This lab involves the qualitative measurement of the changes in carbon dioxide concentration associated with respiration and photosynthesis in the freshwater plant Elodea. Bromthymol blue is used as an indicator for the presence of CO2 in solution. When CO2 dissolves in water, carbonic acid is formed. A bromthymol blue solution, acidified to pH 6.0 by the addition of carbon dioxide produces a yellow color. The blue color is restored when the CO2 is removed and the pH rises above 7.6. Students are responsible for the basic design of this investigation. Given a list of tasks and a background sheet, they are asked to design an experiment which will allow them to demonstrate the use of CO2 by a green plant in photosynthesis and net production of CO2 in the absence of photosynthetic activity.