A yellow free precipitate maelstrom

Drops of a colorless solution are added to a beaker of vigorously stirred and colorless solution, creating a spinning yellow funnel that turns white and then disappears.

Ingredients: potassium iodide, lead nitrate, magnetic stirrer, large beaker

Procedure: A partial recipe follows.

1. Prepare a solution of lead nitrate in a large beaker.

2. Prepare a solution of potassium iodide.

3. Use a magnetic stirrer to create a vortex in the large beaker of lead nitrate.

4. Add drops of the potassium iodide to the beaker and observe.

5. Repeat addition of drops again and again.

Understanding: Drops of a colorless solution of potassium iodide are added to the deep maelstrom formed within the beaker of a colorless solution of soluble lead(II) nitrate. The potassium iodide is "trapped" and spun within the whirling vortex, where a yellow precipitate of lead(II) iodide forms.

Pb2+(aq) + 2 I-(aq) → PbI2(s) (Yellow)         Ksp = aPb2+ (aI-)2

The yellow precipitate is contain in the vortex, but slowly dissipates over time. What is left is a ghostly and glistening shadow of the formerly bright yellow whirlpool.

The concentration of lead ion is uniform throughout the beaker of lead nitrate. However, within the volume of the vortex the concentration of iodide is quite high - high enough that the reaction quotient exceeds the solubility product and precipitate is formed.

Over time, the precipitate is slowly freed from the maelstrom, entering a region of the solution where the overall iodide ion concentration is much lower. The reaction quotient drops below the value of the solubility product, causing the majority of the yellow precipitate to dissolve.

Bucky Badger, Chemistry, and Boston University

Question: What is the connection between Bucky Badger and Boston University?

Bucky Badger is the mascot of the University of Wisconsin and named after Wisconsin's lead miners who, like badgers, often lived in dugouts in the sides of hills. Here we find Bucky Badger pouring a colorless solution of potassium iodide into a beaker of colorless lead nitrate solution. The bright yellow lead(II) iodide precipitate forms. Interestingly, there is a connection between Bucky Badger, chemistry, and Boston University.

Bassam Shakhashiri is Professor of Chemistry and the William T. Evjue Distinguished Chair for the Wisconsin Idea at the University of Wisconsin in Madison, Wisconsin. He is a widely recognized master of chemical demonstrations, having authored the encyclopedic Chemical Demonstrations: A Handbook for Teachers of Chemistry. For decades he has hosted the popular holiday show Once Upon a Christmas Cheery in the Lab of Shakhashiri. Professor Shakhashiri is also a BU alum (CLA '60), having received his A.B. in Chemistry from Boston University in 1960.

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