Which of the following is used for writing on glass ?

1. Basics of Chemical Compounds in Everyday Life (basic)

Welcome to your first step in mastering everyday chemistry! To understand how chemicals interact with the world around us, we must first distinguish between elements, mixtures, and compounds. While an element is a pure substance that cannot be broken down further, a compound is formed when two or more elements chemically combine in a fixed proportion Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.131. Unlike a simple mixture, where components retain their individual properties, a compound possesses entirely new characteristics. For instance, while oxygen supports burning and hydrogen is highly flammable, their compound—water (H₂O)—is used to extinguish fires Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.132.

In our daily lives, one of the most stable compounds we encounter is Silicon Dioxide (SiO₂), the primary component of glass. Most acids are unable to react with glass because the bonds between silicon and oxygen are incredibly strong. however, there is one notable exception: Hydrogen Fluoride (HF). When dissolved in water, it becomes hydrofluoric acid. This chemical has the unique ability to break the silicon-oxygen bonds in glass, reacting to form soluble substances like silicon tetrafluoride (SiF₄) or hexafluorosilicic acid (H₂SiF₆). This process, known as etching, essentially dissolves a controlled layer of the glass surface to create permanent markings or frosted designs.

Substance Type	Key Characteristic	Everyday Example
Element	Pure substance; cannot be broken down chemically.	Iron (Fe), Oxygen (O₂)
Compound	Elements chemically bonded in fixed ratios; new properties.	Water (H₂O), Salt (NaCl)
Mixture	Physical combination; components keep their properties.	Air, Brass, Saltwater

Understanding these chemical behaviors is not just academic; it has shaped human history. For example, Acharya Prafulla Chandra Ray, the Father of Modern Indian Chemistry, used his deep understanding of chemical compounds to establish India's first pharmaceutical company, bridging the gap between theoretical science and practical application Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.17.

Sources: Science, Class VIII . NCERT(Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.131; Science, Class VIII . NCERT(Revised ed 2025), Nature of Matter: Elements, Compounds, and Mixtures, p.132; Science-Class VII . NCERT(Revised ed 2025), Exploring Substances: Acidic, Basic, and Neutral, p.17

2. Silicon and Silicates: The Building Blocks of Glass (basic)

To understand the chemistry of glass, we must first look at the element **Silicon (Si)**. Silicon is the second most abundant element in the Earth's crust, but you won't find it sitting around in its pure form. Instead, it has a high affinity for oxygen, forming **Silicon Dioxide (SiO₂)**, also known as **Silica**. When silica combines with other elements like aluminium or magnesium, it forms **silicates**, which are the fundamental building blocks of most rocks and minerals. For instance, our continents are largely made of lighter silicates (Silicon + Aluminium, often called *Sial*), while the ocean floors consist of denser silicates (Silicon + Magnesium, or *Sima*) Physical Geography by PMF IAS, Earths Interior, p.53.

Glass is essentially a high-purity form of silica that has been melted and cooled in a way that prevents it from forming a crystalline structure. This material is incredibly valued in chemistry because of its **chemical stability**. In its solid state, silica has a fixed chemical composition Science, Class VIII. NCERT, Nature of Matter, p.130. The bonds between Silicon and Oxygen are exceptionally strong, making glass resistant to almost all common chemical reactions and acids. This is why we use glass beakers and flasks in laboratories — they don't react with the substances we put inside them.

However, every "fortress" has a weakness. While glass resists standard acids like Hydrochloric or Sulfuric acid, it meets its match in **Hydrogen Fluoride (HF)**. When glass is exposed to hydrofluoric acid, a **chemical reaction** occurs — a process where the original substance is transformed into something entirely new Science-Class VII. NCERT, Changes Around Us, p.60. The HF molecules are strong enough to break the sturdy Silicon-Oxygen bonds, converting the solid silica into soluble substances like Silicon Tetrafluoride (SiF₄). This unique reactivity is what allows artists and engineers to "etch" or permanently mark the surface of glass.

