Chemical Education Journal (CEJ), Vol. 9, No. 2 /Registration No. 9-10/Received December 17, 2005.
URL = http://www.juen.ac.jp/scien/cssj/cejrnlE.html

Is it molecules? Again!
A review of students' learning about particle theory.

Helen Meyer

E-mail: Helen.meyeruc.edu

Abstract:

Is it molecules? Again! I frequently heard this from my school age students as they tried to explain different and seemingly unrelated phenomenon outside of the science classroom. The particulate nature of matter and all this theory explains is a central feature of science and is given a great deal of attention in school science curriculum, yet past and present research indicates that students do not understand it. This article summarizes research from the late 70s and early 80s as the baseline of our knowledge of students' understanding of the particle nature of matter. I then review more current literature in this area, its change in focus and what this new research adds to the knowledge base. From here, I discuss how these knowledge bases indicate that although the particulate nature of matter is a central theory in science, important questions still remain to be answered such as: do students actually understand, believe, and use this theory as to explain science and everyday events? And does science instruction create a nominal and mathematical view of molecules that students use in science classes to answer questions, but do not truly believe to exist? The implications arising from this review include the need for research that includes international comparisons of students' chemistry learning and curriculum organization, and longitudinal studies in order to develop cognitively guided and developmentally appropriate chemistry curriculum.

A great deal of science instruction in the United States and elsewhere is based upon students holding and being able to use a particle view of the world. When I taught school age children, I frequently asked them to explain natural events such as the wind blowing, water evaporating off of dishes, or what is inside bubbles? Eventually my students learned the trick; if they answered the questions saying it has something to do with molecules they had a good chance of being correct. The particle nature of matter is a powerful explanatory theory in science, yet as chemistry educators we seem to take for granted that our students hold this view of matter and can use it productively to explain events in and out side of the science classroom. In this paper, I discuss the considerable evidence that research in learning in science has produced that indicates how faulty this assumption is. I draw from two sets of research evidence in my discussion, the first is research that took place prior to the 1990's and the second is the research that has taken place since that time. Finally, I suggest that we need to reengage in research in learning with particular attention to international trends in science curricula and the developmental appropriateness of these curricula.

Background

In order for students to fully understand chemistry, it is imperative that they understand particle nature of matter and kinetic molecular theory. Obviously this is the foundation of understanding that we classically think of as school chemistry, atomic structure, chemical change, bonding, and so on. However, the centrality of this theory is not confined to chemistry alone. In order for students to understand earth systems, biological systems and physical systems they need to think about the world as being made up of particles. It is the existence and interaction of particles that explains much of what happens in the systems listed above. Given the importance of kinetic molecular theory to science, it would seem clear that teaching about particles and their interactions would be the foundation of much of what happens in school science. It would also seem obvious that as science researchers and educators, we ensure that students are correctly and accurately learning chemistry, and that school curriculum are designed to enhance student learning in these areas. However, years of research in learning chemistry have not born this out.

A brief survey of science texts and standards used in the United States illuminates how we teach science. Johnston's (1982) defined three levels of science explanation. The first level is the descriptive and functional level which focuses on what chemists can see and handle about materials, including how materials changes when they interact with one another. The second level of explanation is representational. In this level chemists develop and use representational systems to record, document, and communicate chemical activities. This includes information such as the periodic table, chemical formula and reactions, and mathematical calculations. Finally we reach the explanatory level. It is at this level that chemists seek out underlying mechanism and build models to provide mental pictures of the descriptive level. When these three levels of explanation are used to analyze texts and standards, we find that until about age 8 we are happy to let students be involved at the descriptive level. However, past age 8 we instruct students almost exclusively at the representational level. Although the language used in this instruction belongs as the explanatory level i.e., atoms, molecules, ions, and such, this language is not used to explain events, rather it is used in representations for students to know and then manipulate.

Johnston's levels of explanation above in conjunction with constructivist learning theory provide insights in to why students struggle to learn chemistry. I have summarized the basic tenets of constructivist learning theory, taken from multiple sources, for clarity in this article. These tenets included the ideas that:

1. Students bring intuitive ideas about how the world works into science classrooms;
2. Students' intuitive conceptions are based in everyday experiences with the natural world, media, and conversations.
3. Students integrate instruction with their existing ideas of how the world works and;
4. New learning is a synthetic result of students' existing knowledge and newly acquired knowledge.

