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

Question

If \(\mathrm{CO}\), 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

(a) formation of water soluble calcium bicarbonate

Step 1: Understanding Lime Water Reaction

When lime water, which is a solution of calcium hydroxide ( Ca(OH)_2 ), reacts with carbon monoxide ( CO ), it typically does not react directly. However, the question likely implies CO_2 (carbon dioxide), a common gas that reacts with lime water. Initially, CO_2 reacts with Ca(OH)_2 to form calcium carbonate ( CaCO_3 ), which is insoluble and creates a milky suspension.

Step 2: Formation of Calcium Carbonate

As CO_2 is bubbled through lime water, the reaction Ca(OH)_2 + CO_2 ightarrow CaCO_3 + H_2O occurs, resulting in calcium carbonate. This calcium carbonate is an insoluble white solid, making the solution appear milky.

Step 3: Effect of Excess CO_2

When an excess of CO_2 is bubbled through the mixture, the initially formed calcium carbonate reacts further. This second reaction is CaCO_3 + CO_2 + H_2O ightarrow Ca(HCO_3)_2 , where calcium bicarbonate, which is soluble in water, is formed. As a result, the milkiness disappears.

Key Concepts

Lime Water Reaction

Lime water is an aqueous solution containing calcium hydroxide or \( \text{Ca(OH)}_2 \). In the context of this exercise, the term 'lime water reaction' usually refers to the interaction between lime water and carbon dioxide \( \text{CO}_2 \), not carbon monoxide \( \text{CO} \), which was mentioned. This is a critical correction because carbon dioxide is the reactive gas that participates in this classic chemistry demonstration.

When \( \text{CO}_2 \) is introduced to lime water, a chemical reaction occurs:

• Initially, the \( \text{CO}_2 \) reacts with \( \text{Ca(OH)}_2 \) in the solution.
• This reaction forms calcium carbonate \( \text{CaCO}_3 \) and water \( \text{H}_2\text{O} \):
  \[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \]

This process is crucial for understanding why the solution appears milky at first. The calcium carbonate formed is an insoluble precipitate, causing the characteristic milkiness associated with this reaction. For many students, seeing this change is an exciting introduction to the world of chemical reactions.

Calcium Carbonate Formation

The formation of calcium carbonate \( \text{CaCO}_3 \) is a key part of the lime water reaction. This compound is a white, insoluble solid. So when it is produced, it causes the once clear lime water solution to become cloudy or "milky" in appearance. This phenomenon is used as an indicator in various chemistry demonstrations and experiments.

• Calcium carbonate itself is a common substance found naturally. It's present in rocks like limestone and chalk, as well as in seashells and eggshells.
• The creation of \( \text{CaCO}_3 \) can be explained by considering the balanced chemical equation:
  \[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \]
• This process illustrates how a gas can react with a liquid to produce a solid precipitate, a key concept in chemistry known as a precipitation reaction.

Understanding these reactions helps learners grasp the complexities of solubility, precipitation, and gas reactions in solutions.

Calcium Bicarbonate Solubility

Excess carbon dioxide introduces an interesting twist in the lime water reaction. When \( \text{CO}_2 \) is in surplus, it causes calcium carbonate to react further, forming calcium bicarbonate \( \text{Ca(HCO}_3)_2 \), which is soluble in water. This subsequent reaction explains why the initial milkiness caused by \( \text{CaCO}_3 \) disappears.

• In the chemical context, this reaction is represented by:
  \[ \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(HCO}_3)_2 \]
• The resulting compound, calcium bicarbonate, dissolves in the solution, making it clear once more.
• This process illustrates important concepts of solubility and the dynamic nature of chemical equilibria.

Learning about the solubility of calcium bicarbonate helps in understanding other natural phenomena, such as the dissolution of limestone in water, which is vital in geological and environmental studies.