Students’ understanding of the particulate nature of matter
Singer, Jonathan E
The particulate nature of matter is identified in science education standards as one of the fundamental concepts that students should understand at the middle school level. However, science education research in indicates that secondary school students have difficulties understanding the structure of matter. The purpose of the study is to describe how engaging in an extended projectbased unit developed urban middle school students’ understanding of the particulate nature of matter. Multiple sources of data were collected, including pre- and posttests, interviews, students’ drawings, and video recordings of classroom activities. One teacher and her five classes were chosen for an indepth study. Analyses of data show that after experiencing a series of learning activities the majority of students acquired substantial content knowledge. Additionally, the finding indicates that students’ understanding of the particulate nature of matter improved over time and that they retained and even reinforced their understanding after applying the concept. Discussions of the design features of curriculum and the teacher’s use of multiple representations might provide insights into the effectiveness of learning activities in the unit.
The particulate nature of matter is identified in science education standards as one ofthe fundamental concepts that students should understand at the middle school level (American Association for the Advancement of Science [AAAS], 1993). However, empirical studies in science education indicate that secondary school students have difficulty understanding the structure of matter (Ben-Zvi, Eylon, & Silberstein, 1986; Krajcik, 1991) and hold several alternative conceptions of atoms and molecules (Griffiths & Preston, 1992). For example, some students maintain a cloud or vapor model of air (Krajcik, 1989), attribute the properties of substances to an isolated atom (Ben-Zvi et al., 1986), and believe that a molecule changes its size and shape at different temperatures (Griffiths & Preston, 1992). Additional studies show that students have difficulty relating macroscopic properties to the movement and arrangement of particles, even after engaging in substantial chemistry instruction (Ben-Zvi, Eylon, & Silberstein, 1987; O’Connor, 1997). The purpose of this study is to describe how urban middle school students’ understanding of the particulate nature of matter is developed by engaging in an extended project-based unit.
The reported research is part of a large-scale urban reform effort involving collaboration between a large midwestern urban center and a large midwestern university. In order to facilitate students’ understanding of particulate nature of matter, our research group developed a 9-week curriculum entitled, “What Affects the Quality of Air in My Community?” (referred to in this article as “air quality unit”). The intent of this unit was to provide a range of learning opportunities for students to explore their prior ideas, communicate their ideas through the use of multiple representations, and investigate concepts of atoms and molecules. The curriculum materials address science content associated with the particulate nature of matter, phase changes, and the chemical processes involved in the formation of pollutants. To examine how the curriculum engages students in constructing understanding ofthe particulate nature of matter, this study is guided by the following question: How do proj ect-based classroom experiences and materials shape and promote urban students’ understanding of the particulate nature of matter? Findings of the study will extend educators’ understandings of curriculum design and students’ learning of the fundamental scientific concepts.
Fundamental characteristics of project based science (PBS) include providing a context that engages students in extended authentic investigations through using a driving question, promoting collaboration, using cognitive tools, and communicating their ideas (Singer, Marx, Krajcik & Chambers, 2000; Krajcik, Blumenfeld, Marx, & Soloway, 1994; Marx, Blumenfeld, Krajcik, & Soloway, 1997). The purpose ofthe driving question is to link concepts and principles and drive activities and investigations. The question may be teacher or student generated and must provide sufficient creative latitude for multiple avenues of problem solving (Krajcik, Czerniak, & Berger, 1999). Projects are designed to foster student collaboration within a learning community. Students communicate with each other, teachers, community members, and scientists to find information and solutions to their questions and to discuss their findings and understandings. Projects are designed to extend student learning experiences beyond the classroom by posing driving questions that situate the science with issues that are likely to be of interest to scientists, community-based organizations, and families. Collaboration during investigations and lessons involve students interacting with peers in small groups or as part of large class discussions, or students are given opportunities to interact with more knowledgeable community members.
Cognitive tools facilitate students in the collection and analysis of data. These tools also provide an environment in which students can construct artifacts and discuss their emerging understanding of concepts and processes related to the driving question. These characteristics of PBS lead to the promotion of student scientific understanding that is embodied in artifacts. Artifacts are representatives of the students’ problem solving solutions that reflect emergent states of knowledge. Because artifacts are concrete and explicit (e.g., a model, report, videotape, computer program) they can be shared and critiqued (Blumenfeld, Marx, Patrick, Soloway, & Krajcik, 1997). Students maybe required to explain how their artifact is related to the driving question or subquestion or how it represents a specific concept. By promoting public sharing, critiquing, and revision of artifacts, active construction of student understanding is fostered.
