When excess carbon dioxide is passed through lime water, the milkiness first formed disappears due to-

• A the reversible reaction taking place
• B formation of water soluble calcium bicarbonate
• C huge amount of heat evolved during the reaction
• D formation of a water soluble complex of calcium.

When excess carbon dioxide is passed through lime water, the initial milkiness disappears primarily due to the formation of water-soluble calcium bicarbonate. This process occurs as carbon dioxide reacts with calcium hydroxide in the lime water, leading to the dissolution of the precipitate that caused the milkiness.

Key Points to Remember

• Reaction Type: The reaction is reversible.
• Formation of Bicarbonate: Calcium bicarbonate is soluble in water.
• Heat Generation: The reaction does not involve a significant amount of heat.
• Complex Formation: A complex is not primarily responsible for the change.

Thus, the correct answer is B: formation of water-soluble calcium bicarbonate.