Acids, Bases, Salts & Everyday Chemistry

Overview

Acids, bases, salts, and everyday chemistry are high-yield topics for UPSC Prelims. Questions test conceptual understanding of the pH scale, properties of common chemicals, and practical applications in daily life. This chapter covers definitions, indicators, neutralisation, important salts, soaps and detergents, food chemistry, and fire extinguishers — all areas that appear regularly in the General Science section.

Acids and Bases — Definitions

Exam tip: The Arrhenius definition works only in water. The Bronsted-Lowry definition is more general and applies to non-aqueous solvents as well. Water itself can act as both an acid and a base (amphoteric).

Strong vs Weak Acids and Bases

Key distinction: Strength (degree of dissociation) is different from concentration (amount of solute per unit volume). A dilute solution of HCl is still a strong acid.

The pH Scale

The pH scale measures how acidic or basic a solution is, ranging from 0 to 14. The term "pH" stands for "potential of hydrogen" (or "power of hydrogen").

• pH < 7 — Acidic
• pH = 7 — Neutral (pure water at 25 °C)
• pH > 7 — Basic (alkaline)

pH of Common Substances

Exam tip: Human blood is maintained at a narrow pH range of 7.35 to 7.45 through buffer systems. Even a small deviation can be life-threatening — this is why the body has multiple mechanisms (lungs, kidneys, blood buffers) to regulate pH.

pH in Real-World Applications

• Agriculture: Most crops grow best in soil with a pH of 6.0 to 7.5. Below pH 5.5, essential nutrients become less available; farmers add lime (CaO or CaCO₃) to raise soil pH. Tea and blueberries prefer acidic soil (pH 4.5-5.5).
• Acid rain: Normal rain is slightly acidic (pH ~5.6) because atmospheric CO₂ dissolves to form carbonic acid. Rain with pH below 5.6 is classified as acid rain, typically caused by SO₂ and NOₓ emissions. Acid rain in industrial areas often has a pH of 4.2 to 4.4.
• Swimming pools: Pool water is maintained at pH 7.2 to 7.8 for safety and effective chlorine disinfection.

Buffer Solutions

A buffer solution resists changes in pH when small amounts of acid or base are added. The most important biological buffer is the bicarbonate buffer system in blood: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. This system maintains blood pH at 7.35-7.45 by adjusting the ratio of bicarbonate (HCO₃⁻) to carbonic acid (H₂CO₃), which must be approximately 20:1 for normal pH. The lungs regulate CO₂ removal (seconds to minutes), while the kidneys adjust bicarbonate levels (hours to days).

Indicators

Indicators are substances that change colour depending on whether a solution is acidic or basic.

Exam tip: Litmus paper is the most commonly asked indicator. Red litmus turns blue in a base; blue litmus turns red in an acid. Litmus does not change colour in a neutral solution.

Neutralisation

When an acid reacts with a base, they neutralise each other to form a salt and water.

General equation: Acid + Base → Salt + Water

Everyday examples of neutralisation:

• Antacids (Mg(OH)₂ or NaHCO₃) neutralise excess stomach acid (HCl)
• Lime (CaO) is added to acidic soils to neutralise them for agriculture
• Wasp stings (alkaline) are treated with vinegar (acid); bee stings (acidic) are treated with baking soda (base)

Important Salts

Exam tip: Plaster of Paris gets its name from the large deposits of gypsum found in Montmartre, Paris. When mixed with water, it quickly hardens back into gypsum — this setting reaction is exothermic.

Soaps vs Detergents

How soap cleans (micelle formation): Soap molecules have a hydrophilic (water-loving) head and a hydrophobic (water-repelling) tail. The hydrophobic tail attaches to grease/dirt, while the hydrophilic head stays in water. Many soap molecules surround a dirt particle, forming a spherical structure called a micelle, which gets washed away with water.

Hard Water

Water containing dissolved calcium and magnesium salts is called hard water. It does not lather easily with soap.

Exam tip: Temporary hardness can be removed by boiling; permanent hardness cannot. Washing soda and ion-exchange methods work for both types.

