Teacher’s Instructions:
Who’s Polluting the River?
Recommended grade levels: 7-12, estimated time 1 hour
Summary:
Students will act as investigative chemists in this lab experiment to
determine who is polluting the Kennebec River in Waterville Maine. Using
river water samples students will experiment, observe, record, and identify
trends to determine the unknown culprit. Common relatively inexpensive
materials available from hardware and drug stores are used so that this
activity may be conducted on a limited budget. A precipitation reaction,
three color change indicators, and a spectrophotometer are used to identify
the culprit(s). After collecting data based on their spot plate experiments
students are then expected to name the culprit and justify their accusations.
Figure 1: Diagram of Kennebec River and suspected polluters with sample
collection locations.
Teaching Objectives:
 Communication: students will communicate effectively in the applications of
science and technology.
 Scientific reasoning: students will learn to formulate and justify ideas and to
make informed decisions.
 Inquiry and problem solving: Students will apply inquiry and problemsolving approaches in science and technology by observing and collecting
data from their own experimental design
 Structure of Matter: Students will understand the structure of matter and the
changes it undergoes
 Describe how physical properties of objects change during chemical
reactions.
Who’s Polluting the River?, Ben Bricker, Colby College, January 2012
 Explain how matter changes in both chemical and physical ways
Concept Background:
pH is a widely used scientific scale used to measure how acidic or basic a
solution is. Solutions with a pH less than seven are acidic. Common examples
include vinegar and lemon juice. Solutions with a pH greater than seven are basic.
These solutions include laundry detergent, ammonia, and bleach. The most
common method for calculating the pH of a solution takes the negative log of the
concentration (mol/L) of H+ ions. In equation form:
pH=-log10([H+])= log10(1/[H+])
For this experiment red cabbage juice is used as a pH indicator. Red cabbage
juice contains anthocyanin, a pigment that changes color due to pH. Red cabbage
juice is blue in strongly basic pH, bluish-greenish slightly basic pH, purple in neutral
pH, and red in acidic pH.
Spectrophotometers are also utilized in this experiment. A
spectrophotometer consists of two main parts, a spectrometer and a photometer.
The spectrometer produces light of specified wavelength. The photometer
measures the intensity of the light that reaches it. A space is left between the
spectrometer and photometer where a cuvette can be placed. The spectrometer
produces light of various wavelengths that passes through the cuvette and sample to
the photometer. The photometer measures the amount of light that reaches its
detector from the spectrophotometer, and therefore the absorbance of the sample in
the cuvette. In this experiment an ocean optics spectrophotometer is used with
Vernier data acquisition software.
Materials List:
Corn Starch
Iodine
Available at most grocery stores in the baking isle.
Available at most grocery stores as an antiseptic in the
pharmacy.
Acetic Acid
White vinegar is available at most grocery stores with
the condiments.
Magnesium Sulfate Commonly known as Epsom Salts, available at most
drug stores. Sold as a topical anesthetic, laxative, or
bath additive.
Sodium Carbonate Washing soda is a common detergent-booster sold at
most grocery stores in the laundry isle.
Calcium Chloride
Available at most hardware stores. Sold as ice-melt,
de-icer, or rock salt (not to be confused with NaCl,
which is also sold as rock salt).
Red Cabbage Juice Red cabbage from grocery store boiled for
approximately 30 minutes and then strained to isolate
the reddish-purple juice. Keep refrigerated. Used as a
pH indicator.
Antifreeze
Available at most auto parts store
Blue food coloring Available at most grocery stores in the baking isle.
Who’s Polluting the River?, Ben Bricker, Colby College, January 2012
Spectrophotometer A scientific instrument for measuring the absorbance of
a sample.
UV light
A simple small black light from a party supply store can
be used in place of the more expensive scientific
handheld UV light.
Spot plates
5mL Pipettes
Can be substituted with test tubes, cleaned out
children’s paint sets, plastic ice cube trays, or plastic egg
boxes.
Disposable pipettes are available cheaply in bulk from
from any scientific equipment provider. Can also be
replaced by straws.
Safety Notice:
All materials listed are safe to flush down the drain at the end of lab.
Vinegar and red cabbage produce strong odors in larger amounts. After preparing
solutions do not leave any of the listed materials in unsealed containers. The fumes
are not harmful, but the smells will linger.
Iodine and red cabbage juice will both stain clothing, and may cause skin
discoloration. If a student does accidentally get iodine or red cabbage juice on their
skin simply rinse off with tap water. Iodine drops may also leave stains on
table/desk tops.
Antifreeze is toxic if ingested. Antifreeze alos has a sweet flavor that commonly
makes it attractive to small children and pets. If any antifreeze is ingested contact
local poison control. Generally, antifreeze must be safely stored and recycled, but
due to the relatively small amounts used in this experiment it is safe to flush down
the drain.
Preparing the Solutions
The hardware store is adding calcium carbonate (a common deicer) to the river.
100 mL of water + 22.19g of CaCl2 will yield a solution with a pH of 10
Use the pH indicator red cabbage juice to identify the basic pH. The juice will go
from dark purple to light blue-green if the pH is greater than seven.
The winery is dumping acetic acid (vinegar) into the river.
100 mL of water + 10mL of white vinegar= pH of 3
Use the pH indicator red cabbage juice to identify the acidic pH. The juice will go
from dark purple to dark red. Adding more vinegar will result in a lighter red as the
acidity in the solution increases.
Who’s Polluting the River?, Ben Bricker, Colby College, January 2012
The day spa is dumping Epsom salts into the river.
