Silver articles become black after some time when exposed to air because

1. Understanding Oxidation and Reduction (Redox) (basic)

At its simplest level, chemistry is about the movement of atoms and electrons between substances. Two of the most fundamental processes are oxidation and reduction. Historically, these terms were defined by the movement of oxygen: if a substance gains oxygen during a reaction, it is oxidized; if it loses oxygen, it is reduced Science, Class X (NCERT 2025 ed.), Chapter 1, p.12. Because one substance cannot gain oxygen unless another gives it up, these two processes always occur together, forming what we call a Redox reaction (Reduction-Oxidation).

Consider the heating of copper in air. The reddish-brown metal reacts with atmospheric oxygen to form a black coating of copper(II) oxide (2Cu + O₂ → 2CuO). In this case, the copper has been oxidized Science, Class X (NCERT 2025 ed.), Chapter 3, p.41. However, oxidation isn't just about oxygen. In everyday life, we see this principle applied when metals react with other non-metals. For example, silver jewelry often turns black over time. While silver is a noble metal that doesn't easily react with oxygen, it has a high affinity for sulfur. It reacts with traces of hydrogen sulfide (H₂S) in the air to form a thin, black layer of silver sulfide (Ag₂S). Even without a simple gain of oxygen, the silver is undergoing a similar chemical transformation—losing its metallic state to form a compound.

Process	Classic Definition (Oxygen-based)	Everyday Example
Oxidation	Gain of Oxygen	Copper turning black when heated (forming CuO)
Reduction	Loss of Oxygen	Extracting pure metal from an ore (removing oxygen)

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.41-42

2. The Reactivity Series of Metals (basic)

In the world of chemistry, not all metals are created equal. Some, like Potassium, are highly "extroverted" and react explosively even with cold water, while others, like Gold, are "noble" and remain unchanged for centuries. The Reactivity Series (or Activity Series) is a vertical ranking of metals arranged in the order of their decreasing chemical activities Science, Class X (NCERT 2025 ed.), Chapter 3, p.45. This series is not arbitrary; it is built upon displacement experiments where a more reactive metal "kicks out" a less reactive metal from its salt solution.

To understand this practically, imagine a competition. If you place an iron nail into a solution of copper sulfate, the Iron (Fe) will displace the Copper (Cu) because Iron sits higher in the series. The blue solution will fade as Iron takes Copper's place Science, Class X (NCERT 2025 ed.), Chapter 3, p.46. This hierarchy helps us predict how metals will behave with water, acids, and oxygen. Metals at the very top are so reactive they are never found alone in nature, while those at the bottom are frequently found in their native or free state Science, Class X (NCERT 2025 ed.), Chapter 3, p.49.

Position	Metal	Common Behavior
Top	Potassium (K), Sodium (Na)	React violently; stored under oil.
Middle	Zinc (Zn), Iron (Fe), Lead (Pb)	Moderately reactive; react with acids or steam.
Bottom	Silver (Ag), Gold (Au)	Highly stable; found in free state in nature.

One interesting feature of this list is the inclusion of Hydrogen [H]. Even though it is a non-metal, it serves as a crucial reference point. Metals above Hydrogen can displace it from dilute acids to release hydrogen gas, whereas metals below it, like Copper and Silver, cannot Science, Class X (NCERT 2025 ed.), Chapter 3, p.45.

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

3. Introduction to Corrosion (basic)

Corrosion is a naturally occurring process that converts a refined metal into a more chemically stable form, such as an oxide, hydroxide, or sulfide. In simpler terms, it is the gradual 'eating away' of metals when they are attacked by substances in their environment, such as moisture, oxygen, and acids Science, Class X (NCERT 2025 ed.), Chapter 1, p.13. While we often use the term 'rusting' for iron, corrosion is a broader phenomenon that affects many different metals in unique ways based on their chemical reactivity.

The primary driver of corrosion is the metal's desire to return to its original, low-energy state (like the ores they are extracted from). For instance, when Iron is exposed to moist air for a long time, it acquires a coating of a brown, flaky substance called rust. However, different metals react with different atmospheric components. Copper, for example, reacts with the moist carbon dioxide in the air, losing its shiny brown surface to gain a distinctive green coat known as basic copper carbonate Science, Class X (NCERT 2025 ed.), Chapter 3, p.53.

A particularly interesting case is Silver. You might notice that silver ornaments turn black over time. Unlike iron, silver does not react easily with oxygen. Instead, it reacts with sulfur compounds (like Hydrogen Sulfide, H₂S) present in the air to form a thin, black layer of Silver Sulfide (Ag₂S) Science, Class X (NCERT 2025 ed.), Chapter 3, p.53. This demonstrates that corrosion is not a single reaction, but a set of varied chemical interactions between a metal and its specific environment.

