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Osmosis (Solution, Evaporation, Diffusion and Osmosis)

Grade Level

Prospective and Practicing K-8 Teachers; may be adapted for use in elementary classes.

Time

Exercises 1-4 take approximately 2 1/2 hours.

To Ponder

1. Our cells need a constant supply of oxygen and of water. They are also continuously producing carbon dioxide which needs to be removed. How do these materials get into and out of a cell?
2. How does water pass from the roots of a plant to its highest leaves?
3. How does water we pour into our gastrointestinal tracts actually get inside our bodies?

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Supplies

Per Group (unless otherwise noted)

1 triple beam or other balance
1 100 ml beaker
1 250 ml beaker
1 100 ml graduated cylinder
1 250 ml graduated cylinder
5 50 ml beakers
2 glass stirring rods
4 4-inch strips of 1-inch flat dialysis tubing
waxed dental floss, several containers
2 disposable pipettes
paper towels
table sugar - 5 pounds per class
tape
bottled water

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Objectives

Once you have completed these exercises you should be able to:

1. Distinguish between concentrations of solute and solvent.
2. Calculate the concentration of and be able to describe solutions.
3. Predict net movement of molecules across a semipermeable membrane.
4. Define what molecules move during osmosis.
5. Use the terms hypotonic, hypertonic, and isotonic.

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Background
Information

Movement of Matter

Movement is a characteristic which we most readily attribute to living organisms. Animals move around, seeking their prey. Vines twist and contort to maintain a grip on the tree trunk on which they are growing. Flowers open and close. Movement, however, is also a characteristic of the non-living world. Differential heating of air produces wind. Differential heating of water produces currents.

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Powerful Idea

Movement is a pervasive characteristic of matter. All matter is made of molecules. All molecules are constantly in motion. To repeat, molecules and atoms are constantly in motion throughout the natural world.

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To Ponder

1. What kinds of evidence have you seen so far that supports the idea that molecules are constantly in motion?
2. If you are a coffee or tea drinker, and if you use sugar, you know that sugar will dissolve faster in hot tea or coffee than it will in iced tea or coffee. That is, a given amount of sugar will dissolve faster in a given volume of water at higher temperature than at lower temperature. This is an example of rate of diffusion, which is the amount of solute that goes into solution per unit time.
3. Four of the many factors that affect the rate of diffusion are listed below. Thinking back to Lesson 1 and drawing on your everyday knowledge, identify one or more examples you have seen of the effects of each of these factors.

  • temperature,
  • size of molecules diffusing,
  • the magnitude of the charge(s) on, or polarity of, the diffusing molecule, and
  • size of concentration gradient over which diffusion occurs.
4. Design a new experiment which would allow you to test the effect of temperature on the rate of diffusion.

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Exercise 1

Effects of a Concentration Gradient

1. In this experiment you will collect some data which supports the statement that all molecules are in motion. You will also examine some consequences of this which are critical to life. Even though you will be using a non-living system as a model to collect the data, it is important to remember that what you discover is applicable in living systems.
To Do 2. Fill a 100 ml beaker with 80 ml bottled water. Obtain four 4" strips of dialysis tubing from your instructor and place them in the bottled water to soften the tubing. When softened, the dialysis tubing will behave like a semipermeable membrane in a living cell.
To Do 3. Remove the dialysis tubing from the water one piece at a time and tie off one end with waxed dental floss by folding the tubing over the dental floss twice and then tying the dental floss around the fold (see Figure 1). Return each piece of tubing to the 100 ml beaker of bottled water as you complete the task.

Figure 1. Tying off dialysis tubing.

Background 4. The designation used in this lab, e.g., 40% sugar, is a common way to refer to the solute content of a solution. It means that there are 40 grams of sugar per 100 ml of solution (not 40 grams of sugar per 100 ml of water). A 20% sugar solution would contain 20 grams of sugar per 100 ml solution, and so on. For purposes of thinking about osmosis, you can assume that the concentration of water is approximately equal to the difference - that is, about 60% water in a 40% sugar solution.
5. You will now prepare two different solutions of sugar. For each solution, use

  • a clean beaker to hold the sugar,
  • a balance to weigh the beaker alone and then the beaker plus sugar,
  • the beaker for initial mixing of the solution, and
  • a graduated cylinder for measuring the final solution volume (this is because volume measurements are much more accurate in a graduated cylinder than in a beaker).
Make sure that the pan of the balance is dry and that the balance is properly zeroed before weighing. Use the weights on the scale to balance the weight of material.
To Do 6. As noted above, to prepare 100 ml of a 40% sugar solution you would weigh out 40 grams of sugar. However, we will need 200 ml of a 40% sugar solution, so you need to double both the sugar content (to 80 gms) and the solution volume (to 200 ml). Thus, . . .

a.Weigh a 250 ml beaker and record the weight: __________ gms.
b.Add 80 gms to the weight of the beaker to obtain desired weight: __________ gms.
c.Set the weights on the scale to the desired weight in (b).
d.Add sugar until the beaker plus sugar weigh the desired weight.
e.Remove the beaker from the scale and add 150 ml bottled water to the sugar. Stir until dissolved.
f.Pour the sugar solution from the beaker into a 250 ml graduated cylinder.
g.Add bottled water until the total volume of the solution is 200 ml.
h.Pour the 200 ml of solution back into the original beaker.
i.Label the beaker 40 % sugar.

