Which of the following statements with regard to the reaction given below are correct? CaO(s) + H2O(l) → Ca(OH)2 (aq) + Heat 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.

1. Types of Chemical Reactions: Combination and Exothermic (basic)

In chemistry, we often classify reactions based on how the atoms rearrange themselves. One of the most fundamental types is the Combination Reaction. As the name suggests, this occurs when two or more simple substances (reactants) join together to form a single, more complex substance (product). You can think of it as the chemical version of 1 + 1 = 2. A hallmark of this reaction is that only one product is formed Science, Chapter 1, p.14.

While the combination is happening, something else often occurs: the release of energy. This brings us to Exothermic Reactions. These are chemical processes where heat is given out to the surroundings along with the formation of products Science, Chapter 1, p.7. A perfect everyday example is the reaction of Quicklime (Calcium Oxide, CaO) with water. When these two meet, they react vigorously to produce Slaked Lime (Calcium Hydroxide, Ca(OH)₂). If you were to touch the beaker during this reaction, it would feel very hot because it is highly exothermic.

Common Name	Chemical Name	Chemical Formula
Quicklime	Calcium Oxide	CaO
Slaked Lime	Calcium Hydroxide	Ca(OH)₂

Beyond construction and whitewashing, exothermic reactions are vital to life itself. Respiration is a prime example. The food we eat, like rice or bread, is broken down into glucose. This glucose then reacts with oxygen in our cells to provide the energy we need to stay alive. Because this process releases energy, it is scientifically classified as an exothermic process Science, Chapter 1, p.7.

Sources: Science, Chapter 1: Chemical Reactions and Equations, p.14; Science, Chapter 1: Chemical Reactions and Equations, p.7

2. Inorganic vs. Organic Compounds (basic)

Concept: Inorganic vs. Organic Compounds

3. Common Names and Formulas of Calcium Compounds (intermediate)

In the world of everyday chemistry, calcium compounds play a starring role, particularly in construction and historical architecture. We begin with Quicklime (Calcium Oxide, CaO). When Quicklime reacts vigorously with water, it undergoes a combination reaction to produce Slaked Lime (Calcium Hydroxide, Ca(OH)₂). This process is highly exothermic, meaning it releases a significant amount of heat Science, Chemical Reactions and Equations, p.6. While 'Quicklime' is the dry powder, 'Slaked Lime' is the compound used in a solution for whitewashing walls.

The fascinating 'shiny' transformation of a whitewashed wall is actually a slow chemical dance with the atmosphere. Once the Calcium Hydroxide solution is applied to the walls, it reacts slowly with the Carbon Dioxide (CO₂) present in the air. This reaction produces a thin, hard layer of Calcium Carbonate (CaCO₃) and water: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. It typically takes two to three days for this reaction to complete, resulting in the characteristic bright, white, shiny finish we see on traditional buildings Science, Chemical Reactions and Equations, p.7.

Interestingly, Calcium Carbonate is a versatile chemical 'chameleon.' It is the primary constituent of limestone, which is a sedimentary rock essential for the cement and iron industries Geography of India, Resources, p.24. It also takes the form of marble, demonstrating how the same chemical formula (CaCO₃) can appear as a common building material or a precious ornamental stone.

Common Name	Chemical Name	Formula	Typical Use
Quicklime	Calcium Oxide	CaO	Precursor for slaked lime; industrial catalyst
Slaked Lime	Calcium Hydroxide	Ca(OH)₂	Whitewashing; neutralizing acidic soil
Limestone/Marble	Calcium Carbonate	CaCO₃	Cement manufacturing; shiny finish on walls

Sources: Science, Chemical Reactions and Equations, p.6; Science, Chemical Reactions and Equations, p.7; Geography of India, Resources, p.24

4. Acids, Bases, and the Nature of Metal Oxides (intermediate)

When we look at the chemical world, most elements don't exist in their pure form; they are often bonded with oxygen as oxides. To understand their chemical nature, we look at how they interact with water or acids. Generally, metallic oxides are considered basic in nature. This is because they react with acids to produce salt and water, a behavior identical to that of a base Science, Class X, Acids, Bases and Salts, p.22. For instance, when you burn magnesium ribbon, it forms magnesium oxide (MgO). If you dissolve this white ash in water and test it with litmus paper, it turns red litmus blue, confirming its basic character Science-Class VII, The World of Metals and Non-metals, p.51.

However, chemistry is full of fascinating exceptions. While most metal oxides are basic, some exhibit a "dual personality" known as amphoteric oxides. Oxides like aluminium oxide (Al₂O₃) and zinc oxide (ZnO) can react with both acids and bases to yield salt and water Science, Class X, Metals and Non-metals, p.41. In contrast, non-metallic oxides (like CO₂) are typically acidic. We know this because they react with bases (like calcium hydroxide) to form salt and water, mimicking the reaction of an acid with a base Science, Class X, Acids, Bases and Salts, p.22.

A classic everyday application of this chemistry is whitewashing. The process begins with quicklime (calcium oxide, CaO). When water is added, it reacts vigorously—releasing significant heat—to form slaked lime (calcium hydroxide, Ca(OH)₂). This is a textbook example of a combination reaction Science, Class X, Chemical Reactions and Equations, p.6. When this slaked lime is applied to walls, it doesn't stay as Ca(OH)₂. Over 2 to 3 days, it reacts with the CO₂ in the air to form a thin, hard layer of calcium carbonate (CaCO₃), which gives the walls a brilliant white, shiny finish.

