Exploring the Phases of Carbon Dioxide and the Greenhouse Effect in

Laboratory Experiment pubs.acs.org/jchemeduc

Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

Exploring the Phases of Carbon Dioxide and the Greenhouse Effect in an Introductory Chemistry Laboratory Jessica C. D’eon,* Jennifer A. Faust,† C. Scott Browning, and Kristine B. Quinlan Department of Chemistry, University of Toronto, 80 St George Street, Toronto, Ontario M5S 3H6, Canada

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S Supporting Information *

ABSTRACT: A safe and inexpensive laboratory for a first-year general chemistry course has been developed which allows students to explore important properties of carbon dioxide, particularly its behavior as a greenhouse gas. Students witness and measure air displacement to compare calculated gas densities and pressures inside a CO2-filled balloon with those of the surrounding air; because the balloon is permeable to CO2, students can determine the average rate of CO2 loss from the balloon. A loosely sealed centrifuge tube containing dry ice allows students to briefly observe liquid CO2 and to map that experience onto the phase diagram of CO2 discussed in lecture. Students simulate the ability of CO2 to act as a greenhouse gas by measuring differences in heat retention in the presence and absence of a CO2-enriched atmosphere within an open beaker containing gravel that serves as a model Earth system. Student reflections indicate substantial gains in a more scientifically rigorous understanding of the role of greenhouse gases in the heating of the atmosphere. KEYWORDS: First-Year Undergraduate/General, Environmental Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Misconceptions/Discrepant Events, Atmospheric Chemistry, Gases, Kinetic-Molecular Theory, Phases/Phase Transitions/Diagrams, Public Understanding/Outreach

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INTRODUCTION

This simple low-cost laboratory is an easy way to introduce this content into an often crowded first-year curriculum. Having students measure an increase in temperature with an increase in greenhouse gas concentration has a tangible “wow factor” that clearly communicates the importance of greenhouse gases on the climate system. In the first part of the laboratory, students explore important qualitative and quantitative aspects of gas behavior and phase changes using dry ice as a convenient gas source. These topics fit well with the early part of the general chemistry curriculum, and the use of dry ice excites and engages students.

The chemistry laboratory is considered a fundamental aspect of chemistry education.1−3 Laboratory curriculum is set by instructors to provide students with practical scientific experience that will build the students’ knowledge base and hone their hands-on skills.4,5 However, despite these best intentions, learning in the chemistry laboratory, particularly in an introductory general chemistry course, can be hindered by anxiety related to time constraints and grades.6−9 In an attempt to alleviate some of these anxieties, a concerted effort has been made to design laboratories at the beginning of the curriculum that are fun and topical, with an emphasis on exploring the phenomenon in question with less pressure to make accurate measurements and calculate graded quantities. Climate change is a key environmental issue facing the world today. The next generation of decision-makers and innovators can be enabled to combat climate change by providing them with a scientifically accurate understanding of the climate system. It is well-established that students enter university with a fragmented understanding of climate science.10,11 As a result, students often create mental models surrounding the climate system that bring several pieces of evidence together but, as a whole, are not scientifically sound.12−16 Embedding climate science into a first-year chemistry curriculum is an effective way to engage a large number of students and battle these misconceptions at the beginning of the students’ university science career. © XXXX American Chemical Society and Division of Chemical Education, Inc.

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EXPERIMENTAL OVERVIEW AND PEDAGOGICAL GOALS This 3 h, cost-effective experiment,17 using typical laboratory glassware, a retort stand, and a thermometer together with a 100 W incandescent light bulb, has been running successfully in the one-semester general chemistry course entitled “Chemistry: Physical Principles” at the University of Toronto since the Fall of 2016, with over 3500 students moving through the first-year curriculum in this time. High school chemistry is the only expected preparation for students entering this class. Results described here were collected from the Winter 2018 iteration of the class, with an initial course enrollment of Received: May 18, 2018 Revised: November 9, 2018

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DOI: 10.1021/acs.jchemed.8b00373 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

approximately 820 students. Students perform the experiment in pairs with one teaching assistant overseeing a group of 24 students or less. This experiment is the second of five experiments in the laboratory curriculum and is separated into two parts. In Part A, simple manipulations and exercises involving dry ice are introduced to achieve the following learning goals with both qualitative and quantitative objectives:

1. Students should be able to describe the principle of blackbody radiation and contrast the electromagnetic radiation emitted by the Sun versus that emitted by the Earth in terms of their relevance for the greenhouse effect. 2. Students should be able to describe the relationship between photon energy (or frequency) and the excitation energy (or frequency) of the molecule. 3. Students should be able to explain that collisional relaxation of vibrating greenhouse gas molecules results in an increase in the kinetic energy of the surrounding gas molecules, resulting in a warmer atmosphere. Students explore the greenhouse effect by measuring temperature changes of a “beaker Earth” with and without additional carbon dioxide added by dry ice sublimation (see experimental setup shown in Figure 1 and full procedure in Supporting Information). Students perform four trials measuring the temperature change over 10 min as follows: 1. Heating from room temperature, lamp on, air-only 2. Heating from room temperature, lamp on, CO2-enriched 3. Cooling from elevated temperature, lamp off, air-only 4. Cooling from same elevated temperature as air-only, lamp off, CO2-enriched Our experimental setup differs in some keys ways from previous demonstrations22−25 in order to address the stated objectives. Keeping the beaker open to the air reinforces that changes in greenhouse gas concentrations (carbon dioxide) alone are responsible for observed differences in temperature (Part B learning objectives 2 and 3), as opposed to the trapping of convectively heated air as is the case in a physical greenhouse, a common misconception.12−16 Exploring temperature changes with (trials 1 and 2) and without (trials 3 and 4) the lamp on allows us to extend our discussion beyond the vibration of greenhouse gases to include different sources of photons in the system (Part B learning objective 1). This focus on photon source is a novel approach to guide students toward a thorough and chemically accurate understanding of the greenhouse effect. The use of dry ice as a source of carbon dioxide is fun, easy, and safe compared to gas cylinders, and it is more reliable than reactions of carbonate salts in aqueous solution, where the phase changes of water complicate temperature measurements. Finally, the use of plastic beakers to simulate the Earth and its atmosphere, as opposed to aquaria, makes implementation feasible in a large undergraduate laboratory or as an outreach activity. The greenhouse gas activity has been successfully used to teach a class of grade six students about climate science.

1. Students should recognize phase changes, including the effect of both temperature and pressure on phase state, and interpret phase diagrams. 2. Students should be able to use experimental mass measurements together with the ideal gas law to calculate gas pressure and density, and they should be able to evaluate their calculations with respect to their physical significance and laboratory observations. 3. Students should develop their sense of scale by describing changes in mass in terms of numbers of molecules. The full, detailed procedure is available in the Supporting Information. Briefly, students first place small pieces of dry ice in warm water to observe its sublimation at ambient pressure and then place dry ice pieces in a sealed centrifuge tube to observe melting at elevated pressure (Part A learning objective 1). Previously reported experiments have used CO2 to explore phase diagrams18,19 and used liquid CO2 for extractions,20,21 and their procedures were modified for use in this experiment. Mass measurements of the centrifuge tube alone and after all CO2 was transformed into gas allowed students to use the ideal gas law to calculate an approximate pressure inside the tube and see for themselves whether it was consistent with the minimum pressure required to achieve the liquid state according to the phase diagram of CO2 (Part A learning objectives 1 and 2). These practices also effectively prepare students for working with dry ice to deliver CO2 gas by sublimation when exploring the greenhouse gas effect in Part B. Next, students add small pieces of dry ice to an empty balloon and take a mass measurement as soon as the balloon is tied. As the balloon inflates from the sublimation of CO2, air is displaced from the balance pan, and to the surprise of some students, the mass reported by the balance decreases! The students take a second mass measurement of the balloon once all of the CO2 has sublimed. These measurements establish the mass of displaced air, and together with the circumference of the (assumedly spherical) balloon, the density of air can be calculated. Students also calculate the density of gas-phase CO2 using the initial mass of dry ice added and the volume of the balloon (Part A learning objectives 1 and 2). Students measure the change in mass approximately 10 min after complete inflation of the balloon to quantitatively investigate an event that every student has experienced outside of the lab (the escape of gas from an inflated balloon) explicitly expressed here in units of molecules per second (Part A learning objective 3). Part B of the experiment was designed to help students develop a chemically accurate understanding of how greenhouse gases warm the Earth. To this end, we developed three learning objectives for this part of the laboratory and associated activities, including use of online simulations, that we believe facilitate the development of a clear mental model of the greenhouse effect:

Figure 1. Apparatus to study the greenhouse effect demonstrating how CO2 is added to the “beaker Earth” using dry ice in an Erlenmeyer flask immersed in a beaker of warm water. The beaker is filled with carbon dioxide gas in approximately 30 s, and students confirm the presence of CO2 by extinguishing a lit barbeque lighter. B