Students’ Understanding of the Particulate Nature of Matter
Within the scope of this air quality unit, one of the major concepts for students to understand was the particulate nature of matter. In defining the ideas that are associated with this concept, the unit developers focused on two benchmarks articulated by the AAAS (1993).
All matter is made up of atoms, which are far too small to see directly through a microscope. The atoms of any element are alike but are different from atoms of other elements. Atoms may stick together in well-defined molecules or may be packed together in large arrays. Different arrangements of atoms into groups compose all substances. (p. 78)
Atoms and molecules are perpetually in motion. Increased temperature means greater average energy of motion, so most substances expand when heated. In solids, the atoms are closely locked in position and can only vibrate. In liquids, the atoms or molecules have higher energy, are more loosely connected, and can slide past one another; some molecules may get enough energy to escape into a gas. In gases, the atoms or molecules have still more energy and are free of one another except during occasional collisions. (p. 78)
The particulate nature of matter provides a foundation for understanding other chemistry concepts. To explain changes in state, chemical reactions, and behaviors of gases, students must have an understanding of the particulate nature of matter. However, many secondary school students have alternative conceptions ofthe structure of matter. For example, in Krajcik’s study (1989), students represented the molecular view of air as wavy lines and clouds. Their drawings and descriptions indicated that most ofthem held a continuous model of matter. In a separate study (Gabel, Samuel, & Hunn, 1987), even when students drew a particulate model and were asked what exists between the particles, their answer was “more air particles.”
Among many alternative conceptions identified by the literature in the context ofthis study, five aspects of the particulate nature of air are considered: (a) the composition of air, (b) the arrangement and movement of particles in the three phases (Griffiths & Preston, 1992; Johnson, 1998), (c) the energy aspect of phases (Griffiths & Preston, 1992; Krajcik, 1991), (d) the nature ofthe particles themselves (Ben-Zvi et al., 1986; Ben-Zvi et al., 1987), and (e) the symbolic representations of molecules and atoms (Ben-Zvi et al., 1986; Ben-Zvi et al., 1987).
The five aspects of the nature of air can be categorized into three levels of chemical understanding: the macroscopic, microscopic/molecular, and symbolic levels (Label, 1998; Label et al., 1987). Chemical representations at the macroscopic level refer to diagrams and pictures that illustrate observable phenomena. Chemistry at the microscopic level refers to the arrangement and motion of molecules used to explain and predict the structure of matter. Chemistry at the symbolic level refers to the chemical symbols, formulas, and structures used to represent atoms, molecules, and compounds. in the air quality unit, to understand the composition of air, the arrangement of particles in phases, and the energy aspect of phases, students must be able to explain visible phenomena by applying concepts and models associated with kinetic molecular theory. Additionally, students are encouraged to relate the phenomena to collective behaviors of atoms and molecules at the microscopic level and to move frequently back and forth between these two levels. The use of multiple representations aims to develop deeper understanding through supporting abstractions and the establishment of concrete relations among representations at three levels (Ainsworth, 1999; Ainsworth, Bibby & Wood, 1997). Furthermore, through using symbolic representations of particles across various tasks, students learn the language (Hoffmann & Laszlo, 1991) and mediational tools of chemistry (Kozma, Chin, Russell, & Marx, 2000) and construct chemical meanings underlying it. By emphasizing the five aspects of the particulate nature of air and three levels of understanding, this unit is focused on developing both students’ understanding of the particulate nature of matter and their abilities to use representations to describe relationships and connections among the three levels.
Drawing upon the curriculum design features of PBS (Blumenfeld et al., 1991; Singer et al. 2000;) and from the science education research base concerning student understanding of the particulate nature of matter (Gabel et al., 1987; Ben-Zvi etal., 1987; Griffiths & Preston, 1992; Johnson, 1998), the instructional sequence described in this study highlights four fundamental components. These ideas include (a) providing a context for the concepts being addressed, (b) imbedding the use of learning technologies, (c) constructing activities that utilize multiple representations across levels (e.g., macroscopic, microscopic/molecular, and symbolic) to illustrate the targeted physical science concepts, and (d) sequencing the learning events in a logical manner that allows student knowledge to be applied. These four components are imbedded within two main enactment phases of the curriculum, a perspective phase and a content phase. The perspective phase includes learning events associated with the introduction and closure of the unit. This phase of the unit is primarily concerned with the contextual nature of the project and is focused on establishing and responding to the driving question (What affects the quality of air in my community?). The content phase includes learning events that are focused upon understanding the underpinning concepts and processes associated with the driving question. Appendix A contains a brief overview ofthe key instructional events of this curriculum project.
