Arrange the following bases in increasing order of their basic strength: 1. Sodium hydroxide 2. Magnesium hydroxide 3. Aluminium hydroxide 4. Ammonium hydroxide Select the CORRECT answer using the Code glVen below: Codes :

1. Defining Bases: Arrhenius, Brønsted-Lowry, and Lewis Theories (basic)

To understand what a base is, we must look at how chemical definitions evolved to capture the behavior of different substances. The most fundamental definition, provided by Svante Arrhenius, states that a base is a substance that dissociates in water to produce hydroxide ions (OH⁻) Science, Class X, Acids, Bases and Salts, p.24. For instance, Sodium Hydroxide (NaOH) is a classic Arrhenius base because it releases OH⁻ when dissolved. We call these water-soluble bases alkalis. The strength of such a base depends entirely on how many OH⁻ ions it can liberate; a strong base ionizes completely, while a weak base does so only partially Science, Class X, Acids, Bases and Salts, p.26.

However, the Arrhenius definition is limited because it only applies to aqueous (water-based) solutions and substances containing OH⁻. To broaden our scope, the Brønsted-Lowry theory defines a base as a proton (H⁺) acceptor. In this view, any substance that can "grab" a hydrogen ion from another molecule is a base. This explains why Ammonia (NH₃), which doesn't have an OH⁻ group in its formula, still acts as a base—it accepts a proton from water to become NH₄⁺. Remember, a hydrogen ion (H⁺) is essentially a bare proton and cannot exist alone in water; it always associates with molecules to form species like the hydronium ion (H₃O⁺) Science, Class X, Acids, Bases and Salts, p.23.

The most inclusive definition is the Lewis Theory, which moves the focus from protons to electrons. A Lewis base is defined as an electron-pair donor. Because atoms often seek a stable configuration by sharing or transferring valence electrons Science, Class X, Carbon and its Compounds, p.59, a substance with a "lone pair" of electrons can donate that pair to form a bond with an electron-deficient species (the Lewis acid). This definition is vital in organic chemistry where many reactions don't involve hydrogen ions at all.

Theory	Definition of a Base	Scope
Arrhenius	Produces OH⁻ ions in water	Limited to aqueous solutions
Brønsted-Lowry	Proton (H⁺) acceptor	Focuses on proton transfer
Lewis	Electron-pair donor	Broadest; includes non-proton reactions

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.24; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.26; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.23; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59

2. The pH Scale and Degree of Ionization (basic)

To understand why some substances are corrosive while others are mild, we must look at the pH scale and the concept of ionization. The term pH originates from the German word 'potenz', meaning power, and it specifically measures the power or concentration of hydrogen ions in a solution Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.25. While we often think of acids and bases as opposites, the pH scale provides a unified mathematical way to measure them on a spectrum from 0 to 14.

The scale is logarithmic, which is a crucial point for competitive exams. This means that each whole pH value below 7 is ten times more acidic than the next higher value. For instance, a solution with a pH of 4 is ten times more acidic than one with a pH of 5, and a hundred times (10 × 10) more acidic than one with pH 6 Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.102. In pure water, only one in ten million molecules dissociates into ions (10⁻⁷), giving us the neutral baseline of pH 7 Geography of India, Majid Husain (McGrawHill 9th ed.), Soils, p.3.

The strength of an acid or base is defined by its degree of ionization—how readily it breaks apart into ions when dissolved in water. If you take equal amounts of Hydrochloric acid (HCl) and Acetic acid (CH₃COOH), the HCl will produce far more H⁺ ions because it ionizes almost completely. This makes it a strong acid. Conversely, acids that produce fewer H⁺ ions are weak acids Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.26. Similarly, strong bases like Sodium Hydroxide (NaOH) release a high concentration of OH⁻ ions, pushing the pH toward 14.

pH Value	Nature of Solution	Ion Concentration
0 to < 7	Acidic	High H₃O⁺ (Hydronium) concentration
7	Neutral	Equal H₃O⁺ and OH⁻ concentration
> 7 to 14	Basic (Alkaline)	High OH⁻ (Hydroxide) concentration

Sources: Science, Class X (NCERT 2025 ed.), 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

3. Neutralization Reactions and Formation of Salts (basic)

At its heart, a neutralization reaction occurs when an acid and a base react with each other to cancel out their respective properties. In this process, the hydrogen ions (H⁺) from the acid combine with the hydroxide ions (OH⁻) from the base to form water (H₂O). The remaining parts of the acid and base join together to form a chemical compound known as a salt. The general chemical equation for this is: Base + Acid → Salt + Water. For example, when Sodium Hydroxide (a base) reacts with Hydrochloric Acid, they produce Sodium Chloride (common salt) and water: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) Science, Class X, Acids, Bases and Salts, p.21.

It is a common misconception that every neutralization reaction results in a perfectly neutral solution (pH 7). While the acid and base do "neutralize" or diminish each other's effects, the final pH of the salt solution depends entirely on the relative strengths of the original reactants. These reactions are typically exothermic, meaning they release energy in the form of heat Science-Class VII, Exploring Substances, p.18. This is why a test tube often feels warm during such an experiment.

