Consider the following statements : Glass can be etched or scratched by I. diamond. II. hydrofluoric acid. III. aquaregia. IV. cone, sulphuric acid. Which of these statements are correct ?

1. Physical Properties of Solids: Hardness and the Mohs Scale (basic)

In the study of materials, hardness is a fundamental physical property that describes a solid's resistance to being permanently deformed, indented, or scratched. It is important to distinguish hardness from "toughness"; while a glass window is hard (resistant to scratches), it is not tough (it shatters easily). In mineralogy, we primarily use the Mohs Scale of Mineral Hardness, a relative scale from 1 to 10 that determines which substance can scratch another. For instance, a mineral with a higher ranking will always leave a mark on a mineral with a lower ranking.

At the top of this scale is the diamond, the hardest known natural substance Science Class X, Carbon and its Compounds, p.61. Its extreme hardness is legendary; for example, the famous Panna diamonds are valued globally not just for their brilliance, but for this specific physical durability Geography of India, Resources, p.29. On the other end of the spectrum is talc (rank 1), which is so soft it can be scratched by a fingernail. Common materials like quartz (rank 7) are major components of "acidic rocks" like granite, making these rocks very resistant to physical weathering Physical Geography, Types of Rocks & Rock Cycle, p.170.

The secret behind hardness lies in atomic structure. While diamond and graphite are both made entirely of carbon, diamond’s atoms are locked in a rigid, three-dimensional tetrahedral lattice, making it incredibly hard. In contrast, graphite's atoms are arranged in layers that slide over each other, making it smooth and slippery Science Class X, Carbon and its Compounds, p.61. In everyday chemistry, we apply this knowledge to tasks like glass-cutting: because standard glass usually sits between 5.5 and 6.5 on the Mohs scale, any material ranked 7 or above (like a diamond-tipped tool or specialized quartz) can easily etch or cut it.

Mohs Rating	Mineral/Material	Everyday Comparison
1	Talc	Talcum powder
2.5	Finger-nail	Human touch
~5.5 - 6.5	Glass / Steel knife	Common household items
7	Quartz	Sand/Dust particles
10	Diamond	Industrial cutting tools

Sources: Science Class X, Carbon and its Compounds, p.61; Geography of India, Resources, p.29; Physical Geography, Types of Rocks & Rock Cycle, p.170

2. Introduction to Mineral Acids and Their Strengths (basic)

In our study of chemistry, we often encounter substances known as mineral acids (or inorganic acids). These are acids derived from minerals rather than living matter. Common examples include Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), and Nitric acid (HNO₃). What makes these substances 'acidic' is their ability to generate Hydrogen ions (H⁺) when dissolved in water. However, H⁺ ions cannot exist alone; they associate with water molecules to form Hydronium ions (H₃O⁺). It is the presence of these ions that allows an acid solution to conduct electricity, a property not shared by compounds like alcohol or glucose, which do not ionize in water Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.22, 25.

The strength of an acid depends on its degree of ionization. Mineral acids are typically 'strong acids' because they dissociate almost completely in aqueous solutions, providing a high concentration of H₃O⁺ ions. In contrast, organic acids like Ethanoic acid (vinegar) are 'weak acids' because they only partially ionize Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.74. When we dilute a concentrated acid, we are essentially decreasing the concentration of these Hydronium ions per unit volume Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.25.

Handling these acids requires immense care. The process of mixing a concentrated mineral acid with water is highly exothermic (releases heat). If you add water to acid, the sudden heat generated can cause the mixture to splash out or even break the glass container Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.24. Therefore, the golden rule of the laboratory is to always add the acid slowly to water with constant stirring.

Feature	Mineral Acids (e.g., HCl)	Organic Acids (e.g., CH₃COOH)
Origin	Inorganic/Minerals	Plants/Animals
Strength	Generally Strong (highly ionized)	Generally Weak (partially ionized)
Conductivity	High in aqueous solution	Low in aqueous solution

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.22, 24, 25; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.74

3. Composition and Chemistry of Glass (basic)

At its most fundamental level, glass is not a typical solid but an amorphous solid—often described as a 'supercooled liquid.' Unlike crystals, its atoms are not arranged in a perfect repeating pattern. The primary building block of most glass is Silica (Silicon Dioxide, SiO₂), which is the same substance found in quartz and common sand Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. Because pure silica has an extremely high melting point, manufacturers add 'fluxes' like Sodium Carbonate (Washing Soda) to lower the temperature needed for melting Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32. Other materials like Feldspar or Lime (Calcium Oxide) are added to provide chemical durability and prevent the glass from dissolving in water.

