BharatNotes 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
| Theory | Acid | Base |
|---|---|---|
| Arrhenius (1884) | Substance that produces H⁺ ions in aqueous solution | Substance that produces OH⁻ ions in aqueous solution |
| Bronsted-Lowry (1923) | Proton (H⁺) donor | Proton (H⁺) acceptor |
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
| Category | Definition | Examples |
|---|---|---|
| Strong acid | Completely dissociates in water (100% ionisation) | HCl (hydrochloric), H₂SO₄ (sulphuric), HNO₃ (nitric) |
| Weak acid | Partially dissociates in water | CH₃COOH (acetic/vinegar), H₂CO₃ (carbonic), citric acid |
| Strong base | Completely dissociates in water | NaOH (caustic soda), KOH (caustic potash), Ca(OH)₂ |
| Weak base | Partially dissociates in water | NH₄OH (ammonium hydroxide), Mg(OH)₂ (milk of magnesia) |
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
| Substance | Approximate pH | Nature |
|---|---|---|
| Gastric juice (stomach acid) | 1.0 - 2.0 | Strongly acidic |
| Lemon juice | ~2.0 | Acidic |
| Vinegar | ~2.5 - 3.0 | Acidic |
| Orange juice | ~3.5 | Acidic |
| Black coffee | ~5.0 | Mildly acidic |
| Milk | ~6.5 | Slightly acidic |
| Pure water | 7.0 | Neutral |
| Human blood | ~7.35 - 7.45 | Slightly alkaline |
| Baking soda solution | ~8.5 | Alkaline |
| Soap solution | ~9.0 - 10.0 | Alkaline |
| Household ammonia | ~11.0 | Strongly alkaline |
| Bleach (NaOCl) | ~12.5 | Strongly alkaline |
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.
| Indicator | Colour in Acid | Colour in Base | Type |
|---|---|---|---|
| Litmus (oldest known indicator, from lichens) | Red | Blue | Natural |
| Phenolphthalein | Colourless | Pink/Magenta | Synthetic |
| Methyl orange | Red | Yellow | Synthetic |
| Turmeric (haldi) | Yellow | Reddish-brown | Natural |
| Universal indicator | Red → Orange → Yellow → Green → Blue → Violet (pH 1-14) | Shows full pH range through colour gradient | Mixed (synthetic blend) |
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
| Reaction | Product (Salt) | Application |
|---|---|---|
| HCl + NaOH → NaCl + H₂O | Sodium chloride (common salt) | Laboratory demonstration |
| H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O | Sodium sulphate | Industrial chemical |
| HCl + NaHCO₃ → NaCl + H₂O + CO₂ | Sodium chloride + carbon dioxide | Antacid action in the stomach |
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
| Salt | Chemical Name | Formula | Preparation / Source | Key Uses |
|---|---|---|---|---|
| Common salt | Sodium chloride | NaCl | Sea water evaporation; rock salt mining | Cooking; raw material for NaOH, Cl₂, Na₂CO₃; food preservation |
| Baking soda | Sodium hydrogen carbonate (sodium bicarbonate) | NaHCO₃ | Solvay process | Baking (releases CO₂ when heated); antacid; fire extinguisher (soda-acid type) |
| Washing soda | Sodium carbonate decahydrate | Na₂CO₃·10H₂O | Recrystallisation of soda ash with water | Water softening; laundry; glass manufacturing |
| Bleaching powder | Calcium hypochlorite | CaOCl₂ / Ca(OCl)₂ | Passing chlorine gas over dry slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O | Water purification; bleaching cotton and textiles; disinfectant |
| Plaster of Paris | Calcium sulphate hemihydrate | CaSO₄·½H₂O | Heating gypsum (CaSO₄·2H₂O) at 373 K | Setting broken bones; making moulds, casts, and statues; dentistry |
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
| Property | Soap | Detergent |
|---|---|---|
| Chemical nature | Sodium or potassium salt of long-chain fatty acid (e.g., sodium stearate, C₁₇H₃₅COONa) | Sodium salt of long-chain sulphonic acid or sulphate |
| Made by | Saponification — heating fat/oil with NaOH or KOH produces soap + glycerol | Chemical synthesis from petroleum-derived hydrocarbons |
| Works in hard water? | No — forms insoluble scum (calcium/magnesium salts of fatty acids) with Ca²⁺ and Mg²⁺ ions | Yes — works well in both hard and soft water |
| Biodegradability | Easily biodegradable | Some are non-biodegradable (branched-chain); modern ones are biodegradable (linear-chain) |
| Raw material | Animal fats or vegetable oils (renewable) | Petrochemicals (non-renewable) |
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.