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HAZARDS All parts of the experiment must be carried out wearing safety goggles. Dry ice is extremely cold and, if inappropriately handled, can cause cold burns or frostbite. Insulated gloves or tongs should be used to handle the dry ice. The centrifuge tubes are not airtight,26 so as the dry ice sublimes and pressure in the tube builds, CO2 is slowly leaking out of the tube. Nevertheless, the sealed centrifuge tubes filled with dry ice remain a potential explosion hazard, and therefore, a plastic container for the warm water bath is recommended. No such explosions have occurred over the 1500 tubes used by students since the experiment has been running. Students are also explicitly directed to ensure that the tube is never pointed toward themselves or other students. The heated water bath should be handled with insulated gloves.

Table 2. Calculated Air and CO2 Densities with Corresponding Student Response Calculated Density, g/L

RESULTS AND PEDAGOGICAL OUTCOMES The laboratory preparation, in-lab experiment, and report writing work synergistically to guide students toward meeting the learning outcomes. In Part A of the experiment, students use dry ice to explore phase changes and the ideal gas law. In the Winter of 2018, 63% of students reported successfully observing liquid CO2 in their sealed centrifuge tubes. As previously reported,20,21 the centrifuge tubes leak at higher pressures, and thus, some tubes do not achieve pressures necessary for liquid CO2; such troubleshooting issues are addressed in the Supporting Information. Nevertheless, with students working four pairs to a bench, all students had the opportunity to observe this phase change during the course of the experiment, if not in their own setup then in a neighbor’s. Students were asked to correlate the observed formation of liquid CO2 at elevated pressure and constant temperature of the water bath to the liquid region of the phase diagram of CO2. Students used mass measurements of the centrifuge tubes to calculate the pressure inside and were asked the following question: “Is the pressure inside the pressurized Falcon tube enough to allow liquid CO2 to exist?” Sample student responses provided in Table 1 demonstrate that students are able to explain their experimental observations in light of the phase diagram (Part A learning objectives 1 and 2). Table 1. Calculated Pressure in Falcon Tube with Corresponding Student Response Sample Written Student Response to the Question: Is the Pressure inside the Pressurized Falcon Tube Enough To Allow Liquid CO2 To Exist?

4.50

“No, CO2 cannot exist as a liquid below 5.1 atm (triple point P).” “Yes, this pressure inside the Falcon tube is enough to allow liquid CO2 because the triple point is 5.1 atm according to the phase diagram of CO2 in the lab manual. 7.28 atm > 5.1 atm so there’s enough pressure and since the tubes is in hot water bath, the temperature paired with the pressure allow liquid CO2 to exist which is exactly what we observed.”

7.28

Air

2.26

1.23

2.25

1.39

“The calculated density of CO2 is greater than that of air which makes sense because the CO2(g) spilled out of the beaker and fell onto the counter, sinking below the less dense air.” “Yes, as when the dry ice sublimed and the gas left the beaker of water, it flowed over the lab bench rather than rising, since it is much denser than air as shown in the experimental calculations.”

connect their in-lab measurements and gas law calculations to the qualitative phase change behavior observed in the lab (Part A learning objectives 1 and 2). The rubber from which the balloon is made is not impervious to the loss of CO2. By recording the mass of the balloon 10 min after inflation is complete, the mass of CO2 lost to leakage can be used to determine the rate of CO2 loss in molecules per second. Values in Winter 2018 ranged from 3 × 1016 to 2 × 1020 molecules per second. The idea that small changes in mass are a result of a large number of CO2 molecules escaping the balloon is still not intuitive for many of these introductory chemistry students (Part A learning objective 3). Part A of the experiment illustrates some important aspects of gas behavior (pressure, displacement, effusion) and further serves our objective of creating a fun and relevant learning experience for students. The students enjoy working with the dry ice and the measurements are robust. The calculations are appropriately rigorous, and the results are meaningful. In Part B of the experiment, students graph heating and cooling curves for both air-only and CO2-enriched trials, an example of which is shown in Figure 2. Because the output of the different 100 W incandescent lightbulbs is not identical and the concentration of carbon dioxide in the CO2 trials is not fixed, we do not expect students to measure the same temperature differences; rather, we are looking for students to be convinced that carbon dioxide significantly increases the heating and reduces the cooling of the system, and for them to be able to explain the mechanism responsible for these differences. In the heating curves, 98% of the Winter 2018 students saw more heating in the CO2-enriched trial than in the air-only trial, with 94% of students measuring a difference of 2 °C or greater and 89% a difference of 3 °C or greater. When observing this temperature difference, students often express their surprise and new appreciation for the effect of greenhouse gases on climate. Temperature differences in the cooling trials were, as expected, smaller, but still convincing. 85% of students saw greater cooling in the air-only trials with 67% of students measuring a difference of 0.5 °C or greater (see Figures S1 and S2 in the Supporting Information). Students are challenged to think about sources of photons in the system and the effect of temperature on photon flux through a comparison of the heating and cooling curves. When the light bulb is on, it is responsible for the majority of the photons present in the experimental system. This increase in infrared photons is reflected by the larger temperature difference between air-only and CO2-enriched trials in the heating curves as compared to the cooling curves (Figure 2). Students are asked to explain this difference in their reports.

