Which one among the following is the correct order of strenght of acids?

1. Fundamental Definitions of Acids and Bases (basic)

To understand chemistry at its most fundamental level, we must look at how substances behave when they dissolve in water. At the heart of this behavior are Acids and Bases. An acid is traditionally defined by its ability to release hydrogen ions (H⁺) in an aqueous solution. Conversely, a base is a substance that generates hydroxide ions (OH⁻) in water Science, Chapter 2, p.24. This ion-releasing property is what drives their chemical reactivity, including the classic neutralization reaction where an acid and a base combine to form salt and water.

It is important to distinguish between a general base and an alkali. While many substances can act as bases, not all of them can dissolve in water. An alkali is specifically a base that is soluble in water; they are characterized by a soapy touch, a bitter taste, and corrosive properties Science, Chapter 2, p.24. When we talk about the "strength" of these substances, we aren't just talking about how concentrated they are in a bottle. Instead, strength refers to the extent of dissociation—essentially, how many ions the substance produces. A strong acid, like hydrochloric acid (HCl), dissociates almost completely to yield a high concentration of H⁺ ions, whereas a weak acid, such as acetic acid (CH₃COOH), releases far fewer ions for the same concentration Science, Chapter 2, p.26.

Generally, mineral acids (derived from inorganic minerals) tend to be much stronger than organic acids (found in plants and animals). This difference in ion production is why a 1.0 M solution of a mineral acid is significantly more acidic than a 1.0 M solution of an organic acid.

Feature	Acids	Bases
Key Ion Produced	Hydrogen ions (H⁺)	Hydroxide ions (OH⁻)
Common Taste	Sour	Bitter
Solubility Distinction	Most common laboratory acids are water-soluble.	Water-soluble bases are called Alkalis.

Sources: Science, Chapter 2: Acids, Bases and Salts, p.24; Science, Chapter 2: Acids, Bases and Salts, p.26

2. The pH Scale and Ionization (basic)

To understand the chemistry of everyday life, from the acidity of rain to the fluids in our stomach, we use a specialized yardstick called the pH scale. Developed to measure the concentration of hydrogen ions (H⁺) in a solution, the scale typically ranges from 0 to 14. The "p" in pH stands for potenz, a German word meaning "power," signifying that we are measuring the "power of hydrogen." A solution with a pH of 7 is considered neutral (like pure water), while values less than 7 are acidic and values greater than 7 are basic or alkaline Science, Class X (NCERT 2025 ed.), Chapter 2, p. 25.

The most critical feature of the pH scale for a UPSC aspirant to grasp is that it is logarithmic. This means that each whole number change on the scale represents a tenfold change in the concentration of hydrogen ions. For example, a solution with a pH of 4 is ten times more acidic than one with a pH of 5, and one hundred times (10 × 10) more acidic than one with a pH of 6 Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p. 102. It is an inverse relationship: as the concentration of hydrogen ions (or hydronium ions, H₃O⁺) increases, the pH value decreases.

The strength of an acid or base is determined by its ionization—the process by which molecules break apart into ions when dissolved in water. Strong acids, such as hydrochloric acid (HCl) or sulfuric acid (H₂SO₄), dissociate almost completely, releasing a high concentration of H⁺ ions. In contrast, weak acids, like acetic acid (found in vinegar), only partially ionize, leaving many molecules intact. On the basic side of the scale (pH 7 to 14), an increase in pH represents a higher concentration of hydroxide ions (OH⁻) and a corresponding decrease in H⁺ ions 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

3. Classification: Mineral Acids vs. Organic Acids (basic)

When we classify acids based on their origin, we divide them into two primary families: Mineral Acids and Organic Acids. Mineral acids (also called inorganic acids) are derived from the minerals of the earth. They are typically man-made and do not contain carbon-hydrogen bonds. In contrast, organic acids occur naturally in plants and animals. These are characterized by the presence of carbon, often within a specific group called carboxylic acids Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.73.

The defining difference for a chemist—and for your exams—is how these acids behave in water. The strength of an acid depends on its ability to ionize and produce hydrogen ions (H⁺). Mineral acids like hydrochloric acid (HCl) or sulfuric acid (H₂SO₄) are generally strong acids because they ionize almost completely in solution. Organic acids, such as acetic acid (found in vinegar) or citric acid (found in lemons), are weak acids because only a small fraction of their molecules break apart to release H⁺ ions Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.26.

