Which one of the following elements will replace hydrogen from acids to form salts?

1. Fundamental Properties: Metals vs Non-Metals (basic)

Welcome to your first step in mastering chemical principles! To understand the material world, we begin by classifying elements into two broad categories: metals and non-metals. This classification isn't just about how they look, but how they behave at a fundamental level. Metals are generally lustrous (shiny), malleable (can be beaten into sheets), and ductile (can be drawn into wires) Science, Class X (NCERT 2025 ed.), Chapter 3, p.39. One unique physical property often used to identify metals is sonority—the ability to produce a ringing sound when struck, which is why school bells and ghungroos are made of metal Science - Class VII, NCERT (Revised ed 2025), The World of Metals and Non-metals, p.46.

While physical traits are helpful, the most critical differences are revealed through chemical reactions. A defining characteristic of reactive metals is their ability to displace hydrogen gas from dilute acids. For example, when a reactive metal like Zinc (Zn) reacts with hydrochloric acid (HCl), it "kicks out" the hydrogen to form a salt and hydrogen gas: Zn + 2HCl → ZnCl₂ + H₂. In contrast, non-metals like sulphur (S) or phosphorus (P) generally do not displace hydrogen from dilute acids Science, Class X (NCERT 2025 ed.), Chapter 3, p.56. Instead, non-metals tend to react with hydrogen to form hydrides or react with oxygen to form acidic oxides.

Property	Metals	Non-Metals
Physical State	Mostly solids (except Mercury)	Solids, liquids, or gases
Conductivity	Good conductors of heat/electricity	Poor conductors (except Graphite)
Reaction with Dilute Acids	Displace Hydrogen (if reactive)	Do not displace Hydrogen

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.37, 39, 55, 56; Science - Class VII, NCERT (Revised ed 2025), The World of Metals and Non-metals, p.46

2. Nature of Oxides: Basic, Acidic, and Amphoteric (basic)

To understand how elements behave, we look at their oxides—the compounds formed when an element reacts with oxygen. The chemical nature of these oxides is one of the most reliable ways to distinguish between metals and non-metals. Generally, metals produce basic oxides, while non-metals produce acidic oxides Science, Class X (NCERT 2025 ed.), Chapter 3, p.40. For example, when you burn a magnesium ribbon, it forms a white powder of Magnesium Oxide (MgO). If you dissolve this powder in water and test it with litmus paper, it turns red litmus blue, confirming its basic nature Science-Class VII, NCERT (Revised ed 2025), p.51.

Why do we call them 'basic' or 'acidic'? It comes down to how they react. Basic oxides (like Copper Oxide, CuO) react with dilute acids to produce salt and water, a reaction very similar to how a standard base reacts with an acid Science, Class X (NCERT 2025 ed.), Chapter 2, p.22. Conversely, non-metal oxides like Carbon Dioxide (CO₂) or Sulphur Dioxide (SO₂) react with water to form acidic solutions. However, chemistry always has fascinating exceptions. Some metal oxides, specifically Aluminium Oxide (Al₂O₃) and Zinc Oxide (ZnO), exhibit a 'dual personality.' These are known as amphoteric oxides because they can react with both acids and bases to produce salt and water Science, Class X (NCERT 2025 ed.), Chapter 3, p.41.

Here is a quick comparison to help you categorize them:

Type of Oxide	Formed by	Litmus Test / Reaction	Examples
Basic	Most Metals	Turns red litmus blue; reacts with acids.	MgO, Na₂O, CuO
Acidic	Most Non-metals	Turns blue litmus red; reacts with bases.	SO₂, CO₂, P₂O₅
Amphoteric	Select Metals	Reacts with both acids and bases.	Al₂O₃, ZnO

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.40; Science-Class VII, NCERT (Revised ed 2025), Exploring Substances, p.51; Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.22; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.41

3. The Reactivity (Activity) Series (intermediate)

The Reactivity Series (also known as the Activity Series) is a crucial analytical tool in chemistry. It is essentially a vertical arrangement of metals in the decreasing order of their chemical activities. At the very top, you find highly "aggressive" metals like Potassium (K) and Sodium (Na) that react vigorously even with cold water, while at the bottom reside "noble" metals like Gold (Au) and Platinum (Pt) that are remarkably unreactive Science, Class X (NCERT 2025 ed.), Chapter 3, p. 45. This hierarchy is not arbitrary; it is determined by the ease with which a metal atom can lose electrons to form positive ions.

