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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?

The sugar dissolves and diffuses throughout the cup, moving from areas of higher concentration of sugar to areas of lower concentration, until the concentration is the same everywhere.

2. When you put celery and carrots in some fresh water in the refrigerator, why do they get crisp?

They get crisp because the fresh water enters their cells by osmosis, moving from the higher concentration of water (outside the cell) to the lower concentration of water (inside the cell). The concentration of water is lower in the cell because it contains so many dissolved salts, sugars and other molecules.

3. Why does a plant wilt and go limp when it doesn't have enough water?

Water is continuously evaporating from a plant through openings called stomata. If it is not replenished, then the cells of the plant cannot retain enough water to maintain their turgidity. As the water content of the cells declines, the cell walls become more flaccid.

4.Where does your urine come from? How is it that it can contain alcohol, sugar, drugs, or other substances that you have ingested?

Urine is formed in the kidneys by removal of water and certain dissolved substances from the blood. A major component of urine is urea and other nitrogenous wastes which are produced by the cells of the body and released into the bloodstream. When we ingest an excess of substance such as sugar or alcohol, that substance will be absorbed into the bloodstream from the intestinal tract and can be removed from the blood (in part) by the kidneys.

5. When you sweat, what is evaporating from your skin? What remains behind? What does sweating do for your body?

Sweat is a salty water-based liquid secreted by sweat glands. The water absorbs heat from the body and evaporates, cooling the body in the process. The dissolved salts and other substances remain behind on the skin surface.

6. How do the nutrients you eat get through your intestinal wall and your blood vessels into your bloodstream?

The foods are broken down into molecules by enzymes. The molecules diffuse through the villi of the intestine (especially the small intestine) and into the capillaries that are intimately associated with each villus.

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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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Powerful Idea

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

When 25 ml. of water are added to the beaker, the total weight should be 25 g heavier than the beaker by itself.

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

       

The weight of the salt dissolved into the room temperature water can be determined by the difference in weight as mentioned above. Students should find that they can dissolve more salt in the heated solution. This is due to a greater intermolecular distance between the water molecules (there is more space for the sodium and chloride ions to occupy) and a greater kinetic energy in the solvent (more water molecules are at a high enough energy to react with the salt crystals. There are several reversible reactions occurring dynamically and simultaneously in the students' saline solution.

Water reacts with itself to form ions:
H2O + H2O -> / <- H3O+ + OH-

The H3O+ can also ionize:
H3O+ -> / <- H+ + H2O

The water reacts with salt to form hydrochloric acid and sodium hydroxide:
H2O + NaCl -> / <- HCl + NaOH.

And the acid (HCl) and base (NaOH) ionize.
HCl -> / <- H+ + Cl-, NaOH -> / <- Na+ + OH-

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?

Most of the salt will dissolve into solution.

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.

If the students had added just a little salt beyond the saturation point at room temperature, they would find that the additional salt should dissolve in the water after raising the water to a higher temperature. In general, as the temperature of water increases, the amount of solute that can be dissolved in a solvent increases.

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.

The water molecules are at a higher kinetic energy, more water molecules are reacting with the salt, and there is more space between the water molecules for sodium and chloride ions to fit into.

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?

The temperature at which the salt solution boils will depend on how much salt the students added. Pure water boils at 100o C. As solute is added, the boiling point will elevate. The salt reduces the surface area of the water and therefore fewer water molecules can evaporate at any one time. For this reason, and because salt has a much higher boiling point, the salt will increase the boiling temperature; the more salt, the higher the boiling point.

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.

The bubbles that form and leave the liquid are made of water vapor, H2O molecules in a gaseous form. Some students think the bubbles consist of hydrogen and oxygen gas. However, hydrogen and oxygen gas together are highly explosive. Yet holding a lighted match over the steaming solution should extinguish the match, not produce an explosion. This can be one bit of evidence to help convince these students that the bubbles are really water vapor.

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

The water should boil off as water vapor and leave reconstituted salt crystals.

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?

The water evaporates. Salt (and some water) 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.

Molecules all have measurable kinetic energy. How high or low this value is determines the state (solid, liquid, or gas) of these molecules. The hot plate supplies heat energy to the salt solution and raises the kinetic energy of the molecules within it. When enough energy is absorbed by the solution, the water molecules will evaporate, or change from a liquid to a gas state. When the water vapor reaches the cool air, it condenses temporarily to form a mist of small water droplets or steam. Then the steam evaporates again to form invisible water vapor in the air.

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

Evaporation would still occur without a hot plate, but at a much slower rate. The rate of evaporation increases with increasing temperature. Increasing air movement and decreasing humidity also will increase the rate of evaporation.

