Which one of the following when dissolved in H2O gives hissing sound

1. Fundamentals of Chemical Reactions (basic)

At its most fundamental level, a chemical reaction is the process of breaking old chemical bonds and making new ones to transform starting substances, called reactants, into new substances known as products. It is important to remember that during this process, atoms do not simply vanish or change their identity (e.g., a carbon atom doesn't become an oxygen atom); they are merely rearranged into different structures Science, class X (NCERT 2025 ed.), Chapter 1, p.6. This rearrangement is what drives the diversity of matter we see around us, from the rusting of iron to the digestion of food Science, class X (NCERT 2025 ed.), Chapter 1, p.2.

One of the most straightforward types of chemical reactions is the combination reaction. In this type of reaction, two or more simple substances (elements or compounds) join together to form a single, more complex product. A classic everyday example is the reaction between quicklime (calcium oxide, CaO) and water. When you add water to quicklime, they combine to produce slaked lime (calcium hydroxide, Ca(OH)₂). This specific reaction is often used in construction for whitewashing walls Science, class X (NCERT 2025 ed.), Chapter 1, p.7.

Beyond the substances themselves, we must look at the energy involved. Reactions that release energy into the surroundings, usually in the form of heat, are called exothermic reactions. The reaction of quicklime with water is intensely exothermic; it generates so much heat that the mixture becomes hot enough to turn some water into steam, often producing a characteristic hissing sound. In contrast, endothermic reactions are those where energy is absorbed from the surroundings to make the reaction happen Science, class X (NCERT 2025 ed.), Chapter 1, p.15.

Reaction Type	Energy Change	Example
Exothermic	Releases heat (feels hot)	CaO + H₂O → Ca(OH)₂ + Heat
Endothermic	Absorbs heat (feels cold/requires heating)	Decomposition of Limestone

Sources: Science, class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.6; Science, class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.2; 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.15

2. Exothermic vs. Endothermic Processes (basic)

In the world of chemistry, every reaction involves an energy exchange. Think of it as a balance sheet: to start a reaction, we must break existing chemical bonds (which costs energy), and when new bonds form, energy is released. The net result of this balance determines whether a process is exothermic or endothermic Science, Class X (NCERT 2025 ed.), Chapter 1, p.6.

Exothermic reactions are those in which energy is released into the surroundings, usually in the form of heat. You can identify these because the container often feels hot to the touch. A common example is the burning of natural gas or even the biological process of respiration. During respiration, the glucose we get from food reacts with oxygen in our cells to provide the energy we need to stay alive Science, Class X (NCERT 2025 ed.), Chapter 1, p.7. In extreme cases, like the Thermit reaction (used to join railway tracks), the heat released is so massive that it produces molten iron Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.52.

On the other hand, endothermic reactions are those that absorb energy from their surroundings to proceed. If you were to touch a test tube where an endothermic reaction is occurring—such as the reaction between barium hydroxide and ammonium chloride—it would feel remarkably cold because the chemicals are literally "stealing" heat from your hand Science, Class X (NCERT 2025 ed.), Chapter 1, p.10. Most decomposition reactions are endothermic because they require a constant supply of energy (like heat or light) to break down a complex substance into simpler ones Science, Class X (NCERT 2025 ed.), Chapter 1, p.14.

Feature	Exothermic Process	Endothermic Process
Energy Flow	Released to surroundings	Absorbed from surroundings
Temperature Change	Surroundings get hotter	Surroundings get colder
Common Examples	Combustion, Respiration, Slaking of lime	Photosynthesis, Decomposition reactions

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.6, 7, 10, 14; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.52

3. Nature of Metal Oxides and Carbonates (intermediate)

To understand the chemistry of the world around us, we must first look at the nature of **metal oxides**. Generally, metals are electropositive, meaning they like to give away electrons. When they combine with oxygen, they form oxides that exhibit a **basic nature**. This is proven by the fact that they react with acids to produce salt and water—a classic neutralization reaction Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p. 22. For instance, if you treat black copper oxide with hydrochloric acid, it dissolves to form a blue-green solution of copper(II) chloride (a salt) and water Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p. 21.