| Type | Caused By | Removal Method |
|---|---|---|
| Temporary hardness | Ca(HCO₃)₂ and Mg(HCO₃)₂ (bicarbonates) | Boiling (precipitates CaCO₃); adding slaked lime — Clark's method |
| Permanent hardness | CaSO₄, MgSO₄, CaCl₂, MgCl₂ (sulphates and chlorides) | Adding washing soda (Na₂CO₃); ion-exchange resin method; reverse osmosis; distillation |
Exam tip: Temporary hardness can be removed by boiling; permanent hardness cannot. Washing soda and ion-exchange methods work for both types.
Water Purification
| Method | Principle | What It Removes |
|---|---|---|
| Chlorination | Adding Cl₂ or NaOCl; chlorine kills pathogens by disrupting their cell processes | Bacteria, viruses; does not remove dissolved salts |
| UV treatment | UV light at 253.7 nm wavelength damages microbial DNA, preventing reproduction | Bacteria, viruses; no chemical by-products; does not remove dissolved impurities |
| Reverse Osmosis (RO) | Water forced through a semi-permeable membrane under pressure | Dissolved salts, heavy metals, bacteria, viruses — removes 90-99% of dissolved ions |
| Boiling | Raising temperature to 100 °C kills most pathogens | Bacteria, protozoa; does not remove chemical contaminants |
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
| Preservative | Chemical Name / Nature | Used In |
|---|---|---|
| Salt (NaCl) | Sodium chloride | Pickles, cured meat, fish |
| Sugar | Sucrose | Jams, jellies, preserves |
| Vinegar | Acetic acid (CH₃COOH) | Pickles, sauces, chutneys |
| Sodium benzoate | C₆H₅COONa | Soft drinks, fruit juices, sauces |
| Potassium sorbate | C₆H₇KO₂ | Cheese, baked goods, wine |
| Citric acid | C₆H₈O₇ | Canned fruits, beverages |
Artificial Sweeteners
| Sweetener | Sweetness (vs sugar) | Key Fact |
|---|---|---|
| Saccharin | ~300 times sweeter | Oldest artificial sweetener; discovered accidentally in 1879 at Johns Hopkins University; heat-stable |
| Aspartame | ~200 times sweeter | Discovered in 1965; not heat-stable (cannot be used in baking); carries phenylalanine warning for people with phenylketonuria (PKU) |
| Sucralose | ~600 times sweeter | Made from sugar; heat-stable; widely used in baked goods |
Food Preservation Techniques
| Technique | How It Works |
|---|---|
| Salting | High salt concentration dehydrates microorganisms through osmosis, inhibiting their growth |
| Pickling | Immersion in vinegar (acetic acid) creates a low-pH environment hostile to bacteria |
| Pasteurisation | Heating milk to 72 °C for 15 seconds (HTST method) or 63 °C for 30 minutes kills harmful bacteria without significantly altering taste; named after Louis Pasteur (1860s) |
| Sugar preservation | High sugar concentration in jams and jellies draws water out of microbial cells, preventing spoilage |
| Refrigeration | Low temperature slows microbial metabolism and enzymatic reactions, delaying spoilage |
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
| Reaction | Chemical Process | Observation |
|---|---|---|
| Rusting of iron | 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ → 2Fe₂O₃·xH₂O (hydrated iron oxide) | Requires both oxygen and moisture; accelerated by salt and acid rain; prevented by painting, galvanising (zinc coating), or oiling |
| Fermentation | C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (glucose → ethanol + carbon dioxide, catalysed by yeast enzymes) | Used in bread-making (CO₂ causes dough to rise) and alcohol production |
| Photosynthesis | 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in presence of sunlight and chlorophyll) | Plants convert light energy into chemical energy; produces the oxygen we breathe |
Chemicals in Daily Life