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Calculated Pressure, Atm

CO2(g)

Sample Written Student Response to the Question: Does the Calculated Difference in the Density of Air and CO2(g) Agree with Your Observations When Dry Ice Was Placed in Water?

Students were able to accurately measure and calculate that air is less dense (mean Winter 2018: 1.31 ± 0.49 g/L; cf. 1.23 g/L at 15 °C27) than CO2(g) (mean Winter 2018: 2.30 ± 0.65 g/L; cf. 1.86 g/L at 15 °C28), using mass and volume measurements of a CO 2-filled balloon. Students were challenged to place the calculated density values in context with their observations of the fog that results from the addition of dry ice to a beaker of warm water. Sample responses are provided in Table 2 and demonstrate the students’ ability to C




Two responses from the Winter 2018 semester are shown below: “The ΔT value of the heating curve is greater than the ΔT of the cooling curve. This is because the lamp emits more infrared photons than the gravel does. Also, during heating, photons are being absorbed by the greenhouse gases (CO2) as well as the gravel. While, during the cooling, fewer photons are emitted and any photons not absorbed escape the system resulting in low temperatures in both trials.” “When heating, there are two sources of photons and therefore heat energy. The lamp and the rocks are both active but when the cooling trial runs, only the rocks can emit radiation. Since there are more sources of photons in the heating system, the temperature difference is greater.” These student responses demonstrate a clear understanding of photon sources and fluxes within the experimental setup. Students are then asked to build upon this understanding to the actual Earth−Sun system by commenting on the relevance of incoming photons from the Sun versus outgoing photons from the Earth to the greenhouse effect. Two responses from the Winter 2018 semester are provided below: “It depends more on absorption of outgoing photons because it is these that, when trapped, lead to the greenhouse effect or, if not absorbed, lead to cooling through their release. The in-coming photons from the Sun that aren’t absorbed in the atmosphere can be absorbed by the Earth, warming it in this way separate from the greenhouse effect.” “Greenhouse gases which are able to absorb outgoing radiation from the Earth have a greater effect on climate. This is because only a small fraction of the in-coming solar radiation are at the IR wavelengths, but almost all the energy the Earth radiates out is in the IR region. This radiation when absorbed by the greenhouse gases, traps the heat in the atmosphere and prevents it from being lost to space. As a result, the Earth keeps receiving energy from the Sun, but cannot lose it, leading to a temp increase.” The extensive prelab preparation, which includes exploration of an online blackbody spectrum simulator29 (details in Supporting Information), the design of the experiment itself, and the follow-up report questions leave students with a clear and scientifically accurate mental model of relevant photon fluxes within the Earth system (Part B learning objective 1). To further assess student learning gains and to provide students with the opportunity to reflect on their learning, students were asked “How does the greenhouse effect warm the Earth?” before and after completing the experiment. The responses were open-ended and not for credit. 581 students completed both the before and after reflections in Winter 2018. Representative student responses are given in Table 3. Prior to the experiment, many students respond to this question with relatively simple explanations that greenhouse gases trap heat without any detail of how this occurs. After completion of the experiment, 87% of students had refined their answer to give a more chemically accurate, often particulate-level, mechanism of the greenhouse effect. 78% of the postexperiment responses stated that greenhouse gases absorb photons, and that this is the mechanism for warming of the Earth. Almost half of the postexperiment responses mentioned vibration of carbon dioxide or other greenhouse gases. These responses are strong evidence that this laboratory experience largely accomplishes Part B learning objectives 2 and 3.

Figure 2. Representative (a) heating and (b) cooling data from a pair of Winter 2018 students.

