Article pubs.acs.org/jchemeduc
Roles of Terminology, Experience, and Energy Concepts in Student Conceptions of Freezing and Boiling Paul G. Jasien* Department of Chemistry and Biochemistry, California State University San Marcos, San Marcos, California 92096, United States ABSTRACT: A discussion of some student conceptions of the solid−liquid and liquid−vapor phase transitions is presented. Data from open-ended, short-answer questions were collected from first-semester general chemistry students and then discussed in the context of previous studies. The responses gave insight into the various student conceptions about these phase changes. Student misunderstandings were most often related to (i) contextual difficulties with the terms f reezing and boiling associated with their lack of experience with different substances, (ii) misinterpretation of physical and chemical phenomena, either from everyday experience or classroom demonstrations, and (iii) the inability to understand energy transfer processes. Specific implications for the teaching of topics related to f reezing and boiling are also addressed. KEYWORDS: First-Year Undergraduate/General, Misconceptions/Discrepant Events, Phases/Phase Transitions/Diagrams, Professional Development
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matter. de Vos and Verdonk9 concluded that although students are able to answer direct questions about the structure of atoms, “many are unable to integrate this information with the picture they have of chemical and physical processes.” Bridle and Yezierski10 examined phase transitions directly and developed an inquiry-based instructional approach to address the lack of microscopic understanding of these phenomena. The conclusion of their study was that such an instructional approach leads to a better understanding of phase changes. Kikas11 presented another aspect of this problem when she examined teachers’ potential misconceptions of phase transitions as they describe what happens during freezing. In this report, one prominent misconception was that the sizes of atoms change during a phase transition. This is one of the many particulate-based difficulties that Ö zmen12 addressed in his work using computer animation to teach phase transitions to sixth grade students. He concluded that this type of intervention was effective at improving student understanding in both the short and long-term. Not all conceptual problems are related to a particulate description of matter. Osborne and Cosgrove13 reported on an investigation in which they asked children from 8 to 17 years old to describe what was happening during commonly observed physical phenomena. One of their findings demonstrated the inability of students to correctly identify the composition of the bubbles in a boiling liquid. These same results were later seen for chemistry graduate students, as reported by Bodner.14 A review of alternate conceptions by Kind15 noted two particular issues associated with phase change phenomena. She pointed out that novices in chemistry have limited experience with a variety of different substances and that the nature of matter before and after the phase transitions was particularly problematic for students. Corroborating evidence for the effect of limited experience was noted by Costu and Ayas.16 In their study of student understanding of evaporation, they found that a number of individuals only associated this phase transition with water and not with other liquids. In addition, a number of
INTRODUCTION Instructors have begun to realize that even the simplest of chemically related processes discussed in the classroom can cause confusion in the minds of students. The reasons for any misunderstanding can be attributed to many different factors. One of these may be related to confusion with respect to terminology. Herron1 devoted an entire chapter in his book on the chemistry classroom to obstacles in learning caused by terminology. He discussed both cultural and contextual difficulties that students may have in determining word meaning and how it relates to a particular concept. The relationship of word use and understanding is also described in detail in a number of other reports2−5 that demonstrate this important connection for learners. In a series of papers,6−8 the current author examined student misconceptions that could be directly relatable to terminology, in which colloquial and scientific word meanings presented a cognitive conflict for the student. For example, the term strong used in the context of “strong electrolyte”, “strong acid”, or “strong cup of coffee” was studied.7 It was found that the colloquial and scientific meanings of these terms were often intermixed and an incorrect scientific interpretation was often associated with “powerful” or “concentrated”. This may be the reason for some of the commonly encountered errors made by students when interpreting the meaning of a strong acid. In the author’s previous work, the source of contextual understanding and lack of precision in student terminology was generally separable from other sources of learning difficulties. Herein, the phrase “contextual understanding” refers to the ability to correctly assign meaning to a multiple-meaning term from the situation in which it is used. Terminology is obviously not the sole reason for learning difficulties. As has been reported previously, problems related to the correct interpretation of physical phenomena are also important. Although the literature on this is immense, only selected articles relating to the major topic of phase transitions will be discussed here. A number of investigations have focused on the connection between student misconceptions of phase transitions and their understanding of the particulate nature of © 2013 American Chemical Society and Division of Chemical Education, Inc.
