Which one of the following compounds is NOT considered an acid ?

1. General Properties of Acids and Bases (basic)

Welcome to your first step in mastering chemical principles! To understand chemistry, we must first distinguish between acids and bases. Traditionally, we identify them by physical properties: acids taste sour and turn blue litmus red, while bases taste bitter, feel soapy, and turn red litmus blue. However, for the UPSC, we need to look deeper into their chemical behavior. There are three main ways scientists define these substances:

• Arrhenius Definition: The simplest view. Acids release hydrogen ions (H⁺) in water (like HCl), and bases release hydroxide ions (OH⁻) (like NaOH).
• Brønsted-Lowry Definition: This focuses on protons. An acid is a proton (H⁺) donor, and a base is a proton acceptor. For example, in a reaction, Ammonia (NH₃) often acts as a base because it "picks up" a proton to become NH₄⁺.
• Lewis Definition: The most broad view. A Lewis Acid is a species that can accept an electron pair (often because it is electron-deficient, like BF₃ or AlCl₃), while a Lewis Base is a species that can donate an electron pair (like the lone pair on the Nitrogen in NH₃).

In our daily lives, we encounter organic acids which are generally weak acids, meaning they do not fully ionize in water. This is distinct from mineral acids like Hydrochloric acid (HCl), which are strong and highly corrosive Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.73. One of the most important reactions to remember is neutralization: when an acid and a base react, they cancel each other out to produce salt and water Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21.

Many common items around you are acidic or basic due to these chemical properties. For instance, the sting of an ant contains methanoic acid, and the tang of a lemon comes from citric acid Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28.

Natural Source	Acid Present
Vinegar	Acetic acid (Ethanoic acid)
Curd (Sour milk)	Lactic acid
Tamarind	Tartaric acid
Lemon/Orange	Citric acid

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.73; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.21; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28

2. The pH Scale and Its Practical Applications (basic)

To understand chemistry in everyday life, we use a ruler called the pH scale. The term 'p' in pH stands for 'potenz', a German word meaning power, specifically referring to the power or concentration of hydrogen ions (H⁺) in a solution Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.25. The scale typically ranges from 0 to 14. It is important to remember an inverse relationship here: the higher the concentration of hydronium ions, the lower the pH value. A solution with a pH of 7 is considered neutral (like pure water), while values below 7 are acidic and values above 7 are basic or alkaline Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.34.

Our biological systems are incredibly sensitive to these numbers. For instance, the human body functions optimally within a very narrow pH range of 7.0 to 7.8. Even the environment reflects this sensitivity; when the pH of rainwater drops below 5.6, it is classified as acid rain, which can be devastating for aquatic life in rivers Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.26. In our own digestive system, the stomach maintains a highly acidic environment to kill bacteria and break down proteins, while the pancreas secretes basic juices to neutralize that acid as food moves into the small intestine Science-Class VII, Life Processes in Animals, p.125-126.

pH Value	Nature of Solution	Ion Concentration Trend
0 to < 7	Acidic	High H⁺ (Hydronium) concentration
7	Neutral	Balanced H⁺ and OH⁻ ions
> 7 to 14	Basic (Alkaline)	High OH⁻ (Hydroxyl) concentration

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.25; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.26; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.34; Science-Class VII (NCERT Revised ed 2025), Life Processes in Animals, p.125; Science-Class VII (NCERT Revised ed 2025), Life Processes in Animals, p.126

3. Salts and Their Commercial Uses (intermediate)

When we think of salts, common table salt (Sodium Chloride, NaCl) is usually the first thing that comes to mind. However, in chemistry, salts are a massive family of ionic compounds formed when an acid reacts with a base. These substances are not just kitchen staples; they are the backbone of modern industry. Common salt acts as a primary raw material for manufacturing essential chemicals like sodium hydroxide, bleaching powder, baking soda, and washing soda Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30.

Two of the most frequently confused salts in the UPSC syllabus are Baking Soda (Sodium Hydrogen Carbonate, NaHCO₃) and Washing Soda (Sodium Carbonate, Na₂CO₃.10H₂O). While they sound similar, their industrial applications are distinct. For instance, Washing Soda is critical for the glass, soap, and paper industries and is the go-to chemical for removing the permanent hardness of water Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32. Baking soda, conversely, is used as an antacid to neutralize stomach acid and as a constituent of baking powder to make cakes fluffy.

Common Name	Chemical Formula	Primary Commercial Use
Bleaching Powder	CaOCl₂	Disinfecting drinking water and bleaching textiles Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30.
Washing Soda	Na₂CO₃.10H₂O	Manufacture of Borax; removing water hardness.
Plaster of Paris	CaSO₄.½H₂O	Supporting fractured bones and making decorative casts Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.33.