Feature	Mineral Acids	Organic Acids
Source	Earth minerals (Inorganic)	Living organisms (Plants/Animals)
Carbon	Usually do not contain carbon	Always contain carbon
Strength	Generally Strong (High H⁺ yield)	Generally Weak (Low H⁺ yield)
Examples	HCl, H₂SO₄, HNO₃	Acetic acid, Citric acid, Lactic acid

Nature is full of these organic acids! For instance, methanoic acid is found in ant stings and nettle leaves, while tartaric acid gives tamarind its characteristic tang Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28. Even though we consume many organic acids in our food, mineral acids are often too corrosive and reactive for such uses due to their high degree of ionization.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.73; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.26; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28

4. Chemical Properties: Salts and Neutralization (intermediate)

At its heart, a neutralization reaction is a chemical 'handshake' between an acid and a base. When these two substances interact, they effectively cancel out each other’s extreme properties (acidity and alkalinity) to produce a salt and water. This process is generally exothermic, meaning it is accompanied by the evolution or release of heat Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.18. From a molecular perspective, the hydrogen ions (H⁺) from the acid combine with the hydroxide ions (OH⁻) from the base to form H₂O, while the remaining ions aggregate to form the salt.

The general equation for this transformation is: Base + Acid → Salt + Water. For instance, when Sodium hydroxide (NaOH) reacts with Hydrochloric acid (HCl), the result is Sodium chloride (NaCl) and water (H₂O) Science, class X (2025 ed.), Acids, Bases and Salts, p.21. It is important to note that the term 'salt' in chemistry does not just refer to the table salt we eat; it describes a whole class of ionic compounds. Some salts are produced through specific industrial processes, such as bleaching powder [Ca(OCl)₂], which is manufactured by the action of chlorine gas on dry slaked lime [Ca(OH)₂] Science, class X (2025 ed.), Acids, Bases and Salts, p.30.

Neutralization is not just a laboratory concept; it is a vital tool for managing day-to-day problems. For example, the sting of a red ant contains formic acid, which can be neutralized using a mild base like moist baking soda. Similarly, if the soil becomes too acidic due to excessive use of chemical fertilizers, farmers treat it with bases like quicklime (calcium oxide) or slaked lime to restore the pH balance necessary for plant growth Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.19.

Sources: Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.18-19; Science, class X (2025 ed.), Acids, Bases and Salts, p.21, 30

5. Chemistry in Everyday Life: pH Sensitivity (intermediate)

To understand pH sensitivity, we must first look at the scale itself. The term 'pH' comes from the German word potenz, meaning power, and it measures the concentration of hydrogen ions (H⁺) in a solution. It is a logarithmic scale ranging from 0 to 14: a pH of 7 is neutral, while anything below 7 is acidic and above 7 is basic (alkaline). Because it is logarithmic, a single unit change represents a ten-fold difference in acidity; for example, a solution with pH 4 is ten times more acidic than one with pH 5 Environment, Shankar IAS Academy, Environmental Pollution, p.102. In living systems, pH isn't just a number—it is a critical regulator of biological functions.

Our bodies maintain specific pH levels to facilitate life processes. In the stomach, gastric glands release Hydrochloric acid (HCl), which creates a highly acidic environment. This acidity is essential for two reasons: it kills harmful bacteria and, more importantly, it activates the protein-digesting enzyme pepsin. To prevent this strong acid from damaging our own tissues, the stomach wall secretes mucus to protect its inner lining Science Class X, NCERT 2025 ed., Life Processes, p.85. When this balance is disrupted, we experience 'acidity' or indigestion, often requiring a mild base (antacid) to neutralize the excess.

Another critical area of pH sensitivity is our mouth. Tooth enamel is made of calcium hydroxyapatite, the hardest substance in the human body. While it doesn't dissolve in plain water, it begins to corrode (demineralize) when the pH in the mouth falls below 5.5. This happens because bacteria break down sugar and food particles into acids. To counter this, our saliva acts as a natural buffer, but after meals, the acid production can overwhelm it. This is why using alkaline toothpaste is vital—it neutralizes the residual acid and prevents tooth decay Science Class X, NCERT 2025 ed., Acids, Bases and Salts, p.27.

Sources: Environment, Shankar IAS Academy, Environmental Pollution, p.102; Science Class X, NCERT 2025 ed., Life Processes, p.85; Science Class X, NCERT 2025 ed., Acids, Bases and Salts, p.27

6. Measuring Acid Strength: Ka and pKa Values (intermediate)

To understand why some acids are more 'aggressive' than others, we must look at their degree of dissociation—essentially, how many H⁺ ions they release when dissolved in water. A strong acid, like Hydrochloric acid (HCl) or Sulfuric acid (H₂SO₄), is like a generous donor that dissociates almost completely, flooding the solution with hydrogen ions. In contrast, a weak acid, such as Acetic acid (CH₃COOH), is 'stingy'; only a small fraction of its molecules break apart to release H⁺ ions Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p. 26. While the pH scale measures the current concentration of H⁺ ions in a specific solution, the Ka (Acid Dissociation Constant) measures the inherent strength of the acid itself regardless of concentration.

