Research: Science and Education
Chemical Education Research
edited by
Diane M. Bunce The Catholic University of America Washington, D.C. 20064
Using a Computer Animation to Improve Students’ Conceptual Understanding of a Can-Crushing Demonstration Michael J. Sanger,* Amy J. Phelps, and Jason Fienhold Department of Chemistry, University of Northern Iowa, Cedar Falls, IA 50614-0423; *
[email protected] Many articles and presentations at Division of Chemical Education sessions have extolled the virtues of developing students’ understanding of concepts that complement their ability to solve numerical problems (1–12). Often this conceptual understanding requires students to use the particulate nature of matter to explain what is going on in a chemical system at the microscopic level (13–19). Researchers have shown that instruction using the particulate nature of matter can improve students’ understanding of chemistry (16, 17, 19), helping them make the connection between the macroscopic, microscopic, and symbolic representations used by chemists (20). In spring of 1997, one of us (AJP) instructed 70 students enrolled in a second-semester introductory chemistry class about the behavior of gases, including static chalkboard drawings based on the particulate nature of matter (kinetic molecular theory). These students viewed a can-crushing demonstration in which a soda can containing a small amount of water was heated on a hot-plate to boil the water, removed from the heat, and sealed by inverting over a container of cold water (21–24). This demonstration was explained and illustrated in terms of the kinetic molecular theory. To determine whether students understood the experiment at the macroscopic and microscopic levels, they were given a demonstration-based quiz in the subsequent class period (5). They were asked to predict what would happen when a toner can filled with a small amount of water was heated on a hotplate, removed from the heat, and covered by putting on the can’s cap (21, 22). They were also asked to explain what was happening in this system at the molecular level. While these students did reasonably well on the quiz, some of the things they wrote while explaining the behavior of gas molecules were discouraging. The responses were analyzed to identify correct conceptions and common misconceptions. Evaluation of Student Responses One of the most striking observations was that even though some students correctly predicted that the can that was removed from the heat and sealed would collapse, many of them seemed to be blindly applying the gas laws to arrive at this conclusion rather than considering the molecular behavior specifically: The molecules of gas are hot and excited so they are moving around really fast. When you put the cap on, the pressure would increase, so the volume would then decrease, shrinking the can. Since the temp. is decreased by removing it from the heat, the molecules slow down and the volume also decreases. (Spring 1997)
[in margin] V ∝ T V ∝ 1/P Since the temperature would decrease, the volume would decrease, creating a need for higher pressure. Since the temperature ↓, volume ↓ the molecules are slowing down having lower energy so they are not filling the container full (less bumping into one another) so the container would cave in. (Spring 1997)
Interestingly enough, students who believed that the can would expand were just as likely to blindly apply the gas laws as those who thought it would collapse: The gas molecules are moving around very quickly and bouncing off the sides of the can quickly, causing expansion. They are moving quickly due to the temperature and because as temperature increases, so does volume. (Spring 1997) The water vapor inside it would continue to press out against the can due to Charles Law, and the can would possibly explode if the pressure was large enough. (Spring 1997)
In total, 23 of the 70 control students quoted gas laws in their responses. A more alarming student response was also identified from two control students’ responses: they expressed the belief that the molecules themselves are capable of changing their size and shape. The gas molecules when heated are expanded and when they cool they “shrink” back not taking up as much space as before. (Spring 1997) The gas molecules take the shape and volume of the container they are in. They would be moving around quickly because of the heat, and would create pressure to get more space. (Spring 1997)
Other students wrote comments that seem to suggest that energy behaves like a form of matter on the macroscopic level—that it could be trapped in a sealed container: Since the molecules were taken off the heat, they would be moving slower than before. They would also be colliding with each other less. But since it was capped, there would be no place for the excess energy to go and the can would get bigger. (Spring 1997)
With some responses, it was not so much what the students wrote but what they failed to write. Several students did not mention the changes in pressure or volume or the role of the liquid in any way, while others were very vague about how these changes in pressure or volume actually occurred: When the water in the can is heated, the molecules are moving very fast and bumping into the walls a lot which
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Research: Science and Education keeps the can in its usual shape. When the can is capped and cooled, the molecules aren’t moving as fast and can’t keep the sides up. The molecules in the can aren’t bumping into the sides very hard. (Spring 1997) When the molecules inside the can are hot they are farther apart or expanded. As the temperature in the can decreases the molecules move closer together. The can’s shape will change to accommodate the amount of space needed. (Spring 1997) The can’s walls would be sucked inward. The gas molecules are moving around everywhere very quickly, bouncing off of everything. When they do this, they exert a certain pressure. Because the can is a closed system, the temperature also changes but the pressure inside remains the same. The pressure differences suck in the walls of the can. (Spring 1997)