| Product | Active Chemical | How It Works |
|---|---|---|
| Toothpaste | Calcium carbonate, sodium fluoride, baking soda | Mildly basic (neutralises mouth acids that cause tooth decay); fluoride strengthens enamel |
| Antacids | Mg(OH)₂ (milk of magnesia), NaHCO₃, Al(OH)₃ | Neutralise excess HCl in the stomach |
| Disinfectants | Phenol, chlorine (Cl₂), potassium permanganate (KMnO₄) | Kill bacteria and pathogens; chlorine is widely used for water purification |
| Antiseptics | Dettol (chloroxylenol), iodine (tincture of iodine), hydrogen peroxide (H₂O₂) | Applied on living tissues to prevent infection |
| Bleach | Sodium hypochlorite (NaOCl) | Oxidises coloured compounds, removing stains; also disinfects |
| Baking powder | NaHCO₃ + tartaric acid (cream of tartar) | Releases CO₂ when moistened and heated, causing dough to rise; tartaric acid neutralises the bitter taste of Na₂CO₃ formed |
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.
| Type | Extinguishing Agent | How It Works | Best For | Not Suitable For |
|---|---|---|---|---|
| Water | Water (H₂O) | Absorbs heat, cools the burning material below ignition temperature | Class A — solid combustibles (wood, paper, cloth) | Electrical fires (water conducts electricity); oil/grease fires (water splashes burning oil) |
| Carbon dioxide (CO₂) | Compressed CO₂ gas | Displaces oxygen around the fire; also cools slightly | Electrical fires; Class B (flammable liquids) | Open/outdoor fires (CO₂ disperses); metal fires |
| Foam (AFFF) | Aqueous film-forming foam | Forms a blanket over the fuel, cutting off oxygen supply | Class A and B fires (solids and liquids) | Electrical fires; cooking oil fires |
| Dry chemical powder (DCP) | Mono-ammonium phosphate or sodium bicarbonate | Interrupts the chemical chain reaction of fire | Class A, B, and C (solids, liquids, gases) — most versatile | Enclosed spaces (reduces visibility); sensitive electronic equipment |
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
Recent Developments (2024–2026)
FSSAI and Food Safety Chemistry — Regulating pH and Chemical Additives (2024–25)
The Food Safety and Standards Authority of India (FSSAI) updated its Food Safety and Standards (Food Products Standards and Food Additives) Regulations in 2024, tightening limits on artificial preservatives, food colours, and acidity regulators. FSSAI's 2024–25 enforcement drive against adulteration (acidic adulterants in milk, harmful colourants in spices) directly applies acid-base chemistry knowledge. India's mandatory iodisation of salt (connecting iodine chemistry to public health) was reinforced through the National Iodine Deficiency Disorders Control Programme.
UPSC angle: FSSAI regulations and food safety chemistry connect everyday acid-base chemistry (pH, preservatives, food additives) to public health policy — a recurring GS3 Prelims theme.
India's Effluent Treatment Challenges — Industrial Acid-Base Pollution (2024)
India's textile, leather, and chemical industries continued to generate acidic and alkaline effluents in 2024, with the National Green Tribunal (NGT) issuing orders against several industrial clusters for untreated effluent discharge into rivers. The Ganga Rejuvenation Programme and NMCG (National Mission for Clean Ganga) mandated Effluent Treatment Plants (ETPs) for all industries within the Ganga basin by 2024, requiring chemical neutralisation of acidic/basic effluents before discharge.
UPSC angle: Industrial acid-base chemistry connects directly to India's river pollution problem — relevant for GS3 environment and chemistry questions.
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.
