A laser beam is passed through bottles containing two similarly colored liquids, one containing a 0.1 M solution of cobalt(II) chloride in water and the other containing a suspension of iron(III) oxide in water. No "beam" is observed in the solution, but a "beam" of light is visible in the suspension.
This demo can be used a general chemistry course when mixtures and solutions are being discussed. It might also be useful in a general chemistry lab course when precipitation reactions are being discussed, because many precipitates are observed initially in suspension. This demo takes about two minutes to perform.
The particles of a suspension are large enough to scatter light. Thus, when a beam of light strikes the particles in a solution, enough light is scattered in the direction of the observer's eyes that the beam becomes visible. This scattering of light does not occur in a true solution.
- two 250 ml square bottles with caps, one containing a 0.1 M solution of cobalt(II) chloride in water and the other containing a suspension of 0.5 g iron(III) oxide in water
- a red laser pointer
- Darken the room.
- Hold the laser pointer up to the side of the bottle containing the cobalt chloride and turn the laser on. No beam should be visible.
- Shake the bottle containing the iron oxide to disperse the solid.
- Hold the laser pointer up to the side of the bottle containing the iron oxide and turn the laser on. A beam should be visible.
- If desired, the laser beam can be passed through the solution into the suspension.
- Laser light should never be shone directly into someone's eye as it could damage the retina.
- Cobalt chloride is toxic and carcinogenic. It should be cleaned up and disposed of appropriately in the event that the container fails.
These bottles are already prepared so that they do not need to be prepared again for each demo. They are stored in the prepared demos cabinet. The laser pointer is kept with the teacher's supplies.