Addressing Students' Misconceptions about Gases ... - ACS Publications

Research: Science and Education

Addressing Students' Misconceptions about Gases, Mass, and Composition Kristin Mayer High School Chemistry Teacher, Franklin High School, 3013 South Mount Baker Boulevard, Seattle, Washington 98144, United States [email protected]

In the last 10-20 years, much research has been conducted to determine students' misconceptions about science concepts. The American Association for the Advancement of Science (AAAS) published a report titled “Science for All Americans” (1). The report identified which science concepts students should understand, whether a science major or not. As part of the research to support the development of these benchmarks, the AAAS cited literature about students' misunderstandings of the structure of matter (2) and stated “Middle and high-school students are deeply committed to a theory of continuous matter” (2). Students do not have a concrete idea of what the particulate nature of matter means, so they cannot manipulate this theory and therefore do not understand its implications for ideas such as gases. Additionally, although students in fifth grade begin to understand conservation of mass for physical changes between solids and liquids, they do not carry this over to ideas about gases (2). Not only do students fail to understand conservation of mass in relation to gases, but also studies have shown students have developed a “negative mass” conception of gases. “Children aged 9-13 tend to predict that gases have the property of negative weight and hence that the more gas that is added to a container, the lighter the container becomes” (3). These ideas are rooted in students' experiences with gases; helium balloons float, so they seem lighter or “to weigh less”. These misconceptions about gases seem to be universal, not just a product of American education. Hwang's (4) research shows middle, high school, and college students have similar misconceptions in Taiwan. Also, a study in France and Spain showed students in those countries do not understand gases in terms of the particulate nature of matter either (5). The observation that students around the world struggle with these ideas seems to indicate the misconceptions stem from their experiences in life and not poor education. Indeed, Mas (5) has shown that students' ideas about gases reflect the ideas of Aristotle, an early scientific theorist. Mas (5) and Driver (3) both emphasize how important it is to consider students' misconceptions when designing instruction. “The pupils' previous knowledge has a great influence on the process of learning and not only affects the interpretation of phenomena, but also sometimes makes the subject incomprehensible” (4, p 618). Many researchers have chronicled the misconceptions students have and how important it is to address these misconceptions, but it is difficult to find any research that suggested exactly how to address them. Thus, there is a need for research that tests various ways of addressing students' misconceptions so they can fully understand the material. In this action research, labs were designed to directly contradict students' misconceptions about gases and data was collected to understand how these labs affected the students' understanding.

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Classes This research was done with three high school chemistry classes. At this public, inner-city high school, all 10th graders were required to take chemistry. The students came from various middle schools that did not have a standardized science curriculum, so the students had various backgrounds in science classes. All students had a life science class during their ninth grade year. The 10th grade chemistry was their first chemistry class. The students were divided into three classes based on their grade point average and math class. The group with the highest grades was considered the honors class and the other two classes were the regular chemistry classes. All classes were inclusion classes and had students with special needs, learning disabilities, and behavior disorders. Because of attendance, some students did not take both the pretest and post-test. For the results discussed in this article, only the 63 students who were present for both the pretest and post-test were included. The class sizes ranged from 20 to 30 students. Students had previously covered the structure of atoms; forming ionic and covalent bonds; trends in the periodic table; identifying types of reactions; writing and balancing equations of reactions; and stoichiometry. Research Design Identification of Misconceptions Before beginning instruction about gases and the gas laws, students took a pretest to identify their major misconceptions (see the supporting information). The pretest was created using questions selected from an inventory of chemical concepts published by Mulford and Robinson in this Journal (6). The pretest had a total of seven questions designed to show what ideas students had about gases. The students took this pretest at the beginning of the unit. From the responses to the pretest questions, two misconceptions stood out. Only 14% of students correctly identified that the mass of the gaseous iodine would be the same as the solid iodine in question three. On question five, 92% of students answered that water would decompose when evaporated. Question three and five are shown in Figure 1. For each of these concepts, a lab or demo was developed to contradict the students' preconceived ideas. After the lab or demo, the class discussed the data and the correct theory. Mass of Gas Lab One of the most obvious misconceptions on the pretest was that students believed gases weighed less than solids. This is not surprising, as much of research has cited this as a common misconception. To address this erroneous idea, a lab was needed

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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 88 No. 1 January 2011 10.1021/ed1005085 Published on Web 11/02/2010

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Figure 1. Questions 3 and 5 from the pretest.

