During white - washing of walls, slaked lime reacts slowly with carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Which of the following reactions represents this correctly?
Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l)
The question asks about the chemical reaction that occurs during the whitewashing of walls. Whitewashing typically involves using slaked lime, which is chemically known as calcium hydroxide.
Understanding the Whitewashing Process
When you whitewash walls, you usually apply a solution or suspension of slaked lime (\(\text{Ca(OH)}_2\)). This substance is initially wet on the walls. The atmosphere contains carbon dioxide gas (\(\text{CO}_2\)). Over time, the slaked lime on the wall reacts slowly with the carbon dioxide from the air.
This reaction forms a thin, hard layer of calcium carbonate (\(\text{CaCO}_3\)). Calcium carbonate is the same chemical substance found in limestone and marble. It's this calcium carbonate layer that gives the walls a shiny finish after a couple of days.
Let's identify the substances involved and their states:
- Slaked lime: Calcium hydroxide, \(\text{Ca(OH)}_2\). When applied as whitewash, it's usually an aqueous solution or suspension, so its state is (aq).
- Carbon dioxide: \(\text{CO}_2\). It's a gas present in the air, so its state is (g).
- Calcium carbonate: \(\text{CaCO}_3\). It forms a solid layer on the wall, so its state is (s).
- Water: \(\text{H}_2\text{O}\). It's produced during the reaction and is a liquid, so its state is (l).
Chemical Reaction of Slaked Lime with Carbon Dioxide
The reaction between calcium hydroxide and carbon dioxide can be written as:
Calcium hydroxide + Carbon dioxide \(\rightarrow\) Calcium carbonate + Water
In chemical formula form, with states of matter:
\(\text{Ca(OH)}_2\text{ (aq)} + \text{CO}_2\text{ (g)} \rightarrow \text{CaCO}_3\text{ (s)} + \text{H}_2\text{O}\text{ (l)}\)
This equation shows that aqueous calcium hydroxide reacts with gaseous carbon dioxide to produce solid calcium carbonate and liquid water. This reaction is responsible for the hardening and shining of the whitewashed walls.
Analyzing the Given Options
Let's look at the provided options and compare them with the correct chemical reaction:
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Option 1: \(\text{CaO (s)} + \text{CO}_2\text{ (g)} \rightarrow \text{CaCO}_3\text{ (s)}\)
This reaction involves calcium oxide (\(\text{CaO}\)), which is quicklime, not slaked lime (\(\text{Ca(OH)}_2\)). Quicklime reacts with water to form slaked lime, which is then used for whitewashing. So, this option is incorrect as it doesn't represent the reaction during the drying process on the wall.
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Option 2: \(\text{CaO (l)} + \text{CO}_2\text{ (g)} \rightarrow \text{CaCO}_3\text{ (s)}\)
This option also involves calcium oxide (\(\text{CaO}\)) instead of slaked lime (\(\text{Ca(OH)}_2\)). Furthermore, calcium oxide is a solid at room temperature, not a liquid (l), unless specified otherwise which is not implied in standard whitewashing context. This option is incorrect.
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Option 3: \(\text{Ca(OH)}_2\text{ (l)} + \text{CO}_2\text{ (l)} \rightarrow \text{CaCO}_3\text{ (s)} + \text{H}_2\text{O}\text{ (l)}\)
This option correctly uses slaked lime (\(\text{Ca(OH)}_2\)) and shows the formation of calcium carbonate (\(\text{CaCO}_3\)) and water (\(\text{H}_2\text{O}\)). However, it incorrectly states that carbon dioxide (\(\text{CO}_2\)) is in the liquid state (l). Carbon dioxide in the air is a gas (g). Slaked lime is typically used as an aqueous solution/suspension (aq), though (l) might sometimes be used loosely for a liquid mixture. The state of carbon dioxide is definitely wrong here. This option is incorrect.
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Option 4: \(\text{Ca(OH)}_2\text{ (aq)} + \text{CO}_2\text{ (g)} \rightarrow \text{CaCO}_3\text{ (s)} + \text{H}_2\text{O}\text{ (l)}\)
This option correctly uses slaked lime as an aqueous solution (\(\text{Ca(OH)}_2\text{ (aq)}\)), gaseous carbon dioxide (\(\text{CO}_2\text{ (g)}\)), and shows the formation of solid calcium carbonate (\(\text{CaCO}_3\text{ (s)}\)) and liquid water (\(\text{H}_2\text{O}\text{ (l)}\)). This accurately represents the chemical reaction that occurs on the wall during whitewashing as the slaked lime reacts with carbon dioxide from the air.
Comparing our derived correct reaction with the given options, Option 4 precisely matches the chemical process and the states of matter involved in the whitewashing reaction.
Revision Table: Key Substances in Whitewashing
| Substance | Chemical Name | Formula | Role in Whitewashing | State in Reaction on Wall |
|---|---|---|---|---|
| Quicklime | Calcium oxide | \(\text{CaO}\) | Used to prepare slaked lime by adding water | (s) - initial reactant (before slaking) |
| Slaked lime | Calcium hydroxide | \(\text{Ca(OH)}_2\) | Applied to walls; reacts with CO\(_2\) | (aq) - when applied wet |
| Carbon dioxide | Carbon dioxide | \(\text{CO}_2\) | Reacts with slaked lime from the air | (g) - from air |
| Calcium carbonate | Calcium carbonate | \(\text{CaCO}_3\) | Forms the solid, shiny layer on the wall | (s) - product on the wall |
| Water | Water | \(\text{H}_2\text{O}\) | Used to prepare slaked lime; produced in reaction | (l) - solvent and product |
Additional Information on Calcium Compounds
The process of whitewashing involves a sequence of reactions starting from limestone.
- Formation of Quicklime: Limestone (\(\text{CaCO}_3\text{ (s)}\)) is heated strongly (calcination) to produce quicklime (\(\text{CaO}\)) and carbon dioxide. \[\text{CaCO}_3\text{ (s)} \xrightarrow{\text{Heat}} \text{CaO}\text{ (s)} + \text{CO}_2\text{ (g)}\]
- Slaking of Lime: Quicklime (\(\text{CaO}\)) is reacted with water (\(\text{H}_2\text{O}\)) to produce slaked lime (\(\text{Ca(OH)}_2\)). This process is called slaking of lime, and it releases a lot of heat (exothermic). \[\text{CaO}\text{ (s)} + \text{H}_2\text{O}\text{ (l)} \rightarrow \text{Ca(OH)}_2\text{ (aq)}\] Slaked lime is then mixed with more water to make a solution or suspension for applying to walls.
- Whitewashing Reaction: The applied slaked lime solution (\(\text{Ca(OH)}_2\text{ (aq)}\)) reacts slowly with atmospheric carbon dioxide (\(\text{CO}_2\text{ (g)}\)) to form calcium carbonate (\(\text{CaCO}_3\text{ (s)}\)) and water (\(\text{H}_2\text{O}\text{ (l)}\)). This is the reaction described in the question. \[\text{Ca(OH)}_2\text{ (aq)} + \text{CO}_2\text{ (g)} \rightarrow \text{CaCO}_3\text{ (s)} + \text{H}_2\text{O}\text{ (l)}\] The formation of solid calcium carbonate layer gives the white, smooth finish to the walls.
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