CO2 and its effect on temp and plant growth

[cheerashley]

I am trying to finish my science fair project, but my experiment is not going the way I expected it to based on my research. I created two greenhouses out of soda bottles and placed a small jiffy pot in each of them which I had planted string bean seeds. In one of the bottles right before I sealed it I placed another small container with 1 tsp of baking soda and then added 10ml of vinegar (then sealed the container fast). Since the one bottle has higher CO2 levels, based on my research, the temp should be higher, it is not - it has been about 2 degrees cooler than the other container. I have looked everywhere for an answer to why increased CO2 would cause lower temps but all I can find is why increased CO2 levels would cause higher temps. Any ideas? Also, the plant is growing better in the container with the additional CO2, which I know the plant needs for photosynthesis, but shouldn't increasing the level have a bad affect?

[MelissaB]

A lot of the research you are probably looking at has to do with the greenhouse effect, and I'm not sure it relates directly to your project given the differences in scale (the entire earth vs. a small bottle). There are also other differences between the two bottles--one had a chemical reaction occurring in it, and reactions can take energy which can lower the temperature of its surroundings, for example. However, can you tell us how you are measuring the temperature in the two bottles?

Most research that I've read suggests that plants grow better in higher levels of CO2 although if you keep increasing the CO2 eventually the growth will level off. Try putting 'growth of plants in high CO2' into Google and looking through some of the resulting links--there's lots of research to be found out there. If you need help, post back and let us know.

[cheerashley]

Thank you for the google suggestion. I found what I needed on the plant growth.

Now for the temp, I am using an indoor/outdoor thermometer that I put on the inside of the soda bottle with a suction cup before I sealed the bottle. Neither bottle has been opened since the experiment has started. I just can't figure out why the temp is lower in the CO2 bottle. The CO2 bottle has had less water vapor forming, can that effect temperature?

[MelissaB]

It can--and an additional explanation is just that the thermometers are not perfectly calibrated. When you take them out at the end of the experiment, make sure you put them in several different temperatures (inside/outside, refrigerator, etc.) and see if they give you the same reading at all the different temperatures. Make sure you also keep track of which one is which!