Research in learning in science has made clear that students from a young age seek to understand the world at the explanatory level, thus students create their own mini-theories to explain what they observe in their daily lives, and these explanations impact on how students understand all classroom instruction (Driver, 1983a and b; Driver, Guesne, & Tiberghien, 1985). Given that in the US we focus our chemistry instruction at the representational level, students are left to develop their own explanatory mechanisms for events, and research has demonstrated that student generated explanations do not match what the scientific community believes about chemistry.

What the Research Says

In this review I have divided the literature into two time frames, research that was conducted prior to the 1990s and the research that has been conducted since that time. These two research bases both have important implications for chemistry educators, however, I also find it important to highlight how the shift in focus between the two have left gaps and it is because of these gaps that we find chemistry education remains to a great extent unchanged by 30 years of research.

Piaget's research and writings led to a revolution in how we think about learning and heralded the beginning of constructivist learning theory. His basic ideas lead to Johnstone's more elaborated views as discussed above. In addition to the development of constructivist learning theory, Piaget's research marks the beginning of the field now frequently referred to as the 'learning sciences'. It was not Piaget's work of alone that has led to this revolution, however his works provide a convenient place holder for the shift in how learning and subsequently teaching have changed. What we have learned from this revolution is that students' apparent ability to perform in school on tests and school work is often in conflict with how they understand and explain the world outside of the classroom setting.

Early research students' understanding of particle theory

In science education beginning in the early 1980's an explosion of research on students' learning of science concepts took place. Science education researchers during the 1980 and 1990 explored children's (and in some instances adults) ideas about central concepts and events in science, including chemistry. This research was generally labeled research on student misconceptions, but has also been called naive science, alternate theories, and children's science. This research agenda made us aware that children hold ideas about science that are significantly different from what scientist believe. We learned that these ideas are highly resistant to change even with instruction, and that even after 'successful instruction' (correct performance on tests) outside of the science classroom students are more likely to explain natural events based on their own ideas and not scientific conceptions.

Driver, Squires, Rushmore, and Wood-Robinson (1994) wrote an extensive summary book detailing what was learned about children's ideas of science from this research in the 1980s. This book serves as a guide to educators interested in the array of beliefs about scientific concepts that children hold. Based on this book and other sources, I have summarized the major findings about children's ideas about the particle nature of matter, a central concept in chemistry and the one with which I am most concerned.

The initial misconceptions research focused on how students understood big ideas in chemistry. In this research students' ideas were elicited as they explained a variety of macroscopic changes in matter, such as combustion, dissolving, phase change, and states of matter. The findings of research made clear that students do not attribute macroscopic changes in matter to microscopic bits of matter such as particles. This was shown in extensive research on students' explanations of dissolving (Longden, 1984; Cosgrove & Osborne, 1980; Comber, 1983; Preito, Blanco, & Rodriguez, 1989; Fensham & Fensham, 1987). Researchers found that the youngest students believed that things just disappeared when they dissolve. However, even as students age and gain more instruction in chemistry, researcher still found that a large proportion of students over the age of 15 demonstrated misconceptions about dissolving, specifically that water exists as a continuous material in which tiny bits of sugar become suspended.

These descriptions of dissolving and water as a continuous substance versus a particulate substance were born out by students' beliefs about another central event requiring particles as an explanatory mechanism, the phases of matter. Students were most likely to believe that particles exist in solids indicated by their ideas that a solid could be continually broken into smaller and smaller bits until you get to particles (Stavy & Stachel, 1984; Jones & Lynch, 1989; Jones, 1984). However, students were less likely to believe that liquids and gases are composed of particles (Sere 1986; Osborne & Cosgrove, 1983; Nussbuam, 1985) and students confused the properties of macroscopic examples of substances with the behavior of the substance at the microscopic level.