that would show the students that conservation of mass applies to gases. It was also critical that this lab provided accurate data with little possibility for error. The hope was to create an experiment that would contradict the students' expectations. To mimic the question from the pretest, a lab was developed that would contain gas in a sealed container. Sakar and Fraizer have shown when measuring the mass of gas it is important to keep the volume constant (7). Changes in volume affect the density and buoyancy, which causes the mass to appear to decrease. Because of this apparent loss of mass, labs using balloons or resealable plastic bags to capture and measure the mass of gas often reinforce the misconception that gases weigh less than solids and liquids. Kavanah and Zipp reported success using plastic soda bottles in a lab comparing mass to the amount of gas in the soda bottle (8). Adapting this lab, carbon dioxide from dry ice was contained in 2 L soda bottles. This lab with dry ice was used with the students because it produced accurate results and provided a situation that was similar to the question involving iodine on the pretest. Before starting this lab, the students made predictions about what they thought would happen to the mass after the carbon dioxide sublimed. Again, most of the students expected the mass to decrease. During the lab, the teacher placed between 0.50 and 0.75 g of dry ice (about a pea sized piece) in the 2 L bottles. Students sealed the soda bottles with the dry ice inside and measured the mass of the soda bottles with the solid carbon dioxide. Then, the students allowed the carbon dioxide to sublime and measured the mass of the soda bottle with the gaseous carbon dioxide. As they completed the lab, the students said “But it's the same!” One student even said he needed to do the lab again because something went wrong, the mass did not change; he was given the chance to run the test again. Clearly, the students were surprised to see that the mass was conserved. On the handouts, the students answered questions 112


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that required them to analyze their original expectations, explain what they learned in the lab, and relate it to the situation from the pretest (see the student handout in the supporting information). The students' answers were collected after the lab and their responses' analyzed. Forty-eight percent of the students correctly interpreted the data saying the mass of the gas was the same as the solid; most of the others either left the question blank or simply described their observations without addressing the question. Several students also noted that the bottle “felt harder” or had more pressure after the carbon dioxide became a gas. The next day, the results were discussed. The teacher introduced the idea of conservation of mass and the class discussed the fact that if five molecules of solid carbon dioxide changed to a gas you would have five molecules of gaseous carbon dioxide. The amount of molecules does not change; therefore, logically the mass should not change. Students were able to explain that gas spreads out more, but the amount of molecules, and therefore the mass, stays the same. Students also discussed that because the molecules were more spread out the volume of space occupied by the carbon dioxide increased, which caused the density to decrease, but the overall mass remained the same. Water Vapor Demonstration A macroscopic demonstration that would contradict the students' predictions was needed. It was difficult to show that water does not decompose when it changes to the gaseous state as it is impossible to actually see the atoms or molecules. I remembered my college professors thrilling the class with large explosions by filling balloons with explosive mixtures of gases and igniting them to start the reaction. Because the reaction of 2:1 hydrogen-to-oxygen is extremely explosive and water vapor is not, this was used as a way to show students that water vapor is different than hydrogen and oxygen gas. If water broke apart to

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r 2010 American Chemical Society and Division of Chemical Education, Inc.


hydrogen and oxygen gas as the students' indicated on the pretest, it would be predicted that steam would explode as well. Before class, a balloon was filled with a mixture of hydrogen and oxygen gas (see the supporting information). In class, another balloon was attached to an Erlenmeyer flask containing about 30-50 mL of water. The flask was placed on a hot plate and the boiling water inflated the balloon. As the balloon was inflating, the students drew what they thought the atoms or molecules inside the balloon would look like. The students were informed that the first balloon was filled with hydrogen and oxygen gas. After warning the other teachers at the school, instructing the students to cover their ears, and putting on protective earmuffs, the hydrogen and oxygen filled balloon was ignited using a candle attached to the end of a meter stick. Students said they expected the balloon being filled by the boiling water to explode as well. To test this, heat resistant gloves were used to remove the balloon from the flask of boiling water; this balloon was sealed using a clamp. The water started to condense, and the balloon shrunk rapidly. A small pool of water formed in the bottom of the balloon and this was shown to the students. The candle was used to ignite this balloon as well. The balloon did not pop or explode when heated. In one class, the balloon started to burn; however, it did not explode. The class was led through a discussion about the gas in the two balloons and whether water can break apart into hydrogen and oxygen gas when it boils. The students again answered questions about the demo and analyzed their initial drawing about what they thought the atoms inside the balloon looked like (see the student handout in the supporting information).

Figure 2. Pretest and post-test responses for question three.