While most metal oxides are insoluble in water, a few 'superstar' oxides like sodium oxide (Na₂O) and potassium oxide (K₂O) dissolve to form **alkalis** (water-soluble bases) Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p. 41. A very important everyday application of this is the **slaking of lime**. When water is added to **Calcium Oxide (CaO)**, also called quicklime, it reacts vigorously to form **Calcium Hydroxide (Ca(OH)₂)**, or slaked lime Science, class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p. 6. This reaction is highly **exothermic**, releasing so much heat that it can turn water into steam with a distinct hissing sound.

In contrast, **Calcium Carbonate (CaCO₃)**—found in nature as limestone, chalk, or marble—is relatively insoluble and stable in water. It represents a different stage of the chemical cycle compared to the highly reactive quicklime. Understanding this distinction is key to mastering how construction materials (like cement and mortar) and soil neutralizers work in real-world scenarios.

Compound	Common Name	Nature/Reaction
Calcium Oxide (CaO)	Quicklime	Highly reactive with water; Exothermic.
Calcium Hydroxide (Ca(OH)₂)	Slaked Lime	Product of CaO + H₂O; Basic/Alkaline.
Calcium Carbonate (CaCO₃)	Limestone/Marble	Stable; Insoluble in pure water.

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.21-22; Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.41

4. Industrial and Agricultural Uses of Calcium Compounds (intermediate)

Calcium compounds are the unsung heroes of both the industrial world and our agricultural heartlands. To understand their utility, we must look at the three primary forms: Limestone (Calcium Carbonate, CaCO₃), Quicklime (Calcium Oxide, CaO), and Slaked Lime (Calcium Hydroxide, Ca(OH)₂). The transition between these states is where the 'magic' happens for industry and farming.

In agriculture, the health of the soil is often measured by its pH value. When farmers use chemical fertilizers excessively, the soil can become overly acidic (pH < 7), which stunts plant growth. To remedy this, 'liming' is practiced. Since lime is a base, adding it to the soil neutralizes the excess acidity, restoring a healthy balance for crops. As noted in Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.18, if a farmer's plants aren't growing well due to acidity, treating the soil with lime is the standard scientific remedy.

Industrially, calcium compounds are indispensable:

• Construction: Limestone is the primary raw material for Cement. It is also a key component in the iron and steel industries, where it acts as a 'flux' to remove impurities Geography of India, Resources, p.24.
• Whitewashing: When we mix quicklime with water, it undergoes a vigorous exothermic reaction (releasing heat) to form slaked lime. When applied to walls, this Ca(OH)₂ slowly reacts with carbon dioxide (CO₂) in the air to form a thin, hard layer of Calcium Carbonate. This chemical transition gives walls a shiny, white finish over 2-3 days—the same chemical compound found in marble! Science, class X, Chemical Reactions and Equations, p.7.

Compound	Common Name	Primary Use
CaCO₃	Limestone / Marble	Cement, Steel industry, Whitewash finish
CaO	Quicklime	Manufacturing, Precursor to slaked lime
Ca(OH)₂	Slaked Lime / Lime water	Soil treatment, Whitewashing, CO₂ testing

Sources: Science-Class VII . NCERT(Revised ed 2025), Exploring Substances: Acidic, Basic, and Neutral, p.18; Geography of India ,Majid Husain, (McGrawHill 9th ed.), Resources, p.24; Science , class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.7

5. The Lime Cycle: Carbonate, Oxide, and Hydroxide (exam-level)

The Lime Cycle is one of the most fundamental chemical sequences in both industrial applications and geological processes. It begins with Limestone (Calcium Carbonate, CaCO₃), a naturally occurring sedimentary rock. When limestone is heated intensely in a kiln, it undergoes a thermal decomposition reaction, breaking down into Quicklime (Calcium Oxide, CaO) and releasing Carbon Dioxide gas. This process is crucial for the manufacturing of cement, as highlighted in Science, Class X (NCERT 2025 ed.), Chapter 1, p. 8.

The next phase of the cycle is the transition from Quicklime to Slaked Lime (Calcium Hydroxide, Ca(OH)₂). This is a classic combination reaction where Calcium Oxide reacts vigorously with water. This process, known as slaking, is highly exothermic—it releases a tremendous amount of heat energy, often causing the water to boil into steam with a characteristic hissing sound. In this reaction, two reactants (CaO and H₂O) combine to form a single product, as explained in Science, Class X (NCERT 2025 ed.), Chapter 1, p. 6.

The cycle completes when Slaked Lime is exposed to the atmosphere. It slowly reacts with Carbon Dioxide in the air to form a thin, hard layer of Calcium Carbonate again. This is why whitewashed walls develop a shiny, durable finish after a few days: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. On a geological scale, this same chemistry governs Karst topography, where rainwater (containing dissolved CO₂) dissolves limestone over centuries, as noted in Physical Geography by PMF IAS, Major Landforms and Cycle of Erosion, p. 227.

Form of Lime	Chemical Name	Common Reaction
Limestone	Calcium Carbonate (CaCO₃)	Decomposes when heated
Quicklime	Calcium Oxide (CaO)	Reacts vigorously with water
Slaked Lime	Calcium Hydroxide (Ca(OH)₂)	Reacts with CO₂ to harden

Sources: Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.6, 8; Physical Geography by PMF IAS, Major Landforms and Cycle of Erosion, p.227

6. The Chemistry of Slaking Lime (exam-level)

When we talk about the chemistry of slaking, we are observing a fascinating transformation of matter that has been used by humans for centuries, particularly in construction and whitewashing. The process begins with Quicklime (Calcium Oxide, CaO). When water is added to it, a vigorous and highly energetic chemical reaction occurs. This isn't just a simple mixing; it is a combination reaction, where two distinct substances (Calcium Oxide and Water) join together to form a single product: Slaked Lime (Calcium Hydroxide, Ca(OH)₂) Science, Class X, Chapter 1, p. 6.

One of the most striking features of this reaction is that it is highly exothermic. As the chemical bonds rearrange, a significant amount of heat energy is released into the surroundings. This heat is often intense enough to turn some of the reacting water into steam, producing a characteristic hissing sound. Because of this energy release, the reaction mixture becomes very hot to the touch Science, Class X, Chapter 1, p. 7. If you continue to add water and stir the mixture, you eventually form a clear solution of calcium hydroxide, which we commonly refer to as lime water Science, Class VIII, Chapter 8, p. 118.

It is crucial to distinguish between the various forms of "lime" encountered in chemistry. While they all contain calcium, their reactivity and chemical structures differ significantly:

Common Name	Chemical Name	Chemical Formula	Key Property
Limestone	Calcium Carbonate	CaCO₃	Relatively insoluble; does not react vigorously with water.
Quicklime	Calcium Oxide	CaO	Extremely reactive; used as the starting point for slaking.
Slaked Lime	Calcium Hydroxide	Ca(OH)₂	The hydrated product; basic in nature.

Sources: Science, Class X, Chemical Reactions and Equations, p.6; Science, Class X, Chemical Reactions and Equations, p.7; Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.118

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of chemical equations, this question tests your ability to apply the concept of exothermic combination reactions to real-world substances. In your previous modules, we discussed how certain reactions release energy in the form of heat. To solve this, you must identify which substance reacts with water so vigorously that it physicalizes that energy release through sound. Think of the molecular agitation occurring when a highly reactive oxide meets water; that energy has to go somewhere, and in this case, it manifests as thermal energy and steam.

The correct answer is Quicklime (Calcium Oxide, CaO). When water is added to Quicklime, a process called slaking occurs, forming Calcium Hydroxide. As detailed in Science, Class X (NCERT 2025 ed.), this is a textbook example of a combination reaction where the energy released is so intense that it causes the water to boil and escape as steam, creating that signature hissing sound. Your reasoning should follow this path: High reactivity + vigorous hydration = rapid heat release = steam/hissing.

UPSC often uses the "Lime Family" to create confusion, so don't fall into their traps. Limestone (Calcium Carbonate) is the stable, insoluble rock form that does not react vigorously with water. Slaked lime is the result of the reaction, meaning it has already been hydrated and is therefore chemically "calm." Soda lime is a specialized mixture of Sodium Hydroxide and Calcium Oxide used in labs, but it is not the primary substance associated with this classic NCERT experiment. Always distinguish between the reactant (Quicklime) and the product (Slaked lime) to avoid these common errors.