Metal	Reacts With	Corrosion Product (Color)
Iron	Oxygen + Moisture	Rust (Reddish-Brown)
Silver	Sulfur compounds (H₂S)	Silver Sulfide (Black)
Copper	Moist Carbon Dioxide	Basic Copper Carbonate (Green)

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.13; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.53

4. Rusting of Iron: The Most Common Corrosion (intermediate)

At its core, corrosion is the gradual deterioration of a metal surface caused by its reaction with the environment—specifically air, moisture, or other chemicals. While many metals corrode, the most famous and economically damaging example is the rusting of iron. When iron objects are left exposed to damp air, they develop a flaky, reddish-brown coating called rust. This isn't just a surface stain; it is a chemical change because a new substance, iron oxide, is formed through the reaction of iron with oxygen and water Science-Class VII, Changes Around Us: Physical and Chemical, p.62. Unlike some metals where the oxide layer forms a protective shield, rust is porous and crumbles away, exposing fresh iron to the elements and leading to deep structural damage over time.

The chemical formula for rust is typically represented as hydrated iron oxide (Fe₂O₃·xH₂O). For rusting to occur, both oxygen and moisture are essential requirements. This process is a significant economic burden globally, as enormous sums are spent annually to repair or replace iron structures like bridges, ships, and railings that have succumbed to this slow decay Science-Class VII, The World of Metals and Non-metals, p.50. To balance the chemical equations involved in such reactions, we must ensure the number of atoms for each element is equal on both the reactant and product sides, a fundamental principle of chemical accounting Science, Class X, Chemical Reactions and Equations, p.4.

Because rusting is so destructive, we use several prevention methods to create a barrier between the metal and the atmosphere. These range from simple physical barriers to more advanced electrochemical techniques:

Method	Mechanism	Common Use
Painting/Oiling	Creates a physical barrier to block air and moisture.	Gates, car engines, tools.
Galvanisation	Coating iron/steel with a thin layer of zinc.	Roofing sheets, water pipes.
Alloying	Mixing iron with other elements (like carbon or chromium) to change its properties.	Stainless steel utensils.

A fascinating aspect of Galvanisation is that the article remains protected even if the zinc coating is scratched or broken. This is because zinc is more reactive than iron and 'sacrifices' itself to corrode first, effectively diverting the chemical attack away from the iron Science, Class X, Metals and Non-metals, p.54.

Sources: Science-Class VII, NCERT, Changes Around Us: Physical and Chemical, p.62; Science-Class VII, NCERT, The World of Metals and Non-metals, p.50; Science, Class X, NCERT, Chemical Reactions and Equations, p.4; Science, Class X, NCERT, Metals and Non-metals, p.54

5. Corrosion of Copper: Basic Carbonate Formation (intermediate)

In our journey through everyday chemistry, we often encounter the aging of metals. While we frequently talk about the "rusting" of iron, the corrosion of copper is a distinct chemical process with its own unique signature: a beautiful, yet chemically complex, green patina. Corrosion, in a broad sense, occurs when a metal is attacked by substances in its environment—such as moisture, oxygen, or acids—leading to the degradation of the metal surface Science, Class X (NCERT 2025 ed.), Chapter 1, p.13.

When copper objects are exposed to the atmosphere for a prolonged period, they don't simply turn brown or black like silver; they react specifically with moist carbon dioxide (CO₂). This reaction causes the copper to slowly lose its characteristic shiny brown surface and gain a distinctive green coat. This green substance is chemically identified as basic copper carbonate Science, Class X (NCERT 2025 ed.), Chapter 3, p.53.

It is important to understand the chemistry behind the term "basic." In this context, it refers to the presence of hydroxide ions (OH⁻) alongside the carbonate ions. The green layer is actually a mixture of copper carbonate (CuCO₃) and copper hydroxide (Cu(OH)₂). This layer acts as a protective shield, preventing the underlying metal from further deep corrosion, unlike the flaky rust on iron which allows the metal to continue decaying.

Metal	Primary Reactant in Air	Corrosion Product	Typical Appearance
Copper	Moist Carbon Dioxide (CO₂)	Basic Copper Carbonate	Green Coating
Silver	Sulfur compounds (H₂S)	Silver Sulfide	Black Tarnish
Iron	Moisture and Oxygen	Hydrated Ferric Oxide (Rust)	Reddish-Brown Flakes

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.13; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.53

6. Noble Metals and Chemical Inertness (intermediate)

In the world of chemistry, noble metals—such as gold, silver, and platinum—occupy a prestigious position at the very bottom of the reactivity series. Unlike base metals like sodium or potassium, which react violently even with cold water Science, class X (NCERT 2025 ed.), Chapter 3, p.43, noble metals are characterized by their chemical inertness. This resistance to chemical change stems from their electronic configuration; they have a very low tendency to lose electrons to achieve stability Science, class X (NCERT 2025 ed.), Chapter 3, p.46. This is why these metals do not react with oxygen even when heated to high temperatures, making them the ideal choice for jewellery and ornamental articles that must retain their luster over centuries Science, class X (NCERT 2025 ed.), Chapter 3, p.42.

However, nobility does not mean absolute immunity to the environment. While silver is highly resistant to common corrosive agents like acetic acid (vinegar) or oxygen, it has a specific vulnerability to sulfur-containing compounds. When silver is exposed to the air, traces of hydrogen sulfide (H₂S) or other sulfurous fumes react with the metal surface. This process, known as tarnishing, results in a thin, black coating of silver sulfide (Ag₂S) Environment and Ecology, Majid Hussain, Chapter 9, p.34. This is chemically distinct from the green 'patina' seen on copper or the reddish-brown rust on iron; it is a specific reaction between silver and sulfur in the presence of moisture and oxygen: 4Ag + 2H₂S + O₂ → 2Ag₂S + 2H₂O.

Understanding this distinction is vital for everyday chemistry. For instance, a copper vessel turns black when heated because it forms copper(II) oxide Science, class X (NCERT 2025 ed.), Chapter 3, p.42, but a silver spoon turns black simply by sitting in a room with polluted air or near sulfur-rich foods like eggs. Because gold is even more inert than silver, it does not react with sulfur under normal conditions, which is why it remains bright and untarnished indefinitely while silver requires periodic cleaning.

Sources: Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.42, 43, 46, 57; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Chapter 9: Distribution of World Natural Resources, p.34

7. Chemistry of Silver Tarnishing (exam-level)

When we think of metal corrosion, our minds often jump to the reddish-brown rust on iron. However, silver tarnishing is a unique chemical phenomenon. Unlike iron or copper, silver is a noble metal, meaning it is highly unreactive. In fact, silver does not react with oxygen in the air even when heated to high temperatures Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.42. So, if oxygen isn't the culprit, why do your silver spoons and jewelry eventually turn a dull, brownish-black?

The answer lies in silver's extreme affinity for sulfur. In our atmosphere, there are trace amounts of sulfur compounds, most notably Hydrogen Sulfide (H₂S)—a gas known for its distinct 'rotten egg' smell Science, class VIII (NCERT 2025 ed.), Nature of Matter, p.128. When silver is exposed to air containing H₂S, a chemical reaction occurs on the surface. The silver atoms react with the sulfur to form a thin, hardy layer of Silver Sulfide (Ag₂S), which is black in color. This process is accelerated by the presence of moisture and oxygen in the air Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.53.

The chemical reaction can be summarized as follows:
4Ag + 2H₂S + O₂ → 2Ag₂S + 2H₂O

It is important to distinguish this from other types of metallic changes you might study in chemistry. To help you keep them straight for the exam, let's compare them:

Metal	Reacts With	Product Formed	Visual Change
Silver	Sulfur (H₂S)	Silver Sulfide (Ag₂S)	Black coating
Copper	Moist CO₂	Basic Copper Carbonate	Green coating
Iron	Moist Air	Hydrated Ferric Oxide	Reddish-brown flakes

Sources: Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.42; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.53; Science, class VIII (NCERT 2025 ed.), Nature of Matter, p.128

8. Solving the Original PYQ (exam-level)

This question perfectly bridges the concepts of chemical reactivity and corrosion. While you have learned that silver is a relatively inert or "noble" metal that resists oxidation—unlike iron which rusts—it possesses a specific chemical affinity for sulfur. As detailed in Science, class X (NCERT 2025 ed.), the blackening you see on old coins or jewelry isn't a sign of the metal breaking down due to oxygen, but rather a reaction with trace amounts of hydrogen sulfide (H2S) present in our atmosphere.

To arrive at the correct answer, you must distinguish between different types of surface coatings. While many metals react with oxygen, silver remains stable; however, when it encounters sulfur, it undergoes a reaction to form a thin layer of silver sulfide (Ag2S). Therefore, (C) Silver reacts with sulphur in the air to form a coating of silver sulphide is the correct choice. Think of this as a specific "chemical signature" for silver—just as green denotes copper carbonate, black on silver denotes the sulfide.

UPSC often uses plausible-sounding distractors to test the depth of your conceptual clarity. Option (A) is a common trap because we often associate corrosion with simple oxidation; however, silver oxide is not the primary cause of this specific black film. Option (B) describes the corrosion process of copper, which forms a green layer of basic copper carbonate, not silver. Option (D) is an outlier, as nitrogen oxides typically contribute to acid rain rather than the direct tarnish of noble metals in ambient air. By identifying the specific reactant—sulfur—you can confidently bypass these atmospheric decoys.