If all has gone well, you now have precisely 80 gms of sugar mixed with bottled water to give precisely 200 ml of 40% sugar solution.

7. In a similar manner, prepare 20 ml of a 10% sugar solution. Since a 10% sugar solution would contain 10 gms of sugar per 100 ml solution, then 20 ml solution (1/5 the volume) would contain how much sugar? __________ gms.

a.Weigh a 50 ml beaker and record the weight: __________ gms.
b.Add 2 gms to the weight of the beaker to obtain total desired weight: __________ gms.
c.Set the weights on the scale to the desired weight in (b).
d.Add sugar until the beaker plus sugar weigh the desired weight.
e.Remove the beaker from the scale and add 10 ml bottled water to the sugar. Stir until dissolved.
f.Pour the sugar solution from the beaker into a 50 ml graduated cylinder.
g.Add bottled water until the total volume of the solution is 20 ml.
h.Pour the 20 ml of solution back into the original beaker.
i.Label the beaker 10 % sugar.

If all has gone well, you now have precisely 2 gms of sugar mixed with bottled water to give precisely 20 ml of 10% sugar solution. Take a deep breath and continue.

8. Place these solutions inside the four dialysis sacs according to Table 1, following the directions below.

Table 1. Contents of dialysis sacs.

Sac # Contents
110% sugar
240% sugar
310% sugar
440% sugar

Predict 9. As you are ready for it, remove a piece of dialysis tubing from the water in which it is soaking and blot it dry.
Interpret 10. Use a clean disposable pipette (Figure 2). Add 5 ml of the 10% sugar solution to sac #1. Once the sac is filled, tie the open end with waxed dental floss in the same manner as you did previously. Tie a label to one end of the sac and call it #1 Set the filled and labeled sac on a paper towel. Repeat this procedure described in steps 9 and 10 with a second piece of tubing, adding 5 ml of a 10% sugar solution, and label #3.

Figure 2. Disposable pipette.

11. Obtain a clean disposable pipette. Add 5 ml of the 40% sugar solution to sacs #2 and #4 in the manner described in steps 9 and 10 above, placing each one on the paper towel as completed.
Data
Collection
12. Make sure that the pan of the balance is dry and that the balance is properly zeroed before weighing. Use the weights on the scale to obtain the proper adjustment. Weigh each sac to the nearest 0.1 gm. Record the beginning weights in Table 2 below.
13. In the table below, record the beginning weights, ending weights, and net change in weight for each of your dialysis sacs.

Table 2. Dialysis sacs, weight in grams.

Sac# % Solution Beginning Weight Ending Weight Net Change (+ or -)
1 10%      
2 40%      
3 10%      
4 40%      

Interpret 14. Place 40 ml of bottled water in each of two 50 ml beakers and label them Beaker #1 and #2. Place 40 ml of 40% sugar solution in each of two 50 ml beakers and label them Beaker #3 and #4 (Table 3).

Table 3. Beaker Contents.

Beaker #Contents
1bottled water
2bottled water
340% sugar
440% sugar

15. Simultaneously place each filled and weighed dialysis sac into its corresponding 50 ml beaker (Table 4). Note the time: __________ o'clock. You will remove the sacs after 20 minutes of immersion, at __________ o'clock.

Table 4. Sac and beaker contents.

Sac & ContentsSolution in Beaker
Sac #Contents Beaker #Contents
110% sugar 1bottled water
240% sugar 2bottled water
310% sugar 340% sugar
440% sugar 440% sugar

Predict 16. While waiting, predict what will happen to each sac. Use

'+' to indicate small weight gain,
'++' for large weight gain,
'-' for small weight loss,
'--' for large weight loss, and
'0' for no change.

sac 1 ___________________________________

sac 2 ___________________________________

sac 3 ___________________________________

sac 4 ___________________________________

To Do 17. After 20 minutes have passed, remove the sacs from the solutions in which they are sitting. Pat each sac dry with paper towels. Weigh each sac again (removing the label when you weigh it) and enter this Ending Weight into Table 2 above.
Data 18. In the last column of Table 2, summarize results using the notation described in step 16.
Collection
Results
19. Compare your predictions with your results (Table 5).

Table 5. Your predictions and results: Comparison.

