Today in class, students learned how to develop invisible fingerprints using crystal iodine. It turns out that iodine crystals (I₂) react with the oils on human skin, which can reveal hidden fingerprints on paper or other surfaces.

However, you can’t just go to CVS and buy iodine crystals. So today in class, we learned how to make them. By combining potassium iodide (KI), hydrochloric acid (HCl), and hydrogen peroxide (H₂O₂), we created a chemical reaction. And one of the products of that reaction was a heap of gray iodine crystals, which we captured using filter paper.

Before long, all of the students had dark brown fingerprints that were clearly visible. But there was still a problem. Although we could see the fingerprints clearly, no one knew who in the class they belonged to! It turns out that fingerprints are pretty useless without some sort of database for comparison… and that is tomorrow’s lesson!

December 15 – Fingerprinting with Crystal Iodine (pg325)