JCE Classroom Activities are edited by Erica K. Jacobsen
Instructor Information
JCE Classroom Activity: #97
The Sweeter Side of Density
Photo by Charles Henry
Michael Davis* and Charles Henry Harold Washington College, 30 E. Lake Street, Chicago, IL 60601; *
[email protected] In this Activity, students determine the density of different sugar solutions (0–50%). They then dye the solutions and devise a method to combine the solutions to make a multi-colored, layered heterogeneous mixture. Students can determine the density of solid objects through water displacement, or measurement of the mass and volume of a regularly shaped object, including bowling balls (1). Determining the density of liquids with a graduated cylinder and balance is also straightforward. Based on the densities of room temperature water and oil, one can predict that oil will float on water. This happens regardless of the way they are combined. Examples of miscible liquids forming layers based on their density are rarer. For these liquids, the combination method is important. Quickly pouring a 20% sugar solution on top of a 10% sugar solution mixes the two, forming a homogeneous solution. However, if the solutions are carefully layered, it is possible to form a heterogeneous mixture.
Depositing a solution underneath a less dense solution.
Integrating the Activity into Your Curriculum This Activity could be used in units dealing with measurement or density. The Activity also underscores the idea that less dense objects float relative to their more dense surroundings. Students also create a procedure to meet a challenge. As an extension, students could use standard sugar solutions to determine the sugar content of a commercial beverage (2).
About the Activity Students can work in groups of 2–3. The Activity may be split into two or more units. Prepare the sugar solutions ahead of time, and label them randomly with letters or numbers so students don’t know each solution’s concentration, or its order in the collection of solutions. Weigh out the listed mass (see table) and add water to make 500 g of solution. Water added can be weighed with a balance of adequate capacity, or by using a large graduated cylinder to measure the volume of water and assuming the water’s density is 1.00 g/mL. Color solutions with any water-soluble dye (e.g. food coloring). Fluorescent dyes such as Rhodamine B or fluorescein can provide extra interest. Less than two pounds of sugar are needed to prepare ~500 mL of all six solutions. Students can perform the lab with three to six solutions. Each group needs ~20 mL of each solution. Solutions have been stored for 2–4 weeks without mold growth. Blank data tables are available in the online supplement. Students could evaluate the accuracy of their results by comparing their measured densities with densities obtained by other groups and with literature data (see table). The instructor could also lead a discussion of how well the values agree. Solution concentration (mass/mass %)
0
10
20
30
40
50
Mass of sugar needed/g
0
50
100
150
200
250
1.000
1.0381
1.0810
1.1270
1.1765
1.2295
perforated
Reported density at 25 °C/(g/cm-3) (ref 3)
To make a layered mixture, two procedures are common. (1) Place the most dense solution into the container, then deposit the next less dense solution on top with a dropper. This method can be successful with a careful hand. (2) Layer the solutions in reverse order. Place the least dense solution into the container. Plunge the tip of a dropper filled with a more dense solution to the bottom of the container and slowly squeeze it out underneath the first solution. The dropper must be inserted and withdrawn quickly without allowing air to enter the dropper.
Answers to Questions
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
fold here and tear out
Background
1. Two liquids are miscible if they can spontaneously mix to form a homogeneous solution. 2. The mixture is heterogeneous. Random samples of the mixture would not necessarily have the same sugar content. 3. The sugar solutions are miscible, so there will probably be some mixing between them. In this case, yellow and blue food coloring also mix to make green. 4. The slope represents density (mass/volume). 5. If equal volumes of each solution are used, the density of the mix should be an average of the densities of the solutions. 6. The hydrogen balloon has a lower density than air; it rises. The carbon dioxide balloon has a higher density; it sinks.
