BSCS Biology I
Name:________________
Examining the interactions between Elodea and snails by detecting the presence of CO2.
Objective: to compare the products of photosynthesis and respiration.
Background: In this lab you will use an indicator called bromothymol blue (BTB) to indicate whether
CO2, carbon dioxide, is present. Carbon dioxide reacts with water to form carbonic acid (H2CO3). Then,
carbonic acid immediately forms into hydrogen ion and a bicarbonate ion. See the reaction below:
CO2
H2O
H2CO3
Neutral

H+
+
HCO3 –
Acidic
color change
BLUE
GREEN/ YELLOW
When carbonic acid dissociates into free H+, it lowers the pH of the solution, turning the color of the
BTB from blue to yellow. (Recall that the more hydrogen ions there are, the lower the pH – the more
acidic the solution is). The color change is proportional to the amount of acid (H+) in the solution, which
is proportional to the amount of carbon dioxide produced. In other words, the more CO2 dissolves in
the water, the more acid the solution becomes and the pH of the solution will decrease. Therefore, a
pH indicator such as BTB can indicate the relative amount of CO2 dissolved in water based on the color
of the solution. BTB’s color ranges from blue to yellow in an acidic solution.
Materials:
Test tubes and test tube racks
Rubber stoppers
Aluminum foil
Elodea (water plant) and snails
Bromothymol blue solution: ~30 drops of BTB in 100 mls of water
(may include 1 drop of NaOH).
Before you begin, read the above information and answer the questions below. Please write neatly!
1. When carbon dioxide mixes with water, what ion is produced to make the solution more acidic?
2. What indicator is being used in this lab and what does it indicate?
3. If you were to exhale (high content of CO2) into a solution of bromothymol blue, what color would it
become?
4. Write the chemical equations for photosynthesis and respiration below:
Ps:
Rs:
Elodea/ Snail lab revised by Juo 2/9/16
BSCS Biology I
Name:________________
Procedure: You will be testing for the products of photosynthesis and respiration by setting up test
tubes according to the chart with the materials provided. Each group will be in charge of certain tubes.
1. First, if needed, add the Elodea plant piece and/or snail in the test tube. Then, carefully add the
dilute BTB solution until it is about 3 inches from the top of the tube. Be sure that the Elodea plant
and/or snail are submerged in the BTB solution. Gently put the test tube stopper on the test tube.
2. Place the tubes that should be in the light in the test tube rack under the plant light apparatus. Place
the tubes that go in the dark in aluminum foil so no light can get in, then in the test tube rack that will go
in a dark cabinet.
3. Complete the columns of the chart below labeled “Day 0 color of BTB” and “Predicted color”. You
will complete the chart (last two columns) the next time we meet.
Tube
#
Light
Or
dark?
Elodea/
snail
present?
Day 0
color
of
BTB
Blue
1
Light
No
2
Light
Elodea
only
Blue
3
Light
Snail
only
Blue
4
Light
Blue
5
Dark
Elodea
&
Snail
No
6
Dark
Elodea
only
Blue
7
Dark
Snail
only
Blue
8
Dark
Elodea
& Snail
Blue
Predicted
color of
BTB next
day
Explanation of prediction –
what is your reasoning for your
prediction?
Observed
color of
BTB next
day
Blue
4. Answer the questions below. Please write neatly!
a. Which tube is the negative control for the experiment? Explain.
b. Do we have a positive control (one or more) for the experiment? If so, what is it? Explain.
c. Which tube is the experimental tube? That is, based on what you know about photosynthesis and
respiration, it is still difficult to accurately predict what will happen in this tube? Explain.
c. Write one hypothesis for this lab (in regards to your experimental tube).
Elodea/ Snail lab revised by Juo 2/9/16
BSCS Biology I
Name:________________
Discussion: Please write neatly or type your answers on separate paper!
Analysis Questions:
1. What is the purpose of tubes #1 and #5?
2. Did you predict the change in color correctly for
a) tubes #1-4?
b) tubes #5-8?
If no, explain what was incorrect about your reasoning behind your prediction.
3. a) Is the pH of tube #3 basic, neutral, or acidic?
b) What substance was produced to cause the pH to change?
c) How was the substance produced (name the process)?
4. Did placing the snail in the dark affect the results? Why or why not? (i.e., was there a difference
between tubes #3 & 7?)
5. a) Is the pH of tube #4 basic, neutral, or acidic?
b) Was carbon dioxide present?
c) If yes, where did it come from? (remember the tube is tightly sealed shut).
d) There probably was a difference between tubes #2 and #6 – did you observe this? Even if you
didn’t observe a difference, explain why you would expect a difference – be specific.
6) Explain the interaction between the snail and the Elodea in terms of biochemical processes. (hint:
What gases are being produced and used during respiration and photosynthesis?) Answer this by
showing which gases come from and go into the elodea plant and the snail. Label your picture so that
it is clear that you understand the interaction.
Please type or write neatly on the back:
Sources of Error: Describe in great detail one source of error. Be specific and thorough (don’t forget
to tell how the error affected results AND how you could prevent this error next time). 1 paragraph
Elodea/ Snail lab revised by Juo 2/9/16
BSCS Biology I
Name:________________
Conclusion: What did you learn from this lab about photosynthesis and respiration? 1 paragraph
Elodea/ Snail lab revised by Juo 2/9/16