Question

On passing excess CO2 through limewater, it first turns milky and then becomes colorless. Also, write the chemical equations.

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Answer

When excess carbon dioxide (CO2) is passed through limewater (a solution of calcium hydroxide, [Ca(OH)2]), an interesting sequence of chemical reactions occurs. Initially, the reaction between Ca(OH)2 and CO2 produces calcium carbonate (CaCO3), a white, insoluble solid that causes the solution to turn milky due to its precipitation. The chemical equation for this reaction is: $$ \text{Ca(OH)}_2 (aq) + \text{CO}_2 (g) \rightarrow \text{CaCO}_3 (s) ↓ + \text{H}_2\text{O} (l) $$

As excess CO2 continues to be added, it further reacts with the precipitated calcium carbonate and water to form calcium bicarbonate [Ca(HCO3)2], which is soluble in water. This makes the solution turn colorless, as the precipitate dissolves back into the solution. The equation for this subsequent reaction is: $$ \text{CaCO}_3 (s) + \text{CO}_2 (g) + \text{H}_2\text{O} (l) \rightarrow \text{Ca(HCO}_3)_2 (aq) $$

Thus, the initial milkiness caused by calcium carbonate formation is cleared by the formation of the soluble calcium bicarbonate.


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