Match List-I with List-II and select the correct answer using the code given below the lists: List-I (Substance added) A. Chromium (III) oxide B. Cobalt (II) oxide C. Cuprous oxide D. Manganese dioxide List-II (Colour imparted to glass) 1. Red 2. Violet 3. Green 4. Blue Codes: A B C D

1. Understanding Transition Metals and Their Ions (basic)

To understand why certain stained glass windows or glass bottles look so vibrant, we must first look at the transition metals. In chemistry, elements are the fundamental building blocks of all matter Science, Class VIII NCERT, Nature of Matter, p.123. Transition metals are a specific group of elements (like Iron, Copper, and Cobalt) found in the middle of the periodic table. Unlike simpler metals, they have a unique ability to lose different numbers of electrons to form various ions Science, Class X NCERT, Metals and Non-metals, p.49. These ions aren't just invisible charges; in transition metals, they are the secret behind the world's most beautiful colors. In everyday applications like glassmaking, we add transition metal oxides to molten glass. Because these metals have partially filled electron shells, they absorb specific wavelengths of visible light and reflect others. For instance, while a mixture of iron and sulfur might appear as a simple black and yellow powder Science, Class VIII NCERT, Nature of Matter, p.128, when transition metals are chemically bonded with oxygen in glass, they create a permanent, brilliant hue. The specific color depends entirely on which metal is used and its oxidation state (the number of electrons it has lost). Here is how different transition metal oxides transform the appearance of glass:

Metal Oxide	Resulting Glass Color	Common Use/Note
Chromium (III) Oxide (Cr₂O₃)	Vibrant Green	Commonly seen in green wine or soda bottles.
Cobalt (II) Oxide (CoO)	Deep Blue	A very powerful pigment; even tiny amounts create a rich "Royal Blue."
Cuprous Oxide (Cu₂O)	Red	Note that Cupric Oxide (CuO) produces a different blue-green shade.
Manganese Dioxide (MnO₂)	Violet / Amethyst	In low doses, it acts as a "decolorizer" to neutralize green iron tints.

Sources: Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.123; Science, Class VIII NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.128; Science, Class X NCERT, Metals and Non-metals, p.49

2. Introduction to Glass: Composition and Types (basic)

At its simplest level, glass is not a typical solid but an amorphous solid—often described as a 'supercooled liquid' because its atoms do not follow a rigid, repeating geometric pattern. The primary ingredient in almost all glass is Silica (SiO₂), usually sourced from common sand. However, pure silica has an incredibly high melting point. To make the manufacturing process more energy-efficient, we add 'fluxes' like Sodium Carbonate (Washing Soda), which lowers the melting temperature. This is a critical industrial application of sodium compounds mentioned in Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32. To ensure the glass doesn't dissolve in water, limestone (calcium carbonate) is added, creating the standard 'Soda-lime glass' used in windows and bottles.

While basic glass is transparent, the 'magic' of colored glass comes from adding small amounts of transition metal oxides during the melting process. These oxides absorb specific wavelengths of light, giving the glass its distinct hue. For instance, Chromium (III) oxide (Cr₂O₃) is used to produce vibrant green glass, while Cobalt (II) oxide (CoO) is such a powerful coloring agent that even a tiny amount creates a deep, 'royal' blue. Interestingly, Manganese dioxide (MnO₂) is often used as a 'glassmaker's soap'—it acts as a decolorizer to neutralize the greenish tint caused by iron impurities in sand, though in higher concentrations, it can turn the glass a beautiful violet or purple.

Beyond color, the composition can be adjusted to change the physical properties of glass, such as its weight or how it bends light (refraction). For example, replacing some calcium with Lead oxide produces 'Lead Glass' or 'Crystal.' Lead increases the refractive index of the glass, making it sparkle brilliantly and giving it the 'heaviness' characteristic of high-quality glassware, as noted in Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Distribution of World Natural Resources, p.33. Similarly, adding Boron creates Borosilicate glass, which can withstand rapid temperature changes without cracking, making it ideal for laboratory equipment and kitchenware.

Type of Glass	Key Additive	Primary Property/Use
Soda-Lime Glass	Sodium Carbonate	Common windows, bottles; easy to melt.
Lead Glass	Lead Oxide	High sparkle (refraction); used in decorative 'crystal'.
Borosilicate Glass	Boron Trioxide	Heat resistant; used in labs (Pyrex).

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Distribution of World Natural Resources, p.33

3. Industrial Applications of Metal Oxides (intermediate)

Metal oxides are far more than just the result of corrosion; they are the backbone of various industrial processes. Chemically, most metals react with oxygen to form basic oxides, while specific ones like aluminium oxide (Al₂O₃) and zinc oxide (ZnO) exhibit amphoteric behavior—meaning they can react with both acids and bases to produce salts and water Science, Class X, Metals and Non-metals, p.41. This chemical versatility allows them to serve roles ranging from structural protection to delicate aesthetics.

