18.2 The Chemistry of CO2

Chemical Concept Demonstrated: Chemistry of CO2


The three beakers contain limewater (Ca(OH)2).
  1. A student blows into the first beaker with a straw.
  2. Dry-ice is added to the second beaker.
  3. Club soda is added to the third



1.  A cloudy white precipitate forms.
2.& 3.  At first, a precipitate forms in the beaker.  When additional dry ice and club soda are added, the precipitate redissolves.

Explanations (including important chemical equations):

    Carbon dioxide dissolves in water and slowly reacts with water to produce carbonic acid.

    CO2 (g) + H2O (l) <=> H2CO3 (aq)

    The cloudy white solution observed when CO2 is bubbled into limewater results from a reaction between Ca(OH)2 and either CO2 or H2CO3 to form an insoluble calcium carbonate precipitate.

    Ca(OH)2 (aq) + H2CO3 (aq) <=> CaCO3 (s) + 2 H2O (l)    Ksp = 2.8 x 10-9

    Excess CO2 or H2CO3 , however, reacts with the CO3 2- ion in this solution to form HCO3 -.

    H2CO3 (aq)  + CO3 2- (aq) <=> 2 HCO3 - (aq)

    Since calcium bicarbonate is soluble in water, the CaCO3 precipitate dissolves in the presence of excess carbonic acid.

    CaCO3 (s) + H2CO3 (aq) <=> Ca 2+ (aq) + 2 HCO3 - (aq)