The Ka value is a mathematical ratio of the dissociated ions to the undissociated acid. A high Ka indicates a strong acid. Because Ka values can be extremely small (e.g., 0.0000175 for acetic acid), scientists use the pKa scale, which is the negative logarithm of Ka (pKa = -log₁₀ Ka). This conversion works just like the pH scale: the lower the pKa value, the stronger the acid. For instance, a mineral acid like H₂SO₄ has a very high Ka (and a negative pKa for its first dissociation), making it significantly stronger than organic acids like CH₃COOH, which has a pKa of approximately 4.76.

Metric	Strong Acid	Weak Acid
Dissociation	Nearly 100%	Partial/Very low
Ka Value	Very High	Low
pKa Value	Low (often negative)	High (positive)

In practice, mineral acids (H₂SO₄, HNO₃, HCl) are almost always stronger than organic carboxylic acids. Even among mineral acids, there are gradients; for example, Phosphorous acid (H₃PO₃) is a moderately strong diprotic acid, stronger than the organic acetic acid but weaker than the heavy-hitting sulfuric acid. Understanding these values allows us to predict how a substance will react in chemical processes, soil chemistry, or biological systems Geography of India, Majid Husain, Soils, p. 3.

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.26; Geography of India, Majid Husain (9th ed.), Soils, p.3

7. Comparing Strength: Sulfuric, Phosphorous, and Acetic Acids (exam-level)

To understand why certain acids are stronger than others, we must look at their ability to dissociate—or break apart—to release hydrogen ions (H⁺) when dissolved in water. The strength of an acid is not about how concentrated it is in a bottle, but rather how efficiently it gives up its protons. As we learn in Science, Class X (NCERT), Chapter 2: Acids, Bases and Salts, p. 26, acids that give rise to more H⁺ ions are said to be strong acids, while those that give less H⁺ ions are said to be weak acids.
At the top of the hierarchy sits Sulfuric Acid (H₂SO₄). It is a powerful mineral acid (also known as inorganic acid). In an aqueous solution, its first dissociation is nearly 100% complete, meaning almost every molecule of H₂SO₄ splits to release an H⁺ ion. This makes it significantly stronger than most other substances. In contrast, Phosphorous Acid (H₃PO₃) is also a mineral acid, but it is considered a moderate-to-weak acid because it does not ionize as completely as sulfuric acid. However, because it is an inorganic mineral acid, it still possesses a higher dissociation constant than organic acids.
Finally, we have Acetic Acid (CH₃COOH), the primary component of vinegar. Acetic acid is a typical organic acid (containing carbon). Organic acids are generally much weaker than mineral acids because their molecular structure holds onto the hydrogen atoms more tightly; only a small fraction of acetic acid molecules actually ionize in water. This leads to the following comparison:

Acid Name	Formula	Classification	Relative Strength
Sulfuric Acid	H₂SO₄	Mineral (Inorganic)	Strongest (Complete dissociation)
Phosphorous Acid	H₃PO₃	Mineral (Inorganic)	Moderate/Weak
Acetic Acid	CH₃COOH	Organic (Carboxylic)	Weakest (Partial dissociation)

Sources: Science, Class X (NCERT), Chapter 2: Acids, Bases and Salts, p.26

8. Solving the Original PYQ (exam-level)

Now that you have mastered the concepts of ionization and the distinction between mineral and organic acids, this question serves as a perfect application of those building blocks. The core principle at play here is the degree of dissociation: strong acids release hydrogen ions completely in an aqueous solution, while weak acids do so only partially. By identifying the category of each substance—H2SO4 as a strong mineral acid and CH3COOH as a weak organic carboxylic acid—you can immediately establish the two ends of the strength spectrum.

To arrive at the correct answer, start by identifying the most potent acid. Sulfuric acid (H2SO4) is a staple strong mineral acid that dissociates almost entirely, placing it at the top of our list. Next, look at Acetic acid (CH3COOH); as an organic acid, its covalent bonds hold onto protons much more tightly, making it the weakest of the three. Phosphorous acid (H3PO3), while weaker than sulfuric, is still an inorganic oxoacid and significantly more acidic than the carbon-based acetic acid. This logical sequence confirms that (A) H2SO4 > H3PO3 > CH3COOH is the only arrangement that respects the hierarchy of chemical bonding and proton release.

UPSC often uses reversal traps or symbol confusion to test your attention to detail. Option (C) is a classic example where the order is exactly reversed, designed to catch students who might confuse "acid strength" with "pH value" (where a higher pH indicates a weaker acid). Options (B) and (D) use jumbled sequences to see if you can distinguish between two different inorganic acids. Always remember the general rule: Mineral acids > Organic acids. As noted in Science, class X (NCERT 2025 ed.), the strength of an acid is essentially its ability to produce H+ ions, a property that peaks in mineral acids like sulfuric acid.