An interesting nuance in salt chemistry is the water of crystallisation. In Plaster of Paris (POP), the formula is written with half a molecule of water (CaSO₄.½H₂O). This doesn't mean half a molecule exists in isolation; rather, two formula units of CaSO₄ share one molecule of water. When POP is mixed with water, it rehydrates to form Gypsum (CaSO₄.2H₂O), turning into a hard solid mass used by doctors to set bones Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.33.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.32; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.33

4. Foundations of Chemical Bonding and Electrons (intermediate)

At the heart of all chemical interactions lies a simple quest: the search for stability. Most atoms are naturally 'unstable' because their outermost electron shells are incomplete. To find peace, they strive to achieve a noble gas configuration—a state where their valence shell is completely full, much like Neon or Argon Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46. Atoms achieve this through chemical bonding, either by transferring electrons (ionic bonding) or sharing them (covalent bonding). For example, Carbon has four electrons in its outermost shell; rather than losing or gaining four electrons—which is energetically difficult—it shares them with other atoms to complete its octet Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59.

In covalent molecules, electrons aren't always shared equally, and they don't always come in pairs that are part of a bond. We categorize these as shared pairs (which form the chemical bonds) and lone pairs (valence electrons that are not involved in bonding). For instance, in a Nitrogen molecule (N₂), the atoms share three pairs of electrons to form a triple bond Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60. However, in molecules like Ammonia (NH₃), the nitrogen atom has a 'lone pair' of electrons that is not bonded to any hydrogen. This 'extra' pair is significant because it allows the molecule to interact with other electron-deficient species.

This leads us to the two most critical modern definitions of Acids and Bases. While we often think of acids as 'sour' or bases as 'soapy,' chemists define them by their electronic behavior. A Brønsted-Lowry acid is a substance that can donate a proton (H⁺), whereas a Lewis acid is a broader term for any species that is electron-deficient and can accept an electron pair to fill its octet. Conversely, a Lewis base is a species like Ammonia that possesses a lone pair of electrons ready to be donated to form a new bond.

Theory	Acid Definition	Base Definition
Brønsted-Lowry	Proton (H⁺) Donor	Proton (H⁺) Acceptor
Lewis	Electron Pair Acceptor	Electron Pair Donor

Sources: Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60

5. The Brønsted-Lowry Acid-Base Theory (intermediate)

To understand how chemical reactions work, we must look beyond just what a substance is made of and instead look at what it does. The Brønsted-Lowry Theory provides a functional definition: an acid is a substance that can donate a proton (a hydrogen ion, H⁺), and a base is a substance that can accept a proton. This is why you will often hear this theory referred to as the "proton transfer" theory. Since a hydrogen atom consists of one proton and one electron, losing that electron leaves behind a bare proton (H⁺), which is the primary "currency" exchanged in these reactions Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.26.

One of the most elegant aspects of this theory is the concept of conjugate acid-base pairs. When a Brønsted-Lowry acid loses its proton, the remaining piece of the molecule is capable of taking that proton back—meaning it has become a potential base. For example, when Phenol (C₆H₅OH) acts as an acid and loses its H⁺, it becomes a phenoxide ion, which is its conjugate base. Similarly, when a base like Ammonia (NH₃) accepts a proton, it transforms into the ammonium ion (NH₄⁺), which is its conjugate acid. This relationship shows that acidity and basicity are two sides of the same coin, depending on the movement of a positive charge Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46.

This theory is significantly broader than older models because it doesn't require a substance to be dissolved in water to be classified. It allows us to predict how molecules will behave in gases or organic solvents. While some molecules are strictly donors or acceptors, others are amphoteric, meaning they can act as either an acid or a base depending on what they are reacting with—water is the most famous example of such a "chemical chameleon."

Role	Action	Example
Brønsted-Lowry Acid	Proton (H⁺) Donor	HCl, C₆H₅OH (Phenol)
Brønsted-Lowry Base	Proton (H⁺) Acceptor	NH₃ (Ammonia), OH⁻

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.26; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46

6. Lewis Theory: Electron Pair Donors and Acceptors (exam-level)

In our previous steps, we defined acids and bases by their ability to produce H⁺ or OH⁻ ions in water (Science, Class X, Acids, Bases and Salts, p.24). However, the Lewis Theory provides a much broader perspective by focusing on the "currency" of chemical bonding: electrons. This theory is essential for UPSC aspirants because it explains why certain chemicals act as acids even when they don't have a single Hydrogen atom in them.

