s) + 2H 2O(l) → 2HCl(aq) + Mg(OH)2 (aq)

Figure 2. Oliver shows the partial charges of water attracting the oppositely charged ions. Closed circles = magnesium ions, opens circles = chloride ions.

While Ally concluded that a reaction occurred by writing a balanced equation, other students, like Clare (PC), argued that the macroscopic observation was evidence of a reaction: “Well because there’s heat I think it did some sort of reaction but I don’t know why it would want to do this. The heat indicates a chemical reaction. Like dissolving something shouldn’t create heat. So, yea I’d say it did [a] reaction...the chloride ion can attack the hydrogen and pull it away. So then you end up with OH, HCl, and then that [OH] can attack the positive charge of the Mg.” Ally and Clare each came to the conclusion that a reaction, namely the breaking of covalent bonds in water, had occurred by different means, but their explanations of the temperature change were the same. Like the rest of the students in this group, they stated that breaking the bonds of the reactants released more energy than was required to form the bonds of the products.

“So these umm the oxygen is going to umm is going to bond better with the magnesium because it’s got a positive charge and the oxygen is electronegative, carrying that partial negative charge. And hydrogens, which carry a partial positive charge, are going to interact with these chlorines.” Other students also understood that there are interactions between water and the ions that cause the solid to dissolve, but their descriptions and drawings of the resulting solution differed from the accepted view as explained by Stephanie (GC): S: “The water molecules are bonding on to the [ionic solid] and breaking it apart, and breaking the bond between them.” I: [after examining her drawing and noting the absence of interactions between the ions and the water molecules] “So I guess do you have an idea why the water stops interacting with it once it pulls it apart into ions?” S: “No...sorry” In her explanation Stephanie is aware that water was needed to break apart the ionic solid, but in her drawing of the solution (Figure 3), there were no interactions occurring between the

Group 2. Bond Breaking Is Endothermic, Bond Making Is Exothermic

Only 8 of the 32 students (n = 6 GC, 2 PC/BPC) interviewed were able to explain the observed temperature changes by using the correct ideas that bond breaking is endothermic and bond making is exothermic. This concept was stated clearly by Sara (GC) as she explained the endothermic dissolving process: “Because energy, because to break those bonds in the first place it needed energy but like when it, when they all attracted to form a lot [of energy] was released because the attractions are strong.” However, like the first group, these students held differing ideas when describing the dissolving process. Three themes were found within this group of students based on their explanations of the dissolving process. Theme 2a. Water Causes Dissolving. Three students (n = 2 GC, 1 PC/BPC) said that interactions between water and the ions caused the dissolution process to occur. However, only 2 of the 3 students were able to properly explain both the dissolving process with hydration spheres and the energy changes associated with it. One student incorrectly described the dissolving process as the interactions between water and the ions stopped once the ions were separated in solution. Sara (GC) explained this process that she represented in her drawing (Figure 4): “Umm yea they’re like, this is like the picture of them being attracted and then they become ions after. So this is like after the attraction with water. These are after attractions with water but I don’t really understand like where the waters go.” Theme 2b. Salt Reacts with Water. Three of the students (n = 3 GC) in this group thought that when the ionic salts were placed into water that the compounds reacted as shown in Box 1. These students used the idea that ionic and covalent bonds were broken and new bonds were formed to explain their macroscopic

Figure 3. Final solution included a mixture of ions and water without any interactions between them. Note that water was included in the drawing even though it is not in the student’s key.

water and the ions. She was unable to provide a reason for why this interaction did not exist within the solution. Theme 1b. Salt Reacts with Water. Other students within this group (n = 2 GC, 1 PC/BPC) had very different ideas about what bonds were broken and formed when explaining the temperature changes they observed. Ally (GC) explained what she thought occurred when magnesium chloride was put into water: A: “MgCl (sic) plus H2O...I would think that this would...could I do...hydrochloric acid. Magnesium oxide, something like that. Those are your products.” I: “Umm so why do you think those will be your products?” D