Chemically, glass is famous for its inertness, meaning it does not react with most common substances. This is why it is the preferred material for laboratory beakers and bottles. However, there is a major exception: Hydrofluoric acid (HF). While strong acids like Sulfuric acid or even the powerful Aqua Regia (a mix of Nitric and Hydrochloric acids) generally leave glass untouched, HF chemically 'attacks' the silica structure. It reacts with the silicon-oxygen bonds to form Silicon Tetrafluoride (SiF₄), effectively dissolving the surface. This unique reaction is used in industry for etching designs or labels onto glass surfaces.

Physically, glass is quite hard but brittle. On the Mohs scale of mineral hardness, typical glass sits between 5.5 and 7. This means it can be scratched by harder materials. For instance, minerals like Quartz (found in sand) or Diamond (the hardest natural substance at 10 on the Mohs scale) can easily leave a permanent mark on a glass surface. Understanding this chemistry—that glass is a silica-based network resistant to most acids but vulnerable to fluorine-based compounds—is essential for both industrial applications and everyday safety.

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32

4. Allotropes of Carbon: The Unique Strength of Diamond (intermediate)

To understand why diamond is the hardest natural substance known, we must start with the concept of allotropy. Carbon is a versatile element that exists in several different physical forms called allotropes. While their chemical properties are identical, their physical properties — like hardness and electrical conductivity — vary dramatically because of how their atoms are arranged Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.40.

The secret to diamond's extreme strength lies in its bonding architecture. In a diamond crystal, each carbon atom is linked to four other carbon atoms through strong covalent bonds, forming a rigid, three-dimensional tetrahedral structure Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.61. Unlike graphite, where atoms are arranged in flat, sliding layers, the 3D network in diamond creates a massive molecule where every atom is locked firmly in place. This makes diamond incredibly resistant to deformation, allowing it to have a very high melting point and the ability to scratch almost any other material, including glass Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.40.

Feature	Diamond	Graphite
Bonding	Each C bonded to 4 others	Each C bonded to 3 others
Structure	Rigid 3D tetrahedral network	Flat hexagonal layers (2D)
Hardness	Hardest natural substance	Smooth and slippery
Conductivity	Insulator (no free electrons)	Good conductor of electricity

In India, these remarkable stones are found in specific geological formations like the Bhander and Bijwara Series, particularly in the Panna district of Madhya Pradesh. These natural diamonds are prized for their transparency and brilliance, though they can also be synthesized in laboratories by subjecting pure carbon to extreme pressure and temperature, mimicking the conditions deep within the Earth Geography of India, Majid Husain, Resources, p.29 Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.61.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.61; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.40; Geography of India, Majid Husain, Resources, p.29

5. Specialized Acid Mixtures: Aqua Regia and Noble Metals (intermediate)

In the world of chemistry, some metals are so unreactive that they are termed 'Noble Metals'. These include gold, platinum, and silver, which are prized in jewelry precisely because they do not corrode or react with oxygen or common acids under normal conditions Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.57. Because of this chemical stability, these metals are often found in nature in their pure form as 'placer deposits' in riverbeds, having survived eons without being dissolved by water or environmental acids Contemporary India II: Geography, p.107.

However, there is a legendary chemical mixture capable of conquering even the most 'noble' of metals: Aqua Regia. Meaning 'Royal Water' in Latin, Aqua Regia is a freshly prepared, highly corrosive, and fuming liquid. It is unique because neither concentrated Hydrochloric acid (HCl) nor concentrated Nitric acid (HNO₃) can dissolve gold or platinum on their own; it is only their specific combination that achieves this. The mixture works through a 'pincer movement': the Nitric acid acts as a powerful oxidant, while the Hydrochloric acid provides chloride ions that stabilize the metal ions, allowing them to stay in solution.