According to this theory, a Lewis Acid is an electron-pair acceptor, and a Lewis Base is an electron-pair donor. Think of it as a transaction: the acid is "electron-poor" and looking for stability, while the base is "electron-rich" and looking to share. This behavior is driven by the Octet Rule—the fundamental tendency of atoms to attain a completely filled outer shell to reach a stable noble gas configuration (Science, Class X, Carbon and its Compounds, p.59).

Type	Role	Common Characteristic	Example
Lewis Acid	Accepts Electron Pair	Electron-deficient (incomplete octet)	BF₃, AlCl₃
Lewis Base	Donates Electron Pair	Has at least one "Lone Pair"	NH₃, H₂O

For example, Ammonia (NH₃) is a classic Lewis base. Its nitrogen atom has five valence electrons; three are used for bonding with Hydrogen, leaving one "lone pair" unused. Because it can donate this pair to an electron-deficient molecule like BF₃ (which only has six electrons in its outer shell and needs two more to reach eight), NH₃ acts as the donor. This electronic handshake forms a coordinate covalent bond, allowing both species to achieve stability (Science, Class X, Metals and Non-metals, p.49).

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.24; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.49

7. Organic Compounds: The Acidity of Phenol (exam-level)

To understand why certain organic compounds like Phenol (C₆H₅OH) behave as acids, we must first look at the two primary ways scientists define acidity. The Brønsted-Lowry theory defines an acid as a "proton donor" — a substance that can give up a hydrogen ion (H⁺). In contrast, the Lewis theory defines an acid more broadly as an "electron-pair acceptor." While mineral acids like HCl are strong and dissociate completely in water, organic acids like Ethanoic acid and Phenol are typically weak acids, meaning they only partially ionize Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.73.

Phenol consists of a hydroxyl group (-OH) attached to a benzene ring. It acts as a Brønsted-Lowry acid because the oxygen-hydrogen bond can break, releasing a proton (H⁺) into the solution. Once the proton is lost, it forms a phenoxide ion (C₆H₅O⁻). This process is similar to how other acids produce hydronium ions (H₃O⁺) when they react with water Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.23. Phenol is notably more acidic than regular alcohols because the resulting phenoxide ion is stabilized by the benzene ring, making the "loss" of the proton easier.

However, not all acids possess a hydrogen atom to donate. Molecules like Boron trifluoride (BF₃) and Aluminium chloride (AlCl₃) are classic Lewis acids. These molecules are electron-deficient; they have an incomplete octet and are "hungry" for electrons. They act as acids by accepting an electron pair from another molecule. On the other side of the spectrum is Ammonia (NH₃). Because the nitrogen atom in Ammonia has a lone pair of electrons that it can donate, it functions as a Lewis base. Furthermore, it can accept a proton to form NH₄⁺, making it a Brønsted-Lowry base, the opposite of Phenol.

Substance	Classification	Reasoning
Phenol (C₆H₅OH)	Brønsted-Lowry Acid	Donates a proton (H⁺) to form a phenoxide ion.
BF₃ / AlCl₃	Lewis Acid	Electron-deficient; accepts an electron pair.
Ammonia (NH₃)	Base	Has a lone pair to donate; accepts a proton (H⁺).

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.73; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.23

8. Solving the Original PYQ (exam-level)

Now that you have mastered the dual definitions of acidity—the Brønsted-Lowry (proton-centric) and Lewis (electron-centric) theories—you can see how UPSC tests your ability to apply these beyond simple school-level examples. To solve this, you must apply the building blocks of chemical bonding and molecular structure. This question asks you to identify which substance is not an acid, requiring you to scan each molecule for either a donatable proton or an electron-deficient center that can accept an electron pair.

Walking through the options, BF3 and AlCl3 are quintessential Lewis acids; because their central atoms have incomplete octets, they are "hungry" for electron pairs to achieve stability. Conversely, C6H5OH (Phenol) acts as a Brønsted-Lowry acid because it can readily release a hydrogen ion (H+) to become a stable phenoxide ion. However, NH3 (Ammonia) stands out because the nitrogen atom possesses a lone pair of electrons. This makes it an electron-pair donor—a Lewis base—and it typically accepts protons to form ammonium. Therefore, NH3 is the correct answer as it fundamentally functions as a base in chemical reactions.

A common trap in UPSC General Science is the "hydrogen confusion." Students often mistakenly assume any molecule with hydrogen, like NH3, must be acidic, or that molecules without hydrogen, like BF3, cannot be acids. The exam is testing whether you recognize that acidity is about electron deficiency just as much as it is about proton donation. This conceptual depth, emphasized in NCERT Class 11 Chemistry, is what allows you to bypass these traps and identify the true chemical nature of the compounds provided.