This experiment is hands-down the most beautiful chemistry demonstration I've ever seen. I'll be synthesizing lead(II) iodide, which has a beautiful golden yellow color. This experiment is especially striking because this bright yellow solid is produced from two water-clear solutions.
- 1.0 g Potassium Iodide, KI
- 0.8 g Lead Nitrate, Pb(NO3)2
- 500 mL Distilled Water
Pb(NO3)2 (aq) + 2KI (aq) --> 2KNO3 (aq) + PbI2 (s)
Warning! Lead compounds, especially soluble lead compounds (like lead nitrate) are very toxic. These should only be handled by an experienced chemist with maximum safety precautions taken. I do the dangerous stuff so you don't have to!
The first step is to dissolve both reactants in distilled water - 250mL for each solution. Stoichiometrically, equal amounts of the two reactants are needed for this reaction. For safety reasons, I used an excess of potassium iodide to ensure all of the soluble lead is reacted away.
The potassium iodide is extremely soluble, and will dissolve very quickly. Lead nitrate, on the other hand, will sometimes hydrolyze and leave behind a small amount of white precipitate (likely lead oxide or hydroxide). If this happens, a few drops of nitric acid added to this flask should bring everything into solution.
Now we simply add the two solutions together. I went a few drops at a time to start. As soon as the solutions touch, bright yellow lead iodide is produced. The crystals are so small that it looks silky.
Click the following pictures for much larger detailed views.
Lead iodide is slightly soluble at room temperature, so after the first precipitate is formed a bit of swirling dissolves it back to a clear solution. I then added the rest of the KI solution to the lead nitrate. Pouring the KI solution into the lead nitrate was actually another safety measure - I wanted to minimize handling the lead solution as much as possible.
The fact that a bright yellow precipitate is produced from two clear solutions is striking enough, but the real magic of this demonstration is when it is precipitated from a hot solution. I took the flask containing the lead iodide and heated it to near boiling on a hot plate. It took a little while, but eventually all of the yellow crystals dissolved. This returned the solution to being completely water-clear. This happens because lead iodide is almost ten times more soluble in hot solution than in cold.
I then turned off the heat and allowed the flask to cool to room temperature on its own. This causes the lead iodide to precipitate back out of solution. This time, because of the slow cooling, it forms extremely thin hexagonal plate-like crystals that shine and glitter as they move around the solution. It's incredibly beautiful to watch - if you haven't already I highly recommend watching the video to see it. This part starts at 5:37. Here are a couple of the more beautiful photos I captured of it. Again, click the photos for a much larger view.
After watching the crystals form and float around for a few hours, I placed the flask in the lab fridge overnight to push as much of the compound out of solution as possible. I then filtered it to isolate the lead iodide. All of the tiny crystals fused together on the filter paper to form a very beautiful sheet of golden iodide.
I very carefully collected this and placed it in a vial for display. The final yield was 0.8g PbI2.
Finally, cleanup and disposal of the waste solution is an important point. We want to ensure that all of the soluble lead has been eliminated from everything it may have come into contact with. This can be done simply with baking soda (sodium bicarbonate). I made a hot, saturated solution of bicarbonate and added this to the waste solution that the lead iodide was filtered from. This caused the solution to go cloudy with a lead carbonate precipitate. The iodide is still marginally soluble, but the carbonate is truly insoluble. This precipitate can be filtered off and disposed of as regular garbage. The solution now contains only potassium nitrate and baking soda, and can be poured down the drain with plenty of water.
Anything that came in contact with the lead-bearing solutions should also be treated with baking soda in the same way. Any beakers, funnels, filter paper, stirring rods, etc. that may have droplets of soluble lead compounds must be cleaned up, You don't want to come back later and touch a tabletop contaminated with these toxic salts.