Which one of the following elements can not displace hydrogen gas from a dilute acid?

1. General Chemical Properties of Metals (basic)

To truly master chemistry for the UPSC, we must look beyond how metals look and focus on their chemical personality. At the atomic level, metals are electropositive; they have a natural tendency to lose electrons to form positive ions. This 'giving' nature defines almost all their chemical interactions. For instance, when metals react with oxygen, they generally produce metal oxides which are basic in nature (turning red litmus blue), unlike non-metals which typically form acidic oxides Science-Class VII, The World of Metals and Non-metals, p.54. Some specific metal oxides, like those of Aluminum or Zinc, show both acidic and basic behaviors and are known as amphoteric oxides Science, Class X, Metals and Non-metals, p.55. One of the most important concepts to grasp is the Activity Series (or Reactivity Series). Think of this as a 'power ranking' of metals based on how easily they react with other substances. This series uses Hydrogen as a reference point. When a metal is added to a dilute acid, a 'displacement' battle occurs. If the metal is more reactive than hydrogen, it will displace it, forming a salt and releasing hydrogen gas (H₂). If the metal is less reactive, no reaction occurs. This explains why metals like Magnesium (Mg) and Zinc (Zn) react vigorously with dilute hydrochloric acid, while Copper (Cu), Gold (Au), and Silver (Ag) — which sit below Hydrogen in the series — show no reaction at all Science, Class X, Metals and Non-metals, p.44, 55.

Feature	Metals	Non-Metals
Nature of Oxides	Generally Basic (some Amphoteric)	Generally Acidic or Neutral
Reaction with Dilute Acids	Displace H₂ (if above Hydrogen in series)	Generally do not react to displace H₂
Electron Behavior	Lose electrons (Electropositive)	Gain or share electrons (Electronegative)

Sources: Science-Class VII, The World of Metals and Non-metals, p.54; Science, Class X, Metals and Non-metals, p.44, 55

2. Understanding Acids and Hydrogen Ions (basic)

At its most fundamental level, an acid is defined by its relationship with Hydrogen ions (H⁺). When an acidic substance like Hydrochloric acid (HCl) or Nitric acid (HNO₃) is dissolved in water, it dissociates to release these H⁺ ions. However, a free hydrogen ion is just a bare proton and is too unstable to exist alone in a solution. It immediately bonds with a water molecule to form a Hydronium ion (H₃O⁺). This is why we say that the acidic character of a solution depends entirely on the presence of these ions Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.25.

It is important to understand that having hydrogen in a formula does not automatically make a compound an acid. For instance, glucose (C₆H₁₂O₆) and alcohol (C₂H₅OH) both contain hydrogen atoms, but they do not release them as ions when dissolved in water. Consequently, they do not conduct electricity and do not show acidic properties Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.25. Furthermore, acids only exhibit their "acidic" behavior (like changing the color of litmus paper) in the presence of water. Dry HCl gas, for example, will not change the color of dry litmus paper because it hasn't yet formed the H⁺ ions necessary for the reaction Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.23.

When acids interact with metals, a fascinating "displacement" occurs. If a metal is sufficiently reactive, it can literally push the hydrogen out of the acid, releasing it as Hydrogen gas (H₂) and forming a salt. You can observe this through the formation of bubbles. However, this ability is not universal. The Reactivity Series acts as a leaderboard: metals like Magnesium (Mg), Aluminium (Al), Zinc (Zn), and Iron (Fe) are higher than hydrogen and can displace it. In contrast, metals like Copper (Cu) sit below hydrogen in this hierarchy; they are less reactive and therefore show no reaction with dilute hydrochloric acid, producing no bubbles and no change in temperature Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.44.

Sources: Science, class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.23-25; Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.44

3. Metallurgy: Extraction and Reactivity (intermediate)

In the world of chemistry, not all metals are created equal. Their behavior is dictated by the Reactivity Series (or Activity Series), which is a vertical arrangement of metals in decreasing order of their chemical reactivity. This series is our primary roadmap for understanding how to extract metals from the earth. At the very top, we find "aggressive" metals like Potassium and Sodium that react instantly with air and water. At the very bottom, we find "noble" metals like Gold and Platinum that remain unchanged for centuries Science, Class X, Metals and Non-metals, p.45.

The extraction strategy depends entirely on where a metal sits on this ladder. Metals are generally grouped into three categories based on their position:

Reactivity Level	Representative Metals	Extraction Method
High Reactivity	K, Na, Ca, Mg, Al	Electrolytic Reduction: These metals are so reactive they cannot be reduced by carbon; they require electricity to be separated from their ores.
Medium Reactivity	Zn, Fe, Pb	Reduction with Carbon: Usually found as sulfides or carbonates, they are converted to oxides and then reduced using coke (carbon).
Low Reactivity	Cu, Ag, Au	Native State or Thermal Decomposition: Often found in their free state or extracted by simply heating their ores Science, Class X, Metals and Non-metals, p.50.

