Bromothymol Blue: The Demo We All Do

Often science teachers perform demonstrations only to discover that students have already seen the experiment in a previous course. Teachers should take advantage of these opportunities to showcase the interconnectedness of different science disciplines. One example of a demonstration used across most science disciplines and grade levels involves bromothymol blue (BTB). A few drops of BTB is placed in a 500 mL flask three-quarters full of water; this initial solution is blue. The teacher places a straw in the flask and blows through the straw. The BTB eventually changes from blue to yellow. The speed of the color change depends on the amount of exhaled air into the flask. The BTB changes color because it functions as an acid/base indicator. The chemical reaction involves carbon dioxide from the teacher's lungs dissolving in the BTB solution and the water in the solution forming carbonic (weak) acid. This article presents various tips and techniques for science teachers on how BTB demonstration can be used in different science disciplines.