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1/2 Division of Chemical Education www.JCE.DivCHED.org Vol. 85 No. 8 August 2008 Journal of Chemical Education 1088A

The Sweeter Side of DensityMichael Davis* and Charles HenryHarold Washington College, 30 E. Lake Street, Chicago, IL 60601; *mdavis@ccc.edu

In this Activity, students determine the density o dierent sugar solutions (050%). Tey thendye the solutions and devise a method to combine the solutions to make a multi-colored, layeredheterogeneous mixture.

BackgroundStudents can determine the density o solid objects through water displacement, or measuremento the mass and volume o a regularly shaped object, including bowling balls (1). Determining thedensity o liquids with a graduated cylinder and balance is also straightorward. Based on the den-sities o room temperature water and oil, one can predict that oil will oat on water. Tis happensregardless o the way they are combined. Examples o miscible liquids orming layers based on theirdensity are rarer. For these liquids, the combination method is important. Quickly pouring a 20%sugar solution on top o a 10% sugar solution mixes the two, orming a homogeneous solution.However, i the solutions are careully layered, it is possible to orm a heterogeneous mixture.

Integrating the Activity into Your CurriculumTis Activity could be used in units dealing with measurement or density. Te Activity also underscores the idea that lessdense objects oat relative to their more dense surroundings. Students also create a procedure to meet a challenge. As anextension, students could use standard sugar solutions to determine the sugar content o a commercial beverage (2).

About the ActivityStudents can work in groups o 23. Te Activity may be split into two or more units. Prepare the sugar solutions ahead otime, and label them randomly with letters or numbers so students dont know each solutions concentration, or its order inthe collection o solutions. Weigh out the listed mass (see table) and add water to make 500 g o solution. Water added canbe weighed with a balance o adequate capacity, or by using a large graduated cylinder to measure the volume o water andassuming the waters density is 1.00 g/mL. Color solutions with any water-soluble dye (e.g. ood coloring). Fluorescent dyessuch as Rhodamine B or uorescein can provide extra interest. Less than two pounds o sugar are needed to prepare ~500mL o all six solutions.Students can perorm the lab with three to six solutions. Each group needs ~20 mL o each solution.Solutions have been stored or 24 weeks without mold growth. Blank data tables are available in the online supplement.Students could evaluate the accuracy o their results by comparing their measured densities with densities obtained by othergroups and with literature data (see table). Te instructor could also lead a discussion o how well the values agree.

Solution concentration (mass/mass %) 0 10 20 30 40 50

Mass o sugar needed/g 0 50 100 150 200 250

Reported density at 25 C/(g/cm-3) (re3) 1.000 1.0381 1.0810 1.1270 1.1765 1.2295

o make a layered mixture, two procedures are common. (1) Place the most dense solution into the container, then depositthe next less dense solution on top with a dropper. Tis method can be successul with a careul hand. (2) Layer the solutionsin reverse order. Place the least dense solution into the container. Plunge the tip o a dropper lled with a more dense solu-tion to the bottom o the container and slowly squeeze it out underneath the rst solution. Te dropper must be insertedand withdrawn quickly without allowing air to enter the dropper.

Answers to Questions1. wo liquids are miscible i they can spontaneously mix to orm a homogeneous solution.

2. Te mixture is heterogeneous. Random samples o the mixture would not necessarily have the same sugar content.3. Te sugar solutions are miscible, so there will probably be some mixing between them. In this case, yellow and blue oodcoloring also mix to make green.

4. Te slope represents density (mass/volume).5. I equal volumes o each solution are used, the density o the mix should be an average o the densities o the solutions.6. Te hydrogen balloon has a lower density than air; it rises. Te carbon dioxide balloon has a higher density; it sinks.

References, Additional Related Activities, and Demonstrations1. Holley, Kathleen; Mason, Diana S.; Hunter, Kirk. Bowling or Density!J. Chem. Educ.2004, 81, 1312A1312B.2. Henderson, Susan K.; Fenn, Carol A.; Domijan, John D. Determination o Sugar Content in Commercial Beverages by Density:

A Novel Experiment or General Chemistry Courses.J. Chem. Educ.1998, 75, 11221123.3. Density o Sucrose in Aqueous Solutions, able 88. In CRC Handbook; Weast, Robert C., Ed.; CRC Press, Inc.: Boca Raton,

FL, 19781979; p D-308.