Published: November 12, 2013 1609
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second-year chemical engineering students, Gopal et al.29 also identified a number of misconceptions related to evaporation, condensation, vapor pressure, and temperature. They found that one prevalent idea was that a temperature gradient needed to be present for evaporation to occur. They also noted that presenting the student with physical evidence can help trigger conceptual change and a better understanding of the concepts investigated. Lastly, in the work of Hwang and Hwang,30 as summarized by Chang,27 it was reported that students at grade levels from middle school to university have difficulty understanding the balance of energy when a liquid is boiling and believe that the temperature would keep rising upon heating. Interestingly, the authors also indicated that students believed that evaporation was limited to water or water solutions, as was reported by Costu and Ayas.16 This latter observation is further substantiation that lack of experience with a variety of substances may contribute to student misunderstandings. This point will be discussed in detail later, as will the role that water seems to play in students’ interpretations of phase transitions. Obviously, there are many potential pitfalls for students as they try to describe phase transitions. However, when student difficulties with word context combine with the lack of experience with real-world phenomena and interpretation of the associated energy changes, a “perfect storm” of impediments to student learning may occur. Horton31 noted that when students learn chemistry concepts, they tend to gravitate toward “common-sense reasoning, everyday analogies, visible effects and changes, and common (non-scientific) word usage.” These may not provide the best outcome for learning scientific topics, particularly when an individual’s experience is quite limited. The current work will present general chemistry students’ conceptions of the solid−liquid and liquid−vapor phase transitions and will investigate to what extent students use experience or contextual understandings in their explanations. In addition, examples of students’ interpretations of the energy transfer process and how these relate to phase transitions will be presented and discussed.
students also assumed that a substance such as alcohol had the same freezing point as water. The issue of limited real-world experience with a variety of substances as noted by Kind15 is quite relevant here. This limitation can lead to potential problems interpreting what is occurring during a physical or chemical process and is one of the reasons that a hands-on laboratory experience has been viewed as essential.17,18 Because laboratory course time is limited, an expanded experience for students that shows real-life phenomena is often provided through the use of classroom demonstrations. Although there are many such methods, one successful pedagogical approach is the prediction−observation−explanation (POE) method as has been discussed by Champagne, et al.,19 and White and Gunstone.20 A central purpose of laboratories and demonstrations is to enhance student understanding by expanding their experience with actual phenomena. Recently Costu et al.21 reported on the application of a variant of the POE method in a study of condensation phenomena and found it to be quite effective in encouraging conceptual change in introductory chemistry students at the college level. Another hurdle to understanding phase transitions may be related to a student’s inability to distinguish between physical and chemical processes. Tsaparlis22 investigated this ability among high school and college students. He noticed that “[i]n general, errors are caused by the radical change in the form of the substance in the change in physical state.” This was particularly true when gases were involved. He also noted that although differences in the understanding of the older and younger students were seen, overall “achievement” in these groups was not much different. Adding to the overall difficulty in understanding the solid−liquid phase transition is the potential confusion between dissolution and melting. This was discussed by Goodwin23 in a detailed analysis of the nature of both of these processes and the potential for student misunderstandings. In studying students’ abilities to distinguish chemical and physical changes, Andersson24,25 had previously noted a cognitive process, which he referred to as modif ication. In modif ication, students see a new substance simply as a changed form of the previous substance, whether it is the change in appearance that occurs when grinding up “sugar candy” (physical) or the change in color that accompanies copper oxidation (chemical). Such an erroneous assumption could lead an individual to identify a chemical change as being physical in nature. Another concept often introduced when discussing phase transitions is the associated energy exchange that occurs. Bar and Travis26 studied the progression in understanding of boiling and evaporation of children in grades 1−9. Two findings for younger children that may have relevance for the current work include the following: (i) the processes of boiling and evaporation were not viewed as the same, and (ii) when describing the composition of bubbles, 40% of 13−14 year olds indicated that they contain heat. Work by Chang27 has investigated the conceptual knowledge of evaporation, condensation, and boiling among teachers in Taiwan. Examples of difficulties with understanding the role of heat and temperature were seen even in these individuals with significant chemistry training. A further area of difficulty related to energy exchange in phase transitions was discussed by Viennot.28 He noted that some learning difficulties may be related to the fact that the transfer of energy does not have to lead to an increase in temperature during a phase change and pointed out that one alternative belief is that the maximum temperature for a substance is the boiling point. In a series of interviews with
