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Basic Processes
(Solution, Evaporation, Diffusion and Osmosis)

Grade Level

Prospective and practicing K-8 Teachers; may be adapted for use in elementary classrooms.


Time

Exercises 1-4 take approximately 2 1/2 hours. It will take one or two additional class periods to collect some of the results.

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To Ponder

1. When you put a couple of teaspoons of sugar into your coffee, why does it get sweet everywhere? Why doesn't it just settle into the bottom of your cup?
2. When you put celery and carrots in some fresh water in the refrigerator, why do they get crisp?
3. Why does a plant wilt and go limp when it doesn't have enough water?
4.Where does your urine come from? How is it that it can contain alcohol, sugar, drugs, or other substances that you have ingested?
5. When you sweat, what is evaporating from your skin? What remains behind? What does sweating do for your body?
6. How do the nutrients you eat get through your intestinal wall and your blood vessels into your bloodstream?

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Supplies

#Per Group # Per Class

1   hot plate 3   bxs salt
2   50 ml beakers 1   bttl perfume
2   100 ml beakers 1   gal vinegar
4   250 ml beakers 1   qt ammonia
1   500-1000 ml beaker 1   250 ml grad cyl
1   stirring rod 5 gal   fresh water
1   tongs 1-4   red food color
1   balance  
1   stopwatch  
1   grad cylinder, 50 ml  
1   thermometer, oC  
6   raisins  
1   egg  
2   celery sticks  
2   carrots  
2-3   droppers  

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Objectives

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

1. Define solution, saturated solution, dissolving, evaporating, diffusion, osmosis, semipermeable membrane.
2. Explain how molecules of different types separate when some molecules from a solution evaporate.
3. Describe at the molecular level how ions, dyes and other molecules go into solution or dissolve.
4. Describe some of the essential features of molecules that determine whether materials will be able to dissolve in water.
5. Describe the effect of temperature upon the solution process.
6. Explain in molecular terms why more substance can be dissolved in a given quantity of water at higher temperatures.
7. Describe what the bubbles consist of that form at the bottom of a beaker of boiling water.
8. Describe how molecules diffuse through a body of water.
9. Describe how molecules diffuse through a body of air.
10. Know the composition and structure of air.
11. Define, describe, and recognize instances of osmosis.
12. Predict and explain osmosis in various situations using a molecular model.
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Background

Solution of Liquids

A solution is formed when a (usually solid) solute is dissolved in a liquid solvent. Water is a common and powerful solvent. When a substance is dissolved in water, each atom or molecule of the substance becomes surrounded by water molecules, like a prisoner surrounded by a posse. It is this 'sphere of hydration' that results in the complete, even distribution of dissolved molecules in the liquid.

Figure 1. An individual molecule of the solute, sugar, surrounded by molecules of the solvent, water, to form a sphere of hydration.

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Powerful Idea

Evaporation of Liquids

After a rain, puddles dry up. Where does the water go? It evaporates into the air. When water evaporates it changes from a liquid into a gas, but the molecules retain the same structure and size (H2O). Water in the gaseous state is invisible and is called water vapor. Water molecules in the gaseous state are much further apart and moving much faster than water molecules in the liquid state.





What happens when a sugar solution evaporates? The lighter water molecules will escape to the atmosphere (evaporate), but the heavier sugar molecules remain behind. As evaporation continues, the sugar in the solution becomes more concentrated. Evaporation provides a mechanism for separating the solvent from the solute. The molecules of each substance remain intact.

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Exercise 1

Solution & Effects of Heat Energy

To Do 1. Obtain a hot plate from the shelf, plug it in, and turn it to its highest setting.
2. Obtain a balance, weigh a small (50 ml) beaker, and note how much it weighs:

____________________ grams

3. Place 25 ml. fresh water in the small (50 ml) beaker you just weighed and weigh it again:

_____________________ grams

Question 4. What is the weight of the water in the beaker?

_____________________ grams

To Do 5. Obtain a thermometer and determine the temperature of the water:

_____________________ degrees Celsius.

Predict 6. How much salt (in grams) do you think you would be able to dissolve in these 25 ml. of water?

_____________________ grams

To Do 7. Slowly add the salt to the water above, stirring until dissolved. Continue adding salt slowly until you reach a clear saturation point (when added salt will no longer dissolve and some salt rests at the bottom of the beaker). Record the final weight of the beaker (now filled with water and salt). Subtract the weight of the beaker (filled only with water) from this final weight to obtain the weight of the salt that you added. You can summarize your data in the first line of the table below.

