18.3 The Orange Tornado

Chemical Concepts Demonstrated: Solubility product, ion product, soluble/insoluble complex ions

Demonstration:

A beaker is filled with distilled water. A vortex is created using a magnetic stirring bar. Pb(NO3)2 is added to the beaker. KI is added to the center of the vortex.

Observations:

    As KI is added, the orange vortex forms, but quickly vanishes.  As time goes on, the vortex is present for longer and longer periods of time.   Eventually, the vortex doesn't disappear and fills the entire beaker with precipitate.

Explanations:

    The concentration of the ion product of the salt produced is initially greater than the solubility product's concentration only in the vortex, so the product initially only appears in the vortex.  As the ion product comes in contact with the area outside of the vortex, the solubility product becomes higher in concentration, so the ion product (i.e. the precipitate) dissolves.

    After about 3.5 mL of KI is added, there is enough of a concentration of ion product throughout the solution that the precipitate does not redissolve.