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DATA COLLECTION The information that framed the discussion that follows was collected from 117 students in two different semesters of a firstsemester general chemistry course at a midsized public university. Students responded to a series of short-answer questions specifically designed to probe their understanding of the terms f reezing and boiling. These are listed in Box 1. In one Box 1. Questions Used To Probe Students’ Understanding of Simple Phase Transitions 1. Is freezing an exothermic or endothermic process? Is ΔH positive or negative? 2. Is a substance that is undergoing the phase transition from a liquid to a solid (i.e., freezing) always cold? Explain and give an example. 3. Is boiling/evaporation an exothermic or endothermic process? Is ΔH positive or negative? 4. Is a substance that is undergoing the phase transition from a liquid to a gas (i.e., boiling/evaporation) always warm? Explain and give an example. class, the questions were given as a homework assignment, whereas in the other class, the answers were written during a 1610
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temperatures associated with these phase transitions. It should be noted that, by itself, this information does not provide insight into why students answered correctly or incorrectly, as students could answer correctly for the wrong reason. A discussion based on written student responses will be given later, but some general trends gleaned from the responses are given below. · 53% of the respondents correctly answered both Questions 1 and 3 dealing with the energy changes and sign of ΔH. · 22% of the respondents mentioned water when discussing f reezing. · 37% of the respondents mentioned water when discussing boiling. · In answering Question 2 on whether a freezing substance is cold, 21% of respondents erroneously interpreted crystallization, a chemical reaction, or other process as f reezing either based on a class demonstration or other familiar process. The first result indicates that most of the students who got Question 1 correct also answered Question 3 correctly. The latter three results are consistent with the constructivist view of learning, in which previous experience is important in understanding, and will be discussed in detail in the following sections.
class activity. All student responses were aggregated and corresponded to roughly two-thirds of all enrolled students turning in the work. Questions 1 and 3 probed what students recalled from class about energy changes and sign conventions. Questions 2 and 4 were worded to specifically cause a cognitive conflict between the colloquial and scientific meanings of the terms. The underlying hypothesis was that colloquial usage of these terms could interfere with scientific understanding. All responses were collected after the topics of energy changes in chemical and physical processes and phase changes were explicitly covered in class. Depending on the exact concept, this varied from a few days to weeks after the topics had been discussed. Phase transitions were discussed three times during the semester: first in the context of the microscopic and macroscopic properties of the phases of matter, then when discussing intermolecular forces, and finally during the general discussion of enthalpy changes. The collection of data adhered to the rules of the University Institutional Review Board for Research with Human Subjects as they pertain to small-scale classroom studies with minimal risk for the student. Questions 2 and 4 were developed based on the instructor’s previous work with contextual understanding but were not formally validated. In addition, the student responses were reviewed only by the author, so there was no consistency check on ratings. However, the very short nature of the answers made categorization of the responses fairly straightforward. The preliminary results presented here shed light on the interrelationships of terminology, experience, and energy changes in understanding f reezing and boiling that have not been previously discussed.
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DISCUSSION In reading the student explanations, it appeared as if student difficulties in answering Questions 2 and 4 fell into three basic categories: (i) contextualization and personal experience, (ii) misinterpretation of classroom demonstrations or other observed phenomena, and (iii) misinterpretation of energy transfer ideas. Each of these will be addressed separately.
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SUMMARY OF FINDINGS Given in Table 1 is a summary of the responses for Questions 1 and 3 related to the identification of the phase changes as being
Colloquial and Scientific Context and the Link to Experiences
Table 1. Distribution of Responses for Questions 1 and 3 Q1 response data answer
N (total = 117)
%
N (total = 117)
%
endothermic (+) endothermic (−) exothermic (+) exothermic (−) endothermic (no sign) exothermic (no sign) other
23 12 4 72a 1
19.7 10.3 3.4 61.5a 0.9
67a 3 11 24 6
57.3a 2.6 9.4 20.5 5.1
3 2
2.6 1.7
4 2
3.4 1.7
a
A student’s most common experience with phase transitions is the freezing and boiling of water. This is the origin of the colloquial meaning of the terms f reezing and boiling to mean “very cold” or “very warm” because these are the relative conditions for these phase transitions. A relevant question to ask here is: “Did this familiarity with the properties of water negatively influence students’ responses to Questions 2 and 4?” A total of 26 of 117 student answers (22%) to the f reezing question and 43 of the 117 answers (37%) to the boiling− evaporation question mentioned water. As can be seen from the data in Table 2, few of the students using water as an example were able to correctly answer Questions 2 or 4 on whether f reezing and boiling substances were always cold and warm, respectively. A simple analysis of these data using a χ2 test indicates that in both cases, there is a significant difference (p < 0.0001) in student responses when a student uses water as an example. This implies that falling back on the substance that they are most familiar with is correlated with drawing an incorrect conclusion about all substances. This point is intriguing, but the present data do not allow a determination of the relative importance of contextualization versus lack of experience. The importance of water as an example in students’ views of both phase transitions is consistent with previous work.16 (It should be noted that boiling and evaporation were grouped as a single process in Questions 3 and 4, but previous research26 has indicated that students may not see these processes as the same. However, not investigated here, an
Q3 response data
This is the correct answer.
