Match List-I with List-II and select the correct answer using the code given below the Lists: List-I(Compound) A) Boric acid B) Citric acid C) Magnesium D) Acetic acid List-II(Use) 1) Antiseptic 2) Food preservative 3) Antacid Hydroxide 4) Pickle

1. Understanding the pH Scale and Chemical Nature (basic)

To understand the chemistry of our everyday world, we must first master the **pH scale**. The term 'pH' comes from the German word 'potenz', meaning power, and it specifically measures the 'power of hydrogen' in a solution Science, Class X (NCERT 2025 ed.), Chapter 2, p.25. Simply put, the pH scale tells us how **acidic** or **basic** (alkaline) a substance is. The scale runs from **0 to 14**, where 7 is the neutral midpoint — the pH of pure water. Anything below 7 is acidic, and anything above 7 is basic Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.102. At the molecular level, this scale tracks the concentration of **Hydrogen ions (H⁺)**. There is an inverse relationship here that often trips up students: **as the concentration of hydrogen ions increases, the pH value decreases**. Therefore, a very strong acid like hydrochloric acid might have a pH of 1 or 2, while a strong base like caustic soda, which has a high concentration of **Hydroxide ions (OH⁻)**, would push the scale toward 14 Geography of India, Majid Husain (McGrawHill 9th ed.), Soils, p.3.

Nature of Solution	pH Value	Ion Concentration
Acidic	Less than 7	High H⁺ / Hydronium (H₃O⁺) ions
Neutral	Exactly 7	Balanced H⁺ and OH⁻ ions
Basic (Alkaline)	Greater than 7	High Hydroxide (OH⁻) ions

It is also vital to recognize that the pH scale is **logarithmic**, not linear. This means each whole number change on the scale represents a **tenfold difference** in acidity. For example, a solution with a pH of 4 is ten times more acidic than a solution with a pH of 5, and a hundred times (10 × 10) more acidic than one with a pH of 6 Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.102. Finally, the 'strength' of an acid or base isn't just about its pH value in a moment, but its ability to produce ions; **strong acids** dissociate completely to give more H⁺ ions, while **weak acids** give fewer ions even at the same concentration Science, Class X (NCERT 2025 ed.), Chapter 2, p.26.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.25-26; Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.102; Geography of India, Majid Husain (McGrawHill 9th ed.), Soils, p.3

2. Naturally Occurring Organic Acids (basic)

In our daily lives, we encounter chemistry not just in laboratories, but in our kitchens and the natural world. Most acids found in plants and animals are Organic Acids. Unlike strong mineral acids (like HCl) which ionize completely in water, these are generally weak acids, meaning they only partially dissociate in aqueous solutions Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.73. This characteristic makes them safe enough for us to consume in fruits or use as effective preservatives. Nature produces these acids for various functional reasons—flavoring, preservation, or even biological warfare. For instance, the sharp tang in citrus fruits or the sourness of curd comes from specific organic acids. Below is a list of common natural sources and the acids they contain:

Natural Source	Acid Present
Vinegar	Acetic Acid (Ethanoic Acid)
Orange / Lemon	Citric Acid
Tamarind	Tartaric Acid
Tomato	Oxalic Acid
Sour Milk (Curd)	Lactic Acid
Ant Sting / Nettle Sting	Methanoic Acid (Formic Acid)

Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28 A particularly important application is found in Acetic acid (CH₃COOH). A 5-8% solution of this acid in water is known as vinegar, which acts as a preservative in pickles by inhibiting microbial growth Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.73. Meanwhile, the stinging sensation from an ant bite or a nettle plant is due to Methanoic acid, which the organism uses as a defense mechanism to ward off predators.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.73

3. The Chemistry of Neutralization and Antacids (intermediate)

At its heart, neutralization is a chemical 'handshake' between an acid and a base. When these two meet, they cancel out each other’s extreme properties to produce two relatively harmless substances: salt and water. From a molecular perspective, acids generate hydrogen ions (H⁺) while bases generate hydroxide ions (OH⁻). During neutralization, these ions combine to form H₂O, as detailed in Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.24. This fundamental reaction can be summarized by the general equation: Base + Acid → Salt + Water.