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the dissolving process, they did differ regarding the idea that bond breaking could be exothermic and/or endothermic. Theme 3a. Bond Breaking Is Endothermic or Exothermic. These students (n = 2 GC, 3 PC/BPC) offered inconsistent explanations throughout their interviews as they explained their macroscopic observations. Consider Luna’s (PC) explanation of the exothermic dissolution of magnesium chloride: “Uhh the bonds broke and those bonds stored energy. So when the bonds break the energy goes out as thermal energy.” She reasoned that the solution felt warm as it formed because the bonds store energy and this energy was released when bonds were broken. When Luna dissolved silver nitrate in water later during the interview, she felt the beaker grow cold. Clearly, as her earlier explanation could not be used to explain the endothermic process, she was forced to adapt her explanation: I: “Why would it feel cold?” L: “Because it’s absorbing energy in the form of heat...to break the bonds. I think it has to do with the specific enthalpy of formation and whether it is positive or negative. In this case it takes energy to break the silver nitrate bonds. So it takes not energy [sic] to make them.” Luna did not think that the bond between silver and nitrate had energy stored in it, as she did with the magnesium chloride. Rather, she talked about how some bonds required an input of energy in order to break and that this was related to the enthalpy of formation for the compound. Students like Luna offered inconsistent explanations of the observed temperature changes because they failed to consider the enthalpy change associated with bond formation. Theme 3b. Bond Breaking Is Endothermic and Then Exothermic. Other students in this group (n = 2 GC, 2 PC/ BPC) offered consistent explanations of the observed temperature changes, but they treated bond breaking as a two-step process, in which each step had a different change in energy. Consider Tony’s (GC) description of the endothermic process: “Uhh in this case it would be taking in energy to umm I guess bust apart the bonds...the energy held within the bond umm is lower than the energy needed to break the bond. I don’t know why one bond would need more strength to break then it has within it or vice versa.” Students like Tony thought that in order for a bond to break, an input of energy is needed to start the process, which some students called the activation energy. Then, as the bond breaks, it releases the energy stored within it. Students offered consistent explanations with this reasoning by weighting one of the two steps more heavily to match the observed temperature change.

Figure 4. Top left corner: the attraction between the water and ions during the dissolving process. The final solution, shown at the bottom of the beaker, no longer has interactions between the water and ions.

observations. Ami (GC) described how the second reaction between water and the ionic solid occurred as written in Box 1: “Umm...so the MgCl (sic) had to be strong enough to break the hydrogen bonds in H2O. And so they had to have a high enough charge, a negative, a high enough negative charge to take that hydrogen, the positive hydrogen from H2O. Umm but I know that umm hydroxide is negative and so this [Mg] would have to be positive. And so it had enough of a positive charge to attract the OH.” All students in this theme held the idea that the ions formed attractions with water that were strong enough to break the covalent bond(s) between the oxygen atom and the hydrogen atom(s) in water, which some of them erroneously referred to as the breaking of “hydrogen bonds”. These attractions then lead to the formation of two new compounds that made up the solution, i.e., the products in Box 1. Theme 2c. Water Plays an Unknown Role. Two students (n = 1 GC, 1 PC/BPC) in this group could not explain the role of water during the dissolving process, although they knew that water must play some important role. Daisy’s (BPC) lack of knowledge that interactions must be created between the water and ions hindered her ability to explain the temperature change that she felt during the dissolution of magnesium chloride: “...you have to put energy in to break the bonds. Umm...hmm well I’m not sure why it should be exothermic but if it is that’s great [laughs]. In some way I think [water is involved]. Because they want, I mean they [the solid] didn’t just break sitting on their own when I had them in the plastic umm container but...so I’m sure water is involved in some way I just don’t know which way, yet [laughs].” Even though both students in this group had the correct understanding of the energy associated with bond breaking and making, their lack of knowledge about the formation of attractions involving water to create solvated ions during the dissolving process prevented them from explaining an exothermic solution formation.

Group 4. Idiosyncratic

The final group of students (n = 4 GC, 2 PC/BPC) offered ideas that did not fit into any of the larger groups described above. Some of these students were unable to provide details about what caused the temperature change they observed. This can be seen in an excerpt from Penny’s (GC) interview: P: “There was like heat involved that was formed to help it dissolve very quickly.” I: “So where did this heat come from?” P: “Umm it’s from the characteristics of all the elements when they were being mixed.” I: “So why would them being mixed, you know, release this heat?” P: “Umm...I don’t know.” Other students offered consistent explanations, but they made no mention of ideas related to the energy changes that occur

Group 3. Bond Breaking Is Exothermic and/or Endothermic

The students (n = 4 GC, 5 PC/BPC) in this third group understood that water causes the dissolution of an ionic salt. However, none of these students associated an energy change with the creation of interactions and therefore these were not mentioned in the students’ explanations of the temperature change they felt. Instead, these students focused only on the bonds being broken. These students thought that bond breaking could be either an exothermic process or an endothermic process depending on what species were involved. Two distinct themes were identified. While these two themes did not differ based on their ideas about E



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influence their ideas about bond energy or vice versa. For example, some students who interpreted the temperature change to be evidence of a chemical reaction between water and the salt were in group 1 while others were in group 2. These analyses highlight the difficulties that students had with making correct connections between observations in the macroscopic domain and representations in the symbolic and particulate domains. Even though every student consistently described the dissolving process when forming both solutions, only 23 students offered consistent explanations of the temperature changes throughout the interview. Our findings contradict those of Ebenezer and Fraser, who found that most students in their sample were inconsistent when explaining both exothermic and endothermic processes.11 It is possible that the students in the Ebenezer and Fraser study were more inconsistent because they were interviewed prior to instruction on the topic, whereas our students were interviewed post-instruction and post-assessment.

when bonds are broken or formed. Gavin (PC), for example, explained that the temperature change was related to the movement of the ions: “So before they were sort of locked into a rigid state but when they dissolve they’re more free to move around...causing essentially them to move, since that’s what temperature is related to.”

