How to Reveal Fingerprints With Iodine Crystals

How to Reveal Fingerprints With Iodine Crystals

Fingerprints on glass, metal, and other surfaces can be revealed by dusting them with powder. I tried it twice already but it was very hard, so I found this method in my chemistry book.

Position the paper in the beaker like this:

And put a gram of iodine crystals, that should be enough. It was already reacting and making stains. But It stopped, so I turned on the heat. About 10 seconds, vapors are forming quickly. (Becareful, if you wash your hands before touching paper, it’ll not work. Sweaty hands will work. Soap removes all of the oils and salts on the fingers.)

I love the vapors, it looks so beautiful.

Here’s the paper:

I’m actually quite surprised that it worked so well! Definitely better than using powder.

It so clear that you could see it! But I had another idea. What would happen if I sprayed a little bit of cornstarch? Starch will turn iodine into an intense “blue-black” color (iodine-starch reaction). So I added a gram of cornstarch into 20 ml of water and put it in a spray bottle.

It works, but I didn’t spray more because the prints are already clear 🙂

How it works? Very simple. Iodine fuming was the first method to reveal fingerprints on surfaces. As the iodine is heating, purple vapors start to form. The iodine condenses to a solid and sticks fast to oils present in fingerprints, and of course, it reveals the fingerprints.

I hoped you enjoyed. This is now one of my favorite experiments.

17 Replies to “How to Reveal Fingerprints With Iodine Crystals”

  1. 18thTimeLucky – Hi I am Samuel, a 19-year old undergraduate student in the UK who just finished A level, having studied biology, chemistry and mathematics, and am currently on the first year of my Masters of Chemistry degree at the University of Warwick with the aim to eventually become an academic and public speaker in the field. I have a deep passion for a variety of hobbies, including writing, ant keeping, growing plants, and, of course, home chemistry and science communication!
    18thtimelucky says:

    Now I know how to find out who keeps stealing cookies from the cookie jar… great experiment!

  2. I love seeing what you are doing next!

  3. Robyn Haynes – Australia – Robyn enthusiastically pursues a green and writerly life on the north coast of New South Wales, Australia, where her background as a doctor of social anthropology equips her with an interesting slant on the human condition. She spends much time indulging a passion for her garden where she ponders life and attempts to stave off existential angst. In her more reflective moments she makes wry observations on courtyard gardening and its parallels with life.
    Robyn Haynes says:

    Great results, Dan. I’ve learned something here

  4. I agree! This is such a cool experiment! 🙂
    I am amazed at how well it works on paper – which I thought was quite hard to get a print from.
    When we were burgled, the finger print expert wasn’t interested in trying to lift prints from any surfaces other than metal eg a knife.
    Just shows you, the old methods still work very well. 🙂

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