Cassandra Krack

Revealing Fingerprints

Science Concept: Polarity, Solubility This experiment demonstrates the process of sublimation, and the theory of "likes dissolve likes.


* 500ml. Erlenmeyer flask

* cork stopper

* iodine crystals

* hot plate

* white paper

* scissors



1. Cut strips of the white paper, and make sure that the strips of paper will easily fit through the neck of the Erlenmeyer flask.

2. Place a spatula full (about 10-20 crystals.) of iodine crystals in the flask and immediately put the cork stopper in the flask.

3. Put the flask on the hot plate until a purple gas can be seen (this should only take about 10-30 seconds to take place).

4. Wipe your finger across your forehead and place a fingerprint on the strip of paper by squeezing the paper.

5. Take the stopper out of the flask and insert the paper with the fingerprint on it, into the flask. Do not let go of the paper.

6. The fingerprint should slowly appear in a brown color. Remove the paper from the flask when the fingerprint becomes visible.

7. Replace the cork stopper into the flask and turn the hot plate off.

8. After 5 minuites or so the brown color will again disappear from the paper.

Introduction: How do you get your fingerprints taken? With ink, right? Not today, I have a new and interesting way to have your fingerprints taken. A mysterious purple gas will take the fingerprints instead of ink.

Explanation: The iodine crystals turn from a solid into a gas, skipping the liquid stage, this process is called sublimation. During sublimation, a solid directly enters the gas state, without passing through the liquid state.

You wipe your finger across your forehead because it ensures that fats and fatty acids, left over from perspiration, are on your finger. Iodine gas is attracted to these fatty substances that are left on the paper from the fingerprint. The paper is polar; the fatty substances, and the iodine gas are non-polar. Since, only "likes dissolve likes", only the iodine will affect the fingerprinted area. This is the reason for why the paper doesn't turn any colors from the iodine gas, but only where the fingerprint was located turns a color. However, the dark brown fingerprint is not permanent. In a few minutes the iodine gas will evaporate, and then the fingerprint will disappear.

Safety: Do NOT touch the hot plate when it is on. The bottom of the flask will also be very hot after being heated. Be Careful!!!

Waste Disposal: Leave the iodine crystals in the flask for later use. Eventually, the iodine can be washed out of the flask with acetone or alcohol. Throw the used paper away.

Reference: Summerlin, L.R, Borgford C.L, Chemical Activities - A Sourcebook for Teachers, American Chemical Society, 1988, p. 280.