What happens when calcium hydroxide solution is left open in air?

Correct option is B

When calcium hydroxide (Ca(OH)₂) solution, also known as lime water, is left open in air, it reacts with the carbon dioxide (CO₂) present in the atmosphere. This results in the formation of calcium carbonate (CaCO₃), which is a white precipitate. The reaction can be represented as:

Ca(OH)₂(aq)+CO₂(g)→CaCO₃(s)+H₂O(l)

Thus, calcium carbonate (CaCO₃) is formed when lime water reacts with carbon dioxide in the air.

•  O₂ is liberated: This does not occur in this reaction. Oxygen gas is not involved in the reaction between calcium hydroxide and carbon dioxide.
•  H₂O (gas) is evolved: Water is formed as a liquid in this reaction, not as a gas.
•  CaO is formed: Calcium oxide (CaO), also known as quicklime, is formed by heating calcium carbonate (CaCO₃), not by leaving calcium hydroxide in open air.

Hence, the correct process is the formation of CaCO₃ (calcium carbonate).