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

Evaporation takes place on external bodily surfaces. We commonly recognize this as sweating. When we exert ourselves, sweat glands release a saline (salt) solution onto the surface of the skin. The sweat absorbs heat from the body's surface and evaporates. Since evaporation requires and takes away heat, it cools the surface of the skin. As the skin is cooled, the blood in adjacent vessels is also cooled and thus the body temperature of the person is lowered.

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

sodium _____ chlorine _____

Sodium has an atomic weight of 23, and chlorine has an atomic weight of 35; the molecular weight of salt is thus: 23 + 35 = 58.

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?

Oxygen has an atomic weight of 16, and each hydrogen has an atomic weight of 1; the molecular weight of water is thus 18, about 1/3 that of salt. Heavier ions or molecules require significantly more energy to go into the gaseous state. In fact, the boiling point of salt is extremely high compared to 100o C for water.

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

Scientists have found the molecular model extremely useful for explaining many different properties of substances. Hopefully your students can begin to see its power as well.

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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 25oC.), 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).

If the water is not disturbed, it may take a few minutes to a half hour or more for the dye to completely diffuse. Some dyes are so heavy they do not diffuse completely by the end of the period. This is especially true of green food coloring. If the water is disturbed, the same effect may be observed almost instantaneously. The cause of the dye's spread throughout the beaker will be different, however. In the first case, only diffusion was taking place. In the latter case, both diffusion and mixing were occurring.

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

Students will hopefully be able to draw upon past experiences to help them with their predictions. We have all seen examples of diffusion, as when a tea bag is placed in a cup of hot water In this case, both dissolving and diffusion are at work.

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)

It should take longer for the red food coloring to evenly distribute itself throughout the new vessel.

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.

The red food coloring molecules added to the graduated cylinder are virtually identical to the molecules added to the beaker. This means that they would have the same kinetic energy and therefore the same velocity. The volume of water is also the same. The shape of the container, however, reduces the likelihood of convection currents in the water so the distribution of the food coloring will be almost entirely due to diffusion alone and will be slower.

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 ____________________

Have the students record the actual time, compare and discuss the results of the two experiments.

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.

The rate of diffusion is identical in both cases. Remind students that the diffusion rate in this case is speed of the movement of food color molecules through cubic centimeters of water per second. Simply because it takes more time for the coloring to spread through containers of different shapes does not mean that the rate of diffusion is different in the two containers. Convection currents must be considered as well, as noted above.

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?

The molecules of food coloring are spreading apart as they move through the water. The molecules are driven by their own kinetic energy and a system-wide increase in entropy or randomness. The food coloring does not float or sink because it is soluble and therefore goes into solution.

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?

As might be imagined, more heat leads to a greater kinetic energy. More kinetic energy will lead to a greater molecular velocity and a shorter time requirement for even distribution.

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:

As the water is being heated, turbulence will be created. Students should note a complex mixing pattern, different from that in unheated water.

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.

Students should be able to draw on past experiences to help them. Remind students that when they use their sense of smell, molecules from the object they are smelling are actually going into their nose by diffusing through the air from their source. Then, mention examples like perfume or baking bread.

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.

Composition of Air
~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?

These gas molecules are not polar. There is absolute equal sharing of electrons between the atoms in nitrogen gas and also in oxygen gas. For carbon dioxide, each carbon-oxygen bond is polar, but the resulting molecule has no net charge. Gas molecules do not interact with one another in the way that water molecules do. They move around, bump into each other, and bounce off of one another.

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

Air molecules are much further apart than water molecules. Your students can think about this in several ways. Gases have a higher kinetic energy than liquids. Higher kinetic energy produces greater movement of molecules, greater entropy or randomness, and greater intermolecular distance. Students may be asked to picture equal masses of water and water vapor. A gram of air (or a gram of water vapor) will occupy a greater volume than a gram of water at equal temperatures.

Question 6. What is between the molecules of air?

Nothing! Empty space exists between the molecules that comprise 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?

Gases will always diffuse faster through other gases than through fluids. Picture yourself a gas molecule in the middle of a classroom. If your objective was to get out of the room (diffuse), would it be easier to do so if there were a few other "molecules" scattered throughout the room (as would be the case if you were to diffuse through a gas) or if there were many molecules in the room in a more ordered formation (as would be the case if you were to diffuse through water)? You would find it easier to diffuse through a gas.

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?

The time required, of course, will depend upon the dimensions of your classroom. Other than that, factors that may be considered include: the substance being released, the temperature of the room, whether the doors/windows are open or closed, and the convection currents that are present.

