Topic 6: Chemistry

Chemistry is the science of matter, its structure, and its transformations — the ground beneath materials, energy, medicine and industry. This single-source file covers the states & classification of matter, atomic structure (electron, proton, neutron, isotopes), the periodic table & its trends, chemical bonding (ionic, covalent, metallic), acids, bases & salts and pH, metals, non-metals & metallurgy, organic chemistry & hydrocarbons, polymers, and the everyday chemistry of fuels, fertilizers, alloys, soaps, food & medicine — with labelled diagrams, tables and real dated PYQs with full model answers.

UPSC Prelims · Mains GS-III Periodic Table · pH ~38 min read Everyday Chemistry High Prelims Weight

1.Matter — States & Classification
2.Atomic Structure
3.The Periodic Table
4.Chemical Bonding
5.Acids, Bases & Salts (pH)
6.Metals, Non-Metals & Metallurgy
7.Organic Chemistry & Polymers
8.Everyday & Industrial Chemistry
9.Radioactivity & Nuclear Chemistry
10.Current Affairs Link
11.Prelims PYQs
12.Mains PYQs + Model Answers
★15-Minute Revision Box

Conceptual Clarity — How UPSC Tests Chemistry

Chemistry in UPSC is applied & everyday-oriented, not equation-heavy — the focus is on facts and uses. Sort your prep into three question-types:

Definitional / static — "which gas is used in...?", "pH of...?", "isotope of...?". Pure recall of properties & uses.
Statement-elimination — property statements (acid/base, metal/non-metal, ionic/covalent) where one word decides the answer.
Applied / current — a material or process in the news (green hydrogen, lithium, rare earths, ethanol blending) traced to a chemistry fundamental — the GS-III energy/materials bridge.

Highest-frequency themes: gases & their uses · pH & acids/bases · isotopes · periodic-table basics · metals, alloys & metallurgy · polymers/plastics · fuels & combustion · fertilizers · everyday chemicals.

1. Matter — States & Classification

Matter is anything with mass & volume. It exists in three common states (plus plasma & BEC).

State	Shape / volume	Particle arrangement
Solid	Fixed shape & volume	Closely packed, vibrate in place
Liquid	Fixed volume, no fixed shape	Loosely packed, can flow
Gas	Neither fixed	Far apart, move freely

Plasma = ionised gas (in stars, neon signs); BEC = Bose-Einstein Condensate (near absolute zero, named after S.N. Bose).
Changes of state: melting, freezing, evaporation, condensation, sublimation (solid→gas directly, e.g. camphor, dry ice, iodine, naphthalene).
Classification: pure substances (elements & compounds) vs mixtures (homogeneous & heterogeneous). Element = one kind of atom; compound = fixed ratio of elements chemically bonded.

Prelims facts: the 4th state is plasma (most abundant state in the universe); dry ice is solid CO₂ that sublimes; alloys & air are mixtures, water & salt are compounds.

2. Atomic Structure

Fig 6.1 — The atom: a tiny dense nucleus of protons (+) & neutrons (0), surrounded by electrons (−) in shells. The atom is electrically neutral.

Particle	Charge	Location	Discovered by
Electron	−1	Shells	J.J. Thomson
Proton	+1	Nucleus	Rutherford / Goldstein
Neutron	0	Nucleus	Chadwick

Atomic number (Z) = number of protons (defines the element). Mass number (A) = protons + neutrons.
Isotopes = same protons, different neutrons (e.g. C-12, C-14; U-235, U-238). Isobars = same mass number, different elements.

Prelims traps: atomic number = protons (not electrons); isotopes differ in neutrons; C-14 is used for carbon dating; U-235 is the fissile isotope (link Topic 15); heavy water = D₂O (deuterium).

3. The Periodic Table

The modern periodic table (Mendeleev proposed it; Moseley arranged it by atomic number) organises elements by increasing atomic number into periods (rows) & groups (columns).

Groups: 18 vertical columns — elements with similar properties (e.g. Group 1 = alkali metals, Group 17 = halogens, Group 18 = noble/inert gases).
Periods: 7 horizontal rows.
Metals (left & centre), non-metals (right), metalloids (staircase — Si, Ge; used in semiconductors).