Feature	Aqua Regia Detail
Composition	Freshly prepared mixture of Conc. HCl and Conc. HNO₃
Ratio	3:1 (3 parts HCl to 1 part HNO₃)
Primary Use	Dissolving Gold and Platinum
Nature	Highly corrosive, yellow-orange fuming liquid Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.44

It is crucial for a UPSC aspirant to distinguish between corrosiveness and specialized reactivity. While Aqua Regia can dissolve gold, it is generally ineffective at etching glass (which requires Hydrofluoric acid). Similarly, while Sulfuric acid is a strong dehydrating agent, it lacks the specific chloride-coordinating ability of Aqua Regia needed to tackle noble metals. Understanding these nuances helps in identifying the correct chemical reagent for specific industrial or laboratory tasks.

Sources: Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.44; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.57; Contemporary India II: Geography, Minerals and Energy Resources, p.107

6. Chemical Etching: The Specific Reactivity of Hydrofluoric Acid (HF) (exam-level)

In the world of chemistry, glass is often seen as the ultimate "neutral" container because of its remarkable resistance to most chemicals. This stability comes from its primary component, Silica (SiO₂). While we learn that metal oxides react with acids to form salt and water Science, Class X, Acids, Bases and Salts, p.21, silica is a non-metallic oxide and is acidic in nature Science, Class X, Acids, Bases and Salts, p.22. Because "like does not react with like," most common acids like Hydrochloric (HCl) or Sulfuric (H₂SO₄) cannot break the incredibly strong Silicon-Oxygen bonds. This is why you see most laboratory acids stored in glass bottles.

However, Hydrofluoric Acid (HF) is the great exception. It possesses a unique, almost "surgical" reactivity toward silica. Instead of a typical acid-base neutralization, HF performs a ligand exchange. The Fluorine atoms have an extraordinary affinity for Silicon, much stronger than the bond Silicon has with Oxygen. When HF touches glass, it reacts to form Silicon Tetrafluoride (SiF₄), which is a volatile gas, and Hexafluorosilicic Acid (H₂SiF₆), which is water-soluble. The chemical equation for this "eating" of glass is: SiO₂ + 4HF → SiF₄ + 2H₂O.

It is important to distinguish this chemical etching from physical scratching. While a diamond can scratch glass because it is the hardest natural material (10 on the Mohs scale vs. 6-7 for glass), HF removes material through a chemical transformation. This is why HF is never stored in glass; it must be kept in specialized plastic (polyethylene) or lead-lined containers. Even Aqua Regia, the famous "king of waters" used to dissolve noble metals like gold, is powerless against the silica structure of glass, highlighting just how specific the HF-Silica relationship is.

Sources: Science, Class X, Acids, Bases and Salts, p.21; Science, Class X, Acids, Bases and Salts, p.22

7. Solving the Original PYQ (exam-level)

This question perfectly synthesizes two fundamental concepts you’ve just mastered: mechanical hardness and specific chemical reactivity. To solve this, you must distinguish between physical "scratching" and chemical "etching." As we covered in the study of carbon allotropes, Diamond sits at the top of the Mohs Scale of Hardness with a value of 10, easily overcoming the structural integrity of glass (which typically ranges from 5.5 to 7). Meanwhile, the chemical etching of glass relies on the unique ability of Hydrofluoric acid (HF) to break the strong Silicon-Oxygen bonds in silica (SiO2) to form volatile silicon tetrafluoride, a property not shared by most other mineral acids. This makes statements I and II the pillars of Option (C).

When walking through the reasoning, think like a scientist in a lab: why do we store most acids in glass bottles? We do so because glass is remarkably inert. Aqua regia (a potent mixture of nitric and hydrochloric acids) is famous for dissolving "noble" metals like gold, but it is stored in glass precisely because it cannot attack the silica lattice. Similarly, Concentrated Sulfuric Acid is a powerful dehydrating agent and a strong acid, but it lacks the specific chemical "key" required to dissolve or etch the silicate matrix. These two options are classic UPSC distractors designed to trap students who equate general acid "strength" with the ability to corrode any surface.

The key takeaway for your exam strategy is to prioritize specificity over intensity. UPSC often tests whether you understand why a reaction happens rather than just knowing that a substance is powerful. While Aqua regia and Sulfuric acid are "stronger" in many chemical contexts, they are useless for glasswork. Always look for the specific relationship between the reagent and the substrate. For a deeper dive into the properties of silicates and the halogen group, you can refer to the detailed chapters in NCERT Class 12 Chemistry: The p-Block Elements.