Todd Flomberg, Teacher Return to Physics Menu
The Nightingale-Bamford School, Manhattan
Summer 2001
Polarity and Dissolving
Objectives: 1. To have students practice preparing both non-polar and polar solutions.
2. To have students develop patterns that can predict solubility.
Background: Solutions are homogenous mixtures of two compounds, often a liquid vent and a solid solute (but not necessarily). Not all materials form solutions readily. Sand will not dissolve in water and oil and water do not mix. Solids such as wood and sand do not dissolve in water because the forces holding the molecules together in the wood and sand are stronger than the attraction those molecules have for water. For a solution to form, there must be some attraction between the atoms in the solute and those in the solvent.
You may recall that all compounds have ionic or covalent bonds. Ionic bonds, formed between metal ions (+) and nonmetal ions (-) are said to be polar. Covalent bonds are found between two non-metallic elements (they share the electrons) and are said to be non-oolar. The most important rule for governing whether a solution will form is summarized by the expression "like dissolves like." Polar solvents dissolve polar solutes (this includes ionic bonds and polar-covalent bonds) and non-polar solvents dissolve non-polar solutes.
Without a lot of chemical training, it can be difficult to tell which substances will be polar, which will be non-polar and which will have mixed properties. There are two guides that you can use to help you generalize: the chemical formula and what else it mixes with. Polar compounds often have metals in the formula. All compounds containing metal(s) are polar. Compounds containing oxygen tend to be polar or have mixed properties. Compounds that have lots of carbon atoms (organic compounds) tend to be non-polar or have mixed properties.
Materials: safety goggles, apron, latex gloves
test tubes, test tube racks, filter paper, spatulas, pipettes, rubber stoppers
solutes: permanent marker, "Flair" markers, iodine crystals, copper sulfate, sucrose, sodium chloride.
Solvents: deionized water, isopropyl alcohol, benzene (or toluene), corn oil
Procedure:
1. In test tube A, add 3 mL of corn oil, then add an equal volume of water. Cover the tube with the rubber stopper then shake the mixture. Place the tube back in the rack for several minutes. Observe what happens to the mixture and record your observations.
2. In test tube B, add 3 mL of water and a small amount of sodium chloride (less than pea-size). Cover the tube with the rubber stopper then shake the mixture. Place the tube back in the rack and observe what happens to the mixture and record your observations.
3. In test tube C, add 3 mL of alcohol and the same amount of salt you added to tube B. Cover the tube with the rubber stopper then shake the mixture Place the tube back in the rack and observe what happens to the mixture and record your observations.
4. In test tube D, add 3 mL of water and a small amount of sucrose (less than pea-size). Cover the tube with the rubber stopper then shake the mixture. Place the tube back in the rack and observe what happens to the mixture and record your observations.
5. In test tube E, add 3 mL of methanol and a small amount of sucrose (less than pea-size). Cover the tube with the rubber stopper then shake the mixture. Place the tube back in the rack and observe what happens to the mixture and record your observations.
6. In test tube F, add 3 mL of water and a small amount of copper sulfate (less than pea-size). Cover the tube with the rubber stopper then shake the mixture. Place the tube back in the rack and observe what happens to the mixture and record your observations.
7. In test tube G, add 3 mL of methanol and a small amount of copper sulfate. Cover the tube with the rubber stopper then shake the mixture. Place the tube back in the rack for several minutes. Place the tube back in the rack and observe what happens to the mixture and record your observations.
8. In test tube H, add 3 mL of water and a very small amount of iodine (a few crystals). Place the tube back in the rack for several minutes. Cover the tube with the rubber stopper then shake the mixture. Place the tube back in the rack and observe what happens to the mixture and record your observations.
9. In test tube I, add 3 mL of methanol and a very small amount of iodine (same as in Step 8). Cover the tube with the rubber stopper then shake the mixture. Place the tube back in the rack for several minutes. Place the tube back in the rack and observe what happens to the mixture and record your observations.
10. Cut a small piece of filter paper into two long strips (thin enough to fit inside the test tube). Side by side and about 4 cm above the end of the filter paper, draw a small box using permanent marker. Next to that box, draw another box with the Flair pen. Repeat the drawing on a second strip of filter paper.
11. In test tube J, add 3 mL of water and add one of the strips. Place the tube back in the rack, allowing the fluid to rise through the stip, and observe what happens to the mixture and record your observations.
12. In test tube K, add 3 mL of methanol and then add the other strip. Place the tube back in the rack, allowing the fluid to rise through the stip, and observe what happens to the mixture and record your observations.
13. In test tube L, add 3 mL of benzene (or toluene) and water. Then add a small amount of copper sulfate. Cover the tube with the rubber stopper then shake the mixture. Place the tube back in the rack for several minutes. Place the tube back in the rack and observe what happens to the mixture and record your observations.
14. In test tube M, add 3 mL of benzene and water. Then add a few crystals of iodine. Cover the tube with a rubber stopper and then shake the mixture. Place the tube back in the rack and observe what happens to the mixture and record your observations.
15. In test tube N, mix a solute and solvent that have not been mixed together. Predict the results and record your observations.
16. Clean up your test tubes completely and clean your work area.
Data and Observations
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Tube |
Contents of the Mixture |
Observations |
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A
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B
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C
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D
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E
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F
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G
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H
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I
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J
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K
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L
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M
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N
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Questions and Analysis:
· What is the name of the kind of mixture you formed in test tube A? It was not a solution.
· Based on your observations, which substances behaved as if they were polar, which behaved as if they were completely non-polar and which (if any) showed mixed properties?
· How do you think methanol (CH2OH) would behave? Polar, non-polar or mixed. Explain your answer.
· Why do you think the permanent marker is called "permanent'?
· If the two tubes, L and M were mixed, would the two different colors blend? What do you think would happen and explain why based on polarity?
· Were you able to predict the results you saw in test tube N? Why or why not? Explain.
Standards based on National Science Education Standards, National Research Council
Standards met by the lesson by subgroup.
I. Teaching Standards
· Standard A: The activity involves data collection and inference making.
· Standard B: Learning is facilitated through student/teacher interactions.
· Standard C: Student responses to questions generate information needed to assess learning and guide teaching.
· Standard D: Students are provided with resources, space and time to learn science.
· Standard E: The activity provides the structure for collaboration between students.
· Standard A: The teacher learned about phospholipids and misciblity tendencies through his participation in the Summer Research Program.
· Standard B: Teacher gets information needed to assess learning.
· Standard C: Students are given multiple opportunities to demonstrate learning.
· Standard D: The activity and assessment tasks are fair to all students.
IV. Content Standards: 5-8
· Standard A: Students are given opportunity to do and understand inquiry.
· Standard B, Physical Science
§ Students develop an understanding of properties and changes in matter.
§ Students develop understanding of energy.
§ Students develop a better understanding of bonding and electron transfer.
§ Students build upon understanding of forces and motion.
· Standard G: Students do inquiry as a "human endeavor" through their cooperation and learn about nature of scientific knowledge.