endo- or exothermic, as well as the sign of ΔH. Table 2 gives data relevant to Questions 2 and 4 on whether the processes are always “cold” and “warm”, respectively, and whether water was specifically mentioned in the student’s response. The results in Table 1 indicate that about 60% of the students are able to identify the terminology and sign convention corresponding to the enthalpy change in the phase transitions. This relatively high percentage is not surprising based on the fact that this topic was directly discussed in class and simple recall could be used by the students. The more telling results from Questions 2 and 4 are given in Table 2. These questions required students to think critically about whether generalities can be made about the 1611
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Table 2. Distribution of Responses for Questions 2 and 4 Q2 response data
a
Q4 response data
answer
N (total = 117)
%
water mentioned
N (total = 117)
%
water mentioned
yes no sometimes/maybe no answer
48 58a 4 7
41.0 49.6a 3.4 6.0
22 2 0 2
58 43a 2 14
49.6 36.8a 1.7 12.0
36 3 0 4
This is the correct answer.
based on the misinterpretation of a precipitation of a solid from a solution. Almost all of these students referred to the sodium acetate demonstration and identified this as a f reezing process. This confusion between precipitation and freezing is consistent with the ideas presented by Goodwin.23 Two illustrative student comments are given below. No, a substance that is undergoing the phase change from a liquid to a solid is not always cold. An example of this would be the liquid heating packs that were shown to us. The packs go from a liquid to a solid but stay very warm. (Student #33)
interesting follow-up study would be to examine whether students’ understandings of boiling versus evaporation affect their answers to Questions 3 and 4). The vast majority of students who correctly answered “no” to Questions 2 and 4 invoked explanations indicating a greater range of experience with, or at least knowledge of, different substances. A total of 30 of the 43 students (70%) who stated that a boiling−evaporating substance does not have to be warm did so using examples of substances they knew about. The other students who answered “No” to this question had reasons that had little or no relevant explanation associated with their answers. The most common reasons cited by students who correctly answered this question centered on ideas such as: · Water and rubbing alcohol evaporate at room temperature · Dry ice becomes a gas when it is cold · Liquid nitrogen is cold when it boils · Common gases, such as oxygen, have already boiled at room temperature The common feature of all of these answers is the realization that there are many different substances with different boiling points. The results for Question 2 on freezing are more difficult to interpret, as many of the students answering correctly had flawed reasons based on energy exchange or misinterpretation of a demonstration. A number of students used examples of metals, lava, and so on freezing at higher temperatures that would not be considered “cold”. One student answer specifically illustrates this general line of reasoning: No, many things are solid at room temperature. For example, this paper, the desk, etc. So, assuming that cold means below room temperature, not all things that change from a liquid to a solid are cold. (Student #49) This student definitely was able to separate out the colloquial and scientific meaning of freezing because of the ability to recognize the large variety of substances that exist.
