Statement I: At high temperature, hydrogen can reduce PbO to elemental lead. Statement II : Hydrogen has great affinity to oxygen

1. Basics of Redox Reactions: Oxidation and Reduction (basic)

At its heart, a chemical reaction is often a reshuffling of atoms. In Redox reactions (a portmanteau of Reduction and Oxidation), this reshuffling involves the movement of oxygen or hydrogen between substances. According to Science, Class X (NCERT 2025 ed.), Chapter 1, p.12, the simplest way to understand this is by tracking oxygen: if a substance gains oxygen during a reaction, it is said to be oxidised; if it loses oxygen, it is said to be reduced. These two processes are like two sides of a single coin—they almost always occur simultaneously. For instance, when hydrogen gas is passed over heated copper(II) oxide, the black coating turns brown as copper is formed: CuO + H₂ → Cu + H₂O. Here, the copper oxide is losing oxygen (being reduced), while the hydrogen is gaining oxygen (being oxidised). Because one reactant is oxidised while the other is reduced, we call these Redox Reactions Science, Class X (NCERT 2025 ed.), Chapter 1, p.12.

Process	Change in Oxygen	Change in Hydrogen
Oxidation	Gain of Oxygen	Loss of Hydrogen
Reduction	Loss of Oxygen	Gain of Hydrogen

This concept isn't just limited to laboratory beakers; it is fundamental to the world around us. In geology, oxidation is a key part of chemical weathering. When minerals containing iron come into contact with atmospheric oxygen and water, they oxidise to form iron oxides (rust), giving certain soils their characteristic red colour Physical Geography by PMF IAS, Geomorphic Movements, p.91. Conversely, in environments where oxygen is absent, such as stagnant water or deep underground, reduction occurs, often changing those red minerals to a greenish or bluish-grey hue.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.12; Physical Geography by PMF IAS, Geomorphic Movements, p.91

2. Identifying Reducing and Oxidizing Agents (basic)

In the world of chemistry, many reactions involve a "tug-of-war" over oxygen atoms or electrons. To master these, we must identify the players: the Oxidizing Agent and the Reducing Agent. Simply put, if a substance gains oxygen during a reaction, it is said to be oxidised, while a substance that loses oxygen is reduced Science, Class X (NCERT 2025 ed.), Chapter 1, p.12. Because these two processes always occur together, we call them redox reactions.

Think of these agents like roles in a transaction. The Oxidizing Agent is the "oxygen provider"; it gives away its oxygen to another substance. In doing so, it loses oxygen and is itself reduced. Conversely, the Reducing Agent is the "oxygen snatcher"; it takes oxygen away from another substance. By gaining that oxygen, it is itself oxidised. For example, when zinc oxide (ZnO) is heated with carbon (C), the carbon acts as a reducing agent because it strips the oxygen away from the zinc to form carbon monoxide Science, Class X (NCERT 2025 ed.), Chapter 3, p.51.

Agent Type	Action in the Reaction	What happens to the Agent?
Reducing Agent	Removes oxygen from another substance.	It gets Oxidised.
Oxidizing Agent	Supplies oxygen to another substance.	It gets Reduced.

Consider the reaction: CuO + H₂ → Cu + H₂O. Here, Copper(II) oxide (CuO) is losing oxygen to become Cu; therefore, it is the oxidizing agent. Hydrogen (H₂) is gaining that oxygen to become H₂O; thus, it is the reducing agent Science, Class X (NCERT 2025 ed.), Chapter 1, p.12. Identifying these agents is a critical skill for understanding how metals are extracted from their ores and how energy is produced in chemical cells.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.12; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.51

3. The Reactivity Series and Hydrogen's Position (intermediate)

In chemistry, we often need to predict which element will "win" a tug-of-war during a reaction. The Reactivity Series is our primary tool for this—it is a vertical arrangement of metals in the order of their decreasing chemical activity. Metals at the top, like Potassium (K) and Sodium (Na), are so reactive they are never found free in nature, while those at the bottom, like Gold (Au), are noble and unreactive Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.45.