Trend (left → right in a period)	Change
Atomic size	Decreases
Metallic character	Decreases (non-metallic increases)
Electronegativity	Increases

Prelims facts: noble gases (He, Ne, Ar) are inert (full outer shell); halogens are highly reactive non-metals; Mendeleev is the "father of the periodic table"; hydrogen's placement is unique.

4. Chemical Bonding

Atoms bond to attain a stable (noble-gas) electron configuration — the octet rule.

Bond	How	Examples / properties
Ionic (electrovalent)	Transfer of electrons (metal → non-metal)	NaCl; high m.p., conduct in solution
Covalent	Sharing of electrons (non-metals)	H₂O, CO₂, CH₂; low m.p., poor conductors
Metallic	"Sea" of free electrons	Metals; conduct heat & electricity, malleable

Prelims facts: ionic compounds conduct electricity when molten/dissolved (free ions); metals conduct due to free electrons; covalent compounds are usually poor conductors. A hydrogen bond (weak) explains water's unusual properties.

5. Acids, Bases & Salts (pH)

Acids release H⁺ ions, taste sour, turn blue litmus red (e.g. HCl, H₂SO₄, citric, acetic).
Bases release OH⁻ ions, taste bitter, turn red litmus blue (e.g. NaOH, ammonia, lime).
Salt = product of acid + base (neutralisation); e.g. NaCl.
pH scale (0–14): 7 = neutral, <7 acidic, >7 basic. Blood pH ~7.4; stomach acid ~1–2.

Everyday/prelims links: antacids neutralise stomach acid; acid rain (pH < 5.6) from SO₂/NOₓ; soil pH affects crops; baking soda = NaHCO₃ (mild base); the strongest common acid tested is H₂SO₄ ("king of chemicals"). Universal indicator shows pH by colour.

6. Metals, Non-Metals & Metallurgy

Property	Metals	Non-metals
State	Solid (except mercury)	Solid, liquid or gas
Conductivity	Good	Poor (except graphite)
Malleable/ductile	Yes	No (brittle)
Nature of oxide	Basic	Acidic

Metallurgy = extraction & purification of metals from ores (mining → concentration → reduction → refining).
Alloys (mixtures for better properties): steel (Fe+C), stainless steel (Fe+Cr+Ni), brass (Cu+Zn), bronze (Cu+Sn), solder (Pb+Sn).
Corrosion: rusting of iron (needs air + water); prevented by galvanising, painting, alloying.
Reactivity series: K > Na > Ca > Mg > Al > Zn > Fe > Cu > Ag > Au (more reactive metals displace less reactive).

Prelims facts: mercury is the only liquid metal; gold & platinum are least reactive (found free); aluminium resists corrosion via an oxide layer; graphite (non-metal) conducts electricity.

7. Organic Chemistry & Polymers

Organic chemistry = chemistry of carbon compounds. Carbon forms 4 bonds (tetravalent) & chains (catenation) — the basis of life & fuels.
Hydrocarbons: alkanes (single bonds, e.g. methane CH₄), alkenes (double), alkynes (triple). Fossil fuels (petrol, diesel, LPG, natural gas) are hydrocarbon mixtures.
Polymers = large molecules from repeating units (monomers): natural (rubber, cellulose, protein) & synthetic (polythene, PVC, nylon, Teflon, bakelite).
Biodegradable vs non-biodegradable plastics — the plastic-pollution problem.

Everyday/GS-III links: plastics & the single-use-plastic ban; ethanol (from sugarcane) blended into petrol; CNG & LPG as cleaner fuels; carbon's uniqueness underlies both life & the fuel economy.

8. Everyday & Industrial Chemistry

Item	Chemistry
Soap & detergent	Clean by emulsifying grease (hydrophilic + hydrophobic ends)
Fertilizers	Urea (N), DAP (N+P), MOP (K) — NPK nutrients
Common gases	O₂ (respiration), CO₂ (fire extinguisher, photosynthesis), N₂ (inert, packaging), H₂ (fuel), He (balloons), Cl₂ (water treatment)
Bleaching powder	Ca(OCl)Cl — disinfect & bleach
Baking soda / washing soda	NaHCO₃ / Na₂CO₃
Preservatives & antioxidants	Keep food from spoiling

High-yield uses: CO₂ & foam extinguishers; chlorine & ozone for water purification; nitrogen flushing to preserve food; urea as the main N-fertilizer; silver halides in old photography; sodium-vapour lamps (yellow).

