Animated Solution for Chemistry - s and p-Block Elements: What are the products formed in sequence when excess of ${\text{CO}}_2$ is passed in slaked lime?

A Tale of Two Carbonates

Inorganic chemistry is full of beautiful, visual reactions, and few are as iconic as the lime water test. When you pass carbon dioxide gas through slaked lime, you are not just performing a laboratory test; you are recreating the very chemistry that carves out massive underground caves and hardens our groundwater. Let's break down the sequence of events that occurs when we bubble an excess of ${\text{CO}}_2$ through this solution.

A Milky Transformation

We start with slaked lime, which is an aqueous solution of calcium hydroxide, denoted as ${\text{Ca(OH)}}_2$. It is a clear, slightly alkaline solution. As soon as we introduce carbon dioxide gas (${\text{CO}}_2$) into this environment, an acid-base neutralization reaction kicks off.

The carbon dioxide reacts with the calcium hydroxide to form calcium carbonate and water. The chemical equation for this initial step is:

The crucial observation here is the state of the product. Calcium carbonate (${\text{CaCO}}_3$) is highly insoluble in water. Instead of dissolving, it precipitates out as millions of microscopic white solid particles suspended in the liquid. This suspension scatters light, instantly turning the previously clear solution into a milky, opaque liquid. This is the classic, definitive test for the presence of carbon dioxide gas.

The Power of Excess

Many students stop at the milky phase, but the question explicitly states that we are passing an excess of ${\text{CO}}_2$. What happens when we don't stop bubbling the gas?

As more carbon dioxide enters the system, it dissolves in the water to form a weak acid known as carbonic acid (${\text{H}}_2{\text{CO}}_3$). This newly acidic environment attacks the insoluble calcium carbonate precipitate. The solid particles react with the dissolved carbon dioxide and water to form a new compound: calcium bicarbonate.

The equation for this secondary reaction is:

Here lies the magic of the sequence. Unlike calcium carbonate, calcium bicarbonate (${\text{Ca(HCO}}_3{)}_2$) is highly soluble in water. As the reaction proceeds, the solid white particles are consumed and converted into dissolved aqueous ions. Right before your eyes, the milky suspension clears up, returning to a perfectly transparent solution.

By tracing the physical changes—from clear, to milky, and back to clear—we can perfectly map out the chemical sequence. The first product formed is the insoluble calcium carbonate (${\text{CaCO}}_3$), and the final product formed upon adding excess gas is the soluble calcium bicarbonate (${\text{Ca(HCO}}_3{)}_2$).

This exact sequence is not just a textbook curiosity. When rainwater (which naturally contains dissolved ${\text{CO}}_2$ from the atmosphere) seeps through limestone (${\text{CaCO}}_3$) rocks, it performs this exact second reaction, dissolving the solid rock into soluble calcium bicarbonate. This is the primary mechanism behind the creation of spectacular limestone caves and the reason why groundwater often suffers from temporary hardness!