In heavy industry, metal oxides are the critical intermediate stage in metallurgy. Most ores are converted into oxides because it is easier to extract the pure metal from an oxide than from a carbonate or sulfide. For instance, heating zinc oxide with carbon (a reducing agent) allows us to obtain metallic zinc Science, Class X, Metals and Non-metals, p.51. Additionally, the ability of aluminium to form a tough, thin layer of Al₂O₃ on its surface is industrially exploited to protect the metal from further oxidation, a process often enhanced through anodizing.

One of the most fascinating applications is in the glass and ceramics industry. Transition metal oxides are used as coloring agents because their electronic structures allow them to absorb specific wavelengths of light. By carefully controlling the concentration and oxidation state of the metal, manufacturers can produce a vibrant spectrum of glass:

Metal Oxide	Color Imparted to Glass	Industrial Note
Chromium (III) oxide (Cr₂O₃)	Green	Commonly used for wine and beer bottles.
Cobalt (II) oxide (CoO)	Deep Blue	A very powerful pigment; tiny amounts create "Cobalt Blue."
Cuprous oxide (Cu₂O)	Red	Produces a rich, ruby-red or "blood-red" glass.
Manganese dioxide (MnO₂)	Violet / Colorless	Used to "decolorize" glass by neutralizing iron impurities; in high doses, it turns glass purple.

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.41; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.51

4. Connected Topic: Alloys and Their Composition (intermediate)

In the world of metallurgy, an alloy is a homogeneous mixture of two or more metals, or a metal combined with a non-metal. Pure metals often lack the specific physical properties we need for heavy-duty or everyday use; they might be too soft, too reactive, or prone to rust. By creating an alloy, we can engineer materials that are harder, more resistant to corrosion, and aesthetically pleasing. These mixtures are so uniform that they appear as a single substance to the naked eye Science Class VIII, Nature of Matter, p.118.

Historically, India has a rich tradition of using alloys, referred to in ancient texts like the Susruta Samhita as Mishraloha. One prominent example is Bronze (Kamsya), which ancient Indians used not just for tools but also for medicinal purposes to aid digestion Science Class VIII, Nature of Matter, p.118. In modern times, the use of alloys has expanded into specialized fields. For instance, Titanium alloys are critical for aerospace, while Zirconium is utilized in atomic energy applications due to its unique nuclear properties Science Class VII, The World of Metals and Non-metals, p.54.

One of the most important industrial alloys is Stainless Steel. While iron is strong, it rusts easily. By mixing iron with Nickel and Chromium, and a tiny fraction of Carbon, we create a material that resists heat, shock, and abrasion GC Leong, Manufacturing Industry, p.284. The Chromium forms a microscopic, protective oxide layer on the surface, which is why stainless steel does not corrode or rust like ordinary iron Science Class X, Metals and Non-metals, p.55.

To master the basics, you should be familiar with these common everyday alloys:

Alloy	Primary Composition	Key Property/Use
Brass	Copper (Cu) + Zinc (Zn)	Malleable; used in musical instruments and decorative items.
Bronze	Copper (Cu) + Tin (Sn)	Hard and resistant to corrosion; used for medals and statues.
Stainless Steel	Iron (Fe) + Chromium (Cr) + Nickel (Ni)	Rust-resistant; used for surgical tools and kitchen cutlery.

Sources: Science-Class VII . NCERT(Revised ed 2025), The World of Metals and Non-metals, p.54; Science , class X (NCERT 2025 ed.), Metals and Non-metals, p.55; Science ,Class VIII . NCERT(Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.118; Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.), Manufacturing Industry and The Iron and Steel Industry, p.284

5. Connected Topic: Chemical Pigments and Paints (intermediate)

In the world of applied chemistry, color is rarely just an aesthetic choice; it is the result of fascinating interactions between light and transition metal oxides. While dyes are soluble substances that soak into a material, pigments are insoluble solids that are suspended in a medium (like oil, water, or glass) to provide color. Historically, Indian art relied heavily on natural pigments extracted from minerals and plants. For example, the magnificent mural paintings in the Ajanta caves were created by applying natural pigments onto a prepared surface of volcanic rock plastered with vegetable fibers and lime History, class XI (Tamilnadu state board 2024 ed.), Cultural Development in South India, p.128. Similarly, Indigo has been a globally significant natural blue pigment used for centuries in the textile industry Exploring Society: India and Beyond, Social Science, Class VIII, The Colonial Era in India, p.108.