One of the most practical applications of this series is predicting displacement reactions. The fundamental rule is simple: a more reactive metal can "kick out" or displace a less reactive metal from its salt solution. For instance, if you place an iron nail in a copper sulphate solution, the iron (being higher in the series) will displace the copper, turning the blue solution green. Conversely, a less reactive metal cannot displace a more reactive one Science, Class X (NCERT 2025 ed.), Chapter 3, p. 55.

An interesting feature of this series is the inclusion of Hydrogen, a non-metal. Hydrogen serves as a vital benchmark. Metals positioned above hydrogen in the series have the power to displace hydrogen from dilute acids (like HCl or H₂SO₄) to produce a salt and hydrogen gas (H₂). Metals like Zinc (Zn) or Magnesium (Mg) do this readily. However, metals located below hydrogen—such as Copper (Cu), Mercury (Hg), and Silver (Ag)—are not reactive enough to release hydrogen gas from dilute acids Science, Class X (NCERT 2025 ed.), Chapter 3, p. 55. This chemical logic also explains why gold and silver are often found in their "free state" in nature, while highly reactive metals are always found as compounds Science, Class X (NCERT 2025 ed.), Chapter 3, p. 49.

Position	Metal Example	Reactivity Characteristic
Top	Potassium (K), Sodium (Na)	Most reactive; stored under oil; reacts with cold water.
Middle	Zinc (Zn), Iron (Fe)	Moderately reactive; reacts with acids to release H₂.
Bottom	Gold (Au), Silver (Ag)	Least reactive; found in free state; does not react with dilute acids.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.45; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.55; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.49

4. Corrosion and Prevention Strategies (intermediate)

Welcome back! In our journey through chemistry, we often see metals as strong and permanent. However, most metals are naturally 'restless' in their pure form; they have a tendency to return to the stable, combined states (like oxides or sulphides) in which they are found in the earth. This process of gradual deterioration of metal surfaces caused by the action of air, moisture, or chemicals is what we call corrosion Science-Class VII, The World of Metals and Non-metals, p.50.

While we use the general term 'corrosion' for all metals, the most famous example is the rusting of iron. Rusting is a chemical change because it results in the formation of a completely new substance: iron oxide (Fe₂O₃·xH₂O). This reddish-brown flaky substance doesn't just sit on the surface; it eats away at the metal, costing economies billions in repairs to bridges, ships, and cars Science-Class VII, Changes Around Us: Physical and Chemical, p.62. Other metals show corrosion differently: silver develops a black coating of silver sulphide, and copper develops a characteristic green coat of basic copper carbonate when exposed to moist air Science-Class VII, The World of Metals and Non-metals, p.50.

To fight this, we use various prevention strategies. These range from simple barrier methods to sophisticated electrochemical techniques:

Method	How it Works
Barrier Methods	Painting, oiling, or greasing prevents air and moisture from touching the metal surface.
Galvanisation	Coating iron or steel with a thin layer of zinc. Even if the coating is scratched, the zinc protects the iron because zinc is more reactive and corrodes 'sacrificially' Science, class X, Metals and Non-metals, p.54.
Alloying	Mixing a metal with other metals or non-metals to change its properties. For example, pure iron is too soft and stretches when hot, but mixing it with carbon, chromium, and nickel creates stainless steel, which does not rust Science, class X, Metals and Non-metals, p.54.

By understanding these principles, we can manipulate the chemical environment or the material itself to ensure longevity in infrastructure and technology.

Sources: Science-Class VII, The World of Metals and Non-metals, p.50; Science-Class VII, Changes Around Us: Physical and Chemical, p.62; Science, class X, Metals and Non-metals, p.54

5. Alloys and Modern Metallurgy (intermediate)

In its pure state, a metal often lacks the specific physical properties required for modern industrial applications. For instance, pure gold is too soft for jewelry, and pure iron is prone to rusting and is relatively soft when hot. Alloying is the process of mixing a metal with one or more other elements (metals or non-metals) to create a substance with enhanced properties. An alloy is a homogeneous mixture, meaning the components are so uniformly distributed that they cannot be seen individually Science, Class VIII, p.118. By introducing different-sized atoms into the metal's crystal structure, we can increase hardness, resistance to corrosion, or even alter electrical and thermal conductivity.

