Which of the following statements with regard to the reaction given below are 1. Quicklime is used for white- washing of walls. 2. The solution of slaked lime is used for whitewashing of walls. 3. CaO reacts slowly with CO2 in air to form a thin layer of CaCO3 on walls. 4. Calcium hydroxide called 'slaked lime' is an inorganic compound. Select the correct answer using the code given below.
correct?
CaO (s) + H2O(l) → Ca(OH)2(aq) + Heat
Understanding the Quicklime and Water Reaction
The question asks us to identify the correct statements regarding the reaction between quicklime and water, which is given as:
\(\text{CaO (s) + H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq) + Heat}\)
Here, \(\text{CaO}\) is calcium oxide, commonly known as quicklime. When it reacts with water (\(\text{H}_2\text{O}\)), it forms calcium hydroxide (\(\text{Ca(OH)}_2\)), which is known as slaked lime. This reaction is exothermic, meaning it releases heat.
Let's evaluate each statement provided in the question.
Statement 1: Quicklime is used for white-washing of walls.
This statement claims that quicklime (\(\text{CaO}\)) is used directly for whitewashing. While quicklime is the starting material in preparing the whitewashing solution, it is not typically applied directly to walls. Quicklime reacts vigorously and exothermically with water. The substance actually used for whitewashing is a solution or suspension of slaked lime (\(\text{Ca(OH)}_2\)) in water.
Therefore, Statement 1 is incorrect.
Statement 2: The solution of slaked lime is used for whitewashing of walls.
This statement says that the solution of slaked lime (\(\text{Ca(OH)}_2\)) is used for whitewashing. Slaked lime, when mixed with water, forms a suspension called milk of lime or lime water (a clear solution if dissolved). This is indeed the traditional material used for whitewashing walls. When applied, the slaked lime solution reacts with carbon dioxide in the air over a few days to form calcium carbonate, which gives a smooth, white finish.
The reaction on the wall surface is:
\(\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}\)
Therefore, Statement 2 is correct.
Statement 3: CaO reacts slowly with CO2 in air to form a thin layer of CaCO3 on walls.
This statement claims that quicklime (\(\text{CaO}\)) reacts with \(\text{CO}_2\) to form \(\text{CaCO}_3\) on walls during whitewashing. As explained above, the substance that reacts with \(\text{CO}_2\) from the air to form the calcium carbonate layer on the walls is slaked lime (\(\text{Ca(OH)}_2\)), not quicklime (\(\text{CaO}\)). Although \(\text{CaO}\) can react with \(\text{CO}_2\), this is not the reaction that creates the white finish on walls after applying slaked lime solution.
Therefore, Statement 3 is incorrect.
Statement 4: Calcium hydroxide called 'slaked lime' is an inorganic compound.
This statement identifies calcium hydroxide as 'slaked lime' and classifies it as an inorganic compound. Calcium hydroxide (\(\text{Ca(OH)}_2\)) is indeed commonly called slaked lime. Inorganic compounds are generally defined as compounds that do not contain carbon-hydrogen bonds, with some exceptions like carbonates and oxides of carbon. Calcium hydroxide is formed from a metal (Calcium), oxygen, and hydrogen, and it does not contain carbon. It is classified as an inorganic base.
Therefore, Statement 4 is correct.
Summary of Statement Analysis
- Statement 1: Incorrect (Quicklime not directly used)
- Statement 2: Correct (Slaked lime solution used)
- Statement 3: Incorrect (\(\text{Ca(OH)}_2\) reacts with \(\text{CO}_2\), not \(\text{CaO}\), on the wall)
- Statement 4: Correct (\(\text{Ca(OH)}_2\) is slaked lime and inorganic)
Based on the analysis, the correct statements are 2 and 4.
Revision Table: Key Substances and Reactions
| Substance | Chemical Formula | Common Name(s) | Role in Whitewashing |
|---|---|---|---|
| Calcium Oxide | \(\text{CaO}\) | Quicklime | Reacts with water to produce slaked lime. |
| Calcium Hydroxide | \(\text{Ca(OH)}_2\) | Slaked lime | Solution/suspension used for whitewashing; reacts with \(\text{CO}_2\) to form \(\text{CaCO}_3\). |
| Calcium Carbonate | \(\text{CaCO}_3\) | Limestone, Marble, Chalk | Forms the hard, white layer on walls after whitewashing. |
| Water | \(\text{H}_2\text{O}\) | Water | Reacts with quicklime; used to prepare slaked lime solution. |
| Carbon Dioxide | \(\text{CO}_2\) | Carbon Dioxide | Present in air; reacts with slaked lime on walls. |
Additional Information: Exothermic Reactions and Whitewashing
The reaction between quicklime and water is highly exothermic. This means it releases a significant amount of heat. This is why care must be taken when preparing slaked lime from quicklime; the process can cause the water to boil.
Whitewashing using slaked lime is a traditional method for painting walls. The process involves:
- Mixing quicklime (\(\text{CaO}\)) with water to produce slaked lime (\(\text{Ca(OH)}_2\)).
- Diluting the slaked lime with more water to form a suspension or solution.
- Applying this solution to the walls.
- The applied slaked lime reacts slowly with atmospheric carbon dioxide over several days to form a durable layer of calcium carbonate (\(\text{CaCO}_3\)). This layer gives the characteristic white finish.
Calcium hydroxide (\(\text{Ca(OH)}_2\)) is an inorganic compound. Inorganic compounds are substances that do not contain carbon atoms, or if they do, the carbon atoms are not bonded to hydrogen atoms (like oxides of carbon, carbonates, etc.). Organic compounds, on the other hand, are typically based on carbon-hydrogen bonds.
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