Can CO2 Trap Heat?
The greenhouse theory claims that, for life to exist, atmospheric levels of carbon dioxide (CO2) must fall within a fairly narrow range. Too much CO2, and the biosphere would sizzle; too little, and it would freeze.
This laboratory activity focuses on one non-controversial aspect of the greenhouse theory: the ability of CO2 to trap heat. At the same time, this activity will try to help you gain a better appreciation for the puzzle-solving aspect of science. To achieve this second goal, here and there we shall ask you to stop reading this manual and to figure out things on your own.
ò And here is the first pause: 1. How can it be shown that CO2 traps heat?
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Developing a Sound Experimental Design
In a laboratory setting, we need to come up with a miniature version of reality. Instead of sunlight, we can employ an artificial light source. Instead of studying the behavior of CO2 in the atmosphere, we can study its behavior in a confined space.
On first sight, the ideal experiment appears simple enough. You take a glass jar and cover its internal surface with light-absorbing black paper. You then place a thermometer and some CO2 inside the jar, shine a strong light from above, and monitor the jar's internal temperature.
ò This is a good first approximation, but it will fail to give us a clear answer. 2. Why?
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Even if this setup works, it doesn't show that CO2 traps heat. For instance, the jar might have gotten warmer because it is a miniature greenhouse, not because it contains CO2.
ò 3. Can you design a better experiment?
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We can run two experiments. In the first, we shine the light for a fixed period of time on an air-containing jar and monitor the jar's internal temperature. In the second, we replicate the first experiment after placing CO2 in the jar.
In this design, the first experiment serves as a control for the second. An ideal control would be identical to the actual test in all respects except one. Thus, if in our setup the two experiments are equal in all things except the presence of additional CO2, any temperature difference between them must be traceable to CO2.
ò Ideally, all things but one would be equal in the control and experimental treatments. 4. Does our revised setup fulfill this requirement?
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It doesn't, because the time difference leads to a number of complications. For instance, one might wonder whether the lamp was hotter in the second period than in the first, whether the classroom got warmer or colder in the second period, whether the second period inadvertently lasted longer than the first. It would seem that our experimental design still requires some fine tuning.
ò 5. Any ideas?
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It might be better, it seems to us, to conduct the control and experimental operations simultaneously. For one thing, this would save us time. For another, this would allow us to overcome some of the complications above. To be sure, in the new setup we use two different jars, but this difference is more tolerable, and more readily overcome, than the time difference. So, for the time being we shall proceed with the simultaneous observations of air-containing and CO2-filled jars.
An Experimental Test
Materials Needed: Alka-Seltzer tablets, two identical quart-sized mason jars, glass marker pen, 150W reflector lamp, two 4" X 1/4" plastic vials (each provided with a rubber stopper with hole in the center and with 8"-inch plastic tubing with pipet pieces inserted in both ends), large bowl filled with cold tap water for rinsing vials, a beaker filled with cold tap water, black paper, scissors, glue, two thermometers, a powerful blow dryer with a cool cycle.
Note: Alka-Seltzer contains aspirin, baking soda, and citric acid. When placed in water, it releases CO2.
Procedure A: The Key Experiment
|(a) Divide into groups.
(b) Glue identical pieces of black paper to the internal surface of both jars.
(c) Label one jar "control" and the other "experimental."
(d) Place the two jars side by side, just short of coming in direct contact.
(e) Place the reflector lamp some 3" above the jars, equidistant from the centers of both.
(f) Place a thermometer in each jar.
(g) Turn on the lamp and wait until temperatures in both jars stop rising.
Notes: (i) Make sure that temperatures of the two jars are roughly equal. If they differ by more than 1° C, try adjusting the position of the jars or adding black paper to one of them. (ii) While waiting for the jars' internal temperatures to stabilize, study the remainder of this manual.
(h) Pour about 12 ml of water (1/2" from the bottom of the vial) into each of the two vials.
(i) Gently slide one tablet into each vial. Plug each vial with the black rubber stopper, making sure that one end of the tubing snugly fits the stopper while the other end almost touches the bottom of the experimental jar.
Notes: (i) Because CO2 is about 1.5 times heavier than air, some of it will remain at the bottom of the jar for a while. (ii) Don't let the vial's bubbling mixture enter the tubing. If the liquid rises too high in the vial, remove the stopper for a second, let the bubbling subside, then replace the stopper. If this persists, put less water in the vial next time.
(j) In a minute or two, when most of the effervescence subsides, remove the tubing from the jars.
(k) Record the internal temperature of both jars.
(l) Remove the stoppers from both vials and rinse the vials.
(m) Repeat steps (h)-(l) above four more times (for a total of ten tablets).
(n) Turn off the light.
Closing a Few Loopholes
Although this last procedure lends considerable support to the hypothesis that CO2 traps heat, the case is far from being closed. Among the many objections that could be raised, here we shall only touch upon three.
A skeptic might argue that the CO2 added extra warmth to the experimental jar because this CO2 was warm to begin with, not because the CO2 trapped heat after arriving in the jar.
ò 6. Which of your previous observations could safely dispose of this objection?
ò 7. Can you design a simple test which will conclusively rule out this objection?
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Procedure B: Was the CO2 Warm to Begin with? Please perform this test.
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Our results, another skeptic might say, do not show that CO2 traps heat more effectively than other atmospheric gases. By adding CO2 we have merely increased the number of molecules in the jar. And it was this greater number which trapped the extra heat.
ò 8. Based on your observations, and perhaps also upon your knowledge of the behavior of gases, can you answer this objection?
ò 9. Can you devise an experimental test which would rule it out?
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ò 10. Could we use a variation of our experimental design to show that nitrogen is less effective than carbon dioxide in trapping heat?
ò 11. Could we show that methane and CFCs are more effective than carbon dioxide?
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Another skeptic might argue that the control jar happened to radically differ from the experimental jar and that it was this difference+not the extra CO2+that led to the higher temperatures in the experimental jar.
ò 12. Can you reason this objection away?
ò 13. Also, can you devise an experimental variation which would rule it out?
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Procedure C: Dissimilar Jars. If you have time, please perform this variation.
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ò 14. If both jars are held upside down, ventilated for 30 seconds with a blow dryer (cool cycle), then placed under the light again, will their internal temperatures still differ?
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Procedure D: Effect of Ventilation on Temperature. After making your prediction, carry out this experimental variation.
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ò 15. Would the presence of a live mouse exert any effect on the control jar's temperature? 16. Would a live plant exert an effect?
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So here you have it. Your experiences probably support the hypothesis that CO2 traps heat. They suggest, as well, that great caution must be exercised in the design and interpretation of scientific experiments.
Source: Moti Nissani. Permission for the free use of this material is hereby granted.
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