Statement I: Very little hydrogen is produced when sulphuric acid is added to calcium. Statement II: The salt that is produced, calcium sulphate, is insoluble.

1. General Reactions of Acids with Metals (basic)

Welcome to your first step in mastering chemical principles! To understand how the world around us works—from why we don't store pickles in metal jars to how industrial hydrogen is produced—we must look at the fundamental interaction between metals and acids.

At its core, when a reactive metal meets a dilute acid, a displacement reaction occurs. The metal atoms are more "energetic" than the hydrogen atoms sitting inside the acid; they push the hydrogen out and take its place. This interaction can be summarized by a very famous general equation:

Metal + Dilute Acid → Salt + Hydrogen gas Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20.

You can see this in action through effervescence (the formation of bubbles). For example, when Zinc (Zn) reacts with dilute Hydrochloric acid (HCl), it produces Zinc Chloride (ZnCl₂) and Hydrogen gas (H₂). However, not all metals react with the same intensity. In a laboratory setting, you would notice that Magnesium reacts very vigorously, while Iron is much slower, and Copper does not react with dilute HCl at all Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.44. This tells us there is a "hierarchy" of reactivity among metals.

There is a fascinating "catch" to this rule, though. For a reaction to continue, the acid must be able to reach the surface of the metal. If the salt formed during the reaction (like Calcium Sulphate) is insoluble in water, it can settle on the metal's surface like a suit of armor. This solid layer prevents the remaining acid from touching the metal, causing the reaction to grind to a halt almost immediately. This is why some reactions produce very little hydrogen gas even if the metal is technically reactive.

Observation	Scientific Meaning
Formation of bubbles	Evolution of Hydrogen gas (H₂)
Rise in temperature	The reaction is Exothermic (releases heat)
Reaction stops early	An insoluble salt layer has formed on the metal surface

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20, 22, 33; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.44, 46

2. The Metal Reactivity Series (basic)

The Reactivity Series is essentially a 'merit list' of metals, where they are ranked based on how easily they lose electrons to form positive ions. This hierarchy is not arbitrary; it is meticulously built by observing how metals react with water, oxygen, and acids. However, the most definitive way to rank them is through displacement reactions. If Metal A can forcibly remove Metal B from its salt solution, Metal A is crowned the more reactive of the two Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.45. For instance, if you drop an iron nail into copper sulphate solution, the iron 'displaces' the copper, turning the blue solution green — proving iron is more reactive than copper Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46.

While the series tells us which metals are powerful, the physical outcome of a reaction often depends on the solubility of the products. A classic example is the reaction of Calcium with Sulphuric acid (H₂SO₄). Although Calcium is quite high in the reactivity series, its reaction with sulphuric acid produces very little hydrogen gas. This happens because the resulting salt, Calcium Sulphate (CaSO₄), is insoluble in water. It forms a crusty solid layer on the surface of the metal, acting as a physical shield that prevents the acid from reaching the rest of the calcium. In contrast, with Hydrochloric acid, the product is soluble, allowing the reaction to proceed until the metal is consumed.

Metals at the very top of the series, like Potassium and Sodium, are so 'aggressive' that they are never found alone in nature; they are always bound in compounds. Conversely, metals at the bottom, such as Gold and Silver, are 'noble' and non-reactive, often found in their free state in the Earth's crust Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.49. Hydrogen is often included in this series as a reference point: any metal placed above hydrogen can displace it from dilute acids to produce hydrogen gas Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.55.

Reactivity	Metal	Symbol
Highest	Potassium	K
↑	Sodium	Na
↑	Calcium	Ca
Medium	Zinc / Iron	Zn / Fe
↓	Copper / Silver	Cu / Ag
Lowest	Gold	Au

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.45; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.49; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.55

3. Solubility Rules for Common Salts (intermediate)

To understand chemical reactions, we must first understand solubility—the ability of a substance (the solute) to dissolve in a solvent like water. Not all salts behave the same way; while some dissolve completely to form clear solutions, others remain as solids or form a cloudy precipitate during a reaction Science, Class X, Chemical Reactions and Equations, p.14. This distinction is vital in predicting whether a reaction will proceed to completion or be physically blocked by its own products.
General rules help us predict solubility. For instance, most salts containing Sodium (Na⁺), Potassium (K⁺), and Nitrates (NO₃⁻) are highly soluble Science, Class X, Acids, Bases and Salts, p.28. However, Sulphates (SO₄²⁻) present an interesting pattern. While magnesium or copper sulphate dissolve easily, Barium Sulphate (BaSO₄) is famously insoluble and appears as a white precipitate in double displacement reactions Science, Class X, Chemical Reactions and Equations, p.12. Similarly, Calcium Sulphate (CaSO₄) is considered sparingly soluble—it dissolves so poorly that it often settles out as a solid layer.
This physical property has significant consequences in laboratory settings. In a precipitation reaction, two soluble compounds in aqueous solution exchange ions to produce an insoluble salt Science, Class X, Chemical Reactions and Equations, p.11. If an insoluble salt like calcium sulphate forms on the surface of a reacting metal, it acts as a physical barrier, preventing the surrounding liquid from reaching the rest of the metal. This can prematurely halt a chemical process that would otherwise continue.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28-29; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.11-14

4. Industrial Significance of Sulphuric Acid (intermediate)

Often referred to as the "King of Chemicals," Sulphuric acid (H₂SO₄) is the backbone of modern industry. Its consumption is frequently used as an index of a country's industrial prosperity because it is essential in manufacturing everything from fertilizers and detergents to explosives and lead-acid batteries. At its core, H₂SO₄ is a strong mineral acid that exhibits three key properties: it is a powerful oxidizing agent, a strong dehydrating agent, and a source of hydrogen ions in chemical reactions.

