yellow as CO₂ is added). A common demonstration of BTB's pH indicator properties involves exhaling through a tube into a neutral solution of BTB. As CO₂ is absorbed from the breath into the solution, carbonic acid forms and change the solution color from green to yellow. Thus, BTB is commonly used in science classes to demonstrate that the more that muscles are used, the greater the CO₂ output.

Bromothymol blue has been