Chemical tan for a skeleton

How do evil spirits sunbathe on Halloween?

Evil spir­its sun­bathe even on Hal­loween – be­cause they know their chem­istry! Find out which re­ac­tion is key to their sum­mery glow!

Safe­ty pre­cau­tions

Warn­ing! Only un­der adult su­per­vi­sion.


  • glass bowl;
  • so­lu­tions of cop­per(II) sul­fate and potas­si­um io­dide;
  • plas­tic toy skele­ton.

Step-by-step in­struc­tions

Pour so­lu­tions of cop­per(II) sul­fate and potas­si­um io­dide in a bowl with a plas­tic toy skele­ton. The mix­ture will dark­en abrupt­ly. Leave it for 10 hours – the skele­ton turns brown.

Process de­scrip­tion

The re­ac­tion be­tween cop­per(II) sul­fate and potas­si­um io­dide yields molec­u­lar io­dine:

4KI + 2Cu­SO₄ → I₂↓ + 2K₂­SO₄ + 2CuI↓

The io­dine mol­e­cules eas­i­ly pen­e­trate be­tween the plas­tic mol­e­cules and stay in their struc­ture, thus chang­ing the col­or of the plas­tic.

A sim­i­lar ex­per­i­ment is in­clud­ed in the “Chem­istry of ma­te­ri­als” set from the MEL Chem­istry sub­scrip­tion.