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CONCLUSIONS This study identified three different categories that capture how students explained observed macroscopic temperature changes (one exothermic, one endothermic) at a particulate level during the dissolving of an ionic compound. Some of these students offered the well-documented misconception that bond breaking is exothermic and bond making is endothermic.11,13−15 Other students concluded that bond breaking can be either exothermic or endothermic or can be first endothermic and then exothermic. Of the 32 students interviewed, only 8 students correctly described the energy changes associated with bond breaking and making. Eight students did not associate energy with bond making at all. Nearly half of the students (n = 15) were able to properly explain the dissolution process in terms of bonds breaking and attractions forming. However, only 2 students (n = 1 GC, = 1 PC/BPC) in the entire sample were able to correctly describe both the dissolving process and the macroscopic energy changes they observed by touching beakers during the dissolution of ionic salts. No students used the concept of potential energy to explain why bond breaking and formation was exothermic or endothermic. Interestingly, none of the students used terms such as lattice energy or enthalpy of hydration to describe the steps that occur during dissolving, despite the textbook using these concepts. Several misconceptions about the dissolving process were identified. A few students discussed the interactions that occurred between the water and ions, but they held the idea that these interactions stopped once the ions were no longer a visible solid. Other students believed that a chemical reaction took place and reasoned this by writing balanced equations that showed the formation of an acid and either a metal oxide or a metal hydroxide. Some students could not explain how the salt broke into ions once it was put into the water; they knew that water played an important role because the solid did not break apart before it was placed in the water, but they did not know what the role of water was. Not one student in the entire sample discussed that hydrogen bonds between the solvent water molecules must be broken as part of the dissolution process. The only instances of students mentioning the breaking of bonds within the solvent were the breaking of covalent bonds within water molecules. While reactions between ionic salts and water have been reported before,2−4,8 what makes our findings novel is that many students who felt a temperature change with their own hands were convinced that a chemical reaction must have occurred which they attributed to the breaking of covalent bonds within water (to be clear we are not focused on chemical versus physical changes, but the dissociation of salts causing the breaking of covalent bonds in water). Often students are taught a list of macroscopic indicators for a chemical reaction, one of these being a temperature change. Students seemed to be using this rule to explain their macroscopic observations and then crafted a symbolic and/or particulate representation to correspond. Despite students holding very similar ideas about the dissolving process, their ideas about dissolving did not seem to

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IMPLICATIONS FOR RESEARCH AND TEACHING This research has implications for both the chemistry classroom and future chemistry education research studies. Our findings show that students could readily make macroscopic observations but that they struggled to explain what caused these changes and to represent those changes in either symbolic or particulate form. Future research studies should further explore the nature of student thinking about connections between the macroscopic domain and the particulate domain. A few laboratory experiments have been reported that allow students to make connections to the macroscopic domain by using senses other than sight, such as by making direct observations through smell and through touch as a part of the data collection process.26−28 Such experiments demonstrate the usefulness of being able to connect sensory observations with the particulate. While much chemistry education research to date has explored connections between the symbolic domain and the particulate domain, the connections to the particulate domain and sensory observations made directly by students remain relatively unexplored by comparison. In the classroom, teachers should provide opportunities for their students to make these sensory observations and require students to craft particulate explanations that are consistent with these macroscopic observations. Cooper and Klymkowsky argue that the lack of explicit connections made between macroscopic and particulate level bond energy changes in the classroom may be contributing to some of the misconceptions reported above.29 Some curricula have been designed to help students make these connections by constructing knowledge,30 but future research is needed on instructional design to address the findings of this research. Several experiments and demonstrations related to the dissolution of ionic salts and temperature changes have been published in this Journal.31−38 Although these demonstrations may afford students the opportunity to develop their understanding of how changes they observe with their senses are due to changes at the particulate level of molecules and ions, we suggest that instructors who use these or similar experiments carefully examine their assessments in light of our findings. Taber has suggested that the symbolic domain cannot be separated from the macroscopic and particulate domains as it is always used to represent one of those two domains.39 While this may be true for experts, our findings suggest that novices, like Ally for example, may use the symbolic domain devoid of connections to the macroscopic or particulate domains. These students appear to be manipulating the symbols without consideration of what they represent. Future research should investigate novices’ F



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understandings of the role of symbolic representations and how they relate to the particulate and macroscopic domains.

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LIMITATIONS This research reported herein certainly has some limitations. This paper examines students’ ideas about only the enthalpy changes involved in dissolution processes, but the authors certainly recognize that the concepts of entropy and Gibb’s free energy are essential in providing complete explanations of dissolution. Although data regarding students’ invocation of these concepts (or the absence thereof) are not reported in this paper, these data were collected during the interviews. Students’ ideas related to entropy and Gibb’s free energy changes will be presented in a future paper.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Stacey Lowery Bretz: 0000-0001-5503-8987 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based in part upon work supported by the National Science Foundation under Award number 1432466. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. We thank the students who volunteered to participate in this study.

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REFERENCES

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