Calcium oxide reacts vigorously with water to produce slaked lime whose solution is used for white-washing walls. This slaked lime reacts with component (A) in air to form a thin layer of component (B) to give a shiny finish. What are the components (A) and (B) ?
A - CO 2; B - CaCO 3
Understanding the Chemistry of White-washing
The question describes a common chemical process used for white-washing walls. It involves several steps, starting with calcium oxide reacting with water. Let's break down the reactions and identify the components.
Step 1: Reaction of Calcium Oxide with Water
Calcium oxide, also known as quicklime, has the chemical formula $\text{CaO}$. The question states it reacts vigorously with water ($\text{H}_2\text{O}$). This reaction produces slaked lime.
The chemical equation for this reaction is:
$\text{CaO(s) + H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)}$
Here,
- $\text{CaO}$ is Calcium Oxide (Quicklime) in solid state.
- $\text{H}_2\text{O}$ is Water in liquid state.
- $\text{Ca(OH)}_2$ is Calcium Hydroxide (Slaked lime) in aqueous solution.
This reaction is highly exothermic, meaning it releases a significant amount of heat, which is why it is described as vigorous.
Step 2: White-washing with Slaked Lime Solution
The solution of slaked lime, $\text{Ca(OH)}_2\text{(aq)}$, is then used for white-washing walls.
Step 3: Reaction of Slaked Lime with Component (A) from Air
When the slaked lime solution is applied to the walls, it is exposed to the air. The question states that this slaked lime reacts with a component (A) from the air to form a thin layer of component (B), which gives a shiny finish.
Air contains several gases, including nitrogen ($\text{N}_2$), oxygen ($\text{O}_2$), argon (Ar), carbon dioxide ($\text{CO}_2$), and small amounts of other gases. Among these components, carbon dioxide ($\text{CO}_2$) is known to react with calcium hydroxide ($\text{Ca(OH)}_2$).
The reaction between calcium hydroxide and carbon dioxide from the air is:
$\text{Ca(OH)}_2\text{(aq) + CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s) + H}_2\text{O(l)}$
Here,
- $\text{Ca(OH)}_2$ is Calcium Hydroxide (Slaked lime) in aqueous solution.
- $\text{CO}_2$ is Carbon Dioxide from the air in gaseous state.
- $\text{CaCO}_3$ is Calcium Carbonate in solid state.
- $\text{H}_2\text{O}$ is Water in liquid state.
The product, $\text{CaCO}_3$, is calcium carbonate, which is the main component of limestone and marble. When this reaction happens on the wall surface, it forms a thin, solid layer of calcium carbonate. This layer is responsible for the shiny finish that appears after a few days of white-washing.
Identifying Components (A) and (B)
Based on the reaction:
- Component (A) from the air that reacts with slaked lime is Carbon Dioxide ($\text{CO}_2$).
- Component (B) that forms the shiny layer is Calcium Carbonate ($\text{CaCO}_3$).
Comparing with the Options
Let's look at the given options:
| Option | Component (A) | Component (B) |
|---|---|---|
| 1 | $\text{O}_2$ | $\text{CaCO}_3$ |
| 2 | $\text{CO}_2$ | $\text{Ca(OH)}_2$ |
| 3 | $\text{O}_2$ | $\text{Ca(OH)}_2$ |
| 4 | $\text{CO}_2$ | $\text{CaCO}_3$ |
Our identification is Component (A) = $\text{CO}_2$ and Component (B) = $\text{CaCO}_3$. This matches Option 4.
Conclusion
The component (A) from the air that reacts with slaked lime ($\text{Ca(OH)}_2$) during white-washing is carbon dioxide ($\text{CO}_2$), forming a thin layer of calcium carbonate ($\text{CaCO}_3$), which is component (B), giving the walls a shiny finish.
Revision Table: White-washing Chemistry
| Substance | Chemical Formula | Role in White-washing |
|---|---|---|
| Calcium Oxide (Quicklime) | $\text{CaO}$ | Starting material; reacts with water to form slaked lime. |
| Water | $\text{H}_2\text{O}$ | Reacts with quicklime to form slaked lime solution. |
| Calcium Hydroxide (Slaked Lime) | $\text{Ca(OH)}_2$ | Product of $\text{CaO + H}_2\text{O}$; used as the white-wash solution. |
| Carbon Dioxide (Component A) | $\text{CO}_2$ | Present in air; reacts with slaked lime on the wall. |
| Calcium Carbonate (Component B) | $\text{CaCO}_3$ | Product of $\text{Ca(OH)}_2\text{ + CO}_2$; forms the shiny layer on the wall. |
Additional Information: Lime and its Uses
Lime is a broad term referring to calcium-containing inorganic materials. Quicklime ($\text{CaO}$) and slaked lime ($\text{Ca(OH)}_2$) are important lime products with various applications:
- Quicklime ($\text{CaO}$): Used in steel manufacturing, pulp and paper production, water treatment, and as a drying agent. Its reaction with water is called slaking.
- Slaked Lime ($\text{Ca(OH)}_2$): Used in white-washing, mortar and plaster production, water softening, pH adjustment in various processes, and in agriculture to neutralize acidic soils.
- Calcium Carbonate ($\text{CaCO}_3$): Found naturally as limestone, marble, and chalk. Used as a building material, filler in plastics and paper, dietary supplement (as a source of calcium), and antacid. The formation of $\text{CaCO}_3$ from slaked lime and $\text{CO}_2$ is a slow process that happens as the white-wash dries and reacts with air.
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