This unit began with a series of instructional events that set the context for the content being addressed. A walk around the school engaged students in the driving question (What affects the quality of air in my community?) and had them observe their local air quality and identify potential sources (e.g., cars, trucks, smoke stacks, etc.) and effects (e.g., soot, ash, exhaust, etc.) of air pollution. This walk, its subsequent class discussion, and emergent artifacts (observations and questions) constituted the project’s first anchoring event. The walk served as an anchoring event by providing opportunities for students to link their learning to their experience. The observations recorded and questions raised during this walk were revisited throughout the course of the project. Throughout the unit, students looked back at this community walk to provide a reason for learning the various science concepts.
The context of air quality was utilized as a means for introducing key concepts and processes of the project. The chemical composition of “clean and polluted” air was used to introduce the chemical structures and properties of molecules, atoms, elements, and compounds. Sources and effects of pollutants were utilized to introduce the concepts associated with changes in matter. The project concluded with a final response to the driving question by using data sets from the Environmental Protection Agency and synthesizing all of their previous project experiences.
The content phase of the air quality project provided multiple opportunities for students to engage in activities associated with the particulate nature of matter. Student drawings captured their initial conceptions concerning the composition of air. Students then shared and explained their pictures to the class. This pre-assessment was then followed by two sets of content phase learning events. Each of these content phase sets utilized the three levels of chemical understanding (Gabel et al., 1987). The first content phase set focused upon the particulate nature of matter. The second set built upon and required students to apply their understanding of the particulate nature of matter to describe changes in matter.
Set 1 – Particulate nature of matter. Students began by observing and exploring macroscopic properties of air. Using a series of Predict-Observe-Explain cycles, students observed that air takes up space and has mass. Students then investigated the rusting of steel wool within a closed system. Using this system, students began to learn about the composition of air and determined that oxygen is not the only component in the air.
After engaging students in activities that emphasized the macroscopic properties of air, the curriculum proceeded into a series of lessons focusing on the microscopic/molecular level. These lessons utilized multiple representations, including “human models,” gumdrop models, and a visualization tool, eChem, to introduce concepts about phase changes, atoms, and molecules. For example, during the simulation ofa solid, student molecules were packed closely together and students were asked to move very slowly. This process was repeated for the arrangement and movement of particles in liquids and gases. Following this kinesthetic representation, students discussed the differences between the three phases and related the discussion to the arrangement of particles in air.
The microscopic/molecular level continued by using the construction of”gumdrop models” to represent the major constituents of air and air pollutants, such as nitrogen, oxygen, sulfur dioxide, carbon dioxide and monoxide, and ozone. This modeling activity built on what students knew about particles and helped students distingish atoms from molecules. Additional microscopic representations of particles were facilitated with the use of the visualization tool, eChem (Wu, Krajcik, & Soloway, 2000). While this tool allowed students to construct and manipulate a variety of molecules, it increased the additional complexity by representing compounds in acceptable scientific constructs (e.g., oxygen gas is double bonded). Although eChem presented different configurations of atoms, students were not required to learn about chemical bonding theories. The learning objectives ofthe eChem activity were aligned with benchmarks proposed by AAAS (1993) that middle school students should understand that atoms could stick together to form molecules and that different arrangements of atoms compose different substances.
Utilizing discussions associated with the gumdrop and eChem lessons, students were introduced to the symbolic representation of compounds. Using the physical representations in eChem or the gumdrops, students counted the number of different types of atoms and were introduced to the accepted scientific convention (e.g., H2O denotes a compound containing two atoms of hydrogen and one atom of oxygen).
Set 2 – Changes in matter. After providing students multiple learning opportunities to foster their understanding of the particulate nature of matter, the curriculum enactment reinforced and applied this concept by drawing on the structure of matter as a mechanism for changes in matter. Students used gumdrop and human models to represent the movement and rearrangement of particles during phase changes and chemical reactions.
Following the same instructional sequence as Set 1, students began the second instructional phase at the macroscopic level. During these project activities, students engaged in multiple examples of changes in matter (e.g., ice melting, mixing baking soda and vinegar, formation of lead iodide from lead nitrate and potassium iodide). Next students were guided through a series of modeling activities that represented the microscopic/molecular level. For example, gumdrop and human models were used to illustrate the mechanism of several chemical changes that the class experienced. Students used the gumdrop and human models to explain how these changes occur. They created gumdrop models of the reactants and then rearranged the atoms from the reactants to form the products. Human models provided an additional representation for illustrating these phenomena. Students formed molecules of the reactants and then rearranged to form the products. Small groups of students were then challenged to develop a script for how to direct the class through one of the chemical reactions they experienced previously. The second phase ended with students experiencing the symbolic level of representation by creating word and chemical equations of the various chemical reactions.