Modern industrial chemistry uses specific metal oxides to impart consistent colors to glass and ceramics. These transition metals have unique electron configurations that allow them to absorb specific frequencies of visible light. When we add these oxides to molten glass, they act as powerful coloring agents. For instance, a tiny amount of Cobalt (II) oxide (CoO) is enough to produce a deep, royal blue. In contrast, Chromium (III) oxide (Cr₂O₃) is the secret behind vibrant green glass. Interestingly, the oxidation state of the metal matters: Cuprous oxide (Cu₂O) creates a striking red, while Cupric oxide (CuO) tends to produce blue-green shades.

Beyond beauty, pigments and paints serve a vital protective function. We apply paint to iron articles to create a barrier against moisture and air, effectively preventing corrosion (rusting) Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.16. In nature, pigments like chlorophyll are essential for life, though other pigments (red or violet) often hide the green color in some leaves while still assisting in photosynthesis Science-Class VII, Life Processes in Plants, p.142. Even our everyday chemistry utilizes indicators like turmeric or litmus, which are essentially pigments that change color based on the acidity or basicity of a solution Science-Class VII, Exploring Substances, p.19.

Metal Oxide	Color Imparted to Glass
Chromium (III) oxide (Cr₂O₃)	Green
Cobalt (II) oxide (CoO)	Deep Blue
Cuprous oxide (Cu₂O)	Red
Manganese dioxide (MnO₂)	Violet / Amethyst

Sources: Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.19; History, class XI (Tamilnadu state board 2024 ed.), Cultural Development in South India, p.128; Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.16; Exploring Society: India and Beyond, Social Science, Class VIII, The Colonial Era in India, p.108; Science-Class VII, Life Processes in Plants, p.142

6. Glass Coloring Agents: Metal Oxides and Outcomes (exam-level)

At its core, glass is primarily composed of silica (SiO₂), derived from materials like Quartz and Feldspar Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. While pure silica glass is colorless, the vibrant colors we see in stained glass or decorative bottles are achieved by adding specific transition metal oxides during the melting process. These oxides are generally basic in nature Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.41, and they dissolve into the molten glass structure, absorbing specific wavelengths of light to produce distinct colors. The specific color outcome depends heavily on the oxidation state of the metal. For instance, copper can produce two very different results: Cuprous oxide (Cu₂O) creates a deep ruby red glass, while Cupric oxide (CuO) results in blue-green shades. This blue-green characteristic is consistent with how copper(II) compounds behave in other chemical environments, such as when copper oxide reacts with acid to form blue-green solutions Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21.

Below is a quick reference for the most common coloring agents used in the industry:

Metal Oxide	Common Name/Formula	Resulting Color
Chromium (III) Oxide	Cr₂O₃	Vibrant Dark Green
Cobalt (II) Oxide	CoO	Deep Royal Blue
Manganese Dioxide	MnO₂	Violet / Amethyst
Cuprous Oxide	Cu₂O	Red

One of the most fascinating agents is Manganese dioxide (MnO₂). In small quantities, it acts as a "glassmaker's soap"—it is a decolorizer that neutralizes the yellowish-green tint caused by iron impurities. However, when added in higher concentrations, it imparts a distinct purple or violet hue to the glass.

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.41; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21

7. Solving the Original PYQ (exam-level)

Now that you have mastered the chemical properties of transition metal oxides, you can see how they apply directly to industrial chemistry. The color in glass is not random; it arises from the absorption of specific wavelengths of light by the d-electrons of these metal ions. In this question, the building blocks are the unique spectroscopic signatures of each metal: Chromium is synonymous with green (much like the hue in emeralds), while Cobalt is the most powerful and reliable agent for deep blue. Recognizing these two fundamental pairs immediately narrows your choices, demonstrating how conceptual clarity simplifies complex matching tasks.

Let’s walk through the logical elimination used by top scorers. If you pair Cobalt (II) oxide (B) with Blue (4) and Chromium (III) oxide (A) with Green (3), you are instantly led to the correct sequence: (B) 3 4 1 2. To confirm the remaining elements, recall from NCERT Class XII Chemistry that Cuprous oxide (Cu2O) provides a distinct red hue. Finally, while Manganese dioxide is famously used as a "decolorizer" to neutralize iron impurities, in higher concentrations it imparts a beautiful violet or amethyst color, completing the logic perfectly.

UPSC often includes traps to test the depth of your knowledge, such as the confusion between cupric and cuprous oxides. A common mistake is assuming all copper compounds produce blue; however, specifically cuprous oxide is required for red glass. Another trap lies in Manganese; students who only memorize its role as a decolorizer might miss its ability to impart violet. By anchoring your reasoning in these specific oxidation states and their signature color outputs, you can confidently bypass distractors and arrive at the correct answer (B).