From Focus on Corals: Global Climate & Reef Health, Bright Minds™, University of
Queensland, 2004.
Activity # 9
DETECTING CO2*
Lesson at a Glance: Students use an acid-base indicator to test the relative amounts of carbon
dioxide in three or four different air samples.
Suggested Prerequisites: A basic understanding of acids and bases and the use of indicators.
Focus Question: What happens when carbon dioxide is dissolved in water?
Background:
According to the Second Assessment of the UN's Intergovernmental Panel on Climate Change
(IPCC), atmospheric CO2 concentrations have increased more than 37 percent since the
Industrial Revolution. The oldest continuous direct record of atmospheric carbon dioxide
levels comes from Mauna Loa in the Hawaiian Islands, where monitoring began in 1958. The
Mauna Loa data show an 18% increase in the mean annual CO2 concentration, from 316 parts
per million (ppm) 1959 to 373 ppm in 2002. The increase is thought to be largely a result of
human activities such as the burning of such fossil fuels and large-scale changes in land use
patterns.
As part of the global carbon cycle, carbon dioxide gas enters water by means of direct contact
with the atmosphere and through the biological processes of photosynthesis and respiration.
When carbon dioxide reacts with water, it is converted to carbonic acid (H2CO3), most of
which is then converted to bicarbonate (HCO3-) and carbonate (CO32-) ions. These reactions
release hydrogen ions (H+) into the water.
Acids are compounds that break apart in water to form hydrogen ions (H+). The stronger the
acid, the more readily it breaks apart and the higher the concentration of ions. Bases are
compounds that react with hydrogen ions to form negatively charged hydroxide ions (OH-) in
water.
We use a scale called pH to measure acidity or basicity. The pH scale ranges from 0 (strongly
acidic) to 14 (strongly basic), with 7 representing a neutral substance. The scale is exponential
– a substance that has a pH of 1 is ten times more acidic than something with a pH of 2.
The following activity uses an indicator to test different air samples for carbon dioxide. An
indicator is a substance that changes colour when it comes in contact with an acid or base.
The air is bubbled through a water/indicator solution. If you use bromothymol blue, the blue
will change to green or yellow depending on the concentration of carbonic acid. Ammonia (a
base) can then be used to neutralise the solution and return it to its original colour.
Subjects: Chemistry, Earth Science, Geography
From Focus on Corals: Global Climate & Reef Health, Bright Minds™, University of
Queensland, 2004.
Key Syllabus Concepts:
Chemistry – acids and bases
Earth Science – forms of human impact on the environment
Geography – living in physical systems
Assessment: Students will successfully conduct the experiment and offer reasonable
explanations for the differences in the results obtained from the different air
samples.
Key Vocabulary:
acid
base
carbonate
carbonic acid
indicator
ion
neutralise
Time: 1 class period
Materials Needed:
(per team of 3-4 students)
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4 vials or test tubes
test tube rack
measuring cup (marked in millilitres)
measuring spoons
4 plastic straws
3 round balloons (all same size)
6 twist ties
1 - plastic soft drink bottle (1.25 – 2 litre)
150 ml vinegar
10 ml bicarbonate of soda
bromothymol blue or other indicator solution
dropper bottle ammonia (1 part ammonia per 25 parts water)
safety goggles (for all students)
instruction/data sheet
Activity:
1. Give each group of students a set of materials and a data sheet. Allow time for them to
conduct the experiment and record their data.
2. Bring the entire class together to discuss the results.
From Focus on Corals: Global Climate & Reef Health, Bright Minds™, University of
Queensland, 2004.
Discussion Questions:
1. Why didn’t the ambient air sample change the colour of the indicator solution? (The test is
not sensitive enough to react to the small amount of carbon dioxide in the ambient air.)
2. Which air sample caused the greatest change in the acidity of the water?
(vinegar/bicarbonate) Why? (The chemical reaction of the acid and base produced
carbon dioxide gas. This reacted with the water to produce carbonic acid.)
3. What is the major anthropogenic source of atmospheric carbon dioxide? (burning of fossil
fuels)
Adaptations/Extensions:
Have students investigate the following:
1. Test a balloon that is inflated with exhaust from a running automobile. (Caution: Prepare
this balloon for the students. Use a metal funnel and heavy gloves to avoid burns. Do
it in a well-ventilated environment.)
2. The solubility of carbon dioxide in water decreases with increasing temperature. When
you add carbon dioxide the amount of carbonic acid increases and the pH level drops.
When carbon dioxide is removed the pH level rises. Would you get different results in the
previous experiment if you were to change the water temperature? How might this relate
to possible changes in global sea surface temperatures?
3. Normal rain is slightly acidic (pH 5.6-6) because of the carbon dioxide picked up from the
earth's atmosphere. Pure freshwater is neutral (pH 7). Ocean water is slightly basic (pH 8)
because of its salinity. Would your experimental results be different if you used salt water
instead of fresh?
4. Acidity affects the solubility of many minerals, including the calcium carbonate that
makes up the structure of the coral reef. Calcium carbonate dissolves in an acid solution
and precipitates in a basic solution. The solubility of calcium carbonate is also reduced as
water temperature increases. How might these characteristics affect the development of
stony corals if atmospheric carbon dioxide levels and sea surface temperatures continue to
rise?
Activity adapted from “Global Warming & the Greenhouse Effect,” Lawrence Hall of Science, GEMS, 1992.
From Focus on Corals: Global Climate & Reef Health, Bright Minds™, University of
Queensland, 2004.
Detecting CO2
1.
Measure 15 ml of distilled water into each of the 4 test tubes and label them A, B, C, and D.
Test tube D will be your control.
2. Add 10 drops of bromothymol blue to each test tube.
3. Put 100 ml of vinegar in the soft drink bottle. Using the funnel, add 5 ml (1 ½ t) bicarbonate
of soda to the vinegar. Stretch the neck of the balloon over the bottle so that it will inflate.
When the balloon is inflated, take it off the bottle and seal it with a twist tie. Make sure the
twist tie is at least 2 cm from the end of the balloon.
twist ties
balloon
straw
fill
balloon
twist tie
4. Measure the diameter of the inflated balloon. Inflate two other balloons to the same
diameter and seal them. Blow one up by mouth and inflate the other using the air pump. Label
the balloons so that you know which is which.
5. Place one end of a straw in the neck of each of the balloons. Hold the straw in place with
another twist tie. This tie should be tight enough to keep air out, but not tight enough to
crush the straw.
twist tie
insert end
of straw
into balloon
6. Predict what will happen when you bubble the gas through the bromothymol solution. Give a
reason for your prediction:
I predict that:
because:
From Focus on Corals: Global Climate & Reef Health, Bright Minds™, University of
Queensland, 2004.
7. Working with a partner, hold one of the balloons over test tube A. Place the end of the
straw in the liquid, then partially release the first twist tie. Release it just enough to allow
the air in the balloon to bubble slowly through the solution. Compare the solution with the
liquid in your control test tube, and record your observations.
release
twist tie
place end
straw into
tube
8. Repeat the process with the other two balloons.
9. Add ammonia one drop at a time to each of the samples. Gently shake the test tube after
each drop. Count and record the number of drops needed to return the sample to its original
colour.
Air sample
Balloon 1
Vinegar/bicarb.
Balloon 2
Exhaled air
Balloon 3
Ambient air
Colour change
# drops ammonia
10. Answer the following questions:
a) What happened when you bubbled the air through the solutions?
b) Why?
c) Which air sample had the most carbon dioxide?
d) How do you know?