Organic Chemistry

Effect of Elodea on Carbon Dioxide


Purpose:  To observe the effect of elodea on carbon dioxide in water under various conditions.

Background:  This experiment uses the acid base indicator Bromothymol Blue.  It is used to measure tiny changes around the neutral pH (7.0).  Humans exhale carbon dioxide, which is slightly acidic.  Carbon dioxide is soluble in water; therefore, if someone were to blow into water with Bromothymol Blue in it, the pH would change.  Elodea is a type of algae.  It sits in water and can easily survive with small roots because there is an unlimited supply of water all around it.  It is green and therefore is capable of photosynthesis.


·      Fill a test tube about half full with water

·      Using a pipette, add about 1 mL of Bromothymol Blue

·      Using a straw, gently blow into the solution

·      Continue blowing until the solution stops changing color

·      Add a small piece of elodea plant and completely submerge it in the solution

·      Make a test tube with water and Bromothymol Blue

·      Make a test tube with water, Bromothymol Blue, and elodea

·      Make a test tube with water, Bromothymol Blue, and carbon dioxide

·      Make a test tube with water, Bromothymol Blue, carbon dioxide, and elodea and put it in darkness


            As the solution was breathed into, the turquoise-like color of the Bromothymol Blue in the water started to turn green.  Slowly, the green became fairly yellowish and then stopped changing color.


                                                 0 hours                    1 hour                    6 hours                    24 hours

Water, Bromothymol          Green-Yellow         Green-Yellow             Teal                     Blue
Blue, Carbon 
Dioxide, Elodea                                                                                                                                            

Water, Bromothymol          Blue/turquoise      Blue/turquoise         Blue/turquoise          Blue (tinge of                                                                                                                                            Blue                                                                                                                       green)                                                                                                                                                                         

Water, Bromothymol          Blue/turquoise        Blue/turquoise         Blue/turquoise       Sapphire Blue
Blue, Elodea

Water, Bromothymol          Green-Yellow          Green-Yellow           Green                Blue/Green
Blue, Carbon 

Water, Bromothymol 
Blue, Carbon                         Green-Yellow          Green-Yellow        Green-Yellow        Green-Yellow
Dioxide, Elodea (dark)

There were also tiny bubbles coming off of the elodea in all of the test tubes with elodea.


This test proved that CO2 is very soluble in water, but also tends to be reabsorbed into the air over time, especially in warm air. The change in pH of the solutions showed that in the light, the elodea must use up CO2, and therefore, photosynthesis was occurring.  The bubbles coming off of the plants also suggested the same thing.  Since the test tube in the darkness did not change in pH, it can be concluded that the elodea was using the light to photosynthesize and use the carbon dioxide to create glucose and oxygen.