Chemical Concept Demonstrated

• Chemistry of CO₂

Demonstration

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

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 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)    K_sp = 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)

Contributors

• Dr. George Bodner (Perdue University)

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