Feature	Sial (Continental)	Sima (Oceanic)
Primary Elements	Silica + Aluminium	Silica + Magnesium
Relative Density	Lighter	Heavier

Sources: Physical Geography by PMF IAS, Earths Interior, p.53; Science, Class VIII. NCERT, Nature of Matter: Elements, Compounds, and Mixtures, p.130; Science-Class VII. NCERT, Changes Around Us: Physical and Chemical, p.60

3. Understanding Acid Strength and Reactivity (intermediate)

In chemistry, the "strength" of an acid is not about how concentrated it is in a bottle, but how effectively it releases hydrogen ions (H⁺) when dissolved in water. When an acid like Hydrochloric acid (HCl) enters water, it ionizes completely, meaning almost every molecule splits to release H⁺. These are known as strong acids. In contrast, organic acids like Ethanoic acid (commonly known as acetic acid or vinegar) only partially ionize. Even at the same concentration, acetic acid produces fewer H⁺ ions, making it a weak acid Science, Class X (NCERT), Acids, Bases and Salts, p. 26.

While strength tells us about ion concentration, reactivity tells us how an acid interacts with specific materials. Most acids are famous for corroding metals, but they are often stored in glass bottles because glass—primarily made of silicon dioxide (SiO₂)—is incredibly stable and non-reactive with common mineral acids. However, Hydrofluoric acid (HF) is a unique exception. Despite being classified as a weak acid in terms of ionization, it possesses a specific affinity for silicon. It chemically attacks the silicon-oxygen bonds in glass to form soluble silicon tetrafluoride (SiF₄) or hexafluorosilicic acid (H₂SiF₆). This unique reactivity is the basis for glass etching, allowing us to create permanent frosted designs or markings on glass surfaces.

Feature	Strong Acids (e.g., HCl)	Weak Acids (e.g., Acetic Acid)
Ionization	Complete dissociation in water	Partial dissociation in water
Natural Sources	Mainly industrial/mineral	Vinegar, Curd, Citrus fruits Science, Class X (NCERT), Acids, Bases and Salts, p. 28

Interestingly, pure acetic acid has a melting point of 290 K, which is high enough that it often freezes into ice-like crystals in cold climates. This is why pure ethanoic acid is frequently referred to as glacial acetic acid Science, Class X (NCERT), Carbon and its Compounds, p. 73. Understanding these variations in strength and reactivity helps us choose the right acid for the right job—whether it is preserving pickles with mild vinegar or etching industrial glass with specialized hydrofluoric acid.

Sources: Science, Class X (NCERT), Acids, Bases and Salts, p.26; Science, Class X (NCERT), Acids, Bases and Salts, p.28; Science, Class X (NCERT), Carbon and its Compounds, p.73

4. Carbon Allotropes: Graphite and Diamond (intermediate)

In the world of chemistry, allotropy is a fascinating phenomenon where a single element can exist in two or more different physical forms. Carbon is the ultimate master of this. Even though Diamond and Graphite are made of nothing but pure carbon atoms, their physical properties are worlds apart because of how those atoms are arranged spatially Science, Class X, Metals and Non-metals, p.40.

In a Diamond, each carbon atom is covalently bonded to four other carbon atoms, creating a rigid, three-dimensional tetrahedral structure Science, Class X, Carbon and its Compounds, p.61. This massive network of strong bonds makes diamond the hardest natural substance known. Because all four valence electrons of carbon are locked in tight bonds, there are no free electrons to carry a charge, making diamond a non-conductor of electricity Science, Class X, Metals and Non-metals, p.40.