Table 1. Quantity of Salt Added to Solution

Column 2 3 4 5
Temp. of Water Weight of Empty Beaker (gms) Weight of Beaker + H2O (gms) Weight of Beaker + H2O + Salt (gms) Difference: Column 4 - Column 3 = Weight of Salt
Room Temp.        
Heated        
         

To Do 8. Place the beaker containing salt water on the hot plate and heat the water. Watch closely.
Predict 9. What do you predict will happen to the salt on the bottom of the beaker in the saturated solution as you heat it?





Interpret 10. Does all the salt dissolve as the solution heats up? __________
What does this tell you about the effect of temperature on a solution?




As the temperature of water increases, the amount of solute that can be dissolved in the water:

(a) increases, (b) decreases, (c) stays the same. Explain.




To Do 11. Obtain more salt if necessary, weigh it, and add salt to the heating water until you create a saturated solution again.
Question 12. What additional weight of salt did you add to recreate a saturated solution? ____________________. You can determine this by weighing the salt remaining and subtracting that from the weight of salt you obtained.

________________________________________
[weight of additional salt] -[weight of remaining salt] =
____________________
[weight of salt added]

Question 13. What is the temperature of this new saturated solution?

__________ oC

Interpret 14. Explain why more salt can go into solution at higher temperatures.







Question 15. Continue heating the solution until it begins to boil. At what temperature do you predict boiling will begin to occur in the salt solution? (greater than or less than 100o C?) Why?







To Do 16. Notice when boiling begins to occur and measure the temperature immediately. __________ oC.

Allow the boiling to continue until the beaker is nearly but not completely dry. What is the boiling temperature of the boiling solution? __________ o C.

Interpret 17. Explain what is in the bubbles that form at the bottom of the beaker and rise to the top.







Predict 18. What will happen to the water molecules and salt ions in a saturated salt solution as the solution is boiled?

Figure 2. Salt Solution on Hot Plate







Results 19. What do you observe as the boiling continues? Does the entire solution evaporate? If not, what part(s) evaporate and what remain behind?







Interpret 20. Explain. Does this match your prediction?







To Do 21. Remove the beaker from the heat as soon as the solution is almost (but not quite) dried. (Be careful - the beaker may crack if you leave it on the heater too long!)
Question 22. What is the importance of the heat energy supplied by the hot plate for the evaporation process? In particular, explain how it affects the molecules.







23. Would evaporation occur without a hot plate or other heat source? __________
At the same rate? __________ Explain.







24. Name several places in the human body where evaporation occurs and describe what purpose(s) it serves.







To Do 25. Find sodium and chlorine (the two atoms that compose salt) in the periodic table and obtain their atomic weight.

sodium _____ chlorine _____

Interpret 26. What is the mass of the two hydrogens and the oxygen atom in water?

hydrogen __________ oxygen __________ water __________

How do their relative masses affect the behavior of sodium chloride and water?







Question 27. Do you find the molecular model useful in understanding and explaining your observations?







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Exercise 2

Diffusion of Red Food Coloring in Water

Powerful Idea

Diffusion

Diffusion is the movement of molecules through a liquid or gaseous medium from an area of high concentration to an area of low concentration due to the kinetic energy of the molecules. In these experiments we will observe diffusion in water.
Predict 1. If you were to drop three drops of red food coloring in a 250 ml. beaker filled with fresh water at room temperature (about 25 oC.), how many minutes, hours or days do you think it would take for the red food coloring to become evenly distributed throughout, if the water is not disturbed.

____________________ (minutes/hours/days).

Interpret 2. Explain what thinking or prior experiences went into making your prediction.







Predict 3. If you were to drop three drops of red food coloring into a 250 ml graduated cylinder filled with fresh water at room temperature (about 25 oC.), how long do you think it would it take for the red food coloring to become evenly distributed throughout, if the water is not disturbed?

____________________ (minutes/hours/days)

Explain 4. Explain how you arrived at your prediction. If you think different times will be involved in the two situations, the 250 ml beaker and the 250 ml graduated cylinder, explain why.







Results 5. Observe as the instructor performs this experiment with a 250 ml graduated cylinder for the class. How long does it actually take for the red food coloring to become evenly distributed throughout? How does that compare with your prediction? How does it compare with the movement of red food coloring in the 250 ml beaker?

Time for even distribution, beaker ____________________

Time for even distribution, grad cyl ____________________

Predicted time, grad cyl ____________________







Interpret 6. Explain any differences you observe in the rate of diffusion between the beaker and graduated cylinder. Have you considered the effects of convection currents (movement of or currents in the water) in the two containers.