100 mL of water + 24.84g of Epsom salts
Add 1 Tbs of washing soda to 100mL of water. Add a few drops of this solution to
Epsom salt solution to create a white, cloudy precipitate
The dry cleaners are adding starch to the river.
10 mL of water + a sprinkling of corn starch
Add a drop of iodine to turn the solution dark purple to confirm the presence of
starch.
The autobody repair business is dumping antifreeze into the river. Antifreeze is
colored with a florescent dye that appears to glow under UV light. Expose sample to
a black light in a dark space to confirm the presence of antifreeze.
100mL of water + 2mL of antifreeze
The textile factory is dumping old blue dye in the river. Use the spectrophotometer
to confirm the presence of blue dye.
100 mL of water + 1 drop of blue food coloring
The sample spectra will show a significant absorbance peak near 627nm.
Figure 2: Example of spectra from a sample containing deionized water with 1 drop
of blue food coloring. Note the significant peak close to 627nm.
Experimental Setup:
Give each group of 3-5 students 15mL test tubes with ~10mL of river water
sample. For a simple, quick experiment a single test tube with a river water sample
down stream from all suspects can be provided. For a more lengthy procedure,
multiple river water samples from different locations may be provided. Teachers
can choose which culprit is polluting the river and prepare the river water samples
to match (examples on last page). Three test tubes with 2-3mL of iodine, washing
soda, and red cabbage juice should also be provided to each group. Preparing test
tube racks with the necessary test tubes can be time consuming for the instructor
Who’s Polluting the River?, Ben Bricker, Colby College, January 2012
(an additional 20-30 minutes due largely to test tube labeling), but cuts down the
amount of time needed for the activity in the classroom while also limiting the
potential hazard of students dropping, spilling, or confusing chemicals. Each group
should also be provided with at least one spot plate to conduct their investigations
and multiple disposable pipettes.
Beginning the Experiment:
Begin by instructing students that today they are working as investigative
chemists attempting to determine who is polluting a specific section of the
Kennebec River. As chemists they will be working with chemicals and although
everything they will be using is very diluted and relatively harmless, safety
precautions must be observed. Everyone must wear safety goggles to protect their
eyes and remember to handle their samples carefully. Demonstrating how to use
the pipette to transfer a small amount of liquid from a test tube to the spot plate
may be necessary if the class is unfamiliar with pipetting. Reminding students that
although one pipette may be used to dispense river water into multiple spot plate
wells, each reactant must have it’s own pipette. Additionally, if the pipette touches
the surface of two samples it cannot be reintroduced to any of the test tubes. For
instance, a pipette was used to add red cabbage juice to a spot plate well already
containing river water. If the same pipette was then dipped back into the red
cabbage juice test tube it may contaminate the stock cabbage juice. Pipettes are
meant for one solution only, no mixing. For a one hour experiment students should
be given samples from three different sites on the Kennebec. The first sample is
upstream of all suspects. This sample is the control. The second sample is
downstream of the first drycleaner, the winery, the autobody repair shop, and
textile factory. The last sample is downstream of all previous businesses in addition
to a day spa, a hardware store, and another drycleaner. Once a group of students
believes they have identified the culprit(s) ask them to explain their findings based
on experimental evidence.
Reactions:
Epsom salts and washing soda dissolved in water produce a precipitate, magnesium
carbonate.
MgSO4(aq) + Na2CO3(aq) ===> MgCO3(s)
Corn starch and iodine react to produce a dark blue complex.
Beta amylose + I2(aq) ==> I5-1 (a dark blue complex)
Red cabbage juice is a natural pH indicator due to the presence of anthocyanin. Red
cabbage juice is blue in strongly basic pH, bluish-greenish slightly basic pH, purple
in neutral pH, and red in acidic.
Experimental Setup Examples:
Example 1)
Who’s Polluting the River?, Ben Bricker, Colby College, January 2012
The winery, day spa, autobody repair shop, and second drycleaner are polluting the
river.
1st river water sample is clean
2nd river water sample contains antifreeze and vinegar
3rd river water sample contains antifreeze, vinegar, starch, and Epsom salts
Example 2)
The hardware store, first drycleaner, and textile factory are polluting the river.
1st river water sample is clean
2nd river water sample contains blue dye and starch
3rd river water sample contains blue dye, starch, and calcium chloride (making it
basic)
Example 3)
The day spa, textile factory, and second dry cleaner are polluting the river.
1st river water sample is clean
2nd river water sample contains blue dye
3rd river water sample contains blue dye, starch, and Epsom salts.
References:
Caprette, David R. "Principles of Spectrophotometry." 19 may 2005. Experimental
Biosciences Introductory Lab. Januaray 2012
<http://www.ruf.rice.edu/~bioslabs/methods/protein/spectrophotometer.
html>
Colby College. “Ocean Optics for CH142, 2010.” February 2010. Chemistry 142
General Chemistry Laboratory Materials. January 2012
<http://www.colby.edu/chemistry/CH142L/CH142Lab.html>
US Department of Energy. Newton Archives. April 2011. January 2012
<http://www.newton.dep.anl.gov/>
Who’s Polluting the River?, Ben Bricker, Colby College, January 2012
Millard, Jullie. "CH151: k-8 Chem Outreach." January 2012. Colby College
Department of Chemistry Course Web Pages. January 2012
<http://www.colby.edu/chemistry/CH151_current/CH151_current.html>.
Who’s Polluting the River?, Ben Bricker, Colby College, January 2012