Orange solution in a 2L volumetric flask filled with orange solution is exhibits a less intense color in the neck of the flask than in the body.

Curriculum Notes 
This demo is ideal for introducing absorption spectroscopy.

One day of lead time is required for this project.

Discussion 
Absorbance varies linearly with both the cell length path (l) and the analyte concentration (c). These two relationships can be combined to yield a general equation called Beer's Law:

A = ε l c

The quantity ε is the molar absorptivity. The molar absorptivity varies with the wavelength of light used in the measurement.¹ 

Preparation 

• Fill 2L flask about halfway with water.
• Add 3 drops each of red and yellow food coloring.
• Swirl to mix.
• Add more water until it fills most of the neck.

Materials 

• 2L volumetric flask with stopper filled with orange solution.
• White background.
• Lamp to illuminate the demo.

Procedure 

• The flask is placed in front of the background with the lamp illuminating it.
• The difference in color intensity is pointed out to the students.

Safety Precautions 
Uh, don't drop the flask?