Lab 5

Water is a Polar Molecule – Lab/Activity 5

Water, More Than Just Wet, It’s Unique

Florida Marine Science Standards SC.912.N.1.1Define a problem based on a specific body of knowledge, for example: biology, chemistry, physics, and earth/space sci-ence, and do the following: SC.912.L.18.12Discuss the special properties of water that contribute to Earth's suitability as an environment for life: cohesive behavior, ability to moderate temperature, expansion upon freezing, and versatility as a solvent.

Objective:

After completing this lab activity you should be able to:

1. Describe the dipolar structure of the water molecule and explain how it relates to hydrogen bonding.

2. Describe the concept of a hydrogen bond using 3-dimensional models as examples.

3. Compare and contrast the differences between water adhesion and cohesion.

4. List examples of how adhesion and cohesion apply to the marine environment.

5. Explain how bubble formation differs in fresh and salt water and how it relates to dissolved oxygen in shoreline breakers.

6. Explain what causes surface tension and explain why it is important to neustonic biological systems.

7. Describe how colligative properties affect surface tension.

8. State in general terms how water density will change based on a given temperature and/or salinity change.

9. Describe how the cohesion and viscosity of water changes with changes in temperature.

Vocabulary:

Adhesion – the attraction of a molecule to a different molecule.

Cohesion – the attraction of a molecule to another of the same type.

Colligative property – the changing of a physical property of a fluid due to the addition of dissolved compounds such as salt, sugar, or alcohol.

Density – the ratio relationship describing the mass per unit volume of a substance, normally expressed in grams/cm3.

Dipolar – sometimes known as polar, a compound or molecule that has two distinct sides exhibiting two distinct electrical charges.

Halocline – a boundary layer caused by the density differences of varying salinity (salt) concentrations.

Hydrogen bonding – the attraction that results from two dipolar molecules in which a relatively weak bond forms to connect them together. This is not a very strong bond compared with covalent or ionic bonds, but strong enough to hold the substance together.

Plankton – organisms that drift in the water environment.

Salinity – the relative concentration of dissolved salts per unit volume.

Surface tension – a phenomenon caused by the cohesive properties of a liquid in which the surface interface turns inward into the liquid resulting in a “skin-like” film at the surface of the fluid.

Thermocline – a boundary layer caused by the density differences of water with two different temperatures.

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Lab 5Viscosity – the property of a fluid to resist flow, sometimes known as internal resistance. This property is caused by the cohesion of the substance.

Safety Issues:

1. Cut Hazard: Use care in putting the molecular models together. The toothpicks are sharper than they look.

2. Fume Hazard: Keep room well ventilated when using the isopropyl alcohol.

3. Flammable Material Hazard: Keep alcohol away from open flames.

4. Staining Warning: Food coloring will stain clothing. Wear a lab apron.

5. Poison Hazard: Rubbing alcohol presents a poisoning hazard. Keep away from face.

6. Post-lab: Wash your hands with soap thoroughly after you complete the activities.

7. Glass Hazard: You will be using some large glassware. Handle with care and report any incidents to your teacher immediately.

Introduction:

What do you think about when you look at the water? Does it make you relax? Does it help you to forget your worries? Most of us have experienced the calming sensation of a lake or the ocean.

Normally, we don’t think about water’s unique properties, or that water in liquid form, as found on Earth, seems to be rare in the universe. It’s easy to take water for granted. It is available to us most of the time in quantities that make it an everyday substance.

What we do not think about is how water differs from other compounds found in nature. Water is polar in nature and this polarity gives water many unique physical properties. As you explore water’s properties, keep in mind that its unusual characteristics result from how it bonds together.

1. Research Question(s):

What are the unique properties of water? How does the structure of water contribute to its unique properties?

2. Literature Review:

The unique properties associated with the water molecule come from its unusual shape, which results from uneven sharing of electrons between the two hydrogen atoms and the oxygen atom.

Because oxygen attracts electrons more strongly than hydrogen, the molecule doesn’t share electrons evenly. This gives the oxygen side a partial negative charge and the hydrogen side a partial positive charge.

Think of it as an atomic version of tug-of-war. Instead of a rope you have electrons. In this case, the stronger oxygen keeps the electrons mostly to itself. Due to the arrangement of the electrons, this “tug-of-war” causes the oxygen and the hydrogen to form a Y-shape molecule. The hydrogen side is positive and the oxygen side is negative.