A curious feature of this series is the inclusion of Hydrogen [H], which is technically a non-metal. It is included because, like metals, hydrogen can lose an electron to form a positive ion (H⁺). More importantly, hydrogen serves as a critical benchmark for reduction reactions. A metal's position relative to hydrogen determines whether hydrogen can strip oxygen away from that metal's oxide. For instance, even though Lead (Pb) is placed just above hydrogen in some contexts, at elevated temperatures (350–800 °C), hydrogen demonstrates a superior affinity for oxygen. This allows hydrogen to act as a reducing agent, converting Lead(II) oxide (PbO) into pure metallic lead:
PbO + H₂ → Pb + H₂O Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12.

The fundamental driver here is the chemical affinity. When we say an element has a high affinity for oxygen, we mean it forms a more stable bond with oxygen than the competing element does. In the case of lead and hydrogen, the formation of the H-O bond in water releases enough energy to "break" the Pb-O bond, effectively displacing the lead from its own oxide Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.49.

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.45; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.49

4. Principles of Metallurgy: Extracting Metals from Oxides (intermediate)

In nature, metals are rarely found in their pure form; they are usually bonded with oxygen because oxygen is highly reactive and abundant in the Earth's crust Science, Metals and Non-metals, p.50. The process of extracting a pure metal from its oxide is essentially a chemical "tug-of-war" for oxygen, known as reduction. In this context, reduction is the removal of oxygen from a compound to leave the elemental metal behind Science, Metals and Non-metals, p.51.

The method we choose for extraction depends entirely on the metal's position in the reactivity series. Metals with low reactivity (like mercury) can often be reduced by heat alone. However, metals with medium reactivity (like zinc, iron, and lead) require a reducing agent—a substance that has a stronger "desire" or affinity for oxygen than the metal does. While carbon (coke) is the most common industrial reducing agent, hydrogen gas is also highly effective for specific metals like lead (Pb) or copper (Cu). When hydrogen reacts with lead(II) oxide (PbO) at high temperatures, it strips the oxygen away to form water vapor, leaving behind pure lead Science, Chemical Reactions and Equations, p.12.

Metal Reactivity	Extraction Method	Common Reducing Agents
Low (e.g., Au, Ag, Hg)	Found free or reduced by heat	None / Heat
Medium (e.g., Zn, Fe, Pb)	Chemical Reduction	Carbon (C), Carbon Monoxide (CO), Hydrogen (H₂)
High (e.g., Na, Al, Mg)	Electrolytic Reduction	Electricity (Carbon cannot reduce these)

It is crucial to understand why carbon cannot reduce highly reactive metals like Aluminum or Sodium. These metals have a much higher affinity for oxygen than carbon does Science, Metals and Non-metals, p.52. In such cases, the bond between the metal and oxygen is too strong for a chemical agent to break, so we must use electrolysis—using electricity to forcibly pull the atoms apart.

Sources: Science, Metals and Non-metals, p.49-52; Science, Chemical Reactions and Equations, p.12

5. Chemical Affinity and Hydrogen's Interaction with Oxygen (exam-level)

At the heart of many chemical reactions lies the concept of chemical affinity—the inherent tendency of different atomic species to react and form compounds. Hydrogen and oxygen possess an exceptionally high affinity for one another. This attraction is so potent that once they combine to form water (H₂O), the atoms become so "tightly attached" that they cannot be separated by any physical means, such as filtration or boiling Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.124. This fundamental drive to bond with oxygen is what makes hydrogen such a significant player in chemical metallurgy.

Because of this high affinity, hydrogen acts as a reducing agent. In chemistry, a substance that "strips" oxygen away from another substance is known as a reducing agent. When hydrogen gas (H₂) is passed over heated metal oxides, such as Copper(II) oxide (CuO) or Lead(II) oxide (PbO), it literally pulls the oxygen atoms away from the metal to satisfy its own affinity. In this process, the metal oxide undergoes reduction (loss of oxygen), while the hydrogen undergoes oxidation (gain of oxygen) to form water vapor Science, Class X NCERT, Chemical Reactions and Equations, p.12.