Water Purification

Exam tip: RO + UV combined systems are considered the most comprehensive for household water purification as they address both chemical and microbial contamination.

Food Chemistry

Common Food Preservatives

Artificial Sweeteners

Food Preservation Techniques

Food adulteration — a frequent UPSC topic: common adulterants include metanil yellow in turmeric, chalk powder in flour, water in milk, and argemone oil in mustard oil. The Food Safety and Standards Authority of India (FSSAI) regulates food safety under the Food Safety and Standards Act, 2006.

Important Chemical Reactions in Daily Life

Chemicals in Daily Life

Exam tip: The difference between a disinfectant and an antiseptic is the surface of application. Disinfectants are used on non-living surfaces (floors, drains); antiseptics are used on living tissues (skin, wounds). The same chemical at different concentrations can serve both roles — e.g., dilute phenol is an antiseptic, concentrated phenol is a disinfectant.

Fire Extinguishers

Fire extinguishers work by removing one or more elements of the "fire triangle" — heat, fuel, or oxygen.

Soda-acid extinguisher (traditional): Contains NaHCO₃ solution and a small bottle of H₂SO₄. When the knob is struck, acid mixes with baking soda, producing CO₂ gas that is expelled under pressure to extinguish the fire.

Exam tip: Water must NEVER be used on electrical fires (risk of electrocution) or oil fires (water sinks below oil, turns to steam, and causes a dangerous flare-up). CO₂ extinguishers are standard in server rooms and laboratories.

UPSC Relevance

Frequently tested areas:

• pH values of common substances (especially blood, stomach acid, and lemon juice)
• Acid rain causes, pH threshold, and environmental impact
• Buffer solutions and blood pH regulation
• Difference between soaps and detergents; why soap fails in hard water
• Hard water — temporary vs permanent hardness and removal methods
• Important salts — baking soda, washing soda, bleaching powder, plaster of Paris (formulas, preparation, and uses)
• Water purification methods — chlorination, RO, UV treatment
• Fire extinguisher types and which fire class each addresses
• Food preservation techniques (pasteurisation, salting, pickling) and adulteration; FSSAI role
• Common chemical reactions — rusting, fermentation, photosynthesis
• Difference between antiseptic and disinfectant

Cross-links:

• Food safety and FSSAI — links to Governance (GS-2)
• Water purification chemistry — links to Environment (GS-3)
• Metals, alloys, and corrosion — Metals, Non-Metals & Alloys

Vocabulary

Titration

• Pronunciation: /taɪˈtreɪʃən/
• Definition: An analytical technique in which a solution of known concentration (titrant) is gradually added to a solution of unknown concentration until the reaction reaches completion, typically indicated by a colour change.
• Origin: From French titrer (to determine the standard strength), from titre (standard, fineness of alloyed gold); the noun titration first appeared in the 1860s.

Indicator

• Pronunciation: /ˈɪndɪkeɪtər/
• Definition: A substance — such as litmus, phenolphthalein, or methyl orange — that changes colour at a specific pH range to signal the endpoint of a chemical reaction or the acidity of a solution.
• Origin: From Late Latin indicātor (one who points out), from Latin indicāre (to make known, point out), from in- (towards) + dicāre (to proclaim); first recorded in English in the 1660s.

Saponification

• Pronunciation: /səˌpɒnɪfɪˈkeɪʃən/
• Definition: The hydrolysis of a fat or oil with a metallic alkali (such as NaOH or KOH) to produce glycerol and the salt of a fatty acid (soap).
• Origin: From French saponification, from Modern Latin saponificāre, combining sapon (soap) + -ficāre (to make, from Latin facere); first recorded in English in 1801.