In a standard laboratory setting, we observe that acids react with active metals to produce a salt and evolve hydrogen gas Science, Class X, Acids, Bases and Salts, p.22. For instance, when granulated zinc is placed in dilute sulphuric acid, it reacts vigorously to form zinc sulphate and hydrogen gas bubbles Science, Class X, Chemical Reactions and Equations, p.2. However, chemistry is full of fascinating nuances where the physical properties of the resulting salt dictate whether a reaction continues or grinds to a halt.

A critical example of this is the reaction between Calcium (Ca) and Sulphuric Acid (H₂SO₄). Unlike zinc sulphate, which dissolves easily in water, the salt produced here—Calcium Sulphate (CaSO₄)—is insoluble or only sparingly soluble. As soon as the reaction starts, a thin, solid layer of calcium sulphate forms on the surface of the metal. This layer acts as a physical barrier, preventing the remaining acid from coming into contact with the unreacted calcium underneath. This process is effectively self-limiting; the reaction stops almost as soon as it begins, producing very little hydrogen gas.

Metal + Acid	Salt Formed	Solubility	Reaction Outcome
Zinc + H₂SO₄	ZnSO₄	Highly Soluble	Proceeds until metal or acid is consumed.
Calcium + H₂SO₄	CaSO₄	Insoluble	Stops prematurely due to a surface coating.
Calcium + HCl	CaCl₂	Highly Soluble	Proceeds to completion.

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

5. Connected Topic: Gypsum and Plaster of Paris (intermediate)

In our study of mineral resources and chemical compounds, Gypsum and Plaster of Paris (PoP) stand out as a classic example of how a simple change in water content can completely transform a material's physical properties. Gypsum is a naturally occurring mineral, chemically known as Calcium Sulphate Dihydrate (CaSO₄·2H₂O). It is found in sedimentary rock beds alongside limestone and sandstone Geography of India, Resources, p.28. In India, Rajasthan is the powerhouse of gypsum production, contributing nearly 99% of the country's total output.

The magic happens when we apply heat. When Gypsum is heated to exactly 373 K (100°C), it undergoes a partial dehydration—a form of thermal decomposition Science class X (NCERT), Chemical Reactions and Equations, p.8. It loses three-fourths of its water of crystallisation to become Calcium Sulphate Hemihydrate (CaSO₄·½H₂O), popularly known as Plaster of Paris. You might wonder: how can half a water molecule exist? In reality, two formula units of CaSO₄ share a single molecule of water between them Science class X (NCERT), Acids, Bases and Salts, p.33. This white powder is famous for its ability to turn back into a hard, solid mass of Gypsum almost immediately upon mixing with water, which is why doctors use it to support fractured bones.

An essential chemical characteristic of Calcium Sulphate to remember for your exams is its low solubility. While most metal salts dissolve easily, Calcium Sulphate is only sparingly soluble in water. This property has a fascinating effect during chemical reactions: for instance, if you attempt to react pure Calcium metal with Sulphuric Acid (H₂SO₄), the reaction starts but quickly grinds to a halt. This happens because the resulting CaSO₄ forms an insoluble layer or coating on the surface of the metal, creating a physical barrier that prevents the acid from reaching the remaining metal. This stands in stark contrast to reactions with Hydrochloric Acid (HCl), where the resulting Calcium Chloride is highly soluble and allows the reaction to finish completely.

Feature	Gypsum	Plaster of Paris (PoP)
Chemical Name	Calcium Sulphate Dihydrate	Calcium Sulphate Hemihydrate
Formula	CaSO₄·2H₂O	CaSO₄·½H₂O
State	Hard, crystalline solid	Fine white powder
Primary Use	Cement, Fertilizer, Soil repair	Bone plasters, Casts, Statues

Sources: Geography of India, Resources, p.28; Science class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32-33; Science class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.8

6. Concept of Passivation and Surface Barriers (exam-level)

In chemistry, we often expect a reaction to continue until one of the reactants is completely used up. However, a fascinating phenomenon called passivation can stop a reaction in its tracks. This happens when the very first moments of a reaction create a product that is insoluble and sticks firmly to the surface of the metal. This thin, solid layer acts as a surface barrier, physically separating the underlying metal from the surrounding chemicals. Think of it like a protective 'skin' that prevents the rest of the metal from being 'attacked' by the reagent.

