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|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.
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)