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Color Solutions
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Developer:
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Great Ideas from DuPont Teaching Fellows
Compiled by the Kinston, NC
DuPont Communication Team
March, 1994
| Charles Scaife
Department of Chemistry
Union College
Schenectady, NY 12308
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Grade Levels:
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6 through 8
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Discipline:
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Physical Science/Chemistry
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Goals:
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Practice manual dexterity in the use of a straw.
Determine the relative densities of four different solutions.
Prepare a density gradient column.
Devise a way to solve a challenging problem.
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Background:
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Pure water has very specific properties. Pure water is colorless, odorless, and tasteless. Pure water freezes at a certain temperature, 0 ° C. Pure water boils at a certain temperature, 100 ° C. Pure water has a certain density (mass divided by volume) equal to 1 gram per milliliter; thus, each milliliter of pure water weighs 1 gram near room temperature.
The density of a solid object compared to the density of water determines whether the solid object will float in the water or
sink into the water. A solid object which is less dense than water floats in the water. A solid object which is more dense than wateto the water. A s object which is less dense than water floats in the water. An object which is more dense than water sinks into the water. A solid object which is about the same density as water barely floats but almost completely under the surface of the water. We very frequently consider a solid object sinking or floating in a liquid such as water.
In this experiment no solids are involved, but several liquids are present. The several liquids are water solutions, that is, pure water with one or more other components dissolved in the water. Dropping one water solution from a sufficient height into the otvigoroher, or vigorous stirring or mixing, causes the solutions to mix thoroughly. All water solutions dissolve in each other although the other components dissolved in the two solutions might cause formation of a solid, a gas, or even an additional insoluble liquid. The several water solutions will be added to each other very carefully in order to keep them from mixing.
Many properties of water change as substances are dissolved in the water. You will observe a change of density
and use that change to form a density gradient column. Different kinds of density gradient columns are discussed by other authors (1, 2, 3).
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Teacher's Notes:
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This experiment requires considerable manual dexterity on the part of students to construct the density gradient column and to prevent contamination of the various solutions. Practice is the key. The solutions are inexpensive, and only a cup at most can be contaminated at a time.
You may want to use additional colored solutions to increase the level of difficulty for older students.
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Materials:
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1 Clear plastic drinking straw for each student
4 Paper cups for each student
Red food coloring
Green food coloring
Blue food coloring
1 1-cup Measuring cup
Table salt (sodium chloride)
4 1-gallon plastic milk jugs with screw-on caps
4 3-pound coffee cans to serve as waste containers
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Explanation:
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Adding table salt (to pure water increases the density of the water solution for the following reasons. The table salt ionizes to positively charged sodium ions and negatively charged chloride ion charged chloride ion ionizes to positively charged sodium ions and negatively charged chloride ions when it dissolves in water. The attractive forces between polar water molecules (having a positive end and a negative end) for each other is less than the attractive forces between polar water molecules and either the positively charged sodium ions or the negatively charged chloride ions. The net result is that the water molecules are drawn marginally closer to each other in the solution of table salt, the volume of the resulting solution decreases compared to pure water, and the density (increased mass from added table salt divided by a smaller volume) causes a larger density.
The order of increasing dng densities is blue, colorless, red, and green, the same order of layers from top to bottom in the density gradient column. Any mixing of solutions can lead to different layers of colors.
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Questions:
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Which solution is the most dense? the least dense?
How would you prepare a solution which has about twice the density of your most dense solution?
Note that the food coloring is added only to assist in observing mixing if it occurs.
If a bottle containing salt water and blue food coloring were inverted mouth- to-mouth over a bottle containing pure water at the same temperature, would you observe mixing between them? Why?
If a bottle containing pure water were inverted mouth-to-mouth over a bottle containing salt water and blue food coloring at the same temperature, would you observe mixing between them? Why? (Note that the food coloring is added only to assist in observing mixing if it occurs.)
If a bottle containing cold water were inverted mouth-to-mouth over a bottle containing hot water and red food coloring, would you observe mixing between them? Why? (Note that the food coloring is added only to assist in observing mixing if it occurs.)
If a bottle containing hot water and red food coloring were inverted mouth-to-mouth over a bottle containing cold water, would you observe mixing between them? Why?
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References:
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Tik L. Liem, Invitations to Science Inquiry, Supplement to First and Second Edition, Science Inquiry Enterprises, Chino Hills, CA, 1991, p. 47.
Mickey Sarquis and Jerry Sarquis, Fun with Chemistry: A Guidebook of K-12 Activities, vol. 1, Institute for Chemical Education, University of Wisconsin, Madison, WI, 1991, pp. 115-118.
Bassam Z. Shakhashiri, Chemical Demonstrations: A Handbook for Teachers of Chemistry, vol. 3, The University of Wisconsin Press, Madison, WI, pp. 229-233.
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Procedure:
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Four groups of three students should prepare the following solutions by adding to a 1-gallon plastic milk container:
BLUE: 12 drops of blue food coloring and sufficient water to fill the container to within one inch of the top
COLORLESS: 2/3 cup of table salt and sufficient water to fill the container to within one inch of the top
RED: 12 drops of red food coloring, 1 1/3 cup of table salt and sufficient water to fill the container to within one inch of the top
GREEN: 12 drops of green food coloring, 2 cups of table salt, and sufficient water to fill the container to within one inch of the top
Screw the caps tightly on the containers, and shake the containers of colored solution gently to dissolve the table salt completely and to distribute the color throughout each container.
Pour each solution into separate paper cups. These will be your stock solutions. Work very carefully so as not to contaminate these stock solutions.
Lower a clear drinking straw about one-half inch deep into one of the solutions. Cover the open end of the straw carefully with your finger hold the solution in the straw as you remove the straw from the first solution. While still holding the
solution in the straw, lower the straw one inch deep into a second solution. Release your finger slowly and carefully long enough for the second solution to enter the straw. Be careful not to contaminate the second solution. Then cover the open end of the straw carefully with your finger to hold the solution in the straw. Do the solutions mix or remain layered? Which one is more dense than the other? Discard the solution into the waste container by removing your finger from the end of the straw.
Repeat the procedure of the previous paragraph with other combinations of pairs of solutions until you know for certain the relative densities of the blue, colorless, red, and green solutions.
Use the relative densities as a basis for devising a procedure to achieve the goal of getting all four colors layered
in the drinking straw at the same time. If done properly, no mixing of the solutions should occur. Be careful not to contaminate the solutions. Show your teacher the layered solutions in the straw when you achieve your objective. Then discard the solution into the waste container by removing your finger from the end of the straw.
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