Fluorescein

10g of fluorescein salt powder in a vial. 

This red powder is a dye that turns an intense yellow-green when mixed with water.

Fluorescein was first made in 1871, and it has since been used in many niche applications thanks to its unique coloring properties and ultraviolet fluorescence. It is commonly used as a medical dye to detect and diagnose various eye injuries and also to trace the flow of streams, groundwater, and glacier water.

When mixing with water, a tiny amount goes a long way. This vial should be enough to dye at least 100 gallons of water.

It is organic and harmless to the environment at these low concentrations.

The fluorescein reaction is pH-dependent. Once it has reacted in water, the color can be altered by changing the acidity of the water.

Even though fluorescein is commonly used in medical applications, please do not consume it.