Acids, Bases, Buffers, Titrations
16.3 Acid Breath
Subjects: Acids/bases, titrations
Description: The instructor breathes into a flask containing a colored solution. After several breaths, the color of the solution will change.
Materials:
- 250 mL Erlenmeyer flask with stopper to fit
- Disposable pipets
- ~50 mL water
- ~10 drops Universal indicator solution‡
- Saturated sodium bicarbonate (if needed)‡
‡Saturated bicarbonate solution is located on the solutions shelf. Universal indicator is located in the flammables cabinet.
Pre-class preparation:
1. Add water and the Universal indicator to the flask.
2. Add 1 drop of saturated sodium bicarbonate solution to produce a slightly basic solution blue/green in color.
3. Stopper the flask until you’re ready to perform the demo.
Procedure:
1. Unstopper the flask.
2. Breathe into the flask several times. Swirl the flask between breaths.
3. The solution will turn orange/yellow after approximately 10 breaths due to a decrease in pH.
Discussion:
After several breaths, there will be enough CO2 dissolved in the solution to produce enough carbonic acid to cause a decrease in the pH of the solution. The equation for the reaction is given below. The decrease in pH produces a color change of the indicator.
CO2(g) + H2O(l) –> H2CO3(aq) <–> H+(aq) + HCO3-(aq)
Safety:
None
Disposal:
Solutions can be flushed down the drain with water.
References:
1. L. Summerlin; C. Borgford; J. Ealy. Chemical Demonstrations: A Sourcebook for Teachers; Volume 1; 1985; p. 40