A crucial benchmark in this series is Hydrogen. Even though it is a non-metal, it is included because it forms positive ions (H⁺) like metals. This serves as a chemical "litmus test": any metal positioned above Hydrogen in the series is reactive enough to displace Hydrogen gas from dilute acids (like HCl or H₂SO₄). For example, if you drop Zinc into Hydrochloric acid, you will see bubbles of Hydrogen gas: Zn + 2HCl → ZnCl₂ + H₂. However, metals positioned below Hydrogen, such as Copper or Silver, are too stable to force the Hydrogen out of the acid, resulting in no reaction Science, Class X, Metals and Non-metals, p.44.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.44; 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.50

4. Corrosion and Protection of Metals (intermediate)

In our study of chemistry, corrosion is one of the most visible ways we see chemical changes impacting the physical world. At its root, corrosion is the gradual deterioration of a metal surface caused by its reaction with environmental factors like air (oxygen), moisture (water), or various chemicals. Think of it as a metal's journey back to its natural, stable state; most metals exist in nature as ores (oxides or sulfides), and when we refine them into pure metals, they 'want' to react and return to those stable forms.

The most famous form of corrosion is rusting, which specifically refers to the oxidation of iron. When iron is exposed to oxygen and water, it forms a flaky, brown substance called hydrated iron oxide (rust). As noted in Science-Class VII, Changes Around Us, p.62, this is a chemical change because a completely new substance is formed. Other metals also 'corrode' but look different: silver develops a black coating of silver sulfide, and copper develops a characteristic green layer of basic copper carbonate when exposed to moist carbon dioxide Science-Class VII, The World of Metals and Non-metals, p.50.

Because corrosion costs economies billions of dollars in damaged infrastructure, we use several protection strategies. These range from simple barrier methods like painting, oiling, or greasing (which prevent air and moisture from touching the metal) to more advanced chemical methods:

Method	How it Works
Galvanization	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' in place of the iron Science, Class X, Metals and Non-metals, p.54.
Alloying	Mixing a metal with other elements to change its properties. For instance, pure iron is too soft and rusts easily, but mixing it with nickel and chromium creates stainless steel, which is hard and resists rusting Science, Class X, Metals and Non-metals, p.54.
Anodizing	Forming a thick, protective oxide layer on metals like aluminum to prevent further reaction.

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

5. Displacement Reactions (intermediate)

At its core, a displacement reaction is a chemical process where a more reactive element "pushes out" or displaces a less reactive element from its compound. Think of it as a game of chemical musical chairs: if a more "powerful" metal comes along, it will take the place of a "weaker" metal that is already part of a salt solution. This "power" is strictly defined by the Reactivity Series—a ranking of metals from most reactive to least reactive. Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.11

A classic demonstration of this is placing an iron nail into a blue solution of copper sulphate. Because iron (Fe) is higher than copper (Cu) in the reactivity series, it displaces the copper. You will observe two distinct changes: the blue color of the solution fades (as green iron sulphate forms) and the iron nail develops a brownish coating of copper metal. The chemical equation for this is:
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) Similar reactions occur with Zinc and Lead, both of which are more reactive than copper and can displace it from its salts like copper chloride. Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.11

Crucially, this principle also explains how metals interact with dilute acids. In these reactions, the metal attempts to displace hydrogen from the acid (like HCl or H₂SO₄). If the metal is positioned above hydrogen in the reactivity series (like Magnesium, Zinc, or Iron), it successfully displaces it, forming a salt and releasing bubbles of hydrogen gas. However, metals positioned below hydrogen (such as Copper, Silver, or Gold) are not reactive enough to do this. Consequently, when you place copper in dilute hydrochloric acid, no reaction occurs—no bubbles are formed, and no temperature change is recorded. Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.44

Metal	Reaction with Copper Sulphate	Reasoning
Zinc (Zn)	Displacement occurs	Zn is more reactive than Cu
Silver (Ag)	No reaction	Ag is less reactive than Cu

Sources: Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.11; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.44

6. The Reactivity (Activity) Series (exam-level)

At its heart, the Reactivity Series (or Activity Series) is a relative ranking of metals based on their chemical 'eagerness' to lose electrons and form positive ions. Think of it as a hierarchy of chemical aggression: a metal higher in the series is more likely to react with other substances compared to a metal lower down Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.45. This series is not arbitrary; it is meticulously built by observing displacement reactions, where a more reactive metal 'kicks out' a less reactive metal from its salt solution or displaces hydrogen from acids Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.55.

