Bleeding Knife Chemistry Trick

Knife Appears to Write in Blood

The bleeding knife chemical reaction is used for Halloween costumes.
No, he is not really bleeding! The bleeding knife chemical reaction is used in theatrical productions and for Halloween costumes. tirc83, Getty Images

Here's a cool chemistry trick, perfect for Halloween! Trace a knife over your skin and leave a message that appears to be written in blood. The project works equally well with a spoon but somehow loses impact. Try it...

Bleeding Knife Materials

You only need a few materials for this project, however, you'll either need access to a lab for the chemicals or else you can order them online.

  • Dull knife (we don't need real blood here)
  • 5 grams ferric chloride
  • 5 grams potassium thiocyanate
  • Water

Prepare the "Magic" Solutions

Prepare saturated solutions. You'll know the solutions are saturated if no additional solid will dissolve in the liquid.

  • Mix a few milliliters of water with the ferric chloride to dissolve it.
  • Separately, mix a few milliliters of water with the potassium thiocyanate to dissolve it.

Perform the Trick

  1. Coat the area of skin to be "bloodied" or written on with the potassium thiocyanate solution. You'll get the best effect (dripping blood) if the skin remains damp, but the color appears just fine even if you let the area dry.
  2. Dip the knife blade in the ferric chloride solution.
  3. Draw on your skin with the dampened knife blade. A deep red liquid resembling blood will appear where the two solutions mix.

How It Works

This chemistry trick is one form of a sensitive test for the ferric ion. A red color is produced by the reaction between the ferric ion and the thiocyanate ion.

Bleeding Knife Clean-Up and Safety

When you're done, rinse the blade and your skin under running water to remove the chemicals. The demonstration is safe to perform, but restrict the project to your arm or hand and avoid eyes, nose or mouth to avoid ingestion of the chemicals or irritation of mucous membranes.

More Halloween Chemistry

Apply chemistry for more Halloween fun. You can make a glow-in-the-dark jack-o-lantern, slime that looks like ghostly ectoplasm, or a mad scientist Halloween costume.