Carbon Dioxide Tests: Part II. Chemical Breath

    Kids, now we can use the limewater made last month to test for carbon dioxide (CO2) gas in your exhaled breath.  

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    You will also need a straw and a pint-sized jar. Fill this jar halfway with the limewater. Use the straw to exhale your breath into the limewater, and continue until the clear solution turns milky. Why does this happen? Limewater always turns milky when CO2 is mixed with it. The chemical in the limewater is dissolved calcium oxide (CaO). This combines with the CO2 gas to form a white powder called calcium carbonate (CaCO3). This is not at all soluble in water, kind of like sand does not dissolve at all. Have you ever heard of limestone? This is what you just made with your own breath! The powdery limestone precipitate should settle to the bottom of the jar after several hours.

    (Notes: do not drink this solution - take care with the straw to only exhale, not inhale).


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    December 1992


    Reference: Janice Van Cleave in "Chemistry for Every Kid", Wiley: NY, 1989.