Chemical Concept Demonstrated: Chemistry of CO2
|The three beakers contain limewater (Ca(OH)2).
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)