Hazards
- Potassium dichromate is toxic and an oxidizer.
- Do not look directly at the laser source. Make sure that the laser is not directed toward the students.
Chemicals and Solutions
- FeCl₃ solution, about 0.1M and filtered or freshly made
- K₂Cr₂O₇ solution, with the same color intensity as the ferric chloride solution
Materials
- Laser
- Lab jack
- Two 4-liter beakers
Procedure
- Set up light and lab-jack in line so that light beam will pass through the center of the vessels when set on lab-jack.
- Plug in the laser. Leave turned off and shuttered.
- Turn on the laser and observe the beam of light in the colloidal dispersion. The beam of light is not seen in the potassium dichromate solution.
Discussion
The Tyndall effect is exhibited by colloids. Ferric chloride hydrolyzes in water to form a colloidal dispersion of ferric hydroxide. Colloidal particles are large enough to scatter and reflect light. In this demonstration ferric chloride hydrolyzes in water to form a colloidal dispersion of ferric hydroxide in water. The dichromate dissolves but does not form a colloid.