The data utilized in this study was based upon the 1999 air quality unit enactment. The previous year the same teachers participated in the project utilizing an earlier version of this curriculum. It was the poor overall achievement of the project population that led to the reorganization of the curriculum for the 1999 enactment. The total effect size of student content gains for the 1998 enactment was .45. Student gains were even less promising concerning their understanding of the particulate nature of matter, with a standard effect of .37. Both the 1998 and 1999 versions ofthe curriculum lasted for the same number of days and addressed the same standards for approximately the same number of instructional days. The 1999 version was actually 9 days shorter, 54 days versus 45 days. The fundamental changes made between the 1998 and 1999 versions of the curriculum included moving the lessons associated with the particulate nature of matter (a) earlier in the project (Days 8-16, 1999, and 31-39, 1998); and (b) prior to the lessons associated with changes in matter (Days 22-29,1999, and 15-20,1998). Our study did not attempt to compare our students with a control group. Rather, the past situation, which showed limited progress in students’ knowledge and understanding, was a reference point at the beginning of our project. The use of such a reference point has been suggested by the design experiment approach (Brown, 1992).
Nine teachers and 900 seventh-grade (age -13) students in Detroit Public Schools participated in our air quality unit. Six of these teachers also participated during the 1998 fall enactment. Approximately 90% of the students were African Americans. Teacher experiences varied in respect to their familiarity with the curriculum (24 years) and with their level of content and pedagogical content knowledge.
We chose one target teacher who taught five classes for an indepth study. She was selected for the study because she had five years of classroom experience, presented good management skills, was familiar with the content, did not teach special honors sections, and had classes performing better than average during the 1998 enactment.
We focused on one of her classes and videotaped maj or classroom activities. This target class was selected randomly by the teacher at the beginning of the school year. Table I illustrates how this class compares to the other four classes. Five students in the target were chosen as target students. These students were average learners, but were vocal and willing to share their thoughts.
The study involved multiple sources of data including pre- and posttests, a midterm quiz, air drawings, interviews, class video recordings, and field notes. Multidimensional perspectives for data collection are justified by the nature of educational settings, which is complex and multidimensional. Therefore, only complex and multilevel instruments can provide the necessary data to answer complex questions (Salomon, 1991; Shulman, 1986). A combined approach is supported by Keeves (1998), who claimed that the research development in multilevel analysis opens up new approaches to research into science classrooms. The multilevel analysis allowed us to address conceptual understanding, in general, (tests) and ontological perspective, in particular, (interviews of students describing particles and their behavior), as well as some aspects, which are on an affective and cognitive continuum, such as the construction of a community of learners.
In order to classify our questions on our pre- and posttest, midterm quiz and interviews, we used Shepardson and Pizzini’s (1991) framework of input, output and processing skills, and adopted the performance expectation used by the Third International Mathematics and Science Study. The analysis of the question level was done initially by three to five science educators and then reevaluated by two of them, who achieved 95% agreement. Items categorized as low level included questions that required students to (a) recall information, (b) understand simple information, or (c) understand complex information. Questions identified as medium level required students to (a) draw/ understand simple relationships, (b) apply knowledge to new/different situations, (c) go between representations: verbal-graphics etc., or (d) identify hypothesis/ procedure/results/conclusion in a multiple choice question. Assessment items classified as high level included items that required studens to (a) describe/analyze data (charts, graphs), (b) draw hypotheses, (c) draw conclusions, (d) define/isolate variables given in a framework story, (e) apply investigation skills, or (f) use concepts in order to explain a phenomena/situation. The data sources, purposes, group size, and analysis methods are summarized in Appendix B.
Data analyses presented in this section represent a general to specific case approach. This illustration method moves from general quantitative measures to more descriptive qualitative methods. We begin by presenting objective type assessments in the form of pretest and posttest data. This initial level of analysis utilizes all students (N=587) who participated in the air quality unit. Moving to a “finer grain” of analysis, the study presents pretest, midterm quiz, and posttest data associated with classes of one teacher (n = 115). The next level of “narrowed-analysis” focused on describing classroom observations and artifacts from one class of the aforementioned teacher (n = 30). Lastly, final interview and artifacts data are reported from five target students enrolled in this class. Two students were interviewed about their drawings.
