The best guide for young detectives

Three ways to reveal fingerprints

Let’s per­form some sim­ple ex­per­i­ments for young de­tec­tives!

Safe­ty pre­cau­tions

Warn­ing! Only un­der adult su­per­vi­sion.

Equip­ment

  • cop­per(II) sul­fate;
  • potas­si­um io­dide so­lu­tion;
  • wa­ter;
  • su­per­glue (con­tain­ing cyanoacry­lates);
  • pen­cil;
  • bowl;
  • white pa­per;
  • tape;
  • sand­pa­per;
  • plas­tic Petri dish;
  • brush.

Step-by-step in­struc­tions

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.

Process de­scrip­tion

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.

Try it