It is a common misconception that while humans breathe in oxygen and breathe out carbon dioxide, plants breathe in carbon dioxide and breathe out oxygen. In fact, plants do use carbon dioxide in order to perform photosynthesis, which converts the sun's energy into usable energy. To actually use this energy, however, plants respire just as humans do. They take in oxygen during the respiration process and produce carbon dioxide. You can explore this process with several similar experiments.
Do Plants Breathe?
The most basic plant respiration experiment will simply demonstrate that plants do respire, thus creating carbon dioxide. To attempt this experiment, you'll need some purple cabbage indicator water, which changes colours when acids and bases are added to it. It turns red when an acid is added to it, and blue when a base is added to it. Pour enough cabbage indicator water over a small aquatic plant in a test tube so that it is covered completely. Next, wrap the test tube in tinfoil to create a dark environment. In several days, you will see that the water has changed colours because the exhaled carbon dioxide has combined with the water to create carbonic acid. (If you'd like to make this an authentic experiment, you can create a control by simply wrapping a test tube containing the indicator water with some tinfoil.)
Note: You can use an oxygen probe instead of indicator water for this experiment. A probe measures oxygen levels in the water surrounding the plant. This can tell you how much oxygen the plant has used up during the respiration process.
Dark and Light
This experiment builds on the previous experiment, and demonstrates whether plants respire more in dark environments or light environments. To demonstrate this, simply add another variable by putting an unwrapped test tube containing the same materials right next to the wrapped one. Observe the two test tubes every 12 hours or so, and compare the differences in the colours of the indicator water. You will find that the dark test tube shows more carbonic acid (that is, a redder colour) than the light one, because plants respire more efficiently in the dark than in the light, and they do not photosynthesise at all.
Sizes and Types of Plants
You can also perform an experiment testing how the size and type of plant can affect how quickly the plant respires. To do this, use different types of aquatic plants, or different sizes of the same aquatic plant, and use cabbage water or an oxygen probe to measure the different rates of respiration for each plant. Make sure to add the same amount of indicator water (or regular water, if you're using a probe) to each plant, even if they are different sizes.