Type of Oxide	Chemical Nature	Example
Metallic Oxide	Basic (Mostly)	MgO, CaO, CuO
Non-Metallic Oxide	Acidic	CO₂, SO₂
Amphoteric Oxide	Both Acidic & Basic	Al₂O₃, ZnO

Sources: Science, Class X, Acids, Bases and Salts, p.22; Science, Class X, Metals and Non-metals, p.41; Science-Class VII, The World of Metals and Non-metals, p.51; Science, Class X, Chemical Reactions and Equations, p.6

5. Atmospheric Interactions: Reaction with CO₂ (intermediate)

In the world of applied chemistry, the process of 'whitewashing' provides a perfect case study of how atmospheric gases transform the materials around us. The journey begins with Calcium Oxide (CaO), commonly known as Quicklime. When water is added to quicklime, a vigorous exothermic combination reaction occurs, producing a solution of Calcium Hydroxide (Ca(OH)₂), or Slaked Lime Science, Class X (NCERT 2025 ed.), Chapter 1, p.6. While people often mistake the initial application for the final result, the chemical 'magic' actually happens over the subsequent 48 to 72 hours. Once the slaked lime is applied to a wall, it begins a slow, invisible interaction with the Carbon Dioxide (CO₂) present in the atmosphere. This process, known as carbonation, results in the formation of a thin, hard layer of Calcium Carbonate (CaCO₃). The chemical equation for this transformation is:
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
This Calcium Carbonate is the same substance that makes up marble, which explains why walls develop a characteristic shiny white finish only after the reaction has had time to complete Science, Class X (NCERT 2025 ed.), Chapter 1, p.6. From a conceptual standpoint, this interaction highlights a fundamental principle of chemistry: the nature of oxides. Calcium hydroxide acts as a base, while Carbon Dioxide (a non-metallic oxide) behaves as an acid. When they meet, they undergo a reaction similar to neutralization, producing a salt (calcium carbonate) and water Science, Class X (NCERT 2025 ed.), Chapter 2, p.22. Understanding this helps us predict how other materials might degrade or stabilize when exposed to our CO₂-rich atmosphere.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.6; Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.22

6. The Chemistry of Whitewashing Walls (exam-level)

To understand why whitewashed walls develop a characteristic luster, we must look at a two-stage chemical transformation involving calcium compounds. The process begins with Quicklime (Calcium Oxide, CaO). When water is added to quicklime, a vigorous and exothermic reaction occurs, producing Slaked Lime (Calcium Hydroxide, Ca(OH)₂). This reaction releases a significant amount of heat and is a classic example of a combination reaction Science, Class X, Chapter 1, p.7. It is this solution of slaked lime—and not the initial quicklime—that is applied to the walls. Once the slaked lime is spread on the wall, the second, slower stage of the chemistry begins. The Calcium Hydroxide reacts with the Carbon Dioxide (CO₂) naturally present in the atmosphere. This process, known as carbonation, produces a thin, hard, and shiny layer of Calcium Carbonate (CaCO₃) Science, Class VIII, Chapter 8, p.119. This is the same chemical compound that makes up marble, explaining why the finish is so durable and reflective. This transformation is not instantaneous; it typically takes two to three days for the reaction to complete and for the white shine to fully emerge Science, Class X, Chapter 1, p.7.

To keep these compounds clear for the exam, remember their transitions:

Common Name	Chemical Name	Chemical Formula	Context
Quicklime	Calcium Oxide	CaO	The precursor (Solid)
Slaked Lime	Calcium Hydroxide	Ca(OH)₂	The solution applied to walls
Limestone / Marble	Calcium Carbonate	CaCO₃	The final shiny coating

Sources: Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.7; Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.10; Science, Class VIII (NCERT 2025 ed.), Chapter 8: Nature of Matter, p.119

7. Solving the Original PYQ (exam-level)

This question integrates the concepts of Combination Reactions and Exothermic Reactions that you have just mastered. The chemical equation provided describes the hydration of Quicklime (Calcium Oxide) to produce Slaked Lime (Calcium Hydroxide). To solve this, you must distinguish between the raw material and the applied product. While Quicklime is the starting point, it is the resulting solution of slaked lime that is actually applied to walls, making Statement 2 correct and Statement 1 a distractor. Furthermore, Statement 4 is a foundational factual check; Calcium Hydroxide is a standard inorganic compound as it does not contain carbon-hydrogen bonds.

The critical 'UPSC trap' lies in Statement 3. It describes the process where a wall gains a shiny finish over 2-3 days, but it incorrectly identifies the reactant. As detailed in Science, class X (NCERT 2025 ed.), it is the Calcium Hydroxide (Slaked Lime) already applied to the wall that reacts slowly with atmospheric carbon dioxide to form a thin layer of Calcium Carbonate, not the original Calcium Oxide. This subtle shift in chemical identity is a classic way examiners test your precision. By recognizing that Statement 3 is false and Statement 1 is an oversimplification, you can eliminate the incorrect codes to arrive at the correct answer: (B) 2 and 4.