That Glass of Water That Glows in the Dark: A Luminol Chemiluminescence Experiment

Some friends have been curious: that glass of water that glows in the dark—what exactly is it?

It’s the luminol chemiluminescence experiment. This substance is often used to detect blood traces at crime scenes—wherever blood is present, it glows. Below I’ve put together some notes on the experiment.

How the Experiment Works

When luminol is dissolved in an alkaline solution, it is deprotonated, forming a reactive anion. After adding hydrogen peroxide (H₂O₂) and a small amount of iron ions (Fe³⁺), the luminol anion is oxidized, producing a short-lived excited-state intermediate.

This excited-state molecule then returns to the ground state and releases energy in the form of blue light (around 425 nanometers). The “glow” we see with our naked eyes is this energy-releasing process.

Note: the reason different colors of light appear in the videos is actually that a fluorescent agent was separately added to the liquid.

Safety Reminder

If you’re one of those friends who’d like to try this yourself, take special care: the materials in this experiment involve sodium hydroxide and hydrogen peroxide, both of which are corrosive and hazardous to a degree. When mixing the solution, be sure to wear gloves and handle everything with care.