Luminol

Luminol is a chemical used by scientists to detect the presence of blood in a crime scene. Even when there appears to be no signs of blood, luminol will exhibit a chemiluminescence with as little as a few hundred red blood cells. So if crime scene has been 'cleaned up' you will get a blue glow wherever there is blood.

The chemical formula for luminol is C₈H₇N₃O₂.

How does it work? It reacts with iron that is present in haemoglobin. Haemoglobin is a globular protein that carries oxygen in red blood cells. Therefore, if haemoglobin is present, the luminol will react with the iron it contains and 'glow'. Luminol requires the presence of an oxidant in order for it to be activated, something like hydrogen peroxide (that's H2O2.) We use a catalyst, usually iron, to speed up the reaction. The hydrogen peroxide decomposes to form oxygen and water.

How long does the luminescence last? Roughly 20-30 minutes. It's pretty cool actually, and it doesn't need to last long, forensic investigators capture the images of the scene with special cameras, so they don't even need it to last.