Tis Activity was supported by National Science Foundation Undergraduate Research Collaborative grant CHE-0629174 and Afer School Matters.

Instructor Information JCE Classroom Activity: #97

Depositing a solutionunderneath a lessdense solution.

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byCharlesHenry

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8/6/2019 Azucar Colores y Dens Ida Des

2/21088B Journal of Chemical Education Vol. 85 No. 8 August 2008 www.JCE.DivCHED.org Division of Chemical Education

JCE Classroom Activity: #97 Student Activity

The Sweeter Side of DensitySolids, liquids, and gases have two relatively simple properties: mass, which is a measure o how much stu , or matter, thereis in the sample, and volume, which is a measure o how much space the sample occupies. With the proper tools, these twoproperties can be measured. In this Activity, you will measure these properties or dierent concentrations o sugar solutions.You will measure mass in grams (g) using a balance and volume in milliliters (mL) using a graduated cylinder. Te ratio ostu (mass) to space (volume) is density. For example, pure water has a density o approximately 1 g/mL at room temperature.Approximately 1 mL o water would have a mass o approximately 1 g. Density is an intensive property. Tat is, no matter

how much water there was, whether 1 mL or 1000 mL, it would always have the same density at a particular temperature.

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Certain liquids simply do not mix. For example, when oil and water are combined, the two substancesremain separated. Te two substances are immiscible and do not orm a homogeneous mixture. Instead,the substances layer according to their densities, with the less dense liquid on top. Other liquids such asplain water and sugar water are miscible and mix easily to orm a homogenous solution. While they havedierent densities they can still mix and orm a solution that is the same throughout. In this Activity, youwill rst determine the densities o various sugar solutions. Ten, your challenge is to take these misciblesolutions and devise a way to combine them so that they remain as separate layers.

Try This

You will need: two 10 mL graduated cylinders, droppers, balance, ood coloring, stirring rod, and threeor more sugar solutions provided by your instructor.

__1. Prepare a table to record the mass and volume o each sugar solution and or calculating each solu-tions density.

__2. Place a 10 mL graduated cylinder on a balance and tare, or zero, the balance.

__3. Remove the cylinder rom the balance and use a dropper to add ~2 mL o your rst sugar solution.Measure and record the mass and volume o the added solution.

__4. Repeat step 3, adding another 2 mL o solution. Repeat this step until you have at least 4 data pointsor the solution.

__5. Repeat steps 24 or any additional sugar solutions. Clean and dry the cylinder between solutions.

__6. Calculate the density or each trial or a particular solution. Average the trials to calculate an average density or each

solution. Share these results with the rest o the class. How do your results compare?__7. Using the calculated densities o the solutions, place the solutions in order o increasing density. Add one or two dropso ood coloring to each solution. Make each solution a dierent color.

__8. Devise a method to layer the solutions so that they remain separate and do not mix in a 10 mL graduated cylinder.

__9. est your method, using equal volumes o each solution or each layer. Show your results to your instructor.

__10. What will happen i you mix the layered solutions? Predict the density and color o the mixed solution.

__11. Mix the layered solutions. Repeat steps 24 to determine the density o the solution. Record your observations, includ-ing the color o the solution.

More Things To TryCan you put a solution between two other solutions? Obtain a small amount o oil and place it in a cylinder above your mostdense solution. Predict what will happen i you place a small amount o a dierent sugar solution on top o the oil. ry it!

Questions1. What does it mean i two liquids are miscible?2. Is the layered mixture produced in step 9 homogeneous or heterogeneous? Explain your answer.3. A student layered a less dense blue sugar solution on top o a yellow sugar solution and noticed a small green area between

them. Explain this observation.4. Using the data rom at least one o the sugar solutions, plot mass as a unction o volume on a graph. Determine the slope

o the line. What does this value represent? How does it compare to the value you calculated earlier?5. Compare your predictions in step 10 to your results in step 11. Explain any dierences.6. Room temperature air has a density o 1.2 g/L. A balloon lled with hydrogen has a density o 0.082 g/L, and a balloon

lled with carbon dioxide has a density o 1.9 g/L. Predict what would happen i both balloons were dropped.

Information from the World Wide Web (accessed May 2008)Densi-ee Experiment. http://dwb.unl.edu/chemistry/beckerdemos/BD025.html

Carefully layeredsugar solutionsin a graduatedcylinder.

Photo

byCharlesHenry

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