Return to Biology Index
Charles, Harold Farragut Career Academy
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
Marker pens K3Fe(CN)6
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
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
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
3. Comparing Agarose (a clearer compound) with nutrient agar using similar chemical
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.