No, because in the lab we did a precipitate experiment and things turned solid along with other experiments we saw solids come out of mixtures and other things and there was no heat just reactions. (Student #6) The incorrect interpretation of the sodium acetate demonstration may seem perfectly consistent with what students have learned, since freezing is an exothermic process which releases heat. This explanation can be easily assimilated into the student’s current schema for f reezing without any cognitive conflict. A demonstration that was used to illustrate an endothermic process and proved problematic for some students was the reaction of solid Ba(OH)2·8H2O with a solid ammonium salt. This demonstration produces a slushy mixture as a result of a neutralization reaction and the release of the waters of hydration. Some students interpreted this chemical process as freezing because the product produced was very cold. This elicited the following statement from one student in which what was observed “verified” that f reezing substances must be cold. An example is when the other day in lecture Dr. [X] showed the video of the 2 liquids being combined to form a “slushy” type of solid compound. The temperature went to (I believe) −25 °C? (Student #32) Despite the class time spent discussing both demonstrations, the misinterpretation of what occurred exemplifies a point made by Roadruck32 when he emphasized the need to account for students’ previous experiences, lest further misconceptions result. As teachers of chemistry we have had many more experiences with, much more knowledge about, and considerably wider connections with the phenomenon being demonstrated. To us it is perfectly transparent. The demonstrations may be opaque, however, to the student whose structures are incomplete, whose experiences are limited and whose knowledge is uncoordinated.32 Attention-grabbing demonstrations, as well as everyday experiences, can have a profound effect on students and cause them to retain ideas for long periods of time. Below are two erroneous interpretations that refer to observations from previous experiences. The first is from a demonstration seen in another course and the second is from an everyday observation.
Misinterpretation of Demonstrations and Natural Phenomena
A number of student responses to Questions 2 and 4 were based on misinterpretation of a physical or chemical process done in the lab, shown as a demonstration, or observed outside of class. Once again, an important link to student experience exists, but in these cases the real-world process is incorrectly interpreted. One demonstration performed in class, which was referred to by a number of students, was the precipitation of sodium acetate in a supersaturated solution and the subsequent increase in temperature. This was shown both occurring in a flask and in a commercial sodium acetate “hand warmer”. Incomplete understanding of the concept of precipitation− solubility involved in this demonstration seems to increase misconceptions related to f reezing. A total of 19 students (16%) stated that a f reezing substance need not necessarily be cold 1612
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chemistry courses or seen in demonstrations, for example, the reaction of barium hydroxide octahydrate and ammonium nitrate, or the effect of liquid evaporation from the skin. Students who were most likely to give this answer really did not have a thorough explanation of this idea. In most cases they simply associate an endothermic process with a “cold” one. Endothermic, because heat is being absorbed to make substance a solid. Example liquid water turning into ice. (Student #93)
These further emphasize the powerful role that visual observations have on students’ understandings of chemical phenomena. No, a phase change from liquid to solid does not have to be cold. I remember this time in a science class the teacher had liquid hydrogen or nitrogen, I do not remember, and when the teacher poured some on the counter it formed little solid balls. In this case it was the process of it warming up that turned into a solid. (Student #91)
Yes, because when a process is endothermic the process will be cold. This will allow a liquid to go to a solid. (Student #94) Freezing Is Exothermic and Therefore the Substance Will Be Warm. This identifies the f reezing process correctly as being exothermic, but erroneously assumes the energy transfer occurs to the substance, not that the energy is dissipated in the surroundings. Therefore it follows that the energy release will warm the substance. Once again, from the student perspective, this seems to be perfectly consistent with what has been taught. It should also be noted that this particular explanation is at odds with the colloquial use of the term f reezing and students’ experiences with the freezing of water. No, a substance that is undergoing the phase change from a liquid to solid is not always cold because heat is being released in the process and energy transfer often cause substances to warm up. (Student #11)
No, Jell-O is an example of a reactant going from a liquid to a solid. The Jell-O is a liquid when boiled, and changes to a solid at room temperature. Jell-O does not have to be “cold” in order to be a solid. (Student #112) The previous discussion is consistent with the work of Kind15 that discusses limitations due to student experiences, but it also brings to light another well-researched problem. This is the confusion in students’ minds of physical and chemical phenomena.22−25 Confusion of the Role of the Energy Changes
In addition to the confusion of ideas previously discussed, there appears to be another influencing factor. Within the context of the questions asked, the responses hint that the incorporation of the concepts of energy exchange and kinetic molecular theory causes problems in answering Questions 2 and 4. The fact that students may “know” that f reezing is an exothermic process can actually give them “scientific” justification that f reezing substances are cold. Conversely, the fact that boiling is an endothermic process leads some students to conclude that this process results in a warm substance. This is consistent with the problems noted by Viennot28 when he discussed the fact that students had problems rationalizing that energy changes could occur without a change in temperature. Interestingly, whether students said the process released or absorbed energy, they could rationalize why a f reezing substance was cold and a boiling−evaporating substance was warm. The most common lines of reasoning are outlined below. Freezing Is Exothermic and Therefore the Substance Will Be Cold. This correctly identifies freezing as exothermic, but erroneously associates the enthalpy change in the phase transition with a process in which no chemical or physical changes are occurring. In those cases, the change in temperature can be associated with q = CpΔT, where q and, subsequently, ΔT would both be negative. A substance that undergoes a phase change from a liquid to a solid is always cold because the KE within the substance slows down and does not produce as much heat. ΔH will always be less than 0 and therefore it is an exothermic process.” (Student #59)