References, Additional Related Activities, and Demonstrations 1. Holley, Kathleen; Mason, Diana S.; Hunter, Kirk. Bowling for Density! J. Chem. Educ. 2004, 81, 1312A–1312B. 2. Henderson, Susan K.; Fenn, Carol A.; Domijan, John D. Determination of Sugar Content in Commercial Beverages by Density: A Novel Experiment for General Chemistry Courses. J. Chem. Educ. 1998, 75, 1122–1123. 3. Density of Sucrose in Aqueous Solutions, Table 88. In CRC Handbook; Weast, Robert C., Ed.; CRC Press, Inc.: Boca Raton, FL, 1978–1979; p D-308. This Activity was supported by National Science Foundation Undergraduate Research Collaborative grant CHE-0629174 and After School Matters. Supporting JCE Online Material at http://www.jce.divched.org/Journal/Issues/2008/Aug/abs1088A.html
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 8 August 2008 • Journal of Chemical Education
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JCE Classroom Activity: #97
Student Activity
The Sweeter Side of Density Solids, liquids, and gases have two relatively simple properties: mass, which is a measure of how much “stuff ”, or matter, there is in the sample, and volume, which is a measure of how much space the sample occupies. With the proper tools, these two properties can be measured. In this Activity, you will measure these properties for different concentrations of sugar solutions. You will measure mass in grams (g) using a balance and volume in milliliters (mL) using a graduated cylinder. The ratio of stuff (mass) to space (volume) is density. For example, pure water has a density of approximately 1 g/mL at room temperature. Approximately 1 mL of water would have a mass of approximately 1 g. Density is an intensive property. That 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. mass stuff Density volume space Photo by Charles Henry
Certain liquids simply do not mix. For example, when oil and water are combined, the two substances remain separated. The two substances are immiscible and do not form a homogeneous mixture. Instead, the substances layer according to their densities, with the less dense liquid on top. Other liquids such as plain water and sugar water are miscible and mix easily to form a homogenous solution. While they have different densities they can still mix and form a solution that is the same throughout. In this Activity, you will first determine the densities of various sugar solutions. Then, your challenge is to take these miscible solutions 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, food coloring, stirring rod, and three or more sugar solutions provided by your instructor. __1. Prepare a table to record the mass and volume of each sugar solution and for calculating each solution’s density. Carefully layered sugar solutions __2. Place a 10 mL graduated cylinder on a balance and tare, or zero, the balance. __3. Remove the cylinder from the balance and use a dropper to add ~2 mL of your first sugar solution. in a graduated cylinder. Measure and record the mass and volume of the added solution. __4. Repeat step 3, adding another 2 mL of solution. Repeat this step until you have at least 4 data points for the solution. __5. Repeat steps 2–4 for any additional sugar solutions. Clean and dry the cylinder between solutions. __6. Calculate the density for each trial for a particular solution. Average the trials to calculate an average density for each solution. Share these results with the rest of the class. How do your results compare? __7. Using the calculated densities of the solutions, place the solutions in order of increasing density. Add one or two drops of food coloring to each solution. Make each solution a different 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. Test your method, using equal volumes of each solution for each layer. Show your results to your instructor. __10. What will happen if you mix the layered solutions? Predict the density and color of the mixed solution. __11. Mix the layered solutions. Repeat steps 2–4 to determine the density of the solution. Record your observations, including the color of the solution.
More Things To Try Can you put a solution between two other solutions? Obtain a small amount of oil and place it in a cylinder above your most dense solution. Predict what will happen if you place a small amount of a different sugar solution on top of the oil. Try it!
Questions 1. What does it mean if 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 of a yellow sugar solution and noticed a small green area between them. Explain this observation. 4. Using the data from at least one of the sugar solutions, plot mass as a function of volume on a graph. Determine the slope of 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 differences. 6. Room temperature air has a density of 1.2 g/L. A balloon filled with hydrogen has a density of 0.082 g/L, and a balloon filled with carbon dioxide has a density of 1.9 g/L. Predict what would happen if both balloons were dropped.
Information from the World Wide Web (accessed May 2008) Densi-Tee Experiment. http://dwb.unl.edu/chemistry/beckerdemos/BD025.html This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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Journal of Chemical Education • Vol. 85 No. 8 August 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
