It's a chemical compound that acts as a pH indicator. This means it changes color depending on the acidity or alkalinity (pH) of a solution.

Here's how it works:

* Acidic solutions (low pH): BTB turns yellow.

* Neutral solutions (pH 7): BTB stays green.

* Alkaline solutions (high pH): BTB turns blue.

BTB is often used in science experiments and demonstrations to visually show changes in pH.