Hazards If too much dry ice is added to the 2 L soda bottles, the increase in pressure could cause the bottles to explode. This explosion can be extremely loud and could cause considerable damage to ears. In addition, when the bottle explodes, sharp plastic pieces projected from the explosion could cause injury. It is important that the soda bottle only contains air and an acceptable amount of carbon dioxide; water or other liquids or solids could greatly affect the build up of pressure from the sublimation of the dry ice. The balloon containing the hydrogen and oxygen should not be larger than a grapefruit, it should not be ignited near fire detectors to avoid setting off the fire alarm, and everyone must protect their ears. Depending on the ratio of hydrogen-to-oxygen in the balloon, this can create a powerful explosion; extreme caution is necessary. Post-Test After the lab and demo, the students were taught about the nature of gases and the gas laws. To ascertain if the students returned to their original misconceptions, the students moved onto another topic to allow some time to pass before assessing the students' learning. About two and a half weeks after the demo about water boiling, the pretest was given again as a post-test. The post-test had the same questions in the same order; however, the title and directions were changed. The pretest had not been returned to the students, so they had not seen the test since the first time they took it; they had not seen the correct answers either. The students did see the test questions on the handout from the lab and demonstration. The students did discuss the questions and how they related to the data from the experiments,


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Figure 3. Pretest and post-test responses for question five.

but the teacher did not provide the correct answer to any of the questions. The pretest data was compared to the post-test data to access what the students learned and remembered. Results The percentage of students who selected the correct answer on questions three and five increased from the pretest. On question three in the post-test, 46% percent of students correctly stated that the mass of the gaseous iodine would remain the same. For question five in the post-test, 48% of students picked the correct picture of water vapor. On the post-test, the correct answer was picked more often than any of the other choices for both question three and five. The results from the pretest and the post-test are compared in Figures 2 and 3. Analysis The purpose of this analysis is to determine if the lab and demonstration helped change the students' preconceived ideas. It is interesting to look at the differences in growth of the various groupings of students, but because each student had a unique set of circumstances, the differences in growth could have been caused by variables other than the participation in the lab or demo. This work was done to address students' misconceptions and track data to ascertain if the approach was successful. Sixty-three students took both the pre and post-test; of these students, 52 had the incorrect answer for question 3 on the pretest. Of these 52 students who initially choose the incorrect


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response, half of them, 26 were absent on the day of the lab. These 26 students who were absent for the lab went from 0% correct to 31% correct on the post-test. The other half who did participate in the lab and had the incorrect response on the pretest, improved from 0% to 54% correct on the post-test. For the question about the composition of water vapor, 57 students had an incorrect answer on the pretest. Of these students, 16 were absent on the day of the balloon demonstration. On the post-test, four of the students who did not see the demonstration choose the correct answer. This is an improvement from 0% correct to 25% correct. Of the 41 students who had an incorrect answer on the pretest and were present on the day of the balloon demo, 23 students choose the correct answer on the post-test. This is an improvement from 0% to 56% correct. For both questions, the students who participated in the lab or demo showed more improvement than those who were absent. One surprising outcome on the post-test was that the number of students who had the correct answer for question six decreased (see the supporting information). This question addressed what happens to mass as an iron nail rusts completely. The number of students who said the rust would weight the same as an iron nail increased while the number of students who chose the correct answer, that rust weighs more, decreased. Perhaps, because conservation of mass was discussed, students were thinking the mass must stay the same rather than thinking of oxygen as one of the reactants. This result seemed to show that students were not thinking in terms of particles or atoms, rather they were incorrectly applying the concepts discussed in class. It is important to note that there was not a significant difference in the responses of the honors class and the regular chemistry classes. Though the honors group typically had higher grades, the honors group responses on the pretest and post-test were similar to the other students. Mulford and Robinson report the results when 1418 firstyear college students were given the same questions before and after two semesters of college chemistry classes for science and engineering majors (6). On the question asking about the mass of solid iodine compared to gaseous iodine, 68% of the college students got the correct answer before the classes compared to 75% correct after. For the question about the structure of water vapor, the college students went from 39% correct to 45% correct. Hake reported that using normalized gain was an accurate way of analyzing growth on the tests even if the groups being compared preformed differently on the pretest (9). The formula for calculating normalized gain is Ægæ ¼

post% - pre% 100% - pre%

where post% is the percent of correct responses on the post-test and pre% is the percent of correct responses on the pretest. On the mass of iodine question, the normalized gain of the college students was 22% improvement. The high school students in this study had a normalized gain of 32%. For the question about composition of water vapor, the college students had a normalized gain of 10% compared to a normalized gain of 43% for the high school students. The high school students had larger normalized gain than the college students. It is important to note the differences in the populations of Robinson and Mulford's study and this study. 114