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?

Gases diffuse in all directions at once, provided there is no physical barrier. Convection currents in the air cause greater movement in some directions than others.

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

The first thing that would have to happen would be for surface molecules from the substance to evaporate. Then, due to the gas molecules' kinetic energy, the molecules will diffuse throughout the airspace in the classroom from regions of higher concentration to regions of lower concentration.

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?

Assuming that the vegetables were fresh, the celery and carrot will swell and become crisp. The water will move from the area of higher concentration (in the fresh water) across the semipermeable membrane into the area of lower concentration of water (inside the veggies). The water is less concentrated in the veggies because of the salts and other molecules in the cells.

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?

A saturated salt solution has a greater concentration of salts (and therefore fewer water molecules per volume) than the veggies. Thus, in the veggies in the saturated salt water, the water will flow from the area of higher concentration of water inside the cells to the area of lower concentration of water outside the cells. The veggies should become limp and pliable.

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.

Raisins differ from the celery and carrot in that raisins are dried fruits containing very little water. For this reason, the raisins should swell in both fresh and salt water.

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.

The results in 8 - 11 hours should be similar to our predictions. Have students examine, compare and discuss their findings.

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.

Because raisins are dried food and contain very little water, water enters the raisins in both the fresh and the saturated salt solution. The raisins usually become plump or swollen in both cases and often rise to the surface or move up and down in the beakers.

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?

Yes, osmosis was involved. Water is the substance that moved across the membrane into or out of cells. The water always moved to the side of the membrane with the lower water concentration. (Larger molecules like salt and sugar are unable to cross the membranes.)

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

Osmosis is a dynamic process that occurs in both directions, with the overall process often being much larger in one direction than the other (indicated by the longer arrow). In fresh water, the concentration of water is highest outside the fruits and veggies. The direction of osmosis of water is therefore primarily into the plant cells. Diffusion stops when the pressure inside the cell walls becomes too great to continue, although there may still be osmotic pressure resulting from higher concentrations of water outside the cells than inside them.

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

In saturated salt water, the concentration of water is higher inside the cells of the fruits and veggies than outside of them. Another way to state this is that the salt concentration is higher in a saturated salt solution than in living tissue. The direction of osmosis of water is therefore primarily out of the celery and carrot cells. In contrast, because of high sugar concentrations in raisins (and low water content), water diffuses across the membranes into the raisin cells even in saturated salt water.

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

Osmosis is a special case of diffusion. In both diffusion and osmosis, molecules are moving from an area of higher concentration to an area of lower concentration of those molecules. Osmosis, by conventional definition, refers only to the movement of water molecules across a semipermeable membrane.

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

Humans cannot survive by drinking ocean water. Ocean water has a higher salt content than human cells. Thus, water will diffuse out of the cells lining the person's digestive tract (where the water concentration is higher) into the salt water in the gut (where the water concentration is lower), leading to serious dehydration. (This does not happen as readily with skin cells when a person swims in the ocean because skin has some protection against such free flow due to the dead layers of skin and oils on the skin.)

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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.

We have all heard the saying, "Put it to the acid test." This saying has its origin hundreds of years ago. When royalty requested the local goldsmiths to forge their crowns, it was impossible to determine whether or not the smiths actually used gold As a solution to this, a chemist discovered (through experimentation, of course) that if you place gold in acid, it will not dissolve. However, other solids would dissolve. Hence, "Passing the acid test" indicates that whatever is being tested is truly "gold". Placing an egg in vinegar (approximately 95% water and 5% acetic acid) will dissolve the shell which is composed largely of calcium carbonate. and release carbon dioxide gas. The egg will be held together by its membrane only, and will swell as water crosses the membrane.

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?

The solvent in this case is the vinegar. The solute is the eggshell. The products are a gas (indicated by the bubbles and a solution.)

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.

Students should be careful with the egg, which will be fairly fragile. This is particularly true if the class only meets once per week. The egg should eventually appear leathery and should be a bit dull in color. The vinegar may be a bit cloudy and will likely be bubbly. Incidentally, what happens to the egg is the same thing that will happen to your teeth if you do not brush. Bacteria in your mouth feed on the same food that you do. As they metabolize this food, they put out acidic waste products. These products can dissolve the enamel in your teeth.

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)?

The egg will no longer be hard on the outside. The shell has disappeared. The egg also is likely to be swollen and turgid due to the flow of water from the liquid into the egg.

Interpret 5. Interpret your observations. What exactly happened?

A reaction occurred between the acetic acid of the vinegar and the calcium carbonate of the eggshell. They exchanged parts and produced CO2 gas.

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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.

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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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