The indicator is usually bromothymol blue, which is blue in base and yellow in acid. If you do this demonstration you may need to add a drop or two of a strong base like a NaOH solution to make the solution basic. Tap water tends to be slightly acidic. Add the least amount of NaOH solution as possible to make the indicator turn blue. It usually takes about 10 drops to 100 mL of water, but you can adjust the number of drops to give the desired color intensity.

Why does the indicator change color?

CO₂ is slightly soluble in water giving carbonic acid when dissolved, the acid soda pop. As CO₂ bubble through the blue, basic solution, the pH drop as the CO₂ dissolves in the water. The solution becomes more acid and the indicator changes to its acid color.

feedback: bill_switzer@ncsu.edu