Lesson 20:

Concepts conveyed | Materials | Procedure | Benefits

Concepts conveyed:

The purpose of this demonstration is to illustrate the effects of heat and concentration on the transport of molecules.

Materials:

• 1 eye dropper
• 1 bottle of food coloring or food dye
• 3, 250 mL beakers
• 2 large test tubes
• 2 test tube racks (or other device to hold the test tubes still)
• 200 mL of water (room temperature)
• 200 mL of water (hot ~ 100 degrees Celsius)
• 200 mL of water (cold ~ 0 degrees Celsius)
• Colorless, transparent hair gel (main gel ingredient is Carbomer or Carbopol)

Procedure:

Obtain a few drops of food dye with the eye dropper. Carefully lower the dropper into the beaker of water which is at room temperature and let one drop of dye flow into the water. Watch as the dye disperses through the water. Repeat this procedure with the beaker of cold water and one of hot water.

Repeat the entire procedure again using many drops of dye (instead of just one). Discuss with the students what occurs.

Fill the test tubes with gel, leaving some room at the top for a cap and for dye. Try to get as many air bubbles out as possible. Add the same amount of dye to the top of the gel in both test tubes. Place one in the refrigerator overnight and place the other at room temperature for the same period. After this time, measure how far from the top of the gel the dye has moved.

Mass Transport vs. Temperature and Concentration
The food coloring will disperse quickly in the hot water and more slowly in the cold water. The inter-molecular forces of water attract the molecules of dye and carry them away. Heat causes the molecules in the beaker to move around at a faster rate. Therefore, the process of molecular transport of the dye is accelerated. The whole process is significantly slowed down when placed in cold water. Increased concentration also appears to speed up the process. There are more molecules of dye to spread out in the same time period. In this experiment, the molecules can move in two main ways: diffusion and convection. Diffusion is Brownian motion, a random walk of molecules through the solution. Convection is stirring, which can be caused by temperature fluctuations or by the motion of a heavier drop through a less dense solution.

What happens in the gel? The transport of dye is much slower in the gel than in pure water because the gel forms a lattice around small pools of water and prevents them from moving under the influence of convection. (The polymer that forms the gel with water is so low in concentration that it really does not interfere with the movement of the dye molecules.) The movement of the molecules in the gel is mainly by diffusion. If you cap off the test tube (to prevent evaporation) and let the dye diffuse through the gel for a day, the distance, x (in centimeters), that the front moves is proportional to the square root of time,

x = (2 D t)^1/2

where D is the diffusion coefficient (a typical value is 5 x 10^-5 cm²/s) and t is the time of the experiment (in seconds). Diffusion, like convection, is also dependent upon the temperature. The diffusion will be slower in a tube left in the refrigerator for a day than one left at room temperature.

Benefits:

• Learning how molecules get from one place to another helps to explain why reaction rates increase when the temperature goes up and the purpose behind stirring a reaction mixture.
• These concepts are important in understanding a wide variety of phenomena, including cooking, transport of water and other substances through the ground, and how the human body works (how chemicals produced at one site cause reactions at another).