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Science - Biology
Standard 2 Objective 2
1 class periods of 90 minutes each
Students will use bromothymol blue as an indicator of carbon dioxide absorption by water plants.
Bromothymol blue is blue in a neutral or basic solution. It turns greenish and then yellow as it is acidified. Photosynthesis in water plants can be observed by acidifying the bromothymol blue with carbon dioxide from breath through a straw to change the solution to yellow. Placing a plant in the solution will remove the CO2 from the water, changing it back to blue. Sunlight works best as a light source and can make the color change in about 20 minutes. If artificial light is used it will take at least 30 minutes if not more, depending on the strength of the bulb.
It is important that the bromothymol blue solution is not too heavily acidified by the students breath. If too much CO2 is added, it will be difficult for the plant to remove it in a timely fashion. You can create a standard color by acidifying a test tube with bromothymol solution and placing a stopper on it. The CO2 will diffuse out of the solution without the stopper. Students can use this solution as a standard to make their own from.
Lesson Design by Jordan School District Teachers and Staff.