Pre- and Posttests
The pre- and posttests included a cluster of five questions assessing students’ understanding of the particulate nature of matter. Due to high mobility of students within and outside the district, approximately 15% ofthe participants did not complete either the pretest or posttest, so overall, 581 matched pretest-posttest pairs were used for the final analysis.
As shown in Table 3, in general, students participating in this study scored significantly better in the posttest. Compared with the entire population, the target teacher’s classes performed at a higher level than the entire population. This finding leads us to examine the teacher’s enactments of the curriculum materials through the analysis of her classroom recordings.
The effect size indicates that the average score on the posttest in these five classes was more than 1.29 standard deviations greater than the average score on the pretest (effect size = 1.29). While the test score of the target class was 4th out of her five classes, the gains between the target class and other four classes were not significantly different. These statistical results show that after experiencing a series of learning activities the majority ofstudents acquired substantial content knowledge.
Comparison Between the Midterm Quiz and the Posttest
As mentioned previously, the posttest was a summative assessment for the whole unit and administered at the end of the unit (day 45), so it also served as a retention measure for evaluating students’ understanding of the particulate nature of matter, while the midterm quiz was a formative assessment, administrated right after students studied the particulate nature of matter. Figure 1 indicates that although students took the posttest 4 weeks after completing the particulate nature of matter section of the unit, there was no decay effect. The mean scores (in percentage) of the particulate nature of matter cluster on the posttests (66% all target teacher classes; 61% target class; see Table 2) were higher than the scores on the quiz (54% all target teacher classes; 48% target class; see Figure 1). This finding indicates that students’ understanding of the particulate nature of matter improved over time and that they retained and even reinforced their understanding after applying the concept.
Classroom Level: Air Drawings
The air drawing assignment was an embedded classroom assessment, which was administered at the beginning and the end of the particulate nature of matter activities. Table 3 shows the results of the air drawing analysis based on the macroscopic, microscopic, and symbolic levels. Before experiencing the learning activities of the particulate nature of matter, over 90% of the target class students’ drawings were coded as macroscopic representations.
Two types of the macroscopic representations were commonly created. In the first type, students illustrated that people could not see anything from the clean air, and they left the paper blank. In the representations of dirty air, they used dots and spots to illustrate dust and smog (see Appendixes C, D, and E). Although we recognized students’ intention of showing “particles,” these dots and spots did not carry meanings of molecules, so they were coded as macroscopic representations. The second type of drawings was a landscape picture that often included clouds, trees, and the sun and did not contain any information referring to particles. In their post air drawings, however, over 60% of the students demonstrated a substantial change of their understanding of atoms and molecules (see Appendixes C, D, and E). Students used balls and sticks to illustrate a microscopic representation or chemical formulas to represent air symbolically. The number of microscopic and symbolic representations on the post air drawings were significantly higher than the number on the pre air drawings (chi^sup 2^ 153, p
Classroom Video Analysis
The purpose of classroom video analyses was to demonstrate how the teacher enacted features of the intended curriculum and how aspects of the particulate nature of matter were addressed. The following narrative of a class event was selected to illustrate patterns that emerged from the data corpus of class video recordings. The narrative represented a 25-minute event that occurred on Day 11 prior to creating gumdrop and computerized models.
Prior to the event, the class had been introduced to phase changes and pollutants. During the event, the whole class reviewed the concepts of phase changes by creating multiple representations. The teacher first asked students, “Who can tell me a property of a solid?” Students volunteered descriptions that in a solid, molecules are close together, move slowly, and keep their shape. A student added that “You can add heat or energy to melt the solid.” The teacher then asked for examples of a liquid that people cannot drink. Students’ responses were oil, bleach, chlorine, acid rain, and acid. The teacher turned to the blackboard and drew three squares (boxes). She then asked for volunteers to draw how molecules of a solid, liquid, and gas would be arranged inside each box. Three students volunteered and drew circles inside the boxes for each phase. The teacher asked the whole class for agreement. Some students said that molecules in a solid shouldbe touching, and the molecules in a gas were too close to the others. And the teacher allowed the students who drew pictures to give explanations. She then expressed that the class would go with the majority opinion, although her ideas may be different from it.
After the discussion of drawings, the whole class reviewed how the molecules are supposed to look by moving their fingers and hands. When she said solid, students were supposed to hold their hands tightly. To show a liquid, students moved their fingers gently to mimic the movement of liquid molecules. For a gas, students moved fingers and shook hands fast to demonstrate the movement of molecules. Most students correctly used their hands to do the physical models. The teacher then used an example of soda to reinforce the idea of phases that the soda can, soda, and carbon dioxide fizz were solid, liquid, and gas, respectively. Students were asked to show hand models of the soda can, soda, and carbon dioxide.
