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The Chemistry of CO2

Chemical Concept Demonstrated

  • Chemistry of CO2

Demonstration

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

Compare.

co2.gif

Observations

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 COis bubbled into limewater results from a reaction between Ca(OH)2 and either COor H2COto form an insoluble calcium carbonate precipitate.

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

Excess COor H2CO, 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 CaCOprecipitate dissolves in the presence of excess carbonic acid.

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

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