JCE Classroom Activity: #69

A Cool Drink! An Introduction to ConcentrationsMindy BedrossianStrongsville High School, 20025 Lunn Rd., Strongsville, OH 44149; Snb1700@aol.comIn this Activity, students investigate concentration levels by using serial dilution to prepare several solutions ofpresweetened powdered drink mix. Students taste the solutions to determine at which concentration they firstdiscern the sweetness. A connection is also made to the concentration of pollutants in air.

BackgroundScientists occasionally use the technique of serial dilution. This method takes aportion of a concentrated solution and dilutes it in several containers in succes-sion. Each prior dilution becomes the source of solution for the next dilution.It can provide better accuracy in measurements, especially when making a di-luted solution from a much more concentrated solution. Instructors may pointout that although some of the drink mix solutions appear colorless, they stillcontain very small amounts of solute.

Integrating the Activity into Your CurriculumThis Activity can be used as a precursor to discussing concentration and dilution, and the idea of parts per million. Alab previously published in the Journal on detecting the sweetness of aspartame may provide further discussion (1). Adescription of how to create a visual demonstration of one part per million is also in JCE (2). A similar demonstrationitem is also available from Flinn Scientific (3). This Activity correlates well with discussions on air and water pollution,which are often measured in terms of “parts per X”. A person breathes in approximately 11,000 liters of air each day(4). Using dimensional analysis, students make a connection that one part per million can still be a dramatic exposurewhen one calculates just how many particles enter the lungs each day.

About the ActivityAll required materials are available at grocery or discount stores. Either powdered Kool-Aid or other presweetenedpowdered drink mixes can be used. Because the Activity involves tasting, it should not be performed in a lab or uselab glassware. The teacher should control the container of Kool-Aid to prevent students from “dipping” into it for asweet taste treat. Flush remaining solutions down the drain and discard paper and plastic goods in the trash. Ifstudents work in pairs, they should divide the prepared contents of each cup in half for the taste test, and shouldpour the second portion into a fresh cup. Students should be careful when transferring spoonfuls from one cup toanother to avoid getting concentrated solutions on the cup rims. This can affect the tasting results. Perform the tastetest from the lowest concentration to the highest. Additional studies might test whether a particular flavor is moreeasily discernible. Similar experiments using plain sugar or lemon juice concentrate may provide a take-home experi-ment for comparison. Variables such as age and gender are available for study if family members participate.

Answers to Questions1. Cup 1 contains one spoonful of Kool-Aid /10 total spoonfuls (one Kool-Aid + nine water), or one part Kool-Aid

per ten parts solution. The remaining numbered cups change by factors of ten, with cup 6 containing one partper million. For example, cup 2 contained one spoonful from the first (one part Kool-Aid/10 parts solution) plusanother nine of water, or [(1 part Kool-Aid/10 parts solution) � (1 spoonful /10 parts solution)] to equal one partKool-Aid/100 parts solution. Cup 1 is most concentrated; cup 6 the least. The intensity of the color decreaseswith the series of dilutions. Note: the total volume after the Kool-Aid dissolves is actually less than ten spoonfulsbecause the sugar molecules become suspended between the water molecules. The reason is probably that forgranules of sugar there is air space in between.

2. Many will taste the sweetness at one part per ten thousand, and all will respond at one part per thousand. Theremay be one or two who insist they can taste it at one part per hundred thousand.

3. A person inhales approximately 2.96 � 1026 particles of air each day. At the EPA’s safe levels for mercury, thatcould include 5.92 � 1017 particles (atoms) of mercury each day, which is 2.0 � 10�4 g Hg.

4. Molarity = 9.83 � 10�7 mol/11,000 L = 8.94 � 10�11 M.

References, Additional Related Activities, and Demonstrations1. Stein, Paul J. The Sweetness of Aspartame: A Biochemistry Lab for Health Science Chemistry Courses. J. Chem.

Educ. 1997, 74, 1112–1113.2. Meloan, Clifton E.; Meloan, Mindy L.; Meloan, John M. Candy Sprinkles to Illustrate One Part Per Million. J.

