Precipitation of Lead Iodide

A flask containing a hot solution of lead iodide is prepared. As the solution cools, crystals of lead iodide form and fall to the bottom of the flask.

Materials

Fluorence Flask with the solution of Lead Iodide in water

Deep-sided Tray with Ice

Procedure

• Before lecture heat the fluorence flask on a big bunsen burner till it starts to boil.
• Stopper the flask and remove from heat. (Most of the crystals should be dissolved).
• During the demonstration the flask can be placed on the tray of ice to make it cool quicker.
• Altrnatively ice can be poured into the fluorence flask for even more rapid cooling.
• As the flask cools the lead iodide crystals will precipitate out of the solution.

Notes:

1. The solubility of lead iodide is 0.41 g/100ml in hot water, and 0.044 g/100ml in cold water.
2. The slower the flask cools the larger the resulting crystals. (without using ice this can take 30 to 50 minutes).
3. The cooled flask can be stoppered and re-used many times by reheating the solution to boiling to redisolve the crystals.