Sac # Prediction (gain, loss) Result (gain, loss)
1  
2  
3  
4  

Interpret 20. Explain any discrepancies.
To Do 21. Finally, compare your results to those obtained by other groups in the class. Are you all in agreement? If not, why not? Is there a logical explanation?

Table 6. Class predictions and results: Comparison.

Group Sac 1 Sac 2 Sac 3 Sac 4
  PredictResult PredictResult PredictResult PredictResult
1        
2        
3        
4        
5        
6        
7        
8        

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Exercise 2

Organizing Your Knowledge

Question 1. When working with sugar dissolved in water, which molecules flow across a semipermeable membrane and which ones do not?
Interpret 2. The water molecules that flow across the membrane do so freely in both directions. However, the net or overall flow will be in the direction of the concentration gradient, from high concentration of that molecule to low concentration of that molecule. What concentration gradient (that is, the concentration of what substance) did you have to pay attention to in order to make your predictions accurately?
To Do 3. When two solutions are separated by a semipermeable membrane, how does the solute concentration of the two solutions affect the direction in which water flows?
Think 4. During osmosis, does water violate or obey the laws of diffusion? Justify your answer with your data.
Interpret 5. Make a general statement relating the direction of water flow to the concentration of solute in solutions separated by a semi-permeable membrane.
Interpret 6. Does osmosis require living tissue to occur? Explain.
Question 7. Do living things require osmosis? Explain.
Interpret 8. Use the terms hypotonic, hypertonic and isotonic to describe the various conditions in the experiment you just performed:

Table 7. Sac and beaker contents.

 Sac & Contents Solution in Beaker
CaseSac # ContentsBeaker # Contents
A1 10% sugar1 bottled water
B2 40% sugar2 bottled water
C3 10% sugar3 40% sugar
D4 40% sugar4 40% sugar

In Case A, the Sac contents were
hypertonic to the solution in the beaker.

In Case B, the Sac contents were
also hypertonic to the solution in the beaker.

In Case C, the Sac contents were
hypotonic to the solution in the beaker.

In Case D, the Sac contents were
isotonic to the solution in the beaker.

Question 9. Describe the sites in which osmosis occurs in human beings within the context of each of the following systems.

digestive system

urinary system

circulatory system

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Exercise 3

Knowledge Integration

Background
Information

1. Suppose you had a U-shaped tube (shown below) that was filled with water as indicated. Let us also suppose that at the bottom of the U-shaped tube there was a semi-permeable membrane that was permeable to both water and blue dye. However, the membrane IS NOT permeable to yellow dye.

Figure 3. U-tube with semipermeable membrane.

Predict 2. Now let us suppose that we add identical concentrations of blue dye to right side of the tube and yellow dye to the left side of the tube. What color will the right and left sides begin to turn as soon as you add the dyes?

right side will turn __________

left side will turn __________

3. As diffusion begins to occur across the membrane, what will happen to the water levels?

a. On the right side? __________

b. On the left side? __________

c. Why do you make these predictions?

4. After letting enough time pass for the system to equilibrate, what will the system look like?

a. What color will the left side be? __________

b. What color will the right side be? __________

c. Why do you make these predictions?




d. How will the water level on the left side have changed? ______________________________

e. How will the water level on the right side have changed? ______________________________

f. Why do you make these predictions?




5. Which processes are occurring in this system?





6. What molecules are moving across the semipermeable membrane in this system?




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Supplementary
Resources

Borovoy, Alexander. 1991. Learning about (not by) osmosis. Quantum. V2(2), 48-51. EJ447781

Sestili, Mary Ann. 1974. An investigative laboratory on diffusion and osmosis. American Biology Teacher V36(8), 492-493.

Related
AAAS
Benchmarks

Chapter 4: THE PHYSICAL WORLD
Section D: The Structure of Matter

Grade 3-5 Benchmark 1 of 4
Heating and cooling cause changes in the properties of materials. Many kinds of changes occur faster under hotter conditions.

Grade 6-9 Benchmark 3 of 7
Atoms and molecules are perpetually in motion. Increased temperature means greater average energy of motion, so most substances expand when heated. In solids, the atoms are closely locked in position and can only vibrate. In liquids, the atoms or molecules have higher energy, are more loosely connected, and can slide past one another; some molecules may get enough energy to escape into a gas. In gases, the atoms or molecules have still more energy and are free of one another except during occasional collisions.

Grade 6-8 Benchmark 4 of 7
The temperature and acidity of a solution influence reaction rates. Many substances dissolve in water, which may greatly facilitate reactions between them.

Chapter 5: THE LIVING ENVIRONMENT
Section C: Cells

Grade 9-12 Benchmark 1 of 8
Every cell is covered by a membrane that controls what can enter and leave the cell. In all but quite primitive cells, a complex network of proteins provides organization and shape and, for animal cells, movement.

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