Hydrogen, though a non-metal, is included in the series as a crucial benchmark. Metals positioned above hydrogen (like Potassium, Sodium, Magnesium, and Zinc) are powerful enough to displace hydrogen gas from dilute acids like HCl or H₂SO₄. Conversely, metals below hydrogen (like Copper, Mercury, Silver, and Gold) are relatively inert and will not produce hydrogen gas when placed in dilute acids Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.44. We can see this in action by watching the rate of bubble formation (H₂ gas): Magnesium reacts vigorously and exothermically, whereas Copper shows no reaction and no temperature change at all.

The series also dictates how we find these metals in nature. The 'kings' at the top (K, Na, Ca, Mg, Al) are so reactive that they are never found as free elements; they are always bound in compounds. Those in the middle (Zn, Fe, Pb) are moderately reactive and usually found as oxides or sulphides. The metals at the very bottom, often called 'noble metals' (Au, Ag, Pt), are so unreactive that they are frequently found in their free, metallic state in the Earth's crust Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.50.

Category	Metals	Chemical Behavior
High Reactivity	K, Na, Ca, Mg, Al	Displace H from water/acids; found only in compounds.
Medium Reactivity	Zn, Fe, Pb	React with acids; found as oxides/carbonates/sulphides.
Low Reactivity	Cu, Ag, Au	Do not displace H from acids; often found in free state.

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.44; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.45; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.50; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.55

7. Metal Reactions with Dilute Acids (exam-level)

When we drop a piece of metal into a dilute acid like Hydrochloric acid (HCl) or Sulphuric acid (H₂SO₄), we are essentially witnessing a chemical competition called a displacement reaction. The metal attempts to displace (push out) the Hydrogen from the acid to take its place. The general rule for this interaction is:

Metal + Dilute Acid → Salt + Hydrogen Gas

However, not every metal is "strong" enough to win this competition. Whether a reaction occurs depends entirely on the metal's position in the Reactivity Series (or Activity Series) relative to Hydrogen. Metals that are more reactive than hydrogen sit above it in the series and can easily displace it from acids to produce bubbles of hydrogen gas Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.44.

The vigor of the reaction provides a clear map of metal reactivity. For instance, Magnesium (Mg) reacts the most vigorously, producing rapid effervescence and a significant rise in temperature, indicating the reaction is highly exothermic. In contrast, metals like Zinc (Zn) and Iron (Fe) react more moderately. To confirm that the gas being released is indeed Hydrogen, we use the "pop test": if a burning candle is brought near the gas, it burns with a characteristic pop sound Science, Class VIII (NCERT Revised ed 2025), Nature of Matter, p.122.

Crucially, metals that are less reactive than hydrogen — such as Copper (Cu), Silver (Ag), and Gold (Au) — cannot displace hydrogen from dilute acids. If you place a copper wire in dilute HCl, you will see no bubbles and feel no change in temperature, proving that no reaction has occurred Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.44. This is because copper is lower than hydrogen in the electrochemical series.

Metal	Observation with Dilute HCl	Reactivity Trend
Magnesium (Mg)	Fastest bubble formation; most exothermic	Highest Reactivity
Aluminium (Al)	Vigorous bubbling	High Reactivity
Zinc (Zn)	Steady bubbling	Moderate Reactivity
Iron (Fe)	Slow bubbling	Low Reactivity
Copper (Cu)	No bubbles; no temperature change	No Reaction

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.44; Science, Class VIII (NCERT Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.122

8. Solving the Original PYQ (exam-level)

This question perfectly synthesizes your recent study of the Reactivity Series and the fundamental chemical properties of acids. As you’ve learned, the displacement of hydrogen from a dilute acid is essentially a competition for electrons. For a metal to successfully displace hydrogen gas, it must be more "active" or electropositive than hydrogen itself. This hierarchy is neatly organized in the activity series where Hydrogen serves as the critical reference point. According to Science, class X (NCERT 2025 ed.), only those metals situated above hydrogen have the chemical potential to reduce hydrogen ions (H+) from the acid into hydrogen gas (H2).

Walking through the reasoning, we evaluate the positions of the four given metals relative to the hydrogen benchmark. Magnesium, Zinc, and Iron are all positioned above hydrogen in the activity series, meaning they are sufficiently reactive to displace it. However, Copper sits below hydrogen in the series. Because it is less reactive, it lacks the "reductive power" to push hydrogen out of its acid bond. Therefore, Copper (B) is the correct answer as it remains chemically inert when placed in a dilute acid, showing no visible bubbles or temperature change under standard conditions.

UPSC often uses common industrial metals like Iron and Zinc as distractors because students frequently associate them with laboratory acid tests, which might lead to confusion if the hierarchy isn't clearly memorized. The common trap here is assuming that because a metal is "solid" or "strong," it must react with acid. In reality, the chemical reactivity is what matters. While Magnesium is highly reactive and Iron reacts moderately, only Copper (along with other "noble" metals like silver and gold) fails the displacement test, emphasizing why identifying the hydrogen benchmark is vital for your exam preparation.