Reactant Combination	Nature of Salt Formed	Example
Strong Acid + Strong Base	Neutral (pH ≈ 7)	Sodium Chloride (NaCl)
Strong Acid + Weak Base	Acidic (pH < 7)	Ammonium Chloride (NH₄Cl)
Weak Acid + Strong Base	Basic (pH > 7)	Sodium Acetate (CH₃COONa)

Science, Class X, Acids, Bases and Salts, p.29

In our daily lives and environment, neutralization is a vital tool. For instance, buffering is the practice of adding a neutralizing agent, like lime (calcium oxide or calcium carbonate), to acidified water bodies to raise the pH and restore ecological balance Environment, Shankar IAS Academy, Environmental Pollution, p.106. Similarly, when you experience acidity in your stomach, you take an antacid (a mild base) to neutralize the excess hydrochloric acid produced by your digestive system.

Sources: Science, Class X, Acids, Bases and Salts, p.21; Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.18; Science, Class X, Acids, Bases and Salts, p.29; Environment, Shankar IAS Academy, Environmental Pollution, p.106

4. Periodic Table Trends: Metallic Character and Electronegativity (intermediate)

To understand how elements behave, we must look at two opposing forces: Metallic Character and Electronegativity. Metallic character is essentially an atom's 'generosity'—its tendency to lose electrons and form positive ions (cations). In contrast, electronegativity is an atom's 'greed'—its tendency to attract shared electrons toward itself. These properties are not random; they follow very predictable patterns across the periodic table based on how tightly the nucleus holds onto its outer electrons.

As you move across a period (from left to right), the number of protons in the nucleus increases, pulling electrons closer and making them harder to lose. Consequently, metallic character decreases while electronegativity increases. This is why elements on the far left, like Sodium (Na), are highly metallic and reactive, while elements on the right, like Chlorine (Cl), are non-metals with high electronegativity. You can observe this change in nature through the Activity Series, which ranks metals by their reactivity Science class X, Metals and Non-metals, p.55. Metals at the top of this list, like Potassium (K) and Sodium (Na), lose electrons so easily they react vigorously even with cold water.

These trends directly dictate the chemical nature of the compounds these elements form, specifically their hydroxides and oxides. A strong metal like Sodium forms Sodium Hydroxide (NaOH), which is a powerful base because it easily releases hydroxide (OH⁻) ions. As we move right to Magnesium, the metallic character slightly dips, making Mg(OH)₂ a milder base. By the time we reach Aluminium, the element is less 'generous' with its electrons; its hydroxide, Al(OH)₃, becomes amphoteric, meaning it can act as both an acid and a base. This transition from basic to acidic character across a period is a fundamental rule of periodic chemistry.

Trend Direction	Metallic Character	Electronegativity
Across a Period (Left to Right)	Decreases	Increases
Down a Group (Top to Bottom)	Increases	Decreases

Sources: Science class X, Metals and Non-metals, p.55; Science class X, Metals and Non-metals, p.37

5. Nature of Oxides and Hydroxides: Basic vs. Amphoteric (intermediate)

To understand the chemistry of materials, we first look at how elements interact with oxygen. Most metals react with oxygen to form metal oxides, which are typically basic in nature. This means that if you react them with an acid, they produce salt and water Science, Class X, Metals and Non-metals, p.41. For instance, when copper is heated, it forms Copper(II) Oxide (CuO), a black basic oxide. Similarly, when these oxides or metals react with water, they can form hydroxides. The strength of these bases depends on how easily they release hydroxide ions (OH⁻) in an aqueous solution Science, Class X, Acids, Bases and Salts, p.24. Soluble bases, like Sodium Hydroxide (NaOH), are known as alkalis and are very strong because they dissociate completely in water.

However, nature isn't always binary. Some metal oxides exhibit a 'dual personality' known as amphoteric behavior. Amphoteric oxides are special because they react with both acids and bases to produce salt and water Science, Class X, Metals and Non-metals, p.41. This is a critical distinction for UPSC as it highlights the transition from metallic to non-metallic character across the periodic table.

Feature	Basic Oxides/Hydroxides	Amphoteric Oxides/Hydroxides
Reaction with Acids	Reacts to form salt + water	Reacts to form salt + water
Reaction with Bases	Generally No Reaction	Reacts to form salt + water
Examples	Na₂O, MgO, CuO, Mg(OH)₂	Al₂O₃, ZnO, Al(OH)₃

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.41; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.24

