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DemonstrationsAcids, Bases, Buffers, Titrations › 16.3

Lecture Demonstrations

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


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