No, it may not be because as bonds are broken down, others may be forming releasing and absorbing heat. (Student #30) The Solid Has Less Kinetic Energy, So It Must Be Colder. Once again, for the student, this may seem to follow logically from the relationship of kinetic energy to temperature. Students may be thinking of the fact that on average, the particles in the solid will have less kinetic energy than those in the liquid. It should be noted that in the statements below, the students make some correct statements, but the application to the phase transition is not exactly accurate. It is always going to be colder as a solid than a liquid because a liquid has more kinetic energy and movement so it has more heat. (Student #72) Yes, because you have to slow the kinetic energy and when the kinetic energy slows the temperature lowers. (Student #5) Yes, because the decrease in heat slows down the particles in the liquid form to create the structured form of solids. (Student #106) The above examples were for the f reezing process, but the same types of confusion of concepts occur for the boiling process. Approximately one-third of the students stated that a boiling or evaporating liquid must be warm because energy or heat is required or absorbed in the process. Another seven students said that the liquid must be warm because energy is released. Some sample explanations are given below. Yes, for example, water when boiling H2O it releases Heat the energy that was put in in the beginning to make it a liquid is the release during boiling or evaporation. Energy transfer between substances always cause substances to warm up ... (Student #3)
Yes, because in order for a phase change from a liquid to a solid to occur the liquid has to release heat. Therefore making it cold. An example of this would be when water in a liquid state is frozen to become a solid. (Student #63) Yes, because the process of changing from a liquid to a solid is an exothermic reaction, so it will always give off heat. The reaction may not always be cold, but it will be cooler than it was initially. (Student #117) Freezing Is Endothermic and Therefore the Substance Will Be Cold. This follows directly from the fact that in an isolated system, the required energy for an endothermic process comes from the thermal energy of the substance and the temperature decreases. For the student, this result may follow logically from ideas that are typically taught in introductory
Yes, because as a substance goes from a liquid to a gas, the intermolecular forces must be over taken by heat intake and as the molecules disperse the stored energy is released and molecules move at a faster rate making it warm. (Student #79) 1613
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terminology, experience, and energy changes introduces a new lens with which to consider student conceptions of phase transitions. If semantics, real-world experience, interpretation of observations, and energy exchange are all problematic, then what can be done? One easily implemented intervention for contextual issues used by the author is to explicitly point out the differences between scientific versus colloquial meanings of specific terms whenever one is encountered. Such an intervention consumes little class time, yet it has been reasonably effective at making students understand the importance of context. As for increasing the experiences of students with the physical world, this is certainly further evidence for increasing the hands-on lab component of science classes. However, the need for introducing as many examples of various substances as is possible to the student is in conflict with trying to delve more deeply into specific phenomenon. This time allocation conflict between more formal laboratories and frequent demonstrations may be solvable by using a method such as POE. For example, showing various organic substances boiling at “low” temperatures would be powerful real-world examples and would be important in illustrating phase changes for substances other than water. A well-designed activity could have a great impact on overcoming student misconceptions.21 Although water is undeniably an important substance in chemistry, continued overuse of it as an example can lead to problems.16 While more frequent demonstrations can facilitate student learning, there are risks. Roadruck’s article32 about the use of demonstrations emphasized that it is important that these be at the correct level for the students to understand and that sufficient time be spent in discussion. Tai and Sadler33 observed high school instructional practices and reported that “most students recall their high school teachers presenting 1−2 demonstrations each week”. On the other hand, 88% and 79% of these students reported that 10 min or less was spent on preand postdemonstration discussion, respectively. Pierce and Pierce34 investigated the effect of classroom demonstration assessments on student learning. When comparing the control and treatment groups, they concluded that there was a significant increase in understanding in the groups with the assessments. They also concluded that the demonstration assessments had a positive effect in both strong and weak students but were most effective when targeting concepts not included in the laboratory curriculum. Widespread use of demonstration assessments that encourage student reflection should be effective in promoting student learning, yet this type of activity can take up considerable lecture time. Lastly, helping students to understand the ideas of energy transfer with respect to phase transitions is a difficult proposition. The common problem seemed to be related to distinguishing systems and surroundings. Perhaps this is an anomaly associated with first-semester chemistry students; however, energy transfer concepts are difficult. These problems have been reported to persist in more advanced students29 and have been observed by the author in upper-division physical chemistry students. This specific concept of energy exchange and phase transitions is currently being investigated in a formal way by the author to determine how students integrate these ideas and how to facilitate a better understanding.