Question 7. What is happening to the molecules of food coloring as they move through the water? How do they move? Why don't they sink to the bottom or float on top?







To Do 8. Draw a picture of the red food coloring molecules in water.

Predict 9. If you drop 3 drops of red food coloring to a 250 ml. beaker filled with fresh water, and in this case the beaker is sitting on a hot plate that has been turned on high, will the color become evenly distributed faster or slower than before? Why?







To Do 10. Perform this experiment. Place a 250 ml beaker containing 250 ml. fresh water on a hot plate. Allow the water to become still. Turn the hot plate on high and add 3 drops of red food coloring to the beaker. Determine the length of time required for the solution to become evenly mixed. Notice and describe the pattern of mixing, especially any differences from the beaker observed without heat.

a. time when hot plate turned on and red food coloring added:

____________________

b. describe the patterns of mixing:







c. time when solution evenly mixed: ____________________

d. time required for mixing in heating solution: ____________________

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Exercise 3

Diffusion of Aromatic Molecules in Air

Predict 1. Can molecules travel through the air as well as through water? Describe any experiences or observations that may inform your answer.







Review 2. You know that water is made up of water molecules, and that the molecules have many rapidly forming and breaking hydrogen bonds between them. You also know that the polarity of the water molecule plays an important role when charged or partially charged molecules of other substances dissolve in the water.
To Do 3. What do you know about the composition of air? Air contains three primary gases (below). You are familiar with the structure of each one. Draw the molecular structures of the three gases below.

~78.00 % nitrogen







~21.00 % oxygen







~0.03 % carbon dioxide







In addition, other gases such as water vapor, helium and carbon monoxide, may be present in small quantities. Water vapor may be present in air in widely varying concentrations. Particulate matter is also often present.
Question 4. Do you think these gas molecules are polar (partially charged) like the water molecule? Explain. If they are not polar, how do gas molecules interact?







Question 5. Are molecules of air closer together or further apart than molecules of water? Explain.







Question 6. What is between the molecules of air?







To Do 7. Below is a flask of ordinary air. There is no stopper so the flask is open to the air in the room. Draw a representation of the three major gases (nitrogen, oxygen, and carbon dioxide) in the flask using three different types of dots as shown. Label the space between the dots. What is there?

Figure 3. Open Flask with Air

Predict 8. Will oxygen and carbon dioxide diffuse faster through air or through water? Why?







Predict 9. How long do you think it would take a substance to diffuse from the front of the classroom to the back? What factors need to be considered in making such a prediction?







To Do 10. An approximate floor plan for a classroom with approximate distribution of students (indicated by numbers) is shown below. Record the time as the instructor sprays a strong perfume at the front of the room. (after-shave lotion, ammonia or another harmless, aromatic liquid could also be used). When the odor reaches you, record the time again.

Time substance was spilled: ____________________

Time odor reached you: ____________________

Total elapsed time: ____________________ min.

Figure 4. Approximate Floorplan of Classroom with Students

Results 11. Record your elapsed time on the floorplan on the blackboard, and record all the other students' times in the floorplan above.
Question 12. Sometimes several areas that are the same distance away receive the odor at different times. How would you account for that?







Results 13. Describe in molecular terms how the substance diffused across the room. What steps were involved? What caused movement of the molecules?







To Do 14. Assume that the flask below is sitting in the room close to the perfume spray or other aromatic molecules. The air in the flask has equilibrated with the air around it. Draw the molecules of gas in the flask now.

Figure 5. Flask in Room With Aromatic Liquid

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Exercise 4

Osmosis

Powerful Ideas

Osmosis is the diffusion of water across a semipermeable membrane from an area of higher concentration to an area of lower concentration of water. It is a tremendously important and common phenomenon in living things.

A semipermeable membrane contains pores (channels) that allow oxygen, carbon dioxide, water, and certain other small molecules to pass freely. However, most large molecules cannot penetrate either the semipermeable membrane itself or the pores within it. The semipermeable membrane that surrounds cells is a phospholipid bilayer. Plastic semipermeable membranes are available as well, from scientific supply houses.

To Do 1. Place about 200 ml fresh water in each of two 250 ml beakers. Add salt to one of the beakers while stirring to make a saturated salt solution (a saturated salt solution will have salt on the bottom of the beaker).
2. Place a fresh stalk of celery (about 3"), a fresh carrot (about 3"), and three - five raisins in each of the two beakers.