This dipolar characteristic allows water molecules to link together. The positive hydrogen side attracts the negative oxygen side of other water molecules. This linking is known as hydrogen bonding, and gives water a tendency to hold together. This allows water to exist as a liquid under very wide temperature ranges, a property normally found with other compounds.

Two major phenomena resulting from water’s dipolar characteristics are cohesion and adhesion. Cohesion is the attraction of water molecules to itself. Adhesion is the attraction of water molecules to other substances that also have dipolar characteristics.

These two phenomena lead to many effects that you will observe during this activity. The cohesion tends to be strong enough that water forms an extra strong skin-like texture to its surface. This is called surface tension, a phenomenon found with all liquids but rarely as strong as with water. With the exception of mercury, water has one of the strongest surface tensions known in nature.

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Lab 5Depending on its strength and the influence of other forces, cohesion and surface tension are strong enough to bring

a fluid into its smallest 3-dimensional surface area – a sphere. This is the reason why water beads into semi-spherical shapes. In space with the influence of gravity removed, water forms spherical droplets. When falling through the atmosphere (like raindrops), the characteristic teardrop shape results from drag with the air altering

the sphere.

If a liquid has a very weak surface tension, you will observe very little beading. Alternatively, water surface tension can be so strong that it supports life forms on its surface. Known as neustonic life, biologists have only begun to study this unusual habitat and its ecological role.

Another physical property associated with cohesion is viscosity. Viscosity is the ability of a fluid to resist flow. A fluid’s viscosity depends upon the cohesion strength of the bonds, as well as temperature. This especially affects smaller marine organisms by changing how much energy they must expend to move.

Most dipolar substances also bond with materials other than water. Sometimes this adhesive polar bond has greater strength than the liquid’s cohesive bond. This is the reason why a meniscus forms in a graduated cylinder. The water has a stronger tendency to “climb” up or adhere to the glass than to cohere.

The addition of dissolved substances such as salt or alcohol to water affects cohesion, adhesion, surface tension, viscosity and density. Called a colligative interaction, dissolved substances tend to weaken surface tension and reduce adhesion with other substances.

Materials:

• Molecular model kit or class-made kit from teacher

• Protractor

• Masking tape

• Indelible pen (Sharpie®)

• Wax paper sections (small 5 in. by 5 in. sections)

• Food coloring (blue, green, and red)

• Eyedroppers (mark some for freshwater and some for alcohol only)

• Water (distilled is preferred but regular tap water will do)

• 70% rubbing alcohol (isopropyl alcohol)

• Two – 150ml beakers (one for the water and one for the alcohol)

• Supply of small plastic bathroom cups

• Class supply of straws (cafeteria straws will do)

• Bubble solution or home-made dish soap bubble solution (30% soap/70% warm water)

• A few bubble wands (you can make them with small paper clips if the wands are not available)

• Supply of salt (grocery store table salt)

• Supply of salt water at 35-40 ‰ (ocean water would do or you can make it up yourself and check it with a hydromter)

• 100 ml graduated cylinder

• Forceps (tweezers)

• Small paper clips (must be clean and dry)

• 4 in. strips of filter paper (coffee filters would work and as a last resort paper towels)

• Toothpicks (wood, preferably the flat type)

• Pennies (a few for each student or group)

• Aluminum foil (use the heavy gauge type if available)

• Small ruler

• Ice water supply (the closer to 0º C the better)

• Heated water up to 70º C

• ThermometersPage 3 of 19

Lab 5• Hydrometers

• Two - 1000 ml beakers

• Two - 1000 ml graduated cylinders (2000 ml graduated cylinders work best, but 1000 ml cylinders will work)

• Stopwatch (a clock or watch with a second hand would work, but a stop watch is more accurate)

• Wire coat hanger or kitchen straining ladle (used as the retrieval system for the viscosity activity)

3. Hypothesis:

Based on the research question(s) and the literature review write your hypothesis/prediction(s) below.

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4. Activity:

Station 1: Dipolar Structure

1. At this station you will find either a molecular model kit or a kit your teacher has put together.

2. Locate two hydrogen atom models and a single oxygen atom model.

3. Attach them together using the bonding sticks.

4. You will need a protractor to confirm that the angle between the two hydrogen atoms and the oxygen atom is 105 degrees.

