The best guide for young detectives
Three ways to reveal fingerprints
Let’s perform some simple experiments for young detectives!
Warning! Only under adult supervision.
- copper(II) sulfate;
- potassium iodide solution;
- superglue (containing cyanoacrylates);
- white paper;
- plastic Petri dish;
Iodine imprint: pour some copper(II) sulfate into a glass container and add a solution of potassium iodide. Observe as the mixture turns brown. Leave a handprint or fingerprint on a white sheet of paper and put the paper on top of the container, print facing down. In 20 minutes, the fingerprints appear!
Sticky fingers: Leave a print on the inner surface of the lid of a Petri dish, and add a few drops of water and superglue to the other half of the dish. Close the lid. After 10 minutes, the print appears.
Prints of graphite: Rub some pencil graphite on sandpaper. Leave a fingerprint on the surface of a glass. Use a brush to apply the resulting graphite dust to the glass. Capture the resulting print with adhesive tape, pressing firmly. Then carefully peel the tape away and transfer it to a white sheet of paper.
As we touch things, we leave our prints on them. These prints consist of traces of sweat and fat. These traces are easily revealed using, for example, iodine vapors, which form during the interaction of copper(II) sulfate and potassium iodide. The vapors dissolve in the traces and turn brown. Imprints can also be detected with water and superglue with cyanoacrylates – the cyanoacrylates evaporate and dissolve in the traces and, upon reacting with water, whiten and harden, thereby revealing any prints. But the simplest way is to use graphite! If you grind some pencil graphite and sprinkle the powder on suspected prints, small particles of graphite will stick to them. These prints are easily captured with tape.
Similar experiments are included in the “Elements” and “Diffusion” kits from the MEL Chemistry subscription.