This chemistry isn't just for the lab; it is happening inside your body right now. Your stomach naturally secretes hydrochloric acid (HCl) to help break down food and kill bacteria. However, when we eat spicy food or face stress, the stomach may produce an excess of this acid, leading to indigestion, pain, and irritation. To counter this, we use antacids—which are essentially mild bases. They work by neutralizing the excess acid, bringing the pH levels back to a comfortable range without harming the delicate lining of the stomach (Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.27).

One of the most common antacids is Magnesium hydroxide [Mg(OH)₂], popularly known as Milk of Magnesia. While some bases are highly corrosive and dangerous to touch (known as alkalis if they dissolve in water), Magnesium hydroxide is a mild base, making it safe for medicinal use. The choice of base is critical; we use mild bases for health applications because a strong base would be just as damaging to our tissues as the acid itself (Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.24).

Reactant Type	Example	Role in Indigestion
Acid	Hydrochloric Acid (HCl)	The cause of pain and irritation when in excess.
Base (Antacid)	Magnesium Hydroxide [Mg(OH)₂]	The remedy that neutralizes excess acid.
Product	Magnesium Chloride + Water	The neutral result of the reaction.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.21; Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.24; Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.27

4. Chemistry in Food Preservation (intermediate)

To understand food preservation, we must first understand why food spoils. Microorganisms like bacteria, fungi, and yeasts are responsible for decay, and like all living beings, they carry out their metabolic activities within an optimal pH range Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.34. Chemistry allows us to manipulate this environment. By significantly lowering the pH (increasing acidity), we create a hostile environment where the enzymes of most spoilage-causing microbes cannot function, effectively 'pausing' the decay process.

One of the most common chemicals used for this purpose is Acetic acid (also known as Ethanoic acid). In everyday life, a 5-8% solution of acetic acid in water is known as vinegar. It is a staple in food preservation, particularly in pickling, because its acidic nature inhibits the growth of microorganisms Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.73. Similarly, Citric acid, found naturally in lemons and oranges, acts as both a flavoring agent and a preservative. These are weak organic acids, meaning they do not ionize completely in water like mineral acids (such as Hydrochloric acid), making them safe for human consumption while still being effective at controlling microbial growth Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.73.

Beyond simple pH alteration, some substances provide preservation through specific chemical properties. For instance, Boric acid is utilized for its mild antiseptic and antimicrobial properties. In the broader context of applied chemistry, we also use bases like Magnesium hydroxide (Milk of Magnesia) to neutralize excess acidity in our own bodies—a process known as neutralization where an acid and base react to form salt and water Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.34.

Below is a quick reference for common natural acids used in or found in foods:

Natural Source	Acid Present
Vinegar	Acetic acid
Lemon / Orange	Citric acid
Tamarind	Tartaric acid
Tomato	Oxalic acid
Sour Milk (Curd)	Lactic acid

Sources: Science , class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28, 34; Science , class X (NCERT 2025 ed.), Carbon and its Compounds, p.73

5. Medicinal and Industrial Uses of Boron Compounds (intermediate)

Boron is a unique metalloid that sits between metals and non-metals on the periodic table, giving its compounds versatile properties used in everything from your medicine cabinet to heavy industry. The most famous boron compound is Borax (sodium tetraborate, Na₂B₄O₇·10H₂O). Interestingly, one of the primary ways we manufacture borax is through the use of sodium carbonate, highlighting how industrial chemicals are often interconnected Science, Class X, Acids, Bases and Salts, p.32.

In medicine, boron takes the form of Boric acid (H₃BO₃). Unlike strong mineral acids like HCl, boric acid is a very weak acid, making it safe for human use in diluted forms. It is widely recognized for its mild antiseptic and antifungal properties. You will frequently find it as a key ingredient in eyewashes to treat minor irritations or in dusting powders for fungal infections like athlete’s foot. While other non-metals like Iodine are also used as antiseptics on wounds Science-Class VII, The World of Metals and Non-metals, p.54, boric acid is preferred for delicate tissues like the eyes due to its gentle nature.

