Expt 027 -- Antibubbles--Solution Densities

A dilute detergent solution is squirted back into itself to form thin, spherical membranes of air known as anti-bubbles. A sucrose density gradient allows antibubbles to be suspended throughout the solution. These anti-bubbles display principles used to separate and purify very large biomolecules.

Chemical Concepts

1. Whether an object floats or sinks in a fluid depends on whether that object's density is less than or greater than the density of the fluid.
2. Objects may be separated by density by floating the objects in a fluid.
3. If a density gradient exists in the solution, objects float at the level where the density of the solution matches the density of the object.

Background

• Detergent forms a film on the surface of liquids. When more solution is added in sharp spurts the surface is distended and may completely surround the added solution with a thin layer of air. The result is an "anti-bubble" of solution.
• A density gradient is created in the solution being used to generate the anti-bubbles. These anti-bubbles move to a level in the solution with a matched density.
• Plastics are recycled by separating the particles by density.
• If LDPE were being processed, the particles are floated on a solution slightly more dense than LDPE. Any particles that sink are discarded. The floating layer is then added to a solution slightly less dense than LDPE. The LDPE sinks. Any floating particles are discarded. The remaining plastic is processed into new materials.
• Very large biomolecules are separated and purified by adding mixtures extracted from cells to centrifuge tubes with a uniform sucrose density gradient. Small color coded plastic balls are also added to the tubes. After centrifuging the molecules and balls have separated by density just as the anti-bubbles in this solution. The plastic balls mark the level which contains the biomolecules of interest. The layer of interest is withdrawn for further purification.
• The anti-bubbles mimic this separation without centrifuging. The burst colored anti-bubble displays the behavior of molecules in a density gradient without the detergent film.

Safety

This experiment requires normal care. All chemicals are safe to use and dispose.

Procedure

1. Fill a 300 mL beaker with water. Prepare a dilute solution of Dawn or another liquid detergent by adding a little less than a 1 mL of detergent. Mix thoroughly by stirring with a plastic pipet. Avoid creating suds across the surface.

   !!!Click here to See Movie.

2. Fill the plastic pipet with the solution. Hold the plastic pipet a few millimeters above the surface of the solution, and expel a small portion of solution with a quick, deliberate squeeze. Anti-bubbles form when the solution is forced out at the proper rate and the proper distance. Change the distance and squeezing force if anti-bubbles are not forming. Anti-bubbles do not form with every squeeze, even using the best technique.

   !!!Click here to See Movie.

3. Air bubbles on the surface interfere. Skim off any air bubbles with your fingers, and put them in the sink. A few air bubbles around the perimeter are normal and do not interfere, but make sure the area where the solution is being squirted is free of suds. Do not try to form anti-bubbles near the perimeter even if it is free of bubbles.
4. The anti-bubbles are slightly less dense than the main body of the solution, and thus tend to float slowly to the surface. You may "blow" them back down and move them around by expelling solution from the pipet downward onto the anti-bubbles. Pop an anti-bubble by touching it with the tip of the pipet, and observe 2-3 miniscule bubbles of air rise quickly to the top (too small and quick to be captured in this movie -- quite visible when done "live").

   !!!Click here to See Movie.

5. Use these anti-bubbles to illustrate separations by density. Pour Karoª syrup into the solution to a depth of 7-8 mm (approx. 20 mL). A distinct layer is visible on the bottom. Stir to establish a gradient. Do NOT mix until the solution is completely uniform. The gradient does not change linearly because the dilute portion near the top mixes more evenly than the viscous concentrated portion at the bottom. The top half of the solution is nearly a uniform density throughout.

   !!!Click here to See Movie.

6. Withdraw a portion of the solution from the bottom of the beaker. Use this high density solution to "blow" some anti-bubbles at the surface. Observe.

   !!!Click here to See Movie.

7. Withdraw samples from different levels in the beaker, and "blow" more anti-bubbles. Observe how they behave.

   !!!Click here to See Picture.

8. Withdraw a portion from just below the middle of the solution. Make some anti-bubbles. Note the level of these anti-bubbles. Note difference in the level of the small ones and large ones prepared from the same solution.

   !!!Click here to See Movie.

9. Add a drop of blue or red food coloring to a test tube. Withdraw samples from the bottom of the sugar solution. Add the samples to the food coloring and mix thoroughly with your pipet.
10. If desired, a second color may be prepared from the solution near the middle of the beaker.
11. Use the colored solution from the bottom to prepare anti-bubbles. Note the intensity of the color of the anti-bubbles. Some colored solution does not form anti-bubbles as it is added to the solution. Note the level and intensity of the coloring in the solution.

    !!!Click here to See Movie.

12. Pop an anti-bubble by touching it gently. Note what happens to the solution that was inside the anti-bubble.

    !!!Click here to See Movie.

13. If a second color was prepared, make anti-bubbles from the second color. Observe.

    !!!Click here to See Movie.

14. Dispose of solutions at the sink. Wash hands.

Questions

1. For the sugar gradient experiment, describe the relative position of the anti-bubbles made from the bottom of the beaker and the anti-bubbles made from solution near the center of the beaker.
2. Explain the layering described in question 1.
3. When no density gradient exists, the anti-bubbles always rise to the top. Explain.
4. Explain why the small anti-bubbles are always less dense than large anti-bubbles prepared with the same solutions.
5. In the experiment where colored anti-bubbles are formed, use your observations of color intensity to explain why the colored solution free in the beaker does not fall to the bottom.
6. Many plastic disposable items have recycling codes. These plastics have characteristic densities.
   • LDPE, density 0.92-0.94 g/mL, yellow plastic from a mustard squeeze bottle.
   • HDPE, density 0.95-0.97 g/mL, blue plastic from a Miracle White¨ bottle.
   • PS, density 1.05-1.07 g/mL, red plastic from a plastic party plate.
   • PETE, density 1.38-1.39 g/mL, green plastic from the clear part of a Mountain Dew¨ soda bottle.

   Predict what would happen if a mixture of small pieces of the following plastics if it were dropped into the sugar gradient. You may wish to try this experiment.

   1. LDPE
   2. HDPE
   3. PS
   4. PETE
7. If a mixture of very large biomolecules were added to the sugar gradient, predict what would happen to this mixture.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 027 Antibubbles--Solution Densities

Watch the movies. Record your observations from the movies.

H1. Describe the anti-bubble motion in the solutions without a gradient.

H2. Describe the anti-bubble motion in the solution with a gradient.

H3. Describe the motion of the blue anti-bubble.

H4. Describe the motion of the liquid when the blue anti-bubble bursts.

Answer the questions.

Curriculum-

This experiment fits into discussions of separations by density. Separations by density are important in recycling plastics and purifying very large biomolecules such as DNA. The experiment fits into a simple discussion of physical separations or a more advanced discussion of biomolecules.

Activity-

Laboratory or Demonstration or Home Experiment

• The procedure is suitable for a student laboratory, but it works well in much less time as a demonstration. If students are doing a laboratory, demonstrate the technique for the formation of the anti-bubbles
• As a demonstration, use the questions as presentation questions.
• The activity is also suitable for an at-home project, for it is safe and requires no special equipment. However, making the anti-bubbles does require some technique.

Time-

Teacher Preparation: 5 minutes

Class Time: 20 minutes for lab (10 minutes as Demo)

Materials-

• 20 mL Corn Syrup
• 1 mL of Detergent - Dawn^® or other dish detergent.
• Food coloring
• 1 plastic disposable pipet
• 1 300 mL beaker, jar or clear cup
• test tube or jar for mixing solution and coloring.

Optional:

• several plastics of different colors and recycling codes. Be sure to have some of the dense plastics. See question 6 above.

Disposal-

Flush the solutions down the sink with water.

Lab Hints-

• Practice making some anti-bubbles. It may seem tricky at first. Try just gently squeezing out a few drops of soapy water and observe as they form beads of water that skim across the surface on a thin layer of air. Then try squeezing a little harder to push these beads under; the air layer protrudes down and then pinches off to form an anti-bubble. If students are doing the experiment, demonstrate making the anti-bubbles.
• You may wish to have the plastics suggested in question 6 on hand for the laboratory or demonstration.

Observations-

• Detergent forms a film on the surface of liquids. When more solution is added in sharp spurts the surface is distended and may completely surround the added solution with a thin layer of air. The result is an "anti-bubble" of solution. In a solution of uniform density the anti-bubbles always rise because of the air in the outer film.
• A density gradient is created in the solution being used to generate the anti-bubbles. The anti-bubbles formed with solution withdrawn from the beaker move to a level in the gradient with a density equal to their own.
• The anti-bubbles made from solution drawn from the bottom of the beaker drop to the bottom, bounce a bit, and finally settle at a level where the density of the solution is the same as the their own. When using one sample of solution, anti-bubbles of the same size settle at about the same level. Smaller anti-bubbles with more surface area/unit of volume have a slightly lower density and thus settle at a somewhat higher level than larger anti-bubbles formed with the same solution.

Answers-

Q1. For the sugar gradient experiment, describe the relative position of the anti-bubbles made from the bottom of the beaker and those made from solution near the center of the beaker.

A1. The anti-bubbles made from the bottom floated just above the level where the solution was collected. Those from the middle of the solution floated in the upper half of the beaker. Anti-bubbles always end up hovering at a level slightly above the solution used to form them.

Q2. Explain the layering described in question 1.

A2. The anti-bubbles float at a level where the density of the anti-bubble matches the density of the solution. More concentrated solution from the bottom settles to a level near the bottom.

Q3. When no density gradient exists, the anti-bubbles always rise to the top. Explain.

A3. The anti-bubbles contain air at the interface which is the bubble. The air has a buoyant effect which slightly decreases the density of the anti-bubble as a whole.

Q4. Explain why the small anti-bubbles are always less dense than larger ones prepared with the same solutions.

A4. The small anti-bubbles have a larger surface area relative to their volumes. Consequently, the small anti-bubbles contain a larger percentage of air which decreases their density.

Q5. In the experiment where colored anti-bubbles are formed, use your observations of color intensity to explain why the colored solution free in the beaker does not fall to the bottom.

A5. When the colored solution is added it mixes with the solution near the top unless it forms an anti-bubble. The mixed solution is less dense than the solution on the bottom.

Q6. Many plastic disposable items have recycling codes. These plastics have characteristic densities.

• LDPE, density 0.92-0.94 g/mL, yellow plastic from a mustard sqeeze bottle.
• HDPE, density 0.95-0.97 g/mL, blue plastic from a Miracle White^® bottle.
• PS, density 1.05-1.07 g/mL, red plastic from a plastic party plate.
• PETE, density 1.38-1.39 g/mL, green plastic from the clear part of a Mountain Dew^® soda bottle.

Predict what would happen to a mixture of small pieces of the following plastics if it were dropped into the sugar gradient. You may wish to try this experiment.

A6.

• LDPE ---Floats at the top
• HDPE ---Floats at the top
• PS ---Floats in the top half the beaker, but not on the surface.
• PETE ---Sinks to the bottom half of the beaker but still floats above the bottom layer of sugar solution.

Q7. If a mixture of very large biomolecules were added to the sugar gradient, predict what would happen to this mixture.

A7. The biomolecules would float at a level that matches the density of the large molecules.

Makeup Ans.-

QH1. Describe the anti-bubble motion in the solutions without a gradient.

AH1. Anti-bubbles always rise to the surface.

QH2. Describe the anti-bubble motion in the solution with a gradient.

AH2. Anti-bubbles float a little above the point where the solution inside the anti-bubble was withdrawn.

QH3. Describe the motion of the blue anti-bubble.

AH3. The blue anti-bubble sinks to near the bottom, bounces, and then floats near the bottom.

QH4. Describe the motion of the liquid when the blue anti-bubble bursts.

AH4. The blue solution flattens out and floats near the point where the anti-bubble was floating before it popped.

Key Words 1-

density, sucrose density, biomolecules, bubbles, anti-bubble