Figure 6. Celery, Carrots and Raisins

3. Put your name on each beaker as well as whether it is salt or fresh water and place the beakers on a tray at the front of the room to go into the refrigerator until the next class meeting.
Predict 4. What will happen to the celery and carrot in fresh water. What texture, shape and qualities do you expect them to have? Explain your thinking. Why does this seem like the correct prediction to you?







5. What will happen to the celery and carrot in saturated salt water. What texture, shape and qualities do you expect them to have? Explain your thinking. Why does this seem like the correct prediction to you?







Question 6. How do the raisins differ from the celery and carrot? Do you expect the raisins to respond in the same way as the celery and carrot? If not, describe what you do expect the raisins to do in fresh and saturated salt water, and why.







Results 7. Next Class Meeting: What happened to the celery and carrot in fresh water? Feel the vegetables, bend them, and describe their texture and shape.







Compare 8. Compare your observations with your predictions about celery and carrot in fresh water and account for any differences using a molecular model of explanation.







Results 9. What happened to the celery and carrot in saturated salt water? Feel the vegetables, bend them, and describe their texture and shape.







Compare 10. Compare your observations with your predictions about celery and carrot in saturated salt water and account for any differences using a molecular model of explanation.







Results 11. What happened to the raisins in fresh and saturated salt water? Observe them in the liquids and feel them in your hand. Describe their size, shape, texture, and location in the beaker.







To Do 12. Compare your observations with your predictions about raisins and account for any differences using a molecular model of explanation.







Question 13. Was osmosis involved in these experiments? If so, what substance moved across the membrane in each case?





In Figures 7 and 8, use a short arrow to represent the direction of a little diffusion and a long arrow to represent the direction of a lot of diffusion of this substance into and out of each vegetable or fruit.

Figure 7. Celery, Carrots and Raisins in Fresh Water

Figure 8. Celery, Carrots and Raisins in Saturated Salt Water

Think 14. Is osmosis a special case of diffusion, or is diffusion a special case of osmosis? Explain.







Question 15. Can humans survive by drinking salt water such as ocean water? Why or why not? Support your answer with a molecular explanation.







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Exercise 5

Dissolving a Solid

Predict 1. We know that some powdery or particulate substances like sugar dissolve easily in water. But is it possible to dissolve other solids? Even if it is part of a living thing such as an eggshell? Describe what you expect to happen when a fresh egg is submerged in vinegar for several days at room temperature.







To Do 2. Submerge an egg in a beaker of vinegar. Label your beaker and place it on a shelf at the side of the room. What is the potential solvent in this case? What is the solute?







Results 3. Next Class Meeting: Observe the egg in vinegar. Touch it, turn it, feel it, very carefully pick it up (it may be fragile). Examine the vinegar. Describe and draw both the vinegar and egg.

Results 4. If the process is not completed, save your egg on the shelf until next time. If it is completed, take the egg out and break it open in a dry beaker. Has the egg changed, and if so in what way(s)?







Interpret 5. Interpret your observations. What exactly happened? An eggshell is made largely of calcium carbonate, CaCO3 which can ionize into Ca2+ and CO32-. Vinegar is acetic acid (CH3CO2H) dissolved in water, which can ionize into H+ and CH3COO-.







Question 6. Would the egg in vinegar reaction have occurred faster or slower if it had been placed in the refrigerator? Explain your reasoning.







7. Note: The calcium can be removed from chicken bones in a similar manner.

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Supplementary
Resources

Some studies of student learning of diffusion and osmosis.

Cocanour, Barbara. (1986). The case of the soft-shelled egg. Science and Children. 23(6), 13-14.

Friedler, Y., Amir, R., & Tamir, P. (1987). High school students' difficulties in understanding osmosis. International Journal of Science Education, 9 (5Z), 541-551.

Odom, A.L & Barrow, L.H. (1995). The development and application of a two-tiered diagnostic test measuring college biology students' understanding of diffusion and osmosis following a course of instruction. Journal of Research in Science Teaching, 32, 45-61.

Odom, L. (In Press). Secondary and college biology students' misconceptions about diffusion and osmosis. American Biology Teacher 57(), 409-415.

Zuckerman, June Trop (1995). Use of inappropriate and innaccurateconceptual knowledge to solve an osmosis problem. School Science and Mathematics. 95(3), 124-130.

Related
AAAS
Benchmarks

Chapter 4: THE LIVING ENVIRONMENT
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 6-6 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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