The confusion of properties at the macroscopic and microscopic levels was probed in greater detail when students were asked to discuss particles at the microscopic level. Students by about the age of 13, in the countries participating in this research, had been exposed to the terms molecules, elements, and atoms in their school science classes. However, there understanding of these terms can be seen to be at Johnston's (1982) representational level only, since in many cases, once the students had been introduced to the idea of elements and atoms they ascribed the traits of macroscopic samples of an element (copper wires) to each individual atom within the substance (Gabel & Samuel, 1987; Novick & Nussbaum, 1981; Driver, 1985; Ryan, 1990). This misconception was eloquently explained in Ben-Zvi, Elon, and Silberstein's (1986) article, ÒIs an atom of copper malleable?Ó The confusion about the relationship between the properties of a particle of a substance and a macroscopic sample of the substance interfered with the students understanding almost all higher levels of chemistry, from physical and chemical changes to bonding and conservation of matter and energy (De Vos & Verdonk, 1987 Pella & Voelker, 1967).

There is an abundant amount of research focused on other areas of chemistry however, what these studies highlight is that fundamentally students do not hold a particle view of the world. They misunderstand the differences between the microscopic and macroscopic world, and that as they progress through schooling they incorrectly apply concepts about properties of matter at the atomic and elemental level to try to explain changes in matter. As chemistry educators we have had this information available to guide curriculum construction and development, however as discussed in the first section of this paper science curriculum, in the US at least, has disregarded this research and maintained its emphasis on representational explanations of chemistry.

Recent research on students' understanding of particle theory

I now shift to the more recent research in chemistry education focused on students' learning of the particle nature of matter. This research falls into two distinctive categories, each of which provides us with useful information about students' learning in chemistry, but once again much of which has been disregarded in school science curriculum. The first category of research I will review is the extension of the misconceptions research begun in the 1980s. The second category that comprises much of the most current research in learning is mental model research.

The current literature in misconceptions research differs from earlier research in two important ways. The first difference is an increased emphasis on instructional strategies to overcome students' misconceptions. There is significantly less research that catalogues students' ideas about chemistry concepts and the research in this area, such as Kikas (2004) and Case and Fraser (1999) has focused on older science learners. Kikas' (2004) study used adult participants who were training to be or were primary level teachers. These teachers' ideas were then compared to the ideas of scientist ideas of particle theory. This worked addressed the same fundamental misconceptions discussed as earlier research, the incorrect application of macroscopic ideas to explain microscopic events, and phase changes, however by working with older participants illuminated the ineffectiveness of years of science instruction. Kikas found that fewer than a third of the teachers explained the events based on correct conceptions particles. Case and Fraser's (1999) research asked chemical engineering students to apply their knowledge of moles to answer conceptual questions. They found that these advanced chemistry students incorrectly applied fundamental chemistry concepts when giving explanations of reactions at the molecular level.

Beyond this, most current misconceptions research has focused on the success of specific instructional strategies to remedy students' misconceptions. Harrison, Grayson, and Treagust (1999) investigated the development of students' conceptions of heat and temperature throughout an instructional sequence. Thomas and Schwenz (1998) studied how various instructional settings affected college physical chemistry students' understanding of chemical equilibrium. Tan, Goh, Chia, and Treagust (2002) developed a research instrument specifically designed to assess and remediate high school students understanding of qualitative analysis, specifically their understanding of cations and anions. And a final example, Nieswandt (2001) designed different instructional activities to monitor the activities effects on extinguishing high school students' confusions with applying the correct particle explanation for chemical changes as compared to physical changes.

One trend of this new research is the increase age and levels of experience with chemistry of the research participants. A second trend is the use of context of specific instructional settings for conducting the research. In these cases, one objective of the research studies was having students obtain the correct scientific view. Finally, these research studies were focused on more discrete ideas in chemistry such as chemical equilibrium and ion formation, both of which are predicated on students holding a particle view of matter at the beginning of instruction. It appears that the underlying assumption in this research is that these older students are developmentally ready to use a particle view of matter to explain chemical events. Therefore what is needed is the refinement of the instruction of specific cases of particle interactions and not the remediation of basic misconceptions. What this research leaves unquestioned is the appropriateness of teaching specific concepts to students who have not developed an accurate conception of particle theory.

