Diffusion

Charles, Harold             Farragut Career Academy
                            1-312-542-3000


Objectives

After the completion of this lesson, the student will be able to: 
1. Understand the movement of liquid molecules in a solid 
2. Explain how molecular weight affects diffusion rates. 

Equipment and Materials

Agar                          Chemical solutions of:
Petri dishes                    AgNO3
Number 5 Cork Borer             NaCl
Dropper                         KBr
Marker pens                     K3Fe(CN)6
Overhead projector

Recommended Strategy

Demonstrate movement of molecules by (1) opening a bottle of perfume in one corner of 
the room and later observe the odor, (2) adding several drops of food coloring in a 
large beaker of warm water and later observe,  (3) adding several sugar cubes in a 
large beaker of water and later observe.  Elicit from the students the conclusion 
that: (1) molecules move because of some force and (2) molecules move from areas of 
greater to lesser concentrations. 

Before class, pour boiled agar into several Petri dishes to a depth of 4 mm, cover 
and allow to cool and solidify overnight.  Punch four holes, l5 mm apart, in agar 
surface by using a #5 cork borer.  Remove each agar plug from borer before making 
another hole. 

Obtain 1N solutions of these chemicals: AgNO3, NaCl, KBr and K3Fe(CN)6.  Number the 
holes then add several drops of each solution to separate holes. Do not allow them to 
overflow.    

After several hours, observe the movement patterns of each chemical.  Calculate the 
diffusion rate by measuring the distance/time.  Then compare the diffusion rate of 
each chemical based on molecular weight. 

In some instances, bands will appear at the interface of different chemicals 
representing a precipitate.  Other color changes may result over longer periods of 
time, due to dilution and equilibrium. 

Variations in this demonstration can include:

l. Comparing agar dishes that were refrigerated versus room temperature. Also, 
   changes that occur as the colder dishes become warmer. 

2. Comparing various chemical mixtures (e.g. dyes, inks) to determine the different 
   diffusion patterns. 

3. Comparing Agarose (a clearer compound) with nutrient agar using similar chemical 
   compounds. 

4. Placing the agar plugs in stacks in an agar base and apply different chemical 
   solutions to determine the rate of diffusion and vertical diffusion 
   chromatography, then the lateral diffusion across the plate.