Chem. Educ. 1994, 71, 658.3. Flinn Scientific. http://flinnsci.com; 800/452–1261. Becker Bottles–“One in a Million”, #AP4559, #AP4854.4. How much oxygen does a person consume in a day? http://science.howstuffworks.com/question98.htm5. EPA–AIR–Basic Information. http://www.epa.gov/air/urbanair/index.html (All sites accessed Nov 2004)

Instructor Information

www.JCE.DivCHED.org • Vol. 82 No. 2 February 2005 • Journal of Chemical Education 240A

photo by J. J. Jacobsen & E. K. Jacobsen

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JCE Classroom Activities are edited by Erica K. Jacobsen and Julie Cunningham

JCE Classroom Activity: #69 Student Activity

A Cool Drink! An Introduction to ConcentrationsYou’ve probably made a refreshing drink from a powdered drink mix such as Kool-Aid on a hot summer day. There is analmost irresistible temptation to stick one’s finger into the concentrated powder and lick the undiluted sweetness.Many kids who make their own Kool-Aid like to add less water than the recipe calls for, justto make a sweeter drink. Parents often try to make the drink less sweet by adding less sugar orsweetened powder to the water. In other words, you probably understand the idea of concen-trations and solutions better than you realize, just from everyday experiences. In this Activity,you will prepare different concentrations of Kool-Aid using a technique called serial dilution,and then observe and taste their differences.

You will also calculate how many parts of Kool-Aid there are in a million parts of solution.This is similar to how scientists measure air and water pollution and then study its effects onthe human body. Scientists can measure how many particles of pollution there are in amillion (or even a billion) particles of air (air consists largely of nitrogen and oxygen). Since each person breathes inabout 11,000 liters of air each day, it becomes possible to estimate how many particles of air enter the lungs daily. Thisis important in chemistry because many particles of pollution can be inhaled over the course of a day, and haveconsequences even though it seems as if their concentration in air is very small.

Try ThisYou will need: six small cups [5 oz. (150 mL) or larger], large cup, disposable plastic spoon or measuring spoon, plasticknife, Kool-Aid or other presweetened powdered drink mix, marker, and tap water.

__1. Using a marker, number six small cups from 1–6. Label a large cup “water” and fill withtap water.

__2. Add one level spoonful of presweetened powdered drink mix to Cup 1. To level thedrink mix, scrape the back of a plastic knife over the surface of the spoonful of mix.Rinse the spoon in the sink. Using the same spoon you used for the powder, take ninespoonfuls of water from the “water” cup and add them to Cup 1. Stir until the powdercompletely dissolves and observe the intensity of the color.

__3. Take one spoonful of solution from Cup 1 and place it in Cup 2. Take nine spoonfuls ofwater from the “water” cup and add them to Cup 2. Stir and observe the intensity of the color.

__4. Take one spoonful of solution from Cup 2 and place it in Cup 3. Take nine spoonfuls of water from the “water”cup and add them to Cup 3. Stir and observe the intensity of the color.

__5. Continue with the dilution method from steps 3 and 4, through Cup 6. This will produce six cups of drink mixsolutions with varying concentrations. Observe the intensity of the color in each cup.

__6. After you have prepared solutions in cups 1–6, take a sip from Cup 6. Can you taste the sweetness of the drinkmix in Cup 6?

__7. Continue tasting the solution in each cup, proceeding in reverse numerical order. Record the cup number of thecup when you can first taste with certainty the sweetness of the drink mix.

Questions1. Determine the concentration of each cup in terms of (parts of solute)/(parts of solution). (In this Activity, “part”

refers to volume.) Which cup contains the most concentrated solution and which cup the least? How does theintensity of the color change with varying concentration?

2. At what concentration did you first taste the sweetness of the drink mix? Compare your results to the class’s results.Did all people begin to taste the sweetness at the same concentration? Calculate the class average and either draw agraph or box plot of the class data.

3. Air pollution levels are often measured the same way that you measured parts of drink mix to parts of solution. TheEPA has set a safety level for mercury at 2 parts per billion (ppb) by volume. If a person inhales 11,000 L of air a dayand 22.4 L contains 6.02 � 1023 particles of air, calculate the number of mercury particles (atoms) a person maysafely inhale daily. From this number of particles, calculate the mass of mercury that could safely be inhaled in a day.

4. The concentration of solutions is often expressed in terms of molarity or moles of solute per liters of solution. Usingyour data from question 3, calculate the molarity of the mercury a person may safely inhale daily.

Information from the World Wide Web (accessed Nov 2004)

1. High School Environmental Center. Search for “water pollution,” and “air pollution”: http://www.epa.gov/highschool2. U.S. Air Quality Gradebook: A Molecular View of Air Quality. http://www.creativemethods.com/airquality/molecules.htm3. Lung volumes. http://www.brainyencyclopedia.com/encyclopedia/l/lu/lung_volumes.html

240B Journal of Chemical Education • Vol. 82 No. 2 February 2005 • www.JCE.DivCHED.org

Be Safe! Do not perform thisActivity in a lab or use bea-kers or measuring devicesthat have been used in a lab.Excess solutions can beflushed down the sink.

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This Classroom Activity may be reproduced for use in the subscriber’s classroom.