Modern engineering relies on specific alloys tailored for unique tasks. The following table highlights some of the most critical alloys used in industry today:

Alloy	Primary Composition	Key Property / Use
Brass	Copper (Cu) + Zinc (Zn)	Highly decorative and corrosion-resistant Science, Class VIII, p.118.
Bronze	Copper (Cu) + Tin (Sn)	Harder than pure copper; used for statues and medals Science, Class VIII, p.118.
Solder	Lead (Pb) + Tin (Sn)	Low melting point; essential for welding electrical wires Science, Class X, p.54.
Stainless Steel	Iron (Fe) + Nickel (Ni) + Chromium (Cr) + Carbon (C)	Does not rust; provides high strength and durability Science, Class VIII, p.118.
Sterling Silver	Silver (Ag) + Copper (Cu)	Hardened silver used for coinage and silverware Environment and Ecology, Majid Hussain, p.34.

Modern Metallurgy has shifted toward precision and efficiency, particularly in steel production. High-quality steel is often produced via the Oxygen Process, where a high-pressure jet of oxygen is blown through molten iron to eliminate impurities Certificate Physical and Human Geography, GC Leong, p.286. For the most specialized, high-grade steels, Electric Furnaces are used. This electrolytic method uses graphite electrodes to pass a current through molten iron, ensuring absolutely no contamination and allowing for the exact chemical tailoring required for high-tech industries Certificate Physical and Human Geography, GC Leong, p.286.

Sources: Science, Class VIII. NCERT (Revised ed 2025), Nature of Matter, p.118; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.54; Environment and Ecology, Majid Hussain, Distribution of World Natural Resources, p.34; Certificate Physical and Human Geography, GC Leong, Manufacturing Industry and The Iron and Steel Industry, p.286

6. Types of Chemical Reactions: Displacement (intermediate)

In the world of chemistry, a displacement reaction is essentially a "test of strength" between elements. It occurs when a more reactive element kicks out a less reactive element from its compound Science, Chemical Reactions and Equations, p.11. You can visualize this like a game of musical chairs where a stronger player arrives and takes the seat of a weaker one, forcing the weaker player to stand alone. This hierarchy of "strength" is determined by the reactivity series.

A classic demonstration of this is placing an iron nail into a blue solution of copper sulphate. Over time, the blue color fades and the nail turns brownish. This is because Iron (Fe) is more reactive than Copper (Cu); it displaces the copper to form iron sulphate, leaving the copper as a solid deposit on the nail Science, Chemical Reactions and Equations, p.11. The chemical equation for this transformation is: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). Similarly, metals like Zinc (Zn) and Lead (Pb) can displace copper because they sit higher on the reactivity scale than copper does.

One of the most important applications of this principle involves dilute acids. Metals that are more reactive than Hydrogen can displace it from acids to produce a salt and hydrogen gas. For instance, when zinc reacts with hydrochloric acid, it displaces the hydrogen to form zinc chloride (a salt) and H₂ gas. However, non-metals like sulphur or phosphorus, or less reactive metals like gold and silver, cannot perform this displacement. Understanding who can displace whom is vital for predicting whether a chemical reaction will actually take place in a test tube or an industrial reactor.

Reaction Type	Mechanism	Example
Single Displacement	One element replaces another in a compound.	Fe + CuSO₄ → FeSO₄ + Cu
Double Displacement	Two compounds exchange ions to form two new compounds Science, Chemical Reactions and Equations, p.14.	Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

Sources: Science, Chemical Reactions and Equations, p.11; Science, Chemical Reactions and Equations, p.14

7. Reaction of Metals with Dilute Acids (exam-level)

When we look at how substances interact, one of the most fundamental reactions in chemistry is the interaction between metals and dilute acids. At its core, this is a displacement reaction. In these reactions, a metal acts like a "chemical bully"—if it is more reactive than hydrogen, it kicks the hydrogen out of the acid and takes its place to form a salt. The displaced hydrogen is then released as hydrogen gas (H₂). The general chemical grammar for this process is:

Metal + Dilute Acid → Salt + Hydrogen gas

For example, when you add zinc (Zn) or iron (Fe) to dilute hydrochloric acid (HCl), you will see bubbles forming. This "fizzing" or effervescence is the visible evidence of gas being produced. In the case of iron, the reaction produces iron chloride and hydrogen gas Science, Class X (NCERT 2025 ed.), Chapter 1, p.15. However, not all elements can do this. Non-metals like sulfur, phosphorus, or silicon typically do not displace hydrogen from dilute acids; this property is largely reserved for metals that sit above hydrogen in the reactivity series.

