Precipitation of Lead Iodide
When a few crystals of lead nitrate and potassium iodide are added to opposite sides of a Petri dish containing DI water, a line of bright yellow lead iodide precipitate forms down the middle of the dish.
This demo is usually performed when solubility rules are being discussed, but it could also be used when the mechanism of dissolution is presented to illustrate that dissolved substance are still present, even though they may become "invisible." This demo takes about 10 minutes to perform, but the line of precipitate does not begin to appear until a two or three minutes have elapsed and it takes a few more minutes for the line to reach across the dish, so the instructor should be prepared to continue with some instruction while the demo proceeds. This demo is best when projected using a document projector. An overhead projector works, but the students cannot aee the vivid yellow color of the precipitate
The solubility rules tell us that salts of group 1 (IA) metals are soluble, salts containing the nitrate anion are soluble, and salts containing the iodide anion are insoluble except for those containing the lead(II) cation (among others). This demo illustrates these rules. The potassium iodide ane lead nitrate dissolve and their constituent ions begin to diffuse through the water. Eventually the lead ions begin to react with the iodide ions and precipitate out where they "meet," the center of the Petri dish. Pb2+(aq) + 2 I-(aq) --> PbI2(s) The Kspof PbI2 is 9.8 X10-9. 1 1http://www.ktf-split.hr/periodni/en/abc/kpt.html
- small bottle of solid potassium iodide
- small bottle solid lead nitrate
- Petri dish
- wash bottle containing DI water
- 2 tweezers
- dark background (optional)
- Place a Petri dish under an document projector camera (preferably on a dark background) or on an overhead projector.
- Pour enough water into the Petri dish to cover the bottom to a depth of 4-5 mm.
- Using the tweezers, place a few crystals of each substance on opposite sides of the dish. Use different tweezers for each substance to avoid contamination.
- After about two or three minutes a bright yellow line of lead iodide precipitate should begin to form down the center of the dish.
Both lead nitrate and potassium iodide are very poisonous. Lead nitrate is also a strong oxidizing agent. Handle them with care.
Be careful when breaking down this demo. The solution in the Petri dish is very "sloshy." It's probably best to drain it into a waste container of some kind before removing the demo.