In contrast, Graphite has a layered structure. Each carbon atom is bonded to only three other carbon atoms in the same plane, forming a hexagonal array Science, Class X, Carbon and its Compounds, p.61. These layers are held together by weak forces, allowing them to slide over one another, which is why graphite feels smooth and slippery—perfect for use as a lubricant or in pencil leads. Crucially, because only three electrons are used for bonding, the fourth valence electron is "free" to move within the layers. This makes graphite an exceptional conductor of electricity, a rare trait for a non-metal Science, Class X, Metals and Non-metals, p.55.

Feature	Diamond	Graphite
Bonding	Each C bonded to 4 others	Each C bonded to 3 others
Structure	Rigid 3D Tetrahedral	Flat Hexagonal Layers
Hardness	Extremely hard	Soft and slippery
Conductivity	Insulator (Non-conductor)	Good Conductor of electricity

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.40, 55; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.61

5. Halogens and their Hydrides (intermediate)

In the world of chemistry, Halogens (Group 17 elements: Fluorine, Chlorine, Bromine, and Iodine) are known as 'salt-formers' because of their high reactivity with metals. When these halogens bond with Hydrogen, they form Hydrogen Halides (HX), which are commonly referred to as halogen hydrides. While these are covalent gases in their pure form, they behave as strong acids when dissolved in water. For instance, Hydrogen Chloride (HCl) becomes hydrochloric acid only in the presence of water, where it dissociates to release H⁺ ions Science , class X (NCERT 2025 ed.) | Acids, Bases and Salts | p.23. Interestingly, as we move down the group from Fluorine to Iodine, the molecular mass increases, leading to a gradation in physical properties such as boiling points Science , class X (NCERT 2025 ed.) | Carbon and its Compounds | p.67.

While most halogen hydrides like HCl and HBr are famous for their industrial uses, Hydrogen Fluoride (HF) holds a unique and somewhat paradoxical position. Despite being a 'weak' acid in terms of its dissociation in water compared to its cousins, it is incredibly aggressive toward certain materials. Specifically, HF has the rare ability to react with Silicon Dioxide (SiO₂), the primary component of glass. While glass is chemically resistant to almost all other common acids (which is why we store HCl or HNO₃ in glass bottles), HF reacts with the silicon-oxygen bonds to form soluble products like silicon tetrafluoride (SiF₄). This specific chemical property makes HF the essential agent for glass etching and industrial surface treatments.

Hydride	Acid Name	Notable Property
HF	Hydrofluoric Acid	Unique ability to etch/dissolve glass (SiO₂)
HCl	Hydrochloric Acid	Highly corrosive; essential for industrial cleaning
HBr	Hydrobromic Acid	Stronger acid than HCl; used in organic synthesis
HI	Hydroiodic Acid	The strongest acid in this group; high molecular mass

Sources: Science , class X (NCERT 2025 ed.), Acids, Bases and Salts, p.23; Science , class X (NCERT 2025 ed.), Carbon and its Compounds, p.67

6. Types of Glass and Industrial Processing (exam-level)

At its heart, glass is an amorphous (non-crystalline) solid primarily composed of Silicon Dioxide (SiO₂). In the natural world, this silica is sourced from minerals like Quartz and Feldspar, which are the building blocks of the Earth's crust Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.175. During industrial processing, pure silica is mixed with other chemicals to lower its melting point and improve its durability. For example, Sodium Carbonate (Washing Soda) is a critical ingredient in the manufacture of common glass Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32. Depending on the additives used, we get different varieties of glass suited for specific needs:

Glass Type	Key Ingredient	Primary Characteristic
Soda-lime Glass	Sodium Carbonate	The most common glass used for windows and bottles.
Borosilicate Glass	Boron Trioxide (Borax)	High resistance to thermal shock; used in lab equipment.
Flint Glass	Lead Oxide	High refractive index; used for optical lenses and prisms.