Identifying the gas produced is a classic laboratory skill. Since hydrogen is highly flammable, we use the "Pop Test": if you bring a burning candle near the mouth of the test tube, the gas will burn with a characteristic pop sound Science, Class VIII (NCERT 2025 ed.), Chapter 8, p.122. This distinguishes it from other gases like oxygen (which makes a flame glow brighter) or carbon dioxide (which extinguishes the flame and turns lime water milky) Science, Class VII (NCERT 2025 ed.), Chapter 5, p.61.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.15; Science, Class VIII (NCERT 2025 ed.), Nature of Matter: Elements, Compounds, and Mixtures, p.122; Science, Class VII (NCERT 2025 ed.), Changes Around Us: Physical and Chemical, p.61

8. Why Non-Metals do not React with Dilute Acids (exam-level)

To understand why non-metals behave differently from metals when facing an acid, we must look at the fundamental nature of chemical displacement. When a substance reacts with a dilute acid, it typically tries to displace hydrogen to form a salt and hydrogen gas (H₂). This process is essentially an electron-transfer (redox) reaction. In a dilute acid, hydrogen exists as positively charged ions (H⁺). To turn these ions into neutral hydrogen gas, electrons must be added to them (a process called reduction).

Metals are naturally electropositive; they have a tendency to lose electrons to achieve a stable electronic configuration Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46. Because metals can "donate" electrons, they can provide the necessary electrons to the H⁺ ions in the acid, converting them into H₂ gas. However, non-metals are electronegative. Their atoms usually have 4 to 7 electrons in their outermost shell and their primary chemical goal is to gain electrons to complete their octet, not lose them Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46. Since a non-metal cannot donate electrons to the H⁺ ions of the acid, it cannot reduce them to hydrogen gas. Consequently, non-metals do not displace hydrogen from dilute acids Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.56.

Instead of displacing hydrogen, non-metals typically interact with other elements by sharing or gaining electrons to form covalent bonds. For example, rather than reacting with the acid to release gas, non-metals may react directly with hydrogen to form hydrides (like NH₃ or H₂S) or react with oxygen to form acidic or neutral oxides Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.56. This distinction is a cornerstone of the Reactivity Series: only elements that are more reactive (better electron donors) than hydrogen can kick it out of its acidic bond.

Feature	Metals (above Hydrogen)	Non-Metals
Electron Behavior	Electron Donors (Electropositive)	Electron Acceptors (Electronegative)
Reaction with Dilute Acid	Displace H₂ gas	No reaction (do not displace H₂)
Reason	Can reduce H⁺ to H₂	Cannot donate electrons to H⁺

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.56

9. Solving the Original PYQ (exam-level)

Congratulations on mastering the building blocks of chemical reactivity! This question perfectly integrates two core concepts you have just covered: the Reactivity Series and the chemical properties of metals versus non-metals. To solve this, you must recall that the displacement of hydrogen from dilute acids is a property unique to metals that are more electropositive than hydrogen itself. As you look at the options, you are essentially scanning for a metal that sits above hydrogen in the activity hierarchy, ready to 'push' it out of its acidic bond to form a salt.

Walking through the logic, Zinc (Zn) is the only element among the choices that fits this criteria. Because Zinc is positioned above hydrogen in the Activity Series, it acts as a reducing agent, donating electrons to the hydrogen ions (H+) in the acid to release hydrogen gas and form a stable salt. This is a classic example of a single displacement reaction that illustrates the fundamental behavior of reactive metals as detailed in Science, Class X (NCERT).

UPSC frequently uses distractors by mixing non-metals with metals to test your conceptual clarity on the periodic table. Sulphur (S), Phosphorus (P), and Silicon (Si) are non-metals (or a metalloid in the case of Silicon). Unlike metals, non-metals do not displace hydrogen from dilute acids because they are electronegative; they seek to gain electrons rather than donate them to hydrogen ions. By recognizing that options A, B, and D lack the metallic character required for this reaction, you can confidently arrive at the correct answer: (C) Zinc (Zn).