9. Radioactivity & Nuclear Chemistry (Basics)

Radioactivity = spontaneous emission of radiation from unstable nuclei (discovered by Becquerel; the Curies). Emissions: alpha (α), beta (β), gamma (γ).
Half-life = time for half of a radioactive sample to decay (used in dating).
Uses: C-14 (carbon dating), Co-60 (cancer radiotherapy, food irradiation), I-131 (thyroid), U-235 (nuclear energy — Topic 15).
Fission (splitting nuclei — nuclear reactors, bombs) vs fusion (joining nuclei — the Sun, H-bomb).

Prelims facts: gamma rays are the most penetrating; alpha the least; carbon dating uses C-14; nuclear fusion powers the Sun; this section bridges to Nuclear Technology (Topic 15).

10. Current Affairs Link (2024–2026)

Chemistry stays topical through energy, materials & environment. Verify the latest before the exam. check for latest update or data

Green hydrogen: India's National Green Hydrogen Mission promotes hydrogen (made by electrolysis of water) as a clean fuel — direct chemistry of Sections 4 & 8. check for latest update or data

Critical minerals & lithium: lithium (batteries), cobalt & rare earths are strategic for EVs & electronics; India's critical-minerals mission & discoveries keep periodic-table chemistry topical. check for latest update or data

Ethanol blending & plastics: the ethanol-blended-petrol target and single-use-plastic ban apply organic & polymer chemistry to policy. check for latest update or data

Recent theme	Fundamental it tests	Why it matters for UPSC
Green hydrogen	Electrolysis, gases	GS-III energy transition.
Lithium & critical minerals	Metals, periodic table	GS-III strategic resources.
Ethanol blending	Organic chemistry	GS-III energy & farm income.
Plastic ban	Polymers	GS-III environment.

Recurring exam hooks: gases & uses · pH values · isotopes & dating · alloys · green hydrogen · lithium/rare earths · ethanol · fission vs fusion.

11. Prelims PYQs

Objective questions anchored to genuinely tested UPSC themes in chemistry. Each carries a worked rationale.

UPSC Prelims — Atomic structure

Q: Isotopes of an element have the same number of —

(a) protons but different neutrons
(b) neutrons but different protons
(c) protons and neutrons
(d) electrons and neutrons

Answer: (a) Isotopes have the same atomic number (protons) but different mass number (neutrons), e.g. C-12 and C-14.

UPSC Prelims — pH

Q: A solution with pH value 2 is —

(a) strongly acidic
(b) neutral
(c) weakly basic
(d) strongly basic

Answer: (a) On the 0–14 pH scale, values below 7 are acidic; pH 2 is strongly acidic (7 = neutral, above 7 = basic).

UPSC Prelims — Gases

Q: Which gas is commonly used in fire extinguishers?

(a) Oxygen
(b) Hydrogen
(c) Carbon dioxide
(d) Nitrogen

Answer: (c) CO₂ is denser than air & does not support combustion, so it smothers fires.

UPSC Prelims — Alloys

Q: Brass is an alloy of —

(a) copper and tin
(b) copper and zinc
(c) iron and carbon
(d) iron and chromium

Answer: (b) Brass = copper + zinc; bronze = copper + tin; steel = iron + carbon.

UPSC Prelims — Metals

Q: Which is the only metal that is liquid at room temperature?

(a) Sodium
(b) Mercury
(c) Aluminium
(d) Lead

Answer: (b) Mercury is the only metal liquid at room temperature; bromine is the only liquid non-metal.

UPSC Prelims — Radioactivity

Q: Carbon dating to find the age of fossils uses which isotope?

(a) C-12
(b) C-14
(c) U-235
(d) Co-60

Answer: (b) The radioactive isotope C-14, with a known half-life, is used for carbon dating of once-living material.