5. Sketch the water molecule model in the data collection area for Station 1.

6. Include the angle in your diagram.

7. Mark a (+) sign on the hydrogen side of the water molecule model and a (-) on the oxygen side.

8. Add the charges to your diagram.

9. Build four more water molecule models.

10. Place the models together based on how the molecule poles attract each other.

Note: When putting the water molecules together, remember that like charges repel themselves as far away from each other as possible.

11. Sketch the model in Station 1: Data Collection. Use dashed lines to represent the attraction forces of the hydrogen bonds between individual water molecules. Write down your observations regarding molecular attractions.

12. When done, disassemble your water molecule models and replace the pieces where you found them.

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Lab 5

Station 1: Data Collection

Observation statement: What do you need to consider when you place the models together based on how the molecules poles attract each other?

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Station 2: A Study of Cohesion

1. You will need a piece of 5 in. by 5 in. wax paper, a supply of water, a supply of rubbing alcohol, and eyedroppers specific to each solution.

2. Place a small piece of the wax paper in front of you.

3. Fill the water eyedropper with water.

4. Carefully and slowly place three or four drops on top of each other to make a large bead of water on the wax paper.

5. Repeat this with the rubbing alcohol using the wax paper. The alcohol bead can be placed near the water, but make sure the two beads don’t touch.

6. Look at both beads of liquid from the side.

7. Diagram what you observe in the space provided.

8. Take the dropper containing the alcohol and place one or two drops on top of the water bead.

9. Observe what happens and record the result. Discard the wax paper when done.

Station 2: Data Collection

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Lab 5

Observation statement: What do you observe when you put a few drops of alcohol on the water bead?

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Station 3: A Study of Surface Tension

1. You will need a bubble wand, bubble solution, a small plastic cup, and salt.

2. Fill the cup about one-third with bubble solution.

3. Using the bubble wand and solution, blow a bubble.

4. Observe the shape and texture of the bubble and record the data below.

5. Add salt (about one tablespoon) to the bubble solution, blow a bubble, observe the shape and texture of the bubble, and record the data below.

6. Write down your observations about differences in blowing a bubble after adding the salt. Discard the cups and solutions and return other materials to their proper places when done.

Station 3: Data Collection

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Lab 5Observation statement: What did you observe when you tried to blow a bubble after adding salt to the bubble solution?

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Station 4: A Study of Adhesion

1. You will need a 100 ml graduated cylinder and water.

2. Fill the 100 ml graduated cylinder to the 100 ml mark with water.

3. Look at the surface of the water (known as the meniscus) from the side.

4. Diagram the meniscus below. Examine the water level on the sides of the cylinder compared to the center and record your observations.

5. Empty the cylinder and return it to its proper place when done.

Station 4: Data Collection

Observation statement: What do you notice about the water level on the sides of the cylinder compared to the center?

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Station 5: A Study of Cohesion and Surface Tension

1. You will need one small plastic cup, water, alcohol, forceps, and a dry clean paper clip.

2. Fill half of the drinking cup with fresh water.

3. Using the forceps, carefully place a small paper clip on the water surface without letting it sink down to the bottom.

4. If you miss on the first attempt try it again with a clean paper clip.

5. Note that the water surface tension is strong enough to hold up the paper clip. Sketch the paper clip held up by the surface tension of the water below.

6. Add drops of alcohol to the water with the paper clip still resting on the surface.

7. Count the number of drops until something happens. (You will know it when it occurs.)

8. Record your observations below. Include the number of drops of alcohol required to make the change you observed. Discard the water and return the other materials to their proper places when done.

Station 5: Data Collection

Observation statement: What happened when you placed 4-7 drops of alcohol in the water supporting the small paper clip?

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Station 6: A Study of Adhesion

1. You will need two pieces of cut filter paper (4 in. strips), two toothpicks, two small plastic cups, food-colored water, and food-colored alcohol.

2. Put about an inch of water in one cup and an inch of alcohol in the other cup.

3. Punch a toothpick through one end of the filter paper strip. You will use the toothpick to hold the paper up in the drinking cup.

4. Repeat with the other strip.

5. Place a hanging filter paper strip in each container so the end is just under the liquid surface.

6. Note the movement of the two fluids up the filter paper. Diagram this in the Data Collection area.

7. After five minutes, compare the heights the two fluids reached as they moved up the paper. Record this below. Discard the cups, toothpicks and solutions when done.

Station 6: Data Collection

Observation statement: What were the relative heights of the two fluids on the filter paper after five minutes?

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Station 7: A Study of Adhesion

1. You will need a piece of wax paper and four flat wooden toothpicks. (This won’t work with plastic toothpicks.)

2. Break each toothpick by folding it over itself, but do not separate the sections.Page 9 of 19

Lab 53. Arrange the four toothpicks in a star-shaped pattern on a piece of wax paper with the breaks in the center.

4. At the center, where all the toothpicks meet, leave a small open space. Sketch this arrangement below.

5. Take a water dropper and place two drops carefully in the middle of the space.

6. Watch what happens. Draw your observations below.

7. After three minutes, discard the toothpicks.

Station 7: Data Collection

Observation statement: What did the water seem to do when you added it to your wooden starfish? Did you notice any of the water actually penetrating the wood?

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Station 8: A Study of Viscosity

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Lab 51. You will need two pennies, some aluminum foil, water at 5°C and 50°C, a stopwatch (or watch with second hand),

and two 1000 ml graduated cylinders. Note: Two 10-gallon fish tanks can replace the 1000 ml graduated cylinder.

2. You will need to make a “penny diver.” Wrap a penny in aluminum foil. The aluminum foil provides some lift. You will need to use the same “penny diver” in all trials. Be sure to complete all the 5°C water trials before putting the “penny diver” in the 50°C water.

3. You will need to use a retriever. A kitchen utensil works well or you may construct your own made from a wire coat hanger and some aluminum foil. Aluminum foil covers the wire coat hanger loop forming a base with a circumference slightly less than the graduated cylinder. Punch holes through the aluminum foil. Place the retriever into the cylinder before you begin. The “penny diver” will land on it, allowing you to pull it back to the surface for the next trial.

4. Fill one cylinder with 50°C water and the second with 5°C water to the 1000 ml mark (store in a refrigerator or cooler).

5. Start with the 5°C water and hold the “penny diver” at the surface. Have someone ready to start and stop the stopwatch.

6. Record the time to the nearest 1/10th of a second that it takes for the “penny diver” to sink.

7. Retrieve the “penny diver” and repeat the run at least 10 times. Record each trial time as you complete a dive on the data table below.

8. Repeat the 10 trials with the 50°C water. Record the descent times of each trial to the nearest 1/10th of a second.

9. Determine the average descent times for the 5°C water and the 50°C water. Record the difference, if any, on the data table. Empty the cylinders and return the cylinders and the other materials to their proper places when done.

Station 8: Data Collection

Observation statement: What difference, if any, did you find between the average times for the “penny diver” to descend through the 5ºC and 50ºC water?

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Station 9: A Study in Density Gradients

1. You will need two 1000 ml beakers, warm fresh water, very cold fresh water, room temperature fresh water, and room temperature salt water.

2. Put six to ten drops of red food coloring in 150 ml of warm fresh water. Stir well.

3. Put ten drops of blue food coloring into 150 ml of very cold fresh water. Stir well.

4. Carefully pour the 150 ml of blue cold fresh water into the 1000 ml beaker. Tip the beaker and pour gently.

5. Gently tilt the 1000 ml beaker and very carefully and slowly add the 150 ml of red warm fresh water.

6. Allow the beaker to sit for one minute. Carefully, without mixing the warm and cold water, take and record the temperature of both layers of water, and then diagram what you observe in the data collection area for Station 9.

7. Rinse the beaker. Repeat the procedure with the room temperature fresh water and room temperature salt water. Use red for the fresh water and blue for the salt water. Before mixing use a hydrometer to measure the salinity of the salt water. If you are not sure how to use or read your hydrometer ask your teacher to show you how before proceeding. Record this in the data collection area for Station 9. Then carefully pour the room temperature fresh into the 1000 ml beaker and slowly add the salt water.

8. Allow the beaker to sit for one minute. Carefully, without mixing the fresh and salt water, take and record the temperature of both layers of water, and then diagram what you observe in the data collection area for Station 9.

9. Once again, use a hydrometer to measure the salinity of the salt water.

10. Rinse the beaker and return it to its proper place when done.

Station 9: Data Collection

Observation statement: What was the salinity of the salt water before mixing and after mixing?

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What similarity, if any, did you find between warm fresh water, very cold fresh water, room temperature fresh water, and room temperature salt water?

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Analysis of Results:

Interpret and analyze your results for each station by answering the following questions.

Station 1: Dipolar Structure

1. Based on what you learned from your model, explain how the dipolar structure of water relates to hydrogen bonding. What forms the hydrogen bond?

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2. Many fluids are similar to water in that they display dipolar characteristics. Based on how water molecules interact with other water molecules to form hydrogen bonds, predict what might happen if you mixed water with other dipolar compounds?

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3. Assume a compound does not have dipolar characteristics (known as a non-polar substance). Predict what would happen if you put a non-polar substance into a beaker containing water.

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Station 2: A Study of Cohesion

1. When the water drop beads up on the wax paper why does it take on the shape of a sphere? Why not a cube, column, or a pyramid? How does the sphere shape differ from all other shapes?

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2. If you placed a drop of liquid mercury on wax paper, you would see an almost perfect sphere form. Why doesn’t this happen with water?

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3. If the cohesion of water forms the bead on wax paper, then why doesn’t alcohol form a bead? What does this tell you about the dipolar characteristics of alcohol compared to water?

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4. When you added a drop of alcohol to the bead of water, the ability of the water to maintain its shape changed. Explain what happened and why?

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5. Why did you need wax paper for this activity? Hint: Wax is a non-polar substance.

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Station 3: A Study of Surface Tension

1. What do your observations about blowing bubbles tell you about the relationship between surface tension and bubble formation?

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2. Contrast the difference between bubble formations in fresh water versus that of bubble formation in the marine environment.

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3. When a wave breaks on a beach, the breaking wave pushes air into the water, resulting in bubbles that add oxygen to the water. The more bubbles, the greater the dissolved oxygen in the water. Assume you have a beach on a large freshwater lake versus an ocean coastline beach. Assuming all conditions are the same except salinity, which beach water would have the higher dissolved oxygen levels? Explain your answer.

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Station 4: A Study of Adhesion

1. What characteristic of water explains the shape of a meniscus? Explain what happens to cause the shape.

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2. Would you expect the meniscus to have a different shape if the beaker were made of a non-polar compound? If so, in what way?

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Station 5: A Study of Cohesion and Surface Tension

1. Explain why water was able to support the paper clip on its surface, but alcohol couldn’t.

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2. Why was it necessary to place the paper clip flat? What would happen if you tried to put the paper clip in the water on its end (straight up and down)?

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3. Some organisms support their mass on surface tension. What would be the best shape for the feet of such an organism? Why?

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4. Dissolved salt affects surface tension much like alcohol does. Would this have any effect on neustonic life living on fresh water versus living on salt water? Explain your answer.

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5. Describe the environmental effect on the neustonic life if someone accidentally introduced a significant amount of salt into a fresh water lake.

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Stations 6 and 7: A Study of Adhesion

1. Contrast the differences between cohesion and adhesion.

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2. Explain why the water “climbs” up filter paper more effectively than does alcohol.

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3. If something were dissolved in the water, what would happen when it moved up the paper? To answer this, imagine you’re climbing a ladder wearing a backpack. Does the backpack’s weight affect how high or fast you can climb?

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4. Research the plant structure known as xylem and apply the concept of adhesion to explain how water moves through a terrestrial vascular plant.

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Station 8: A Study of Viscosity

Use the data collected for Station 8 to construct a scatter graph for the 10 different trials at each of the two temperatures (use a different color for each temperature).

1. You should have found that the “penny diver” sinks through warm water faster than cold water. All plankton have adaptations to prevent them from sinking. How does the tendency for water to cool with depth assist plankton in staying near the top of the water column?

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2. Describe three adaptations that planktonic organisms could have to prevent or slow sinking in the water column.

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3. Predict the swimming efficiency of a fish as it moves through warm and cold-water layers in the ocean. Explain on what basis you make this prediction.

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Station 9: A Study in Density Gradients

1. What property did you find that cold fresh water and salt water have in common?

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2. Contrast the differences between a halocline and a thermocline.

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3. Deep-water fishing is normally better around a thermocline or halocline. Using the information from the past few questions provide an explanation as to why the boundary layers would be great areas for setting a fishing line.

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OVERALL3. Is your original hypothesis/prediction(s) supported by your data? Why or why not?

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4. Write two new research questions based on what you have learned from this activity:

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5. The value and importance of this activity: What are water’s unique properties and how might these properties affect marine life?

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Conclusion:

2-3 sentence summary of lab. Sources of possible error in your results, and was this lab successful for you? Why or WHY not?

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