Key Terms

pH Scale

• Pronunciation: /piː eɪtʃ skeɪl/
• Definition: A logarithmic scale ranging from 0 to 14 that measures the hydrogen ion (H+) concentration in an aqueous solution, indicating its acidity (below 7, higher H+ concentration), neutrality (exactly 7), or alkalinity/basicity (above 7, lower H+ concentration). Because the scale is logarithmic, each unit change represents a 10-fold change in H+ concentration -- a pH 3 solution is 10 times more acidic than pH 4 and 100 times more acidic than pH 5. Values below 0 and above 14 are possible for extremely concentrated solutions but are uncommon.
• Context: Introduced in 1909 by Danish biochemist Soren Peter Lauritz Sorensen at the Carlsberg Laboratory in Copenhagen while studying the effect of ion concentration on proteins in brewing. The p likely stands for the German/Danish Potenz (power/exponent) and H for hydrogen. Key pH values: stomach acid (~1-2), lemon juice (~2), vinegar (~2.5), normal rain (~5.6), pure water (7.0), human blood (7.35-7.45, tightly regulated), baking soda solution (~8.5), milk of magnesia (~10.5), household bleach (~12.5). Acid rain is defined as precipitation with pH below 5.6 (normal rain is slightly acidic at 5.6 due to dissolved CO2 forming carbonic acid). Soil pH critically affects agriculture -- most crops thrive at pH 6-7; acidic soils require lime treatment, alkaline soils require gypsum.
• UPSC Relevance: GS3 (General Science / Environment). Prelims frequently tests pH values -- blood (7.35-7.45), stomach acid (~1-2), lemon juice (~2), pure water (7), and the acid rain threshold (below 5.6). Know that human blood pH is maintained within an extremely narrow range by buffer systems; deviation causes acidosis (<7.35) or alkalosis (>7.45). Mains connects pH to water quality standards (BIS specifies 6.5-8.5 for drinking water), acid rain impact on Taj Mahal ("marble cancer" -- sulphuric acid reacting with CaCO3 marble), soil health for agriculture, and environmental pollution monitoring. Also links to industrial effluent standards and river water quality.

Neutralisation Reaction

• Pronunciation: /ˌnjuːtrəlaɪˈzeɪʃən riˈækʃən/
• Definition: A chemical reaction in which an acid (H+ donor) and a base (OH- donor) combine in stoichiometrically equivalent quantities to produce a salt and water: Acid + Base -> Salt + Water. This is a double displacement reaction. The resulting solution's pH depends on the relative strengths of the reactants: strong acid + strong base yields a neutral salt (pH 7); strong acid + weak base yields an acidic salt (pH < 7); weak acid + strong base yields a basic salt (pH > 7).
• Context: The concept was formalised with the development of Svante Arrhenius' acid-base theory in 1884 (acids produce H+ ions, bases produce OH- ions in solution). Key everyday applications: antacids (Mg(OH)2, Al(OH)3) neutralise excess stomach HCl to relieve acidity; agricultural lime (CaO/Ca(OH)2) is added to acidic soil to raise pH for optimal crop growth; bee stings are acidic (formic acid, treat with baking soda/base) while wasp stings are alkaline (treat with vinegar/acid). Important salts produced by neutralisation that UPSC tests: baking soda (NaHCO3 -- used in fire extinguishers, baking), washing soda (Na2CO3.10H2O -- water softening, cleaning), bleaching powder (CaOCl2 -- water purification), and plaster of Paris (CaSO4.1/2H2O -- surgical casts, construction).
• UPSC Relevance: GS3 (General Science). Prelims tests everyday applications -- antacids neutralising stomach acid, lime added to acidic soil, bee sting (acidic, treat with base) vs wasp sting (alkaline, treat with acid), and tooth decay (bacterial acid dissolves enamel CaHPO4, prevented by fluoride toothpaste). Know important salts: baking soda (NaHCO3 -- formula, used in soda-acid fire extinguishers and baking), washing soda (Na2CO3.10H2O -- water softening), bleaching powder (CaOCl2 -- water purification in India), and plaster of Paris (CaSO4.1/2H2O -- medical and construction use). Also know that water treatment plants use alum (KAl(SO4)2) and chlorine (neutralisation/disinfection), connecting to clean water access and Jal Jeevan Mission.

Sources: US EPA, Chemistry LibreTexts, Biology LibreTexts, FDA, Science History Institute, Britannica, IFSEC, Wikipedia (cross-verified with multiple sources). All facts verified as of March 2026.