Furthermore, the teacher put a clear box on the overhead. The clear box was made of a wood frame, covered by transparencies with wood pieces inside. Through projection, students could see shadows of wood pieces on the screen. She then shook the box to make pieces very close to each other and asked students what phase these pieces were representing. Some students answered, “a solid.” The teacher kept shaking the box to make pieces far apart and students’ response was “liquid.” Finally, the wood pieces moved faster and scattered in the whole box and students answered, “a gas.”
To conclude what the class just modeled and to introduce new concepts, the teacher encouraged students to think about what the process was called in their daily life when a solid changed to a liquid. Several students answered “melting.” She then confirmed the answer and showed a transparency with equation-like and visual representations of liquid and solid.
The class went through sublimation, precipitation, condensation, and evaporation. All these processes were shown by verbal and visual representations. The teacher then had students in each group to choose a phase changing process and to use their hand model to demonstrate the process. The teacher walked around the class to see how students worked together. After the group discussion, the teacher introduced a new activity, and the class moved to a new event to learn about chemical formula and atoms.
As shown in this narrative, a variety of models and representations were used to learn the concept of phase changes. During this 25-minute event, students were repeatedly exposed to the ideas of molecules, phases, movement, energy, and models and learned to use models to represent invisible particles. Additionally, the teacher brought in an example of soda and asked students to apply their model to make explanations. Students not only learned concepts of molecules but also engaged in the activities that provided them with opportunities to reinforce the ideas of particles and move between the microscopic and macroscopic levels. The narrative reveals that this class used representations as resources and applied them to make explanations.
Target Students: Interviews
Five target students were interviewed about their understanding of the particle arrangement and movement in different substances. They were shown a picture with a stove burning wood and a kettle with boiling water and asked to draw how molecules of solid (wood), liquid (water) and gas (vapor) were arranged. All of them provided appropriate descriptions of the movement and arrangement of molecules in three phases. As mentioned previously, the teacher frequently used various types of models to represent the particulate nature of matter in different phases and phase changes. It seemed that repeated exposure of multiple representations helped the students to develop their understanding of particles.
To illustrate how students visualize air as particles, we present works of three students, including their self– explanations of why their post air drawings differed from the previous ones. Two of them were interviewed after they completed their drawings.
Asha was a high achiever in science and actively participated in the class activities. Appendix C shows her pre and post air drawings. As she explained in a written paragraph, when finishing her pre air drawing, she did not recognize that air is made up of particles. In her post air drawing, she listed names of particles and viewed them as molecules, although she used anthropomorphism to describe the molecules as “bad” in the polluted air versus “good” in the clean air. Asha explained her pre air drawings as follows: “Dirty air would have dark black spots and wiggly things in it because it has germs….The reason why my box (clean air) has nothing in it is because if the air is clean there is nothing in there.”
She explained her post air drawings as follows: The difference is that my dirty picture now has molecules representing bad air. My clean air has good molecules of oxygen and nitrogen to represent the clean air. My dirty air has much more stuff in it. More pollutants instead of little wiggly things. Clean air has clean molecules instead of nothing. Bad molecules affect air in a bad way. Good molecules affect air in a good way.
According to Asha’s explanations and drawings, she recognized that air is made of particles (molecules) and presented this idea visually by circles and lines in her post air drawings. After a series of activities about the particulate nature of matter, she was able to identify several substances in the air, give them appropriate chemical names, and draw them as ball-and-stick models represented on eChem and by gumdrops. However, she still kept an alternative conception of the polluted air that had only pollutants. In the follow-up interview, Asha explained how her ideas changed in her two drawings: “At the beginning, I had just dust and dirt [in the dirty air]. Now I have dust, sulfur, acid and I didn’t have a lot of nitrogen and oxygen. In my after picture of clean air, I have lots of oxygen and nitrogen.”
When she was asked about why she chose to draw three O2 and two N^sub 2^ molecules in the clean air, she responded that air is made up of 21% oxygen and 78% nitrogen and that she should have drawn more nitrogen molecules. The interviewer then asked her to explain the reason for not drawing oxygen or nitrogen molecules in the polluted air. She said, “They [nitrogen and oxygen molecules] are there, but there is not a lot of it.” In response to the interviewer’s probe of the percentages of nitrogen and oxygen in the polluted air, she responded that she did not know the percentage, but the dirty air has less oxygen and nitrogen. When asked about what helped her change her ideas, she listed a video the class had watched, eChem, and the models they built.
