Luminol is a soluble, yellowish crystal. It is used in crime scenes to reveal blood.
The reaction of Luminol is several steps. First, the Luminol is put into solution with hydrogen peroxide, causing the Luminol to become a dianion in resonant stabilization. This means the Luminol is in one atomic configuration with two different electron configurations. Next, iron in the blood acts as a catalyst, decomposing the peroxide into oxygen and water. The oxygen reacts with the Luminol dianion, producing an unstable peroxide and nitrogen gas. The unstable peroxide immediately decomposes into 3-aminophthalic acid in an excited state. When the excited 3-aminophthalic acid drops to ground state, a photon is released. This light is why blood turns bright blue.
Luminol is used in forensics where there is suspected blood to determine a bloodstain from a regular stain. Investigators will lightly spray a suspected region with a solution of Luminol and hydrogen peroxide, looking for marks of blood.
Luminol is most often used on dry blood, because Luminol is not readily soluble with water, failing to react when blood is wet. Because Luminol requires very little blood to work, it is very effective even when blood may not be visible or when areas have been washed.
Luminol is unreliable when investigating surfaces with present catalysts, such as copper. Surfaces with other bodily fluids, like urine and feces, will also trigger Luminol.
Here are some sources.
Pictures of Luminol at a crime scene
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