6. Industrial and Medicinal Applications of Hydroxides (exam-level)

To understand the applications of hydroxides, we must first look at their chemical nature. Hydroxides are compounds containing the hydroxide ion (OH⁻). In chemistry, the strength of a base is determined by how easily it releases these ions in an aqueous solution. For instance, Sodium Hydroxide (NaOH) is a powerhouse of the industrial world because it dissociates completely in water, making it a very strong alkali. It is primarily produced through the Chlor-alkali process, where electricity is passed through brine (aqueous NaCl), resulting in the formation of NaOH, chlorine gas, and hydrogen gas Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.30. This strength makes NaOH indispensable for heavy industrial tasks like de-greasing metals, manufacturing paper, and producing soaps and detergents. Moving from the factory to the pharmacy, we encounter Magnesium Hydroxide (Mg(OH)₂), commonly known as Milk of Magnesia. Unlike the harsh NaOH, Mg(OH)₂ is a mild base. This makes it perfect for medicinal use, specifically as an antacid. Our stomachs naturally produce hydrochloric acid (HCl) to aid digestion, but an excess can cause pain and irritation. Because Mg(OH)₂ is mild, it safely reacts with and neutralizes the excess acid in the stomach without damaging the delicate lining Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.27. The effectiveness of these hydroxides is tied to the reactivity series of metals. Magnesium is more reactive than metals like aluminium or zinc, but less aggressive than sodium Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.44. This gradient in reactivity dictates why we use specific hydroxides for specific jobs—you wouldn't want to swallow a strong alkali like NaOH, nor would a very weak, nearly insoluble hydroxide be effective for industrial cleaning.

Hydroxide	Common Name	Primary Application	Nature
NaOH	Caustic Soda	Soap, paper, and dye industry	Strong Base (Corrosive)
Mg(OH)₂	Milk of Magnesia	Antacid (Indigestion relief)	Mild Base
Ca(OH)₂	Slaked Lime	Bleaching powder production	Moderate Base

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.27, 30; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.44

7. Comparing Basic Strength of Period 3 Hydroxides (exam-level)

In chemistry, the strength of a base is determined by its ability to release hydroxide ions (OH⁻) when dissolved in water. Bases that are highly soluble and dissociate completely are specifically referred to as alkalis Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.24. When we look at the third period of the periodic table (Sodium to Chlorine), we see a distinct shift in chemical behavior from strongly basic to acidic.

The basic strength of Period 3 hydroxides decreases as we move from left to right. This happens because the metallic character of the elements decreases across the period. Sodium (Na), being a highly electropositive metal, forms Sodium Hydroxide (NaOH), which dissociates easily into Na⁺ and OH⁻ ions Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.23. As we move to Magnesium (Mg) and Aluminum (Al), the size of the metal cation decreases and its positive charge increases (from Na⁺ to Mg²⁺ to Al³⁺). This higher charge density causes the metal to pull the oxygen of the hydroxide group more strongly, making it harder for the OH⁻ ion to break away and enter the solution.

Hydroxide	Nature	Behavior in Water
NaOH	Strong Base (Alkali)	Dissociates completely; high OH⁻ concentration.
Mg(OH)₂	Weak/Moderate Base	Partially soluble; used as an antacid Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.18.
Al(OH)₃	Amphoteric	Very weak base; can react with both acids and bases.

By the time we reach the non-metals later in the period (like Phosphorus or Sulfur), the "hydroxides" actually behave as oxyacids (e.g., H₂SO₄) because the bond between the non-metal and oxygen is so strong that the molecule releases H⁺ ions instead of OH⁻ ions. Therefore, the trend of decreasing basicity is a direct result of the increasing electronegativity and non-metallic nature of the elements as you move across the period.

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.24; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.23; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.18

8. Solving the Original PYQ (exam-level)

To solve this question, you must synthesize three core concepts: metallic character, charge density, and the extent of ionization. As you move across a period in the periodic table, metallic character decreases, which directly correlates to a decrease in the basicity of hydroxides. Sodium hydroxide (NaOH), being a Group 1 alkali metal hydroxide, dissociates completely in water, making it the strongest base in this set. In contrast, Ammonium hydroxide (NH4OH) is a weak base because it is a non-metallic hydroxide that only partially ionizes, providing the fewest hydroxide ions in solution. This fundamental understanding of bond strength versus ease of dissociation is the key building block for this PYQ.

Walking through the logic, we compare the cations: Na+, Mg2+, and Al3+. As the positive charge increases and the ionic radius decreases, the charge density increases. This creates a stronger pull on the hydroxide ion, making the bond more covalent and less likely to release the ion in water. Consequently, basicity follows the periodic trend: NaOH (1) > Mg(OH)2 (2) > Al(OH)3 (3). Since NH4OH (4) is a weak electrolyte that does not contain a metal cation, it sits at the bottom of this hierarchy. Therefore, the increasing order is 4—3—2—1, making (C) 4-3-2-1 the correct answer. This logic aligns with the principles found in Science, Class X (NCERT) regarding the nature of bases and salts.

UPSC frequently uses order-based questions to catch candidates in a rush. A common trap is Option (D), which presents the decreasing order; students often correctly identify the strongest and weakest bases but select the reverse sequence. Another trap involves the placement of Aluminium hydroxide; because it is amphoteric (reacting with both acids and bases), candidates might struggle to place it relative to Ammonium hydroxide. Remember, while Al(OH)3 is poorly soluble, NH4OH is a weak base by its very chemical nature. Always verify the direction of the order (increasing vs. decreasing) before finalizing your choice.