Industrially, boron is a game-changer in the world of materials science. Its most significant application is in the production of borosilicate glass (commonly known by the brand name Pyrex). By adding boron oxide to glass, manufacturers create a product that has a very low coefficient of thermal expansion. This means the glass does not crack when subjected to sudden temperature changes, which is why your laboratory beakers and oven-safe dishes are boron-based. Additionally, boron is indispensable in the nuclear industry; the isotope Boron-10 is an exceptional neutron absorber, used in control rods to regulate the speed of nuclear fission reactions.

Compound	Primary Application	Key Property
Boric Acid	Eyewashes & Antiseptics	Mild antimicrobial action; low toxicity.
Borax	Cleaning agents & Glass manufacture	Acts as a flux and water softener.
Boron Carbide	Nuclear Control Rods	High neutron absorption capacity.

Sources: Science, Class X, Acids, Bases and Salts, p.32; Science-Class VII, The World of Metals and Non-metals, p.54

6. Applied Everyday Chemistry: A Synthesis (exam-level)

To master applied chemistry, we must understand how specific chemical properties—like pH levels and biological inhibition—are leveraged to solve everyday problems. These applications generally fall into two categories: maintaining our internal biological balance and protecting our external resources (like food) from decay. Our stomach naturally produces Hydrochloric acid (HCl) to aid digestion. However, overproduction leads to indigestion, causing pain and irritation. To treat this, we use antacids, which are mild bases that perform a neutralization reaction. A classic example is Magnesium hydroxide, commonly known as Milk of Magnesia. Because it is a mild base, it safely neutralizes excess acid without damaging the stomach lining Science, Class X, Acids, Bases and Salts, p.27. In contrast, stronger bases would be corrosive and harmful if ingested. In the realm of food safety and hygiene, we rely on the inhibitory properties of acids and mild antiseptics:

• Acetic acid: Also known as Ethanoic acid, this is the primary component of vinegar. It is highly effective in pickling because its acidic environment prevents the growth of spoilage-causing microorganisms Science, Class X, Carbon and its Compounds, p.71.
• Citric acid: Found naturally in citrus fruits like lemons, it serves as both a flavoring agent and a preservative due to its antioxidant properties.
• Boric acid: Unlike the stronger acids used in industry, Boric acid is a very mild antiseptic. Its gentle nature makes it suitable for medical use in dilute forms, such as antimicrobial eyewashes, to treat minor infections without irritating sensitive ocular tissues.

Comparison of Common Applied Compounds

Compound	Nature	Primary Application	Mechanism
Magnesium Hydroxide	Mild Base	Antacid	Neutralizes excess stomach HCl
Acetic Acid	Weak Organic Acid	Food Preservative	Inhibits microbial growth via low pH
Boric Acid	Weak Inorganic Acid	Mild Antiseptic	Antimicrobial action for sensitive areas

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.27; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.71

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental properties of acids and bases from NCERT Class X Science: Chapter 2, you can see how these building blocks converge in real-world applications. This question requires you to apply the concepts of pH neutralization and microbial inhibition. For instance, knowing that Magnesium Hydroxide is a mild base (Milk of Magnesia) allows you to deduce its role as an Antacid (C-3) to neutralize excess stomach acid. Similarly, recognizing Acetic acid as the primary component of vinegar immediately links it to Pickles (D-4), where it acts as a preservative by lowering pH to levels where bacteria cannot survive.

To arrive at the correct answer, (A) A-1 B-2 C-3 D-4, coach yourself through a process of elimination using the most distinct pairings first. Boric acid is a weak acid frequently used as a mild Antiseptic (A-1), especially in eyewashes, due to its gentle antimicrobial properties. Citric acid, naturally found in citrus fruits, is the industry standard Food preservative (B-2) and flavoring agent. Notice the logic: each compound's specific chemical strength dictates its use, ranging from human biological safety (antiseptics) to culinary preservation.

UPSC often sets distractor traps by pairing chemicals with similar functions. In options (C) and (D), the examiner attempts to confuse you by misaligning the acids. While both Citric and Acetic acids have preservative qualities, the specific association of vinegar with pickling is a classic textbook link you must prioritize. Another common trap is confusing the antiseptic property of Boric acid with more corrosive acids. Success in these Match-the-Following questions comes from identifying the most specific application for each chemical rather than its general class.