Chemistry’s Colorful Clue
Problem: Students conduct activities to learn basic concepts about acids, bases, and
indicators while trying to solve a mystery of how the stream presented in the story became
acidic.
Hypothesis: The question to investigate: What are the color changes of bromothymol blue
solution when citric acid is added?
Materials:
 Goggles
 Bromothymol blue indicator in dropper bottle
 Water
 2 small clear plastic cups
 Graduated cylinder
 Citric acid
 Baking soda
 2 flat toothpicks
Procedure:
Make an indicator solution
1. Use a graduated cylinder to add 20 mL of water to a small plastic cup. Add 20 drops
of bromothymol blue indicator and gently swirl to mix.
2. Pour about half the indicator solution into the other small clear plastic cup so that
you divide the solution evenly between the two cups. The solution in both cups
should be the same color blue.
Add citric acid
3. Use a toothpick to pick up a very small amount of citric acid. Try to pick up only
two or three crystals.
4. Add the citric acid to one cup of indicator and gently swirl.
5. Compare the color in this cup to the color of the bromothymol blue solution in the
other cup.
6. Add two or three more crystals of citric acid to the same cup and see if the color
changes again.
Add baking soda
7. Use a toothpick to add a very small amount of baking soda to the citric acid cup.
Gently swirl to mix.
8. If the color does not match the bromothymol blue in the second cup, continue to
add a little more baking soda and swirl until the color matches. Record how many
times you had to add baking soda to the cup.
Questions to answer after lab
What are the color changes of bromothymol blue solution:
When you add just a little citric acid?
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When you add more citric acid?
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Neutralization means a reaction between an acid and a base that produces a neutral solution
(pH=7).
What did you do to neutralize the citric acid?
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You had two cups of indicator solution, but only added substances to one of the cups.
What was the purpose of keeping one solution the original color (indicator mixed with
water0?
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Teacher Demonstration
Problem: Can carbon dioxide from a chemical reaction make a solution acidic?
Hypothesis:
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Materials:
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Goggles
Baking soda
Bromothymol blue indicator
Water
2 small clear plastic cups
2 wide clear plastic cups containing citric acid solution
2 tall clear plastic cups
Small metric measuring cups
Small spoon
Procedure
Make indicator solution
1. Place 20 mL of water in an empty small plastic cup. Add 20 drops of bromothymol
blue indicator and gently swirl to mix.
2. Pour about half the indicator solution into another small clear plastic cup so that you
divide the solution evenly between the two cups.
Make carbon dioxide gas
3. Place one rounded spoonful of baking soda into one of the wide cups that has citric
acid solution in it. Then quickly do steps 4 and 5.
Conduct the tests
4. Stand one small cup of indicator solution into each of the wide cups.
5. Immediately place a tall cup upside down over each wide cup as shown.
6. Gently swirl both sets of cups and watch the color of the indicator solution. Record
observations.
Questions to answer after lab
Can carbon dioxide gas from a chemical reaction make water acidic?
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How do you know that it was the carbon dioxide and not the citric acid solution that made
the bromothymol blue solution change color?
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What is the name of the weak acid formed when carbon dioxide mixes with water?
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Story continued…
Our teacher e-mailed the Department of Natural Resources, who sent a team of biologists
and chemist to investigate the stream. The chemist came to our class and told us what they
checked and what they discovered.
They checked:
They discovered:
Pipes that collect rainwater from streets and
The openings to the sewers were full of
drain into the stream
leaves and pine needles. The water inside was
slightly acidic, but that is normal.
Evidence of containers or some substances
There was one empty soda and three empty
placed in the water
soda cans on the bank of the stream.
The acidity of the water at various points in
The water near the park was just fine.
the stream.
Traveling downstream toward the ice cream
factory, the water was fine. But just at the ice
cream factory, the water was much more
acidic than normal. Continuing downstream
toward our school, the water remained acidic
and became normal not far past our school.
The evidence seems to point to the ice cream factory. The factory lost power a couple of
days ago because of s storm, and the freezers were not able to keep the ice cream frozen.
Rather than let the ice cream melt, workers brought in dry ice, which is frozen carbon
dioxide. They placed the ice cream in insulated containers with the dry ice.
When the power came back on, workers returned the ice cream to the freezers. They put the
containers of dry ice outside behind the factory. The next day when the workers came to get
the containers, they were surprised because all of the dry ice was gone. Could the missing dry
ice have something to do with why the stream is acidic?
There are different kinds of indicators. Some are liquids, while others are on paper. Some
change color with an acid, like bromothymol blue, while others change color with a base.
Some even detect other substances. The golden-colored paper is an indicator that changes
color with a base.
Problem: What color does goldenrod paper become when a base and an acid are added?
Materials:
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Goggles
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Goldenrod indicator paper
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Baking soda
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Citric acid solution
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Small spoon
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Cotton swabs
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Water
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Small plastic cup
Procedure:
1. Make a baking soda solution by adding 10 mL of water and 1 small spoon of baking
soda in a small plastic cup. Stir to get as much of the baking soda to dissolve as
possible.
2. Dip one end of the cotton swab into the baking soda solution. Write your initials, a
smiley face, or some other small design or mark on a section of the paper.
3. Repeat step 2 using the citric acid solution.
Questions to answer after lab
Does goldenrod paper change color with acids or bases?
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What could you use to make the red get lighter or go away?
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Conclusion:
Written in paragraph form and tells the reader what you learned in
this lab.
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Briefly explain what you did in the lab.
Restate your hypothesis. Explain if you were correct
or not.
State your observations and results (refer to specific
data).
What was the answer to the question stated in the
problem?
What were some important sources of error?
What are some practical applications from this
experiment that makes life easier or better?
Sources of Error:
Systematic Error
Systematic errors result from flaws in the procedure. Consider the Battery testing experiment where the
lifetime of a battery is determined by measuring the amount of time it takes for the battery to die. A flaw in
the procedure would be testing the batteries on different electronic devices in repeated trials. Because
different devices take in different amounts of electricity, the measured time it would take for a battery to
die would be different in each trial, resulting in error.
Because systematic errors result from flaws inherent in the procedure, they can be eliminated by
recognizing such flaws and correcting them in the future.
Random Error
Random errors result from our limitations in making measurements necessary for our experiment. All
measuring instruments are limited by how precise they are. The precision of an instrument refers to the
smallest difference between two quantities that the instrument can recognize. For example, the smallest
markings on a normal metric ruler are separated by 1mm. This means that the length of an object can be
measured accurately only to within 1mm. The true length of the object might vary by almost as much as
1mm. As a result, it is not possible to determine with certainty the exact length of the object.
Another source of random error relates to how easily the measurement can be made. Suppose you are
trying to determine the pH of a solution using pH paper. The pH of the solution can be determined by
looking at the color of the paper after it has been dipped in the solution. However, determining the color on
the pH paper is a qualitative measure. Unlike a ruler or a graduated cylinder, which have markings
corresponding to a quantitative measurement, pH paper requires that the experimenter determine the color
of the paper to make the measurement. Because people's perceptions of qualitative things like color vary,
the measurement of the pH would also vary between people.
Random error can never be eliminated because instruments can never make measurements with absolute
certainty. However, it can be reduced by making measurements with instruments that have better precision
and instruments that make the measuring process less qualitative.
Questions from the lab:
What are the color changes of bromothymol blue solution when citric acid is added?
Can carbon dioxide from a chemical reaction make a solution acidic?
What color does goldenrod paper become when a base and an acid are added?
How do you think the dry ice made the stream acidic?