Acids, Bases and Salts

The study of acids, bases, and salts forms a foundational pillar of General Science in the UPSC Civil Services Examination. These chemical compounds are classified based on their characteristic properties, behavior in aqueous solutions, and pH values. Understanding their theoretical frameworks, chemical behaviors, and real-world applications is crucial for both Prelims and Mains examinations.

Chemical Theories of Acids and Bases

Three major theoretical frameworks define the behavior and nature of acids and bases:

• Arrhenius Theory: Defines an acid as a substance that dissociates in water to give hydrogen ions (H^+) or hydronium ions (H₃O^+), and a base as a substance that dissociates in water to give hydroxyl ions (OH^-).
• Brønsted-Lowry Theory: Defines an acid as a proton (H^+) donor and a base as a proton (H^+) acceptor. This theory introduces the concept of conjugate acid-base pairs.
• Lewis Theory: Provides a broader electronic definition. An acid is an electron-pair acceptor (electrophile), while a base is an electron-pair donor (nucleophile).

Acids: Properties, Classification, and Applications

Physical and Chemical Properties

• Taste and Touch: Acids have a sour taste. Strong mineral acids cause severe skin burns.
• Indicator Action: Acids turn blue litmus paper red, keep phenolphthalein colorless, and turn methyl orange to red/pink.
• Reaction with Metals: Acids react with active metals (like Zinc, Magnesium, and Iron) to liberate Hydrogen gas (H₂).
• Reaction with Carbonates: Acids react with metal carbonates and bicarbonates to evolve Carbon Dioxide (CO₂) gas, which turns lime water milky.

Classification of Acids

Acids are classified based on their source, strength, and concentration:

Classification Criteria	Type of Acid	Definition and Characteristics	Examples
Based on Source	Organic Acids	Derived from plants and animals; naturally occurring weak acids.	Citric acid, Acetic acid, Tartaric acid
	Mineral (Inorganic) Acids	Synthesized from minerals; generally strong and highly corrosive.	Hydrochloric acid (HCl), Sulfuric acid (H2SO4), Nitric acid (HNO3)
Based on Strength	Strong Acids	Dissociate completely into ions in an aqueous solution.	HCl, H2SO4, HNO3
	Weak Acids	Dissociate only partially into ions in an aqueous solution.	Formic acid (HCOOH), Carbonic acid (H2CO3)
Based on Basicity	Monobasic	Ionizes to produce one H^+ ion per molecule.	HCl, HNO3, CH3COOH
	Dibasic	Ionizes to produce two H^+ ions per molecule.	H2SO4, H2CO3, Oxalic acid
	Tribasic	Ionizes to produce three H^+ ions per molecule.	Phosphoric acid (H3PO4)

Natural Sources of Common Organic Acids

• Citric Acid: Present in citrus fruits like lemons, limes, and oranges.
• Acetic Acid: The primary constituent of vinegar.
• Tartaric Acid: Found naturally in tamarind, grapes, and unripened bananas.
• Lactic Acid: Formed in sour milk, curd, and during strenuous exercise in human muscles.
• Oxalic Acid: Present in tomatoes and spinach; used for removing ink stains.
• Formic Acid (Methanoic Acid): Injected via ant stings and nettle stings, causing burning sensations.
• Malic Acid: Naturally occurring acid found in apples.

Key Industrial and Domestic Applications

• Sulfuric Acid (H₂SO₄): Known as the “King of Chemicals.” It is extensively used in car batteries, manufacturing fertilizers (like superphosphate), plastics, and synthetic fibers.
• Nitric Acid (HNO₃): Used in the production of fertilizers (Ammonium Nitrate) and commercial explosives like TNT (Trinitrotoluene) and nitroglycerine.
• Hydrochloric Acid (HCl): Secreted naturally in the human stomach to aid digestion. Industrially used for descaling boilers and cleaning metal surfaces (pickling).
• Boric Acid (H₃BO₃): Used as a mild antiseptic for eyes and a grain preservative.

Bases: Properties, Classification, and Applications

Physical and Chemical Properties

• Taste and Touch: Bases have a bitter taste and a soapy, slippery feel to the touch.
• Indicator Action: Bases turn red litmus paper blue, turn phenolphthalein pink, and turn methyl orange yellow.
• Neutralization: Bases react with acids to neutralize their properties, forming salt and water.
• Alkalis: Water-soluble bases are explicitly called alkalis. All alkalis are bases, but all bases are not alkalis. Sodium Hydroxide (NaOH) and Potassium Hydroxide (KOH) are alkalis, whereas Ferric Hydroxide (Fe(OH)₃) is a base but not an alkali.

Classification and Strength of Bases

• Strong Bases: Undergo complete ionization in water to yield a high concentration of hydroxyl (OH^-) ions. Examples include Sodium Hydroxide (NaOH) and Potassium Hydroxide (KOH).
• Weak Bases: Undergo partial ionization in water, resulting in a low concentration of hydroxyl ions. Examples include Ammonium Hydroxide (NH₄OH) and Calcium Hydroxide (Ca(OH)₂).

Key Applications of Common Bases

• Sodium Hydroxide (NaOH): Commonly called Caustic Soda. Used in soap manufacturing, paper making, petroleum refining, and synthetic rayon fibers.
• Calcium Hydroxide (Ca(OH)₂): Commonly called Slaked Lime. Used in whitewashing buildings, manufacturing bleaching powder, and neutralizing acidic soils.
• Magnesium Hydroxide (Mg(OH)₂): Known as Milk of Magnesia. Widely used as an antacid to neutralize excess stomach acidity.
• Sodium Carbonate (Na₂CO₃): Known as Washing Soda. Used in water softening and the glass and paper industries.
• Ammonium Hydroxide (NH₄OH): Used as a household cleaner and a reagent to remove grease stains from clothes.