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The college students were all science and engineering majors and had more background in science than the high school students. The high school classes included a wider variety of students, those who enjoyed science and those who “don't get” or “don't believe in” science. The college students also had less room for growth given they started with a higher percent correct responses on both questions. Because of these differences, no definitive conclusions can be made from this data. It is included here to provide a reference point against which to judge the change in the scores of the high school students. It is interesting that, on the question regarding the composition of water vapor, both the college students and the high schools students in this study ended up with about the same percent correct after lessons despite the differences in percent correct on the pretest and the different levels of interest and background in science. Conclusion After the lab and demo, there was an increase in the number of correct responses. However, the majority of the students did not correctly answer the question. Students' experiences with or “intuitive assumptions” about gas have created strong preconceived ideas. Students had a hard time accepting that gases have the same mass as solids. Though the students experienced this in a lab and were able to discuss the results the day after the lab, the majority did not remember this several weeks later. Instead, a significant number of them held on to the belief that gases weigh less than solids. Perhaps more labs should have been included, for example, a lab that measured mass versus the number of gas molecules in a container (8) and a lab showing that when solids react with gases the mass increases. Similarly, there was a significant improvement on question five; however, the majority of the students did not select the correct molecular representation of water vapor. The students had a hard time accepting the results of the demo and lab. They seemed surprised and some students even thought something must have gone wrong. Perhaps, the students needed more time to discuss the results and reconcile their ideas with the outcome of the lab. To do this, the students may need exposure to more situations showing them how gases behave and given time to discuss the results and how they compare to their original ideas. Though the students analyzed how their expectations matched up with the results, just one exposure did not seem to be enough to overcome their longstanding and powerful preconceptions. Future studies could include several questions, each addressing the same topic. This will differentiate between students who guessed the correct answer and students who understood the concept and were able to consistently apply their understanding. Future research could also involve interviewing students to ascertain why they choose the answer they did. Perhaps rather than addressing individual misconceptions, there may be broader underlying misconceptions. For example, the AAAS (2) stated that students do not understand the particulate nature of matter. Because many students do not think of all matter as made out of atoms, they may not have had a solid context for the activities or discussions done in class. When it was pointed out that gas is simply the same atoms or molecules as the solid only more spread out, the students recognized that the mass should stay the same. However, they may not think of matter in this context on their own, so on the test, they returned to their


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former misconceptions. Perhaps it would have been more effective to infuse the idea of the particulate nature of matter into the class rather than address misconceptions from specific questions as done in this study. At the beginning of this research, the author expected the lab and demo to drastically change the students' ideas. It was expected that showing them an experiment that disproved their ideas would be a powerful learning experience. However, this research shows that students' preconceived ideas are difficult to change. The experiences were powerful for some students, but the lab and demo used here clearly need to be implemented along with other instruction. To determine better ways of addressing students' misconceptions, further research is needed. It would be powerful to look at research that compares how students' understanding changes as the number of labs and demos increase as well as research that looks at how the length and depth of discussions about the labs and demos affects students' understanding and retention.

Literature Cited 1. American Association for the Advancement of Science. Project 2061: http://www.project2061.org/publications/sfaa/default.htm (accessed Oct 2010). 2. Benchmarks on line, Project 2061, AAAS. http://www.project2061.org/ publications/bsl/online/ch15/findings.htm#Ch4 (accessed Oct 2010). 3. Driver, R.; Squires, A.; Rushworth, P.; Wood-Robinson, V. Making Sense of Secondary Science; Routledge: London, 1994; pp 77-84. 4. Hwang, B.-t. Students' Conceptual Representations of Gas Volume in Relation to Particulate Model of Matter [Online] San Francisco, CA, April 1995. Available from ERIC, ED387325. http://eric.ed. gov (accessed Oct 2010). 5. Mas, C. J. F.; Hernandez Perez, J.; Harris, H. H. J. Chem. Educ. 1987, 64, 616–618. 6. Mulford, D.; Robinson, W. J. Chem. Educ. 2002, 79, 739–744. 7. Sarkar, S.; Frazier, R. Science Sampler 2008, summer, 52-55. 8. Kavanah, P.; Zipp, A. P. J. Chem. Educ. 1998, 75, 1405–1406. 9. Hake, R. Am. J. Phys. 1998, 66, 64–74.

Acknowledgment

Supporting Information Available

The author thanks Shawn Mintek and AP Statistics student Peter Ng for their help with the statistical analysis.

Pretest; student handout; instructions for filling hydrogen and oxygen balloon. This material is available via the Internet at http://pubs.acs.org.


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Addressing Students' Misconceptions about Gases ... - ACS Publications

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