Further analysis of these responses showed that, before the experiment, 21% of students conflate ozone depletion with the greenhouse effect and that 15% of students demonstrate a mental model that involves greenhouse gases causing solar radiation to be reflected back into space or reflected back toward Earth. These particular misconceptions have been identified previously12−16 and could stem from a lack of understanding that photons are emitted from sources other than the Sun. If the Sun is the only source of radiation, then increases in temperature could be explained by increased solar radiation entering the Earth system (ozone depletion) or a decrease in the ability of solar radiation to leave (“radiation bouncing”). Our experiment specifically addresses this knowledge gap, which we identify in Part B learning objective 1, and its success in doing so is reflected in the postexperiment responses where discussion of ozone and “radiation bouncing” almost completely disappear.

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CONCLUSIONS This laboratory provides students with the opportunity to explore the greenhouse effect and phases of carbon dioxide in a series of fun and engaging exercises using dry ice. It was developed for a one-semester general chemistry course with high enrollment and few lab periods and could easily be modified to serve other formats, such as a discovery-based laboratory environment or as an outreach activity. Mass measurements made before and after the sublimation of CO2 inside a balloon allow students to witness air displacement and D



to reflect upon their calculated values of relevant gas properties such as pressure, density, and the rate of loss from the balloon due to its permeability. Students safely observe the liquid state of CO2 in a pressurized centrifuge tube and relate its (fleeting) existence to their calculated value of the pressure inside the tube. In studying its greenhouse effect, a simple, yet realistic, experimental setup (Figure 1) provides students with reliable and reproducible results to examine the impact of increased CO2 concentrations in the atmosphere. The prelab preparation, the designed exercises with “beaker Earth”, and the associated report questions collectively guide students to consider the relative importance of the two major infrared sources that contribute to the warming of the atmosphere. Comparison of pre- and postlab reflections show that 87% of students obtain a more refined, chemically relevant understanding of the phenomenon upon completion of the laboratory.

“The ozone layer, O3, is gradually thinning out. This layer serves as a protective “Greenhouse gas molecules increase Earth’s temperature because they vibrate at the same frequency as some of the infrared photons emitted by field against the sun’s rays and as this layer is thinning out, more of the sun’s the Earth’s surface. Not all of Earth’s infrared photons are expelled to space since the CO2 molecules absorb them. Thus, the Earth’s heat and radiation reaches the earth’s surface and warms up the earth.” temperature increases.” “Gases keep the light from the sun inside the ozone layer.” “When photons leave the earth and get lost so cool the earth, however when greenhouse gases are in the atmosphere, they capture these photons and make them vibrate so creating kinetic energy. For this reason greenhouse gases warm the earth or don’t allow to cool properly.” “Radiation coming from the sun is reflected off the Earth’s surface. A percentage “Radiation from sun is absorbed by earth’s surface. Earth’s surface emits IR EMR [electromagnetic radiation] and this is absorbed by CO2 of the reflected radiation is bounced back to Earth and another is lost. The molecules which vibrate and gain energy.” bounced back radiation warms the Earth.” “Gases are trapped in the Earth’s atmosphere, warming up the Earth. These “The photons coming from the Earth’s surface can either escape into space or be absorbed by the green house gas, only if the vibrational gases often arise from pollution.” frequency of the photon matches that of the greenhouse gas. When absorbed, the photon will cause the greenhouse gas molecule to vibrate, increasing the average kinetic energy of the molecules in the atmosphere, thus increasing the temperature and warming the Earth.”

Sample Student Responses to the Question: How Does the Greenhouse Effect Warm the Earth?

Corresponding Response after Completion of the Experiment Response before the Experiment

Table 3. Comparison of Sample Paired-Student Responses before and after Completing the Experiment


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00373.

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Additional details related to data collection and analysis, laboratory instructions to students and teaching assistants, student report sheet, and student selfreflection exercise (PDF, DOCX) Excel spreadsheet to simplify teaching assistant grading (XLSX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jessica C. D’eon: 0000-0001-7448-8828 Jennifer A. Faust: 0000-0002-2574-7579 Present Address †

Department of Chemistry, College of Wooster, 943 College Mall, Wooster, Ohio 44691, USA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank undergraduate students Bessie Xue, Labib Chowdry, and Bianca Bird who helped optimize the experimental procedure through the Research Opportunities Program at the University of Toronto. They also thank laboratory technician Rudolf Furrer and laboratory coordinator Dr. Marvin Morales for help with laboratory implementation and useful discussions. J.A.F. acknowledges the Chemistry Teaching Fellows Program in the Department of Chemistry at the University of Toronto for funding.

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REFERENCES

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