Another difficulty that was apparent in the responses is that many of the students did not recognize the importance of having the can filled with water vapor. In fact, 53 of the 70 students in the control group ignored the condensation of water vapor, many attributing the decreased pressure inside the can solely to the water molecules moving slower as the gas cooled. While the can is being heated the gas particles are moving really fast within the can, and some are coming out. Then when you take the can off the heat and cap it, the molecules are slowing down really quickly and the pressure outside the can is greater than inside so the can will compress. (Spring 1997) When heated the H2O turns into water vapor—as this cools, the gas takes up less space and since it is capped and air can’t enter to even out the pressure on the inside and out. The can must therefore collapse, because the gas fill a small volume when cooled. (Spring 1997)
Although several student misconceptions were reported above, 9 control group students gave a completely correct response. For a response to be deemed completely correct, two major points must have been mentioned: the condensation of water vapor, which results in a lower inner pressure, and the pressure differential between the lowered pressure inside and the constant (atmospheric) pressure outside, which results in the crushing of the can by the molecules outside the can. When the can is heated, the air goes out of the can and it fills with water vapor. When the can is allowed to cool with the cap on, the water vapor condenses and air wants to get back in. But since air can’t, the pressure inside the can is less than outside the can, and the air outside pushes on the can and it crinkles. (Spring 1997)
students’ visualization skills and their ability to think about chemical processes at the molecular level (25–27). Developing the Computer Animation To help students understand the chemical processes that occur when a can filled with water is heated and then sealed and cooled, we developed a computer animation depicting this process. Before the can is heated it contains liquid water, and the air space inside and outside the can contains air molecules (N2, O2, and some H2O). When the can is heated, the water boils and the water molecules enter the gas phase inside the can, forcing most of the nitrogen and oxygen molecules out of the can. When the can is sealed and cooled, the water vapor condenses back to a liquid inside the can. This lowers the pressure inside the can below atmospheric pressure, and the can is crushed by the collisions of outer air molecules with the surface of the can, which is depicted in Figure 1. The animation was carefully constructed so that it would correctly address the misconceptions identified from the students’ responses. For example, after the can was sealed and cooled, the water molecules were shown as moving slower than before; and eventually these molecules returned to the liquid state, which is depicted by water molecules encased in a circular water droplet (Fig. 1). Because the water molecules were depicted as moving slower after the sample was capped and cooled, this animation should dispel the belief that the molecules inside the can are exerting more pressure on the can than the outer molecules (i.e., that there is a pressure buildup inside the can). The animation should also help students realize that the condensation of the water vapor leads to a decrease in the number of gas particles inside the can, which translates into a decreased pressure. The pressure differential between the decreased inner pressure and the constant (atmospheric) outer pressure results in the gas molecules on the outside colliding with the can’s surface more often than the inner molecules, crushing the can. The animation was tested on a second set of 86 students enrolled in a first-semester introductory chemistry class (experimental group), who received the same instruction regarding gas laws and watched the same can-crushing demonstration as the first set (control group), but in addition viewed the computer animation. Both sets of students came from the same population: students (predominantly natural science majors) enrolled in an introductory chemistry course
The steam is what fills it right now, but if that turns back to water the pressure has gone down inside but stayed the same outside. It wants to be equal so it will crush the can to make it equal. (Spring 1997)
The variety of responses led us to wonder if there were not a better way to communicate the particulate nature of matter to students. Instruction using chalkboard drawings and colorful transparencies did not appear to be adequate. We decided that developing an animation depicting the behavior of gas particles in the can-crushing demonstration might be helpful. Several chemical education researchers have shown that computer animations can facilitate the development of 1518
Figure 1. Microscopic view of the can being crushed by the outside air molecules.
Journal of Chemical Education • Vol. 77 No. 11 November 2000 • JChemEd.chem.wisc.edu
Research: Science and Education Table 1. Student Predictions for Sealed and Cooled Can Students Predicting Student Group Control a Experimental
b
Collapse
Expansion
No Choice
No.
%
No.
%
No.