The second category of research around the particle nature of matter since the 1990's is based upon a specific research tool, the use of mental models and student talk aloud protocols. This research was typically conducted with small groups of students who are asked to solve problems by drawing diagrams and explaining their diagrams during a semi-structured interview. This interview based research is similar to the initial research on student conceptions in that it used small groups of students and was focused on how students' explained their ideas. The inclusion of a diagrammatic representation of their ideas added insights into the tools students use to make sense of ideas and because these assimilate instruction into coherent conceptions.

The mental model research on particle theory has tended to focus on students' ideas and representations of atoms and molecules (Harrison and Treagust, 1996) and how these representations enhance or interfere with their ability to explain chemical phenomenon (Taber, 2002; Coll & Treagust, 2003; Coll & Treagust, 2002). This research, similar to the newer misconceptions research was focused on specific concepts in chemistry, all of which are predicated on the students holding an accurate particle view of matter. These studies asked to students to conceptualize and represent bonding patterns based on electronic orbitals. They queried students about what happens at the atomic level during ionic bonding or during the bonding of metals. What this research demonstrated was that students struggle to move from a representational view of chemistry, as taught in science classes, to an explanatory view. The student participants in these studies were able to solve mathematical problems and balance equations involving bonding, a representational activity, however they were less able to diagram and explain bonding when asked to create a model of it. In addition, these researchers also found that students modified scientifically accurate models to accommodate their misconceptions about particles to the point where they will gave up the scientific model in favor of their models that allowed for their inaccurate representations of particles to remain stable.

There are distinct similarities in these new avenues of research about the particle nature of matter. One is the increased age and associated increased instruction of the research participants, and second is the narrowing of the chemistry content studied. In both of these cases, what is left unquestioned is why advanced students still hold fundamental misconceptions about matter and how this reflects on the curriculum that allowed students to arrive in higher level chemistry courses with these misconceptions intact. An important difference between these two research avenues is that the misconceptions research tends to focus on the products of learning episodes, whether it is their ability to remediate misconceptions or in cataloguing of misconceptions. While the mental models research details what tools and thinking processes students bring to the task of learning science.

Where Do We Go from Here?

So where does this new research leave us in relation to the older chemistry education research? The focus on discrete content in the current research has left chemistry educators to continue to deal with finding ways to overcome students' fundamental misconceptions about matter. The early research in chemistry education stressed identifying these misconceptions. The more recent literature make clear that these misconceptions still exits, however rather than addressing how to overcome them, the research shifted to focus on new problems. I would like to suggest that at this time as chemistry education researchers need to think anew about the status of these fundamental misconceptions and why they persist.

In addition to returning to these earlier studied ideas, I believe chemical educators have overlooked two important concepts that are requisite to holding a full particle view of matter. First, we have not effectively addressed students' ability to conceptualize and understand the issues of size and number that chemistry requires students to accept. When students are presented with the idea of billions of atoms that are infinitely small, how are they to conceptualize what this means? Currently there is little research in this area and the research that there is indicates that students tend to believe that invisible (too big or too small) means imaginary (Tretter and Jones, 2003). If students believe atoms are imaginary, then it follows that they may believe they possess properties that are magical or at least inconsistent. The majority of teaching in the US about relative size and number in chemistry is highly abstract and reliant on mathematical calculations of scientific notation and conversions. Very little time is spent enabling students to understand how small the small of chemistry is and how many atoms we are really talking about. If students do not hold a sufficient conception of the relative sizes of atoms, molecules, and subatomic particles they are likely to be forced to accept the representational explanations presented in school when answering school questions, but disregard them when explaining chemistry events out side of school.

Tied to issues of size and number and a discontinuous view of matter is the problem of empty space. If students are going to understand a particulate view of matter, they must be willing to accept that there is empty space between and within these particles. Currently there is an insufficient research base to know when students are able to accept that solid and liquid objects are filled with empty space. And that the apparent empty space of air, is actually filled with billions of particles.