The pH Scale and Indicators

The pH Scale Explained

The pH scale, developed by Sørensen, measures the hydrogen ion concentration in a solution to determine its acidity or alkalinity. It ranges from 0 to 14.

• Mathematical Expression: pH = -log₁₀[H^+].
• Acidic Solutions: pH values less than 7. Lower pH indicates a stronger acid.
• Neutral Solutions: pH value exactly equal to 7 at 25°C (e.g., pure water).
• Alkaline Solutions: pH values greater than 7. Higher pH indicates a stronger base.

Biological and Daily Life Significance of pH

• Human Blood pH: Maintained within a narrow, slightly alkaline range of 7.35 to 7.45.
• Acid Rain: When the pH of rainwater drops below 5.6 due to atmospheric pollutants (SO₂ and NO_x), it becomes acid rain, damaging aquatic ecosystems and monuments like the Taj Mahal.
• Tooth Decay: Begins when the pH inside the mouth falls below 5.5. Bacteria produce acids from sugar degradation, degrading tooth enamel (Calcium Phosphate).
• Soil Chemistry: Plants require a specific pH range for optimal growth. Citrus fruits prefer slightly acidic soils, whereas crops like alfalfa prefer neutral to slightly alkaline soils.

Chemical and Natural Indicators

Indicators change color depending on whether they are placed in acidic or alkaline mediums.

Indicator	Original Color	Color in Acidic Medium	Color in Basic Medium
Litmus	Purple (Neutral)	Red	Blue
Phenolphthalein	Colorless	Colorless	Pink
Methyl Orange	Orange	Red / Pink	Yellow
Turmeric	Yellow	Yellow (No change)	Reddish-brown
Red Cabbage Juice	Purple	Red	Greenish-yellow

Salts: Formation, Types, and Industrial Use

Mechanism of Formation

Salts are ionic compounds formed when an acid reacts with a base in a chemical process known as a Neutralization Reaction. In general terms:

An example of this reaction is:

Classification of Salts

Salts are classified based on the nature of the parent acid and base from which they are derived:

• Normal (Neutral) Salts: Formed by the complete replacement of replaceable hydrogen ions of an acid by a metal or ammonium ion. They do not contain replaceable hydrogen or hydroxyl ions. Examples include Sodium Chloride (NaCl) and Potassium Sulfate (K₂SO₄). Their aqueous solution has a pH close to 7.
• Acidic Salts: Formed by the partial replacement of replaceable hydrogen atoms of a polybasic acid by a metal ion. They contain replaceable hydrogen. Examples include Sodium Bicarbonate (NaHCO₃) and Sodium Bisulfate (NaHSO₄). Their aqueous solution has a pH less than 7.
• Basic Salts: Formed by the partial neutralization of a polyhydroxy base by an acid. They contain replaceable hydroxyl groups. Examples include Basic Copper Carbonate (CuCO₃ · Cu(OH)₂) and Basic Lead Carbonate. Their aqueous solution has a pH greater than 7.

Industrially Important Chemical Salts

Sodium Chloride (NaCl)

Commonly known as Table Salt. It is extracted from seawater and rock deposits. It serves as an essential nutrient for the human body, conducting nerve impulses and maintaining osmotic balance. Industrially, it is the raw material for the Chlor-Alkali process to produce caustic soda, chlorine, and hydrogen.

Sodium Bicarbonate (NaHCO₃)

Commonly known as Baking Powder or Baking Soda. It releases carbon dioxide gas when heated or mixed with an acid, causing bread and cakes to rise. It is also used in soda-acid fire extinguishers and as an antacid.

Sodium Carbonate Decahydrate (Na₂CO₃ · 10H₂O)

Commonly known as Washing Soda. It exhibits efflorescence (loses water of crystallization when exposed to air). It is used for removing the permanent hardness of water, manufacturing glass, and laundering.

Calcium Oxychloride (CaOCl₂)

Commonly known as Bleaching Powder. Produced by the action of chlorine gas on dry slaked lime. It is used as an oxidizing agent in chemical industries, for bleaching cotton and linen, and for disinfecting drinking water.

Calcium Sulfate Hemihydrate (CaSO₄ · 1/2H₂O)

Commonly known as Plaster of Paris (POP). It is obtained by heating Gypsum (CaSO₄ · 2H₂O) at 373 K (100°C). If heated above this temperature, it loses all water molecules to form “dead burnt plaster” (anhydrous calcium sulfate). It is used for setting fractured bones, making casts, and interior decoration.

Core Scientific Trivia for Prelims

• Aqua Regia: A highly corrosive, fuming mixture of concentrated Hydrochloric Acid (HCl) and concentrated Nitric Acid (HNO₃) in a 3:1 ratio by volume. It is one of the few reagents capable of dissolving noble metals like Gold (Au) and Platinum (Pt).
• Water of Crystallization: A fixed number of water molecules chemically combined in a definite crystalline proportion within one formula unit of a salt. For example, Copper Sulfate pentahydrate (CuSO₄ · 5H₂O) is blue due to these water molecules; heating removes the water, turning the substance into a white amorphous powder.
• Buffer Solutions: Solutions that resist any change in their pH value upon the minor addition of an acid or a base. Human blood is an example of a natural buffer solution, sustained primarily by the carbonic acid-bicarbonate (H₂CO₃ / HCO₃^-) system.
• Olfactory Indicators: Substances whose odor changes depending on whether they are mixed with an acidic or basic solution. Examples include onion, vanilla extract, and clove oil, which lose their characteristic smell in strongly basic solutions.

Last Modified: May 25, 2026