As can be seen in other students’ drawings, Asha and her classmates held a conception that polluted air had much less oxygen in it. This alternative conception was enhanced by the teacher, who later realized that she misled her students and corrected herself. However, it seems that the students still kept this conception during the time of making their post drawings.
Gervase generated a similar pre air picture (Appendix D), but his explanation revealed that he had deeper understanding than did Asha in the composition of the polluted air.
The difference between my clean air pictures is that [now] I know more about oxygen and that it takes up to 21% of the air. Without oxygen we couldnot breath ..Another reason why my picture is different is because nitrogen takes up 78% of our air and I did not know that [before]…. The difference in my dirty air pictures is that I did not know that only 1% of our air is other stuff, like sulfur dioxide and bad ozone. That is why my picture is different and my dirty picture has less things in it.
While Gervase’s drawing did not indicate his attempt to illustrate the composition ofthe air; his written explanation addressed the quantitative aspect of the composition. Although he drew few chemical formulas of the pollutants without showing nitrogen and oxygen, he explained that all of these pollutants take only 1% of the polluted air. Therefore, compared to his pre air picture, there were less “things” in the post drawing. Additionally, while Gervase mentioned nitrogen in his written paragraph, in the post clean air drawing he drew “NO” to represent nitrogen. Some other students also demonstrated this type of representational error by using incorrect chemical symbols to represent substances in their drawings. We suspect that the curriculum, as well as the teacher who frequently used the term “nitros” for NO^sub X^, caused students to confuse nitrogen with nitros and NO^sub X^.
In Gervase’s case, his drawing and written paragraph complement each other. These artifact analyses support our approach of including both graphical and verbal representations in students’ artifacts in order to better demonstrate and construct understanding.
Tabari, a student with relatively low achievement in science, was one of the few students who drew particles in the pre air drawing (Appendix E). In his pre air drawings, he identified that both clean and dirty air are made of particles. Additionally, clean air particles are different from dirty air particles, but still exist in the dirty air. His ideas of dirty air particles were similar to the conceptions of pollutants.
However, Tabari’s written paragraph and his interview transcript substantiated a microscopic view about air. He presented relatively low understanding, even after a series of learning activities about the particulate nature of matter. What Tabari wrote in his written explanation was mainly that clean air is clean and dirty air is dirty. During the interview, the interviewer tried to prompt him by asking him about substances in the air. He responded, “The stars represent clean air,” and “You have only 2% of clean air in the atmosphere.” Later on he mentioned that clean air has 15% nitrogen, but could not recall what the other 85% were. Again, Tabari’s artifact shows that the combination of graphic representation and verbal description helps us better understand his ideas.
Discussion and Conclusions
While the lens for this study does not allow for individual learning opportunities to be evaluated and correlated to specific learning outcomes, it does provide insight into the effectiveness of the overall set of learning activities and the fundamental design features that framed them. These design features include supporting an extended inquiry in a meaningful context and providing multiple opportunities of applying the concept.
In the classroom narrative and instructional sequence described previously, a variety of models and representations were used to illustrate and apply the concept of particulate nature of matter. Our intention was not to make comparisons and argue which model is more efficient than others. Previous studies (Copolo & Hounshell, 1995; Wu et al., 2001) have already addressed this issue. Our findings support previous claims by Wu et al. (2001) that various models should be provided in science classrooms, because different models emphasize different aspects of concepts, and different students have preferences for different types of models and symbol systems.
The classroom narrative depicted the teacher engaging the students in a variety of symbolic visualizations (e.g., verbal descriptions, drawings, hand models, and equation-like expressions). During this transcribed event, students were repeatedly exposed to the ideas of molecules, phases, movement, energy, and models and learned to use various models to represent invisible particles. No single method illustrated all of the five aspects of matter, but instead each type of representation highlighted a specific characteristic of the concept.
The design of the curriculum project provides opportunities for multiple representations to be used as formative assessment instruments that are a powerful component of effective teaching (Bell & Cowie, 2001; Black, 1998). By using the hand models and the prepost air drawings to externalize ideas, students’ understanding of molecules was visible to the teacher. For example, when students were asked for hand models, the teacher could easily identify that some of them may not have developed ideas of movement yet. Although some students seemed to copy other students’ responses in the hand modeling activity, students’ performances could still be evaluated through drawings. Thus, the teacher used them to assess students’ understanding formatively and decided whether she needed to go through a specific concept again. This study shows that the enactment of a whole sequence of gumdrop, hand, and human models and then computerized models and drawings had contributed to the continuous shift from one level to another. Various types of representations enable students to make transitions between the macroscopic, microscopic, and symbolic levels. This finding is supported by Dori and Hameiri (1998), who reported higher gains and less frustration of students who had the opportunity to make this transition in a gradual and constructed manner.