%
36
51
28
40
6
9
62
72
19
22
5
6
an
= 70. bn = 86.
at a small Midwestern university. However, the control students received instruction on gas laws while enrolled in a secondsemester course in the spring, and the experimental students received instruction on gas laws while enrolled in a firstsemester course in the fall. The experimental group viewed the animation three times (total time: less than two minutes), and the instructor explained the important and relevant microscopic processes depicted. Both sets of students received instruction at the molecular level (static drawings or animations) and each lesson lasted 25–30 minutes. Both sets of students were given the same quiz during the class period following instruction, and the two sets of responses were compared. Comparison of Student Responses Table 1 shows the number and percentage of students from each group who predicted the can would collapse or expand or who did not make a clear choice. As a result of viewing the computer animation, it is clear that the students in the experimental group were more likely to predict that the can would collapse and less likely to predict that the can would expand than students in the control group ( χ2(2) = 7.14, p = .028). Although the two sets of students were not completely equivalent, the control group had received more formal chemistry instruction. Therefore, any differences in the student populations should tend to favor the control group and not the experimental group. Students who viewed the animations were also much less likely to blindly apply gas laws in making their predictions. While 23 students in the control group (33%) quoted gas laws in their predictions, only 5 students in the experimental group (6%) did so (z = ᎑ 4.38, p < .0001). Students who had viewed the animation were much more likely to discuss the liquefaction (or condensation) of water. Only 17 students in the control group (24%) mentioned the process of condensation, whereas 50 students in the experimental group (58%) suggested that condensation was important (z = 4.25, p < .0001). The following quotes describe how students who saw the animation viewed the role of the condensation of water vapor in this process. The water would begin to cool. The water vapor would start to condense on the walls of the can, the can then should, in theory, collapse. (Fall 1997) The water vapor would be turning back to liquid. The can would crush because molecules outside of the can would crush into the sides, denting it in. (Fall 1997)
The students who viewed the animation provided more responses that were deemed completely correct explanations of the process: 9 of 70 (13%) in the control group vs 29 of 86 (34%) in the experimental group (z = 3.02, p = .0025).
Examples of correct responses are given below. When the can is heated, the air goes out of the can and it fills with water vapor. When the can is allowed to cool with the cap on, the water vapor condenses and air wants to get back in. But since air can’t, the pressure inside the can is less than outside the can, and the air outside pushes on the can and it crinkles. (Fall 1997) As the can cooled, the molecules of water as a gas would liquify. As there began to be less molecules of gas pushing out from within the can, the molecules of gas in the air outside the can would begin to push in on the can more frequently than the molecules inside, causing them to push the sides of the can in. (Fall 1997) When the can was heated the water turned to water vapor filling the can with gas. When the can is taken off and capped to cool, the vapor molecules begin to condense into (liquid) water reducing the pressure inside the can which causes the can to be crushed by the atmospheric pressure trying to equal out the two pressures. (Fall 1997)
Conclusions The purpose of this study was to develop an instructional approach to improve students’ conceptual understanding of the molecular processes occurring when a can containing water is heated, sealed, and cooled. Students who received instruction using static chalkboard drawings and overhead transparencies were generally successful at explaining these molecular processes. However, students who received similar instruction including the use of a computer animation of this process at the molecular level were less likely to quote memorized mathematical relationships and more likely to mention the importance of the condensation of water vapor and the decreased pressure inside the can and the resulting pressure differential inside and outside the can in their descriptions of the can-crushing process. These results are consistent with others (25–27) suggesting that students who receive instruction including computer animations of chemical processes at the molecular level are better able to answer conceptual questions about particulate phenomena. These results may also have implications for instructors struggling to determine how to use technology in the classroom to increase students’ conceptual understanding in chemistry. Acknowledgment Jason Fienhold was supported by the National Science Foundation through the Research Experiences for Undergraduates in Chemistry Program at the University of Northern Iowa (NSF grant number CHE-9531791). Literature Cited 1. Nakhleh, M. B.; Mitchell, R. C. J. Chem. Educ. 1993, 70, 190–192. 2. Zoller, U.; Lubezky, A.; Nakhleh, M. B.; Tessier, B.; Dori, Y. J. J. Chem. Educ. 1995, 72, 987–989. 3. Nakhleh, M. B.; Lowrey, K. A.; Mitchell, R. C. J. Chem. Educ. 1996, 73, 758–762.
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