Problem 21 - A Complete Resource Book in Chemistry for JEE Main

Question

If \(\mathrm{CO}_{2}\) is passed in excess through lime water, the milkiness first formed disappears due to (a) formation of water soluble calcium bicarbonate (b) the solution getting heated by exothermic reaction (c) reversal of the original reaction (d) formation of volatile calcium derivative

Step-by-Step Solution

Answer

The milkiness disappears because calcium bicarbonate, which is water-soluble, is formed.

Step 1: Formation of Calcium Carbonate

When \( ext{CO}_2\) is initially passed through lime water (which is a solution of calcium hydroxide, \( ext{Ca(OH)}_2\)), calcium carbonate \( ext{CaCO}_3\) is formed. This reaction causes the lime water to turn milky: \[ \ ext{Ca(OH)}_2 + ext{CO}_2 ightarrow ext{CaCO}_3 ext{ (milky) } + ext{H}_2 ext{O} \]

Step 2: Reaction with Excess CO2

When excess \( ext{CO}_2\) is continuously bubbled through the solution, the initially formed calcium carbonate reacts further with \( ext{CO}_2\) and water to form calcium bicarbonate, \( ext{Ca(HCO}_3)_2\), which is soluble in water: \[ \ ext{CaCO}_3 + ext{CO}_2 + ext{H}_2 ext{O} ightarrow ext{Ca(HCO}_3)_2 ext{ (aqueous) } \]

Key Concepts

Lime water reaction

When carbon dioxide \(\text{CO}_2\) is bubbled through lime water, a fascinating chemical reaction takes place. Lime water is essentially a solution of calcium hydroxide \(\text{Ca(OH)}_2\). Initially, as the carbon dioxide passes through, it reacts to form calcium carbonate \(\text{CaCO}_3\). This newly formed compound gives the lime water a milky appearance, symbolizing the completion of what's known as the lime water reaction.

Here's how the process works step-by-step:

• Carbon dioxide interacts with calcium hydroxide in the solution.
• This interaction leads to the formation of a white precipitate, calcium carbonate.

Consequently, viewing this milky liquid can be a visual reminder of the lime water reaction in progress.

Calcium carbonate

Calcium carbonate \(\text{CaCO}_3\) is a key player in the lime water reaction. It's what causes the initial milkiness when carbon dioxide is added. This compound is a white, insoluble substance that forms as a result of the chemical reaction between lime water and carbon dioxide.

In terms of its role:

• Calcium carbonate precipitates out of the solution, creating a cloudy appearance.
• It marks the first significant step in the chain of reactions that can occur when \(\text{CO}_2\) is in the mix.

In everyday life, you might encounter calcium carbonate as a primary component in limestone and marble.

Calcium bicarbonate

Calcium bicarbonate \(\text{Ca(HCO}_3)_2\) represents the next phase of the reaction. Once you pass excess carbon dioxide through the lime water, the initially formed calcium carbonate reacts further. This reaction results in calcium bicarbonate, which is different from calcium carbonate in that it is soluble in water.

The steps include:

• Calcium carbonate continues to react with the additional \(\text{CO}_2\) and water.
• This process forms calcium bicarbonate, which remains dissolved in the solution.

Because it is water-soluble, calcium bicarbonate clears up the milky solution, turning it clear again.

Solubility

Solubility is a crucial concept for understanding these chemical reactions. It defines the ability of a substance to dissolve in a solvent, like how calcium bicarbonate dissolves in water, unlike its predecessor, calcium carbonate.

In the context of the lime water reaction:

• Calcium carbonate is insoluble, which explains its milky precipitate.
• Calcium bicarbonate is soluble, allowing it to dissolve and clear the solution.

This transformation highlights how solubility can determine whether a solution remains cloudy or becomes clear.

Exothermic reaction

An exothermic reaction is one that releases heat as it occurs. In the context of these reactions, while heat might be produced, it is not what primarily drives the changes observed in the lime water.

Instead:

• The visual change from milky to clear is a result of further chemical reactions, rather than heat effects.
• Understanding exothermic reactions, in general, can help appreciate how energy changes might occur in other reactions.

Ultimately, while pertinent to chemistry, exothermic reactions aren't a primary factor in the lime water reaction's observed effects, especially regarding solubility changes.