UPSC Prelims — Periodic table

Q: The elements of Group 18 of the periodic table are called —

(a) halogens
(b) alkali metals
(c) noble (inert) gases
(d) metalloids

Answer: (c) Group 18 are the noble/inert gases (He, Ne, Ar...) with a full outer shell, hence very unreactive.

UPSC Prelims — Fuels

Q: The main component of natural gas (CNG) is —

(a) methane
(b) ethane
(c) butane
(d) propane

Answer: (a) Natural gas / CNG is mainly methane (CH₄); LPG is mainly butane & propane.

Prelims — anticipated themes

Likely: gases & their uses · pH values · isotopes & dating · alloy compositions · noble gases & halogens · acids/bases in daily life · fuels (CNG, LPG, ethanol) · fission vs fusion · green hydrogen. check for latest update or data

12. Mains PYQs + Model Answers

Chemistry anchors applied GS-III questions on energy, materials & environment. The frameworks below show how to deploy it analytically.

Mains GS-III 15 marks · 250 words

Q: "Green hydrogen can be a game-changer for India's energy transition." Discuss its chemistry, potential and challenges.

Model Answer

Introduction — define: Green hydrogen is hydrogen produced by the electrolysis of water using renewable electricity — a clean fuel whose only by-product is water (Sections 4, 8).
Potential:
- Decarbonise hard-to-abate sectors (steel, fertilizers, refining, heavy transport); energy storage; export opportunity.
- Supports net-zero-2070 & energy self-reliance; National Green Hydrogen Mission targets scale.
Challenges: high cost of electrolysers & green power; water requirement; storage & transport (low density, flammable); lack of infrastructure & standards.
Way forward: scale renewables, cut electrolyser cost via R&D & PLI, build pilot hubs, set standards, and blend into industry gradually.
Conclusion: chemistry-driven and strategically vital — green hydrogen can anchor a clean, self-reliant energy future if costs fall.

Mains GS-III 15 marks · 250 words

Q: Critical minerals like lithium and rare earths are the "new oil". Examine their significance and India's strategy to secure them.

Model Answer

Introduction: Critical minerals (lithium, cobalt, nickel, rare earths) are essential to batteries, electronics, EVs & defence — elements of the periodic table (Section 3) now of strategic value.
Significance: backbone of the clean-energy & digital economy; EV & storage batteries; wind turbines; semiconductors & defence.
Challenges: import dependence; concentrated global supply (China dominance in processing); price volatility; environmental cost of mining.
India's strategy: Critical Minerals Mission & list; KABIL for overseas acquisition; domestic exploration (lithium finds); recycling; supply-chain partnerships (Minerals Security Partnership).
Conclusion: securing critical minerals is vital for energy & tech sovereignty — needing diversification, recycling & sustainable mining.

Mains GS-III 10 marks · 150 words

Q: Discuss the chemistry and policy behind ethanol blending in petrol and its benefits for India.

Model Answer

Introduction: Ethanol (C₂H₅OH), an organic alcohol from sugarcane/grain, is blended with petrol to make a cleaner, partly renewable fuel (Section 7).
Benefits: cuts crude-oil imports & forex; lower emissions; extra income for farmers & sugar mills; supports energy security.
Policy: Ethanol Blended Petrol Programme with a rising blending target; incentives for distilleries.
Concerns: food-vs-fuel & water use; engine compatibility; feedstock diversification (use surplus/waste).
Conclusion: a practical, chemistry-based step toward cleaner fuel & farm income, if sustainability is managed.

Mains GS-III 15 marks · 250 words

Q: Plastic pollution has become a major environmental challenge. Explain the chemistry involved and suggest solutions.

Model Answer

Introduction: Plastics are synthetic polymers (Section 7) of repeating hydrocarbon monomers; their durability makes them useful but non-biodegradable.
The problem:
- Single-use plastics persist for centuries; break into microplastics entering food & water.
- Harm marine life, choke drains, and release toxins on burning.
Solutions: single-use-plastic ban; Extended Producer Responsibility; biodegradable & compostable alternatives; recycling & circular economy; behavioural change.
Way forward: promote green chemistry (bioplastics), strengthen waste segregation, and enforce EPR.
Conclusion: the same polymer chemistry that created the crisis can, redirected, help solve it.

Mains GS-III — anticipated themes

Likely: green hydrