Expt 026 -- Sucrose Density

A cocktail called Pousse-Cafe is prepared by layering colored liqueurs one on top of the other. This demonstration uses colored sugar solution to achieve the same rainbow effect but with impressively sharp boundaries.

Chemical Concepts

1. Solutions must mix to react.
2. Solutions of different density layer even when the solutions are soluble in one another.
3. The unassisted diffusion of a substance through a liquid is rather slow -- compared, for instance, to diffusion through a gas. This is a result of the relatively small spaces between the liquid particles and the subsequently small mean free path of the diffusing particles.

Safety

Use ordinary laboratory safety procedures.

Procedure

1. Prepare 500 mL of a concentrated sugar solution to serve as the stock solution for dilutions. The sugar requires some time to dissolve.
2. Prepare 150 mL of each solution by diluting the stock solutions. Use beakers or plastic cups to dilute the solutions. Add 150 mL of sugar to the first container. Add 130, 90, 60, 30 and 0 mL to the next 5 containers. Careful measurement is not required; simply estimate even steps in the five containers. Fill all six beakers to the same level with water. Add a different color to each beaker. Mix colors to make violet and orange. Stir each solution until uniform.

   !!!Click here to See Movie. The movie is accelerated 10 times the actual speed.

3. Cut the soda bottle in half to make a funnel. Insert a piece of rigid tubing into the stopper. Use care inserting the tubing. Use plenty of glycerin. Hold the stopper at the side. Stopper the funnel. Connect the flexible tubing to the tube in the stopper. Wrap the narrow tube with electric tape to fit the flexible tubing. Insert the narrow tube into the graduated cylinder. Connect the tubing to the narrow tube. Adjust height of the funnel and narrow tube with the tip just above the bottom of the cylinder. Clamp in place with a ring stand. Attach the screw clamp to the flexible tubing.

   !!!Click here to See Movie. Click |> or <| to step the slides forward or back.

4. Pour the pure water solution into the funnel. Open the screw clamp and try to clear any bubbles. Allow the solution to drain quickly until the funnel is nearly empty. Slow the flow with the clamp, and stop it when the funnel has only a few drops remaining. These few drops mix with the next solution, but that does not disturb the crisp layering effect nearly as much trying to drain the funnel completely and allowing air bubbles to form in the tube.

   !!!Click here to See Movie. Click |> or <| to step the slides forward or back.

5. Add the next liquid, the most dilute sugar solution, to the funnel, then open the screw clamp slightly to allow a very slow trickle of this solution into the bottom of the graduated cylinder.

   !!!Click here to See Movie. Click |> or <| to step the slides forward or back.

6. Once 4-5 mm of this new layer have formed, the flow rate may be increased gradually. Again, once the funnel is nearly empty, slow the flow, then close it off completely just before the funnel runs dry. Then add the next solution, and so on with each of the remaining solutions in the same manner in order of increasing density, ending with the most concentrated sugar solution.

   !!!Click here to See Movie. The movie is accelerated 100 times.

7. Just before the last solution has finished draining, clamp off the tube completely and either leave it that way or very carefully lift the tube straight out, avoiding any sideways movement of the tube that may cause mixing of the layers.

Display

1. Display the finished rainbow to your students.

   !!!Click here to See Movie.

2. Tip the cylinder to illustrate how little mixing takes place even with some motion. Wave action by layer is apparent. The waves in the different layers are not synchronized.

   !!!Click here to See Movie.

   !!!Click here to See Movie.

Questions

1. Predict the result of adding the most dense solution to the top of the cylinder.
2. Predict the effect of adding a bubble of air at the bottom of the cylinder.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 026 Sucrose Density

Watch the movies and answer the questions.

Curriculum-

Use this demonstration when discussing properties of solutions. The demonstration also fits into discussions of the role of mixing on chemical reactions rates. Use in conjunction with the Densi-Tee (BBExperiment 025) to discuss the mean free path in liquids.

Activity-

Demonstration - Student or Teacher

• The activity is best as a demonstration. Make the column ahead and display to all of your classes.
• The demonstration is safe and easy for a student project.

Safety-

Use ordinary laboratory safety procedures.

Time-

Teacher Preparation: 30-40 minutes (Plan other work while you wait.)

Class Time: 5 minutes

Materials-

• 500 mL of 66% sugar solution -- Mix 200 mL of water with 400 mL of sugar
• 1 1-L Graduated cylinder (or similar narrow container.)
• 2 40-cm narrow tubes
• a funnel cut from the top of a 2-L clear plastic soda bottle.
• 1 #3 1-hole rubber stopper to fit funnel
• a screw clamp
• 1 5-cm length of glass or rigid tubing to fit stopper.
• 1 5-cm length of flexible tubing
• electrician's tape

Disposal-

These materials may be disposed of safely by pouring into the sink and flushing.

Lab Hints-

• Prepare the layered solution ahead of time and display it for your classes. You can tend the additions while you do other things. Do not rush the addition at the interface between the two layers.
• Solutions last several days if no one disturbs them. They eventually turn cloudy with mold and other organisms. From this point on, the display becomes a good biology demonstration: which sugar concentration provides the best growing medium. Note: substituting salt for sugar avoids any mold formation, but it will not give the same sharp interfaces obtained with sugar.

Observations-

• Extremely sharp interfaces occur because of the difference in densities between adjacent layers and because of the minimal-mixing technique used to add the layers -- from the bottom up. If liquid is added too fast, some mixing takes place from the eddy currents around the tip of the tube. By limiting the flow rate the turbulence around the tip is minimized.
• Solutions are frequently layered by pouring less dense solutions down the side of the vessel onto the dense solution. The downward momentum of the poured solution, however, causes some mixing which blurs the interface.
• Industrial processes frequently require mixing of large volumes of solution under conditions where mechanical mixing is expensive. The demonstration vividly displays the need to add more dense solutions from the top. Additional mixing is still required, but the correct order of addition is a start, and can greatly reduce operating costs.

Answers-

Q1. Predict the result of adding the most dense solution to the top of the cylinder.

A1. The dense solution will fall to the bottom, but a good deal will mix with the more dilute solutions on the way down. Layers may still exists, but the interfaces will not be as sharp.

Q2. Predict the effect of adding a bubble of air at the bottom of the cylinder.

A2. The air bubble will rise to the surface and initiate currents in the solution as it moves. Some mixing will take place all along the path. The mixed solutions will settle at the interfaces because their densities will be intermediate between densities of the adjacent layers which were mixed by the bubbles ascent.

Key Words 1-

sucrose, sugar, density, rates of reaction, mixing