Additionally, I believe that we if we are to address students' tendencies to incorrectly apply macroscopic properties of matter to individual atoms, we need to gain a clear understanding of when students are developmentally ready to make a shift from macroscopic explanations to microscopic explanations. Similarly, we need to investigate when students are ready to make the developmental shift from understanding matter as a continuous substance to a particulate substance in all phases of matter. In order to know when these developmental shifts take place our research efforts need to be longitudinal and international. Early research demonstrated that there is a link between instruction and conceptual understanding. Without instruction few students will move to a particulate view of matter, therefore school instruction must be a catalyst for this shift to take place. However, at what age and with what sequence of instruction successfully proceeds this shift must be understood. International studies would shine a light on when and how this concept is taught and indicate if earlier or later instruction is more effective.

Finally, if students are going to be able to use a particulate view of matter to explain more advanced chemistry ideas, ranging from phase changes to heat to chemical equilibrium, they must understand and accept the dynamic nature of particles. The early research in this area indicated that students tend to think of particles as static and that chemical reactions always progress to a static end point. These same misconceptions underlie the struggles that students in Harrison, Grayson, & Treagust (1999) faced when dealing with heat and temperature, and Thomas and Schwenz (1998) students faced as they tried to explain problems in chemical equilibrium. We know this misconception interferes with students' learning, but we have not investigated why students hold a static view of matter and when is the optimal time to help students to shift to a dynamic view of particles.

The underlying theme from the collective research in learning about particle theory is that although we know what concepts cause difficulties for students learning chemistry, we have not systematically investigated these issues in relationship to cognitive development and curriculum design. Johnson's (2002) study is one study that considered the impact of cognitive development and curriculum design on students' understanding. Based on his work he pushes for a much later introduction to the formal representational language of chemistry. In his study he found that students at the age of 11 were not ready to differentiate the idea of particles into molecules, elements and atoms. Rather, he asked students to provide explanations of events using the generic term 'substances'. However, students by the age of 14 could sensibly divide substances into the smaller chemical units of molecules and atoms and use these terms in meaningful and accurate ways in their explanations.

Implications for Chemistry Research and Education

Given the knowledge base that we have in chemistry, with its strengths in terms of identifying the major concepts students don't understand and its gaps in terms of development and curriculum design, where do we go? I believe that we need to develop a much stronger international chemistry education research network. This network would allow researchers on different continents to examine the same sets of conceptions within their own educational settings. This international perspective would provide data about the effects of curriculum on learning of fundamental chemistry concepts. We could begin to isolate which chemistry curriculum enable students not just to solve mathematical problems, but also to enable students to develop sufficient explanatory knowledge to answer chemistry questions outside of the school context. This international curriculum research needs to be built from the foundation of research on learning in chemistry that is currently available and would form one rung of a new instructional guide for teaching chemistry.

The second research strand that must be developed are better tools to measure students' understanding of concepts. We know what students do and don't do when creating explanations, however more attention must be paid to why students do not accept or believe in the scientific views of atoms. This moves beyond the cataloguing conceptions and misconceptions, and even beyond the mental models research in which we saw glimpses of how students use different learning tools. We need to consider students in a more holistic fashion that considers their conceptual ecologies and how students' personal beliefs, affective domains, and instructional context enhance or interfere with their acceptance of a particle view of matter.

Finally, we need to return to considering learning science as a developmental process. Much of chemistry is highly abstract, yet we try to teach these abstract concepts to students as young as age eight. Johnston's (1982) levels of science explanation suggest there is a developmental quality to knowing science. Current curriculum, as discussed previously, focuses on the knowing science at the representational level, however much of the research in chemistry education focused on knowing at the explanatory level. Neither the research nor curriculum design spend a significant amount of time with the descriptive level of knowing. As chemistry researchers we need to develop studies that seek to understand how children move into and through these different levels of science knowing. This can be done with cross-sectional as well as longitudinal research. If we find optimal ages when children are able to make these knowledge shifts in their cognitive development, then curriculum and instruction can follow.

The final result of this expanded chemistry education research agenda would be to make constructive use of our current knowledge of students' learning in chemistry to drive curriculum and instruction in chemistry. This would be a shift from having chemistry content drive instruction, followed up by research that indicates the ineffectiveness of the instruction. Similar to what constructivist learning theory tells us about instruction we must place children's learning at the center of our research agenda.

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