The findings summarized in Figure 1 and Table 3 indicate that, although 4 weeks had passed from the day the students finished the particulate nature of matter section of the unit, no decay effect occurred. The data illustrate continued improvement in student scores. While there was not a significant difference in the difficulty level in the design of the two instruments, scores on the posttests were higher than on the midterm quiz, despite the passing of4 weeks. Additionally, when comparing student scores between the 1998 enactment and the reorganized curriculum for the 1999 enactment, significant improvements are illustrated. The 1998 effect size associated with the particulate nature of matter subscore was .33 (all teachers) and .50 for the target teacher. These same measures associated with the 1999 data show marked improvement:.91 and 1.29 (all teachers/target teacher, respectively). This improvement occured despite maintaining the same amount of instructional time associated with the particulate nature of matter and an overall decrease in the total number of days of the curriculum enactment (54 days in 1998 and 45 days in 1999). These findings suggest that the curriculum sequence is an important aspect for supporting deep conceptual understanding. The use of the particulate nature of matter as a mechanism to explain phase and chemical changes is a plausible explanation for the increasing trend on the objective measures.
Sequencing the curriculum and using similar strategies (multiple levels of representation-macro, micro, symbolic; gumdrops, human, eChem models) for learning more complex concepts allow the students to extend their understanding of particulate nature of matter. Anderson, Reder, and Simon (1996) suggested several factors that may influence the degree of transfer. Two of these factors. are consistent with the design of the air quality unit. These factors are that (a) the degree of transfer is influenced by the number of symbolic components that are shared, and (b) transfer may also be facilitated when the cues that signal the relevance of an available skill are emphasized. As students progressed from the segment of the curriculum focusing upon the particulate nature of matter to the segment illustrating chemical and physical changes, several symbolic representations were maintained and the relevant cues were emphasized. For example, classroom instruction associated with phase and chemical changes utilized identical human, gumdrop, and drawing representations, as in the particulate nature of matter project segment.
When the class was engaged with understanding phase changes, human models that emphasized the arrangement, motion of the particles, and the level of energy were constructed to facilitate the change. Each time this activity was conducted, specific cues were highlighted. During the illustration of chemical changes, the classes constructed the compounds associated with the reactants and then rearranged the particles to illustrate the mechanism for chemical changes. The rearrangement of particles was highlighted as the key point of discussion. Anchoring the learning context in the driving question allowed symbolic components between the different project segments to be shared. One such example is the use of similar compounds (air pollutants) in the various example representations of particles in air and in chemical reactions.
In summary, the particulate nature of matter has been recognized by national standards (AAAS, 1993; National Research Council, 1996) and is also known to be particularly difficult for students to comprehend (Ben-Zvi et al., 1986,1987; Krajcik, 1991; Gabel, 1998). This study shows some promising curriculum design features. These features include multiple representations that allow students to experience phenomena at the macro, micro, and symbolic levels. These levels are illustrated through repeated exposure and using various representations that illustrate different aspects of the molecular theory. Sequencing activities in a gradual way enables urban students to integrate concepts into model construction. The extended nature of the project provides urban students with sufficient time and interactions to develop substantial conceptual understanding. Using a high degree of shared symbolic components over time provides opportunities for students to make understanding more robust and provides a mechanism to explain other complicated concepts.
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Editors’ Note: We thank the numerous people associated with the Center for Learning Technologies in Urban Schools who made this work possible. It is their continuing commitment and dedication, which is reflected in the work reported in this article. The research reported was funded with support from the National Science Foundation under the following programs: Research on Educational Policy and Practice (REC-9720383, REC-9725927) and Urban Systemic Initiative (ESR-9453665). All opinions are the responsibility of the authors and no endorsement by the National Science Foundation should be inferred.
Jonathan E. Singer, College of Education, University of South Carolina; Revital (Tali) Tal, Department of Education in Technology and Science; Hsin-Kai Wu, School of Education, University of Michigan.
Correspondence concerning this article should be addressed to Jonathan E. Singer, University of South Carolina, College of Education, 223 Wardlaw Hall, Columbia, SC 29208.
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Jonathan E. Singer
University of South Carolina
University of Michigan
Revital (Tall) Tal
Department of Education in Technology and Science
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