To visualize how these roles swap during a reaction, consider the table below:

Substance	Action	Resulting State
Metal Oxide (e.g., PbO)	Loses Oxygen	Reduced to pure metal (Pb)
Hydrogen (H₂)	Gains Oxygen	Oxidized to water (H₂O)

This reaction typically requires elevated temperatures (often between 350°C and 800°C) to provide the necessary activation energy. Even though hydrogen has a high affinity for oxygen, heat is required to break the existing bonds within the metal oxide so that hydrogen can successfully "displace" the metal and claim the oxygen for itself.

Sources: Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.124; Science, Class X NCERT, Chemical Reactions and Equations, p.12

6. The Reduction of Lead(II) Oxide (PbO) by Hydrogen (exam-level)

In the study of chemical reactions, the interaction between Lead(II) oxide (PbO) and Hydrogen (H₂) is a textbook example of a redox (reduction-oxidation) reaction. When dry hydrogen gas is passed over heated lead(II) oxide—a solid typically yellowish or orange in color—a distinct chemical transformation occurs. The hydrogen effectively 'strips' the oxygen away from the lead, leaving behind shiny, greyish beads of metallic lead (Pb) and producing water vapor (H₂O) as the byproduct. This reaction usually requires elevated temperatures, typically between 350°C and 800°C, to overcome the energy barrier for the process to proceed.

To understand this from first principles, we must look at the dual roles played by the reactants. In this equation, PbO + H₂ → Pb + H₂O, lead(II) oxide undergoes reduction because it loses oxygen. Conversely, hydrogen undergoes oxidation because it gains oxygen. While we often see lead displaced by more reactive metals like zinc or iron in aqueous solutions Science, Chemical Reactions and Equations, p.11, this specific gas-solid reaction highlights hydrogen's role as a potent reducing agent. Hydrogen has a remarkably high chemical affinity for oxygen, which serves as the fundamental driving force for this reaction at high temperatures.

Process	Reactant Involved	Change Observed
Reduction	Lead(II) Oxide (PbO)	Loses oxygen to form metallic Lead (Pb)
Oxidation	Hydrogen (H₂)	Gains oxygen to form Water (H₂O)

This principle of using a reducing agent to extract a metal from its oxide is a cornerstone of metallurgy. While carbon is frequently used in industrial settings for this purpose Science, Chemical Reactions and Equations, p.14, the hydrogen-lead reaction serves as a clean, laboratory-scale demonstration of how a gas can chemically 'rescue' a metal from its oxidized state.

Sources: Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.11; Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.14

7. Solving the Original PYQ (exam-level)

Now that you have mastered redox reactions and the reactivity series, this question brings those building blocks together. You have learned that reduction involves the removal of oxygen from a compound, and in this scenario, lead(II) oxide (PbO) is being reduced to elemental lead. The fundamental principle at play here is chemical affinity; for hydrogen to act as a reducing agent, it must "want" the oxygen more than the lead does. This aligns perfectly with your understanding of how reducing agents function based on their relative attraction to oxygen atoms at specific energy levels, as detailed in Science, Class X (NCERT).

To arrive at the correct answer, let's analyze the causal link. Statement I is a factual observation: at high temperatures, hydrogen gas strips oxygen from PbO. Statement II provides the chemical "why." The term affinity refers to the tendency of an atom to combine with another. Because hydrogen has a great affinity for oxygen at elevated temperatures, it successfully displaces the lead. Therefore, Statement II isn't just a true fact—it is the direct chemical driver of the reaction described in Statement I. This confirms that (A) Both the statements are individually true and Statement II is the correct explanation of Statement I is the correct choice.

UPSC often uses Option (B) as a trap, presenting two true facts that are unrelated. To avoid this, always ask: "If Statement II were false, could Statement I still happen?" Since the reduction depends entirely on hydrogen's attraction to oxygen, the link is undeniable. Another common pitfall is doubting Statement I because lead is close to hydrogen in the reactivity series; however, remember that high temperature provides the necessary activation energy to facilitate the process. Options (C) and (D) are quickly eliminated once you recognize hydrogen's primary role as a reducing agent in basic chemical equations.