Greenhouse effect experiment
Here's the results of a neat little experiment my Environmental Geology students did a couple weeks ago. This is the first time I've run this activity, and I was pleased with the results:
We made a little terrarium out of a transparent plastic box, and set it out in the sunshine. Two probes were inside: one measuring CO2 and one measuring temperature. We had placed in the box two petri dishes: one containing baking soda, and the other containing vinegar. We let the system equilibrate, sort of. But prompted by the setting sun (this is an evening class, and daylight was short), we opened the box, quickly dumped the vinegar into the baking soda, and closed the box again. This shows up in the two plots above as an abrupt decrease in temperature, as ambient air mixes with the trapped air in the box, and then an ensuing rise in CO2 accompanied by a correlated rise in temperature.
Interestingly, the box appears not to have been airtight, as the CO2 level diminishes after its sharp initial rise, and the temperature likewise diminishes.
Then we did it again, and again, each time adding more CO2 to the mix. Each time, you see the box cool down as we open it up to fiddle with the petri dishes, and then warm up to a higher level than it was before. I think I can also see the effect of the setting sun's decreasing energy input in the broad curve on the lower graph (upon which the peaks and valleys are superimposed).
A note on the CO2 units: we failed to properly calibrate the CO2 probe at the begining, so I'm not sure how confident I am in these measurement's accuracy -- but I feel their precision is internally consistent, so they show relative levels of CO2 well, even if that actual ppm may be shifted up or down. (We were supposed to calibrate to 400 ppm, but average atmospheric conditions of ~385 ppm are pretty close to that, I guess...)
Note also that you can translate the vertical axis of the upper plot from ppm to %: The plot ranges from 0% to 10% CO2 gas in the box. The highest value we saw was ~8.5% CO2 in the box.
Pretty cool little demo, eh? I'm looking forward to trying this again with a larger terrarium system, and adding in variables like photosynthesizing plants, etc.
Labels: climate change, CO2, global warming, nova, teaching
7 Comments:
Very cool post... I've passed this on to some science ed friends.
simple enough experiment with clear results. I like it
What were you using, specifically, for measuring the CO2? This topic is relevant to my interests :)
Very cool overall, and I look forward to hearing about how your proposed variants turn out.
Jay --
I was using a Pasco Xplorer, a device to which you can attach a bunch of probes. So I was using their temperature probe and their CO2 probe.
Pretty cool device -- graphs the data as you run the experiment, and later you can download it to your computer, as I did to share this graph here.
C
If you are proposing to have detected the greenhouse effect your methodology is incorrect. You need substantial optical depth of CO2 at a substantially different temperature from the surface for a long time on the order of several days. The warming effect is quite difficult to measure directly in a laboratory setting.
I believe you are measuring the energy released from the chemical reaction, not the greenhouse effect.
Michael - that would probably depend on whether this is an *endo*- or an *exo*- thermic reaction.
So who is incorrect?
Thanks to Michael for his comment. I made up this methodology and have less confidence in it than I would like. However, I've now had a couple of seconds to look into it, and it appears that the vinegar + baking soda reaction is endothermic, not exothermic, so that could not be the source of extra energy to warm up the box. If anything, the reaction alone would lower the temperature of the air in the box, so whatever caused the temperature to go up, it wasn't the reaction. Source 1. Source 2
So unless there's something else going on, I'm sticking with the greenhouse interpretation. If Michael, who speaks with authority (or anyone else) has insights or suggested alternate experiments to demonstrate the greenhouse effect, I would be pleased to hear them.
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