The following students imply that a boiling liquid is warm because the process is endothermic and try to make a connection with kinetic molecular theory. Some of the statements made are correct, but it is not clear from their responses whether they are referring to the actual phase transition at a constant temperature or are simply stating that liquids first have to be raised in temperature to achieve the boiling point. Unfortunately, the information collected from these students is insufficient to distinguish these possibilities and further research is certainly warranted. Yes, since the molecules are going from a slower moving state (liquid) to a faster moving state (gaseous), they contain more energy and would be warmer then they were in the liquid state. An example is liquid water going from room temperature to boiling at 100 °C. (Student #13) Yes, because you have to get the kinetic energy up to go through a phase change so in order to raise the kinetic energy you raise the temp. Boiling water. (Student #5) Yes, the only way to get a substance to boil or evaporate going from a liquid to a gas is to add heat to it. (Student #47) One particular student’s explanation was quite involved and used a number of chemical ideas, some of which were correct, but in total, the explanation was lacking. Yes, because it requires energy to be released to break the bonds b/w [between] the atoms to allow movement in to the gaseous phase. When the bonds are being broken energy is being released and thus giving off heat. If H2O was going from liquid → gas there is need of energy release to allow the molecules to move fast, but also to break bonds. The H-bonds b/w H2O molecules are broken thus allowing the molecules to move freely. (Student #40) Part of the overall problem here may be associated with what students have learned from kinetic molecular theory. The idea that kinetic energy is related to temperature and that gas particles move faster than liquid or solid particles are easily grasped by students. However, this could also be the source for the misconception of f reezing substances being cold and boiling substances being warm. That is, the greater kinetic energy of the gas means that it is warmer than the liquid, and the lesser kinetic energy of the solid means that it is colder than the liquid. This difficulty integrating the ideas of molecular motion was noted by Osborne and Cosgrove13 when they concluded Further, more ideas to do with particles moving and colliding appeared to be understood by older pupils, but sustained probing of these ideas did not produce sound scientific explanations in terms of intermolecular forces or of loss of kinetic energy. It may be too much to expect first-semester chemistry students to understand the interplay of the kinetic and potential energy and the transfer of energy into or out of the surroundings during these phase transitions. Even in more advanced second-year chemical engineering students, Gopal et al.29 identified a number of misconceptions related to evaporation, condensation, and vapor pressure. Similarly, the work by Hwang and Hwang,30 as summarized by Chang,27 reported that students at grade levels from middle school to university have difficulty understanding the balance of energy and the constancy of temperature when a liquid is boiling.
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CONCLUSION A discussion of students’ understandings of phase transitions indicated that although some misconceptions may be related to the inability to contextualize the terms f reezing and boiling,
IMPLICATIONS FOR TEACHING None of the individual misunderstandings outlined in this report are new; however, the discussion of the interrelationships of 1614
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Journal of Chemical Education
Article
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other factors play important roles. One of these is the lack of student experience with a variety of natural phenomena, which caused many students to use water as their only reference point. In addition, misinterpretation of classroom demonstrations and the inability to apply energy transfer concepts can reinforce erroneous ideas about phase change processes. Interestingly, most of the students who were able to correctly describe whether the f reezing and boiling processes must occur when the substance is cold or warm, respectively, used examples of known substances other than water and avoided using energy-based explanations.
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AUTHOR INFORMATION
Corresponding Author
*P. G. Jasien. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The author would like to thank the reviewers for many helpful comments and E. Jasien for assistance in proofreading. REFERENCES
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