The reaction that occurs on passing carbon dioxide gas through lime water is

Question

Prepp Team · Accepted Answer

Reaction of Carbon Dioxide with Lime Water

When carbon dioxide gas ($CO_2$) is bubbled through lime water, a specific chemical reaction occurs. Lime water is the common name for a clear, aqueous solution of calcium hydroxide, which has the chemical formula $Ca(OH)_2$.

Understanding the Chemistry of Lime Water and $CO_2$

The reaction involves the interaction between the dissolved calcium hydroxide in lime water and the incoming carbon dioxide gas. This process results in the formation of calcium carbonate ($CaCO_3$), which is insoluble in water, and water ($H_2O$).

The specific chemical equation representing this transformation is:

$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$

Breakdown of the Reaction:

Reactants: Calcium hydroxide solution ($Ca(OH)_2(aq)$) and Carbon dioxide gas ($CO_2(g)$).
Products: Calcium carbonate solid precipitate ($CaCO_3(s)$) and Water ($H_2O(l)$).

The formation of the solid calcium carbonate ($CaCO_3$) is observed as a white precipitate, causing the lime water to turn milky or cloudy. This observation is a key characteristic test for carbon dioxide gas.

Evaluating Alternative Reactions

Let's examine the other options to understand why they don't represent the primary reaction:

Option 1: $CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2$
This equation describes the reaction that happens when excess carbon dioxide is passed through the lime water *after* the initial formation of calcium carbonate. The calcium carbonate reacts further to form soluble calcium bicarbonate ($Ca(HCO_3)_2$), and the milky appearance disappears. This is a subsequent step, not the initial reaction.
Option 3: $CaO + CO_2 \rightarrow CaCO_3$
This reaction involves calcium oxide (quicklime, CaO) reacting with carbon dioxide. However, lime water is specifically a solution of calcium *hydroxide* ($Ca(OH)_2$), not calcium oxide.
Option 4: $CaCl_2 + H_2O + CO_2 \rightarrow CaCO_3 + 2HCl$
This reaction is chemically incorrect and does not represent the process of passing carbon dioxide through lime water. Calcium chloride ($CaCl_2$) does not react in this manner.