One of the most unique aspects of glass is its chemical inertness. Because it is so stable, it does not react with most common acids, which is why glass is the gold standard for laboratory containers. However, there is a notable exception: Hydrofluoric Acid (HF). While other acids are "repelled" by the silicon-oxygen bonds, HF specifically attacks them, reacting with the SiO₂ to form soluble Silicon Tetrafluoride (SiF₄). This unique chemical reaction allows us to etch or "write" on glass by dissolving controlled areas of the surface to create frosted designs or permanent markings.

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

7. Hydrofluoric Acid: The Glass Dissolver (exam-level)

In our previous discussions on chemical reactions, we saw how acids like dilute sulphuric acid react with metals to produce hydrogen gas Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.2. However, glass is a remarkably different story. Most laboratory acids—hydrochloric, sulphuric, or nitric—are safely stored in glass bottles because glass is primarily composed of Silicon Dioxide (SiO₂), a compound that is chemically inert and highly resistant to most forms of chemical attack. But there is one notable exception that every chemistry student must remember: Hydrofluoric Acid (HF).

Hydrofluoric acid is unique because of the extreme affinity between Fluorine and Silicon. While other acids fail to break the strong silicon-oxygen bonds in glass, HF reacts with the silicon dioxide to form Silicon Tetrafluoride (SiF₄), which is a gas, or Hexafluorosilicic Acid (H₂SiF₆), which is water-soluble. The chemical equation looks like this:
SiO₂ + 4HF → SiF₄ + 2H₂O
This reaction essentially "eats away" or dissolves the surface of the glass, a process known as chemical etching. It is because of this reactivity that hydrofluoric acid can never be stored in glass containers; it must be kept in specialized plastic containers, such as those made of polyethylene or Teflon, which do not contain silicon.

In the world of applied chemistry, this property is harnessed for glass etching and frosting. When HF is applied to glass, it creates a rough, opaque surface by dissolving a thin layer, allowing for the creation of permanent markings, decorative patterns, or the graduation marks you see on laboratory burettes and pipettes. It is far more effective than mechanical scratching because it provides a uniform, permanent finish at the molecular level.

Feature	Standard Acids (HCl, H₂SO₄)	Hydrofluoric Acid (HF)
Reaction with Glass	No reaction (Inert)	Dissolves Silicon Dioxide (SiO₂)
Storage	Glass or Plastic bottles	Strictly Plastic (Polyethylene)
Primary Use in Glass	Safe storage medium	Etching and Frosting

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.2; Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.15

8. Solving the Original PYQ (exam-level)

This question perfectly synthesizes your recent lessons on the chemical properties of Halogens and the structural stability of Silicon Dioxide (SiO2). You have learned that glass is essentially a network of silicon and oxygen bonds, which makes it remarkably resistant to most strong acids like Hydrochloric or Sulfuric acid. However, this question tests your knowledge of the unique exception to that rule. To 'write' on glass in a permanent industrial sense, we require a chemical process called etching, which involves dissolving the surface layer to create a visible mark.

To arrive at the correct answer, (C) Hydrogen fluoride, you must identify the reagent capable of breaking the robust Si-O bond. When hydrofluoric acid (the aqueous form of HF) comes into contact with glass, it reacts to form silicon tetrafluoride (SiF4) or hexafluorosilicic acid. This specific reactivity allows it to 'eat' into the glass, leaving behind a frosted or engraved pattern. Think of this as a chemical drill rather than a simple pen; while other acids remain inert against the glass, the high electronegativity and specific affinity of Fluorine for Silicon allow this reaction to proceed, as noted in ScienceDirect.

UPSC often includes distractors to test the depth of your conceptual clarity. Graphite (B) is a common trap because students associate it with writing (pencils), but it only leaves a superficial carbon trail that can be easily wiped off glass. Silicon (A) is the primary component of glass itself, and Hydrogen iodide (D), though a fellow halogen acid, lacks the specific bond energy and reactivity required to etch silicon dioxide. Therefore, only the unique chemical interaction between fluorine and silicon makes Hydrogen fluoride the indispensable choice for permanent glass marking.