A classic example of this occurs when Calcium reacts with Sulphuric Acid (H₂SO₄). While Calcium is a highly reactive metal, the reaction with sulphuric acid produces very little hydrogen gas. This is because the reaction forms Calcium Sulphate (CaSO₄), which is sparingly soluble in water. As soon as the reaction begins, a layer of solid CaSO₄ coats the calcium metal, shielding it from further contact with the acid. In contrast, when calcium reacts with Hydrochloric Acid (HCl), it forms Calcium Chloride (CaCl₂), which is highly soluble and dissolves away, allowing the reaction to proceed until the metal is gone.

This principle is not always a nuisance; sometimes we use it to our advantage. Aluminium is naturally resistant to corrosion because it forms a thin, tough oxide layer when exposed to air Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.42. We can even enhance this through a process called anodising, where we intentionally use electrolysis to create a thicker, more protective oxide layer on aluminium articles to make them even more durable Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.55.

Reaction Type	Nature of Product Layer	Outcome
Active Reaction (e.g., Fe + HCl)	Soluble or porous	Reaction continues to completion.
Passivation (e.g., Ca + H₂SO₄)	Insoluble and non-porous	Reaction stops prematurely due to a surface barrier.

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.42; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.55

7. Specific Interaction: Calcium vs. Sulphuric Acid (exam-level)

In general chemistry, we learn that when a metal reacts with an acid, it produces a salt and releases hydrogen gas. However, the reaction between Calcium (Ca) and Sulphuric Acid (H₂SO₄) is a fascinating 'exception' that frequently appears in competitive exams. While you might expect a vigorous release of bubbles, the reaction actually grinds to a halt almost as soon as it starts. To understand why, we have to look at the physical properties of the product being formed: Calcium Sulphate (CaSO₄).

When Calcium comes into contact with Sulphuric Acid, the initial reaction is: Ca + H₂SO₄ → CaSO₄ + H₂. Unlike many other metal salts (like Calcium Chloride), Calcium Sulphate is insoluble or only sparingly soluble in water. This lack of solubility is a critical factor in how the reaction progresses—or fails to. As soon as the first few molecules of CaSO₄ are created, they don't dissolve into the liquid. Instead, they precipitate right at the site of the reaction, forming a tough, solid coating over the surface of the calcium metal.

This solid layer acts as a physical barrier (often called a 'passivating' layer). It effectively 'chokes' the metal, preventing the remaining Sulphuric Acid from reaching the unreacted Calcium underneath. Because the acid and metal can no longer touch, the production of Hydrogen gas (H₂) ceases. This behavior stands in stark contrast to the reaction with Hydrochloric Acid (HCl), where the resulting salt (CaCl₂) is highly soluble, leaving the metal surface 'clean' and allowing the reaction to proceed until the metal is entirely consumed.

Acid Used	Salt Formed	Solubility	Reaction Progress
Sulphuric Acid (H₂SO₄)	Calcium Sulphate (CaSO₄)	Insoluble/Low	Stops quickly due to coating
Hydrochloric Acid (HCl)	Calcium Chloride (CaCl₂)	High	Continues until completion

Physical Geography by PMF IAS, Ocean temperature and salinity, p.518 notes that Calcium Sulphate is a constituent of sea salts, but its low solubility compared to chlorides is a defining characteristic in chemical interactions.

Sources: Physical Geography by PMF IAS, Ocean temperature and salinity, p.518

8. Solving the Original PYQ (exam-level)

You have already mastered the foundational rule that Metals react with Dilute Acids to produce a salt and liberate hydrogen gas. However, this UPSC question tests your ability to move beyond general rules and apply Solubility Rules to chemical kinetics. While Calcium is a highly reactive alkaline earth metal, the physical properties of the resulting salt—Calcium Sulphate—completely change the reaction's outcome. This is a classic example of surface passivation, where the product of a reaction prevents the process from continuing, much like the concepts you studied in NCERT Class 10 Science: Metals and Non-metals.

To arrive at the correct answer, reason through the physical interaction happening at the molecular level. When the reaction begins, hydrogen is indeed produced, but because Calcium Sulphate is insoluble, it does not dissolve away into the solution. Instead, it forms a tough, solid coating around the remaining calcium metal. This layer acts as a physical barrier, shielding the metal from the surrounding sulphuric acid. Because the acid can no longer touch the metal, the chemical reaction grinds to a halt almost immediately. Therefore, Statement II provides the specific mechanism that explains the observation in Statement I, leading us directly to (A) as the correct answer.

UPSC often uses Option (B) as a trap, presenting two scientifically accurate facts that have no causal relationship. To avoid this, always ask: "If Statement II were false, would Statement I still happen?" If the answer is no, a causal link exists. Another common pitfall is Option (C), where students might wrongly assume that because Calcium is highly reactive, it must produce a lot of hydrogen regardless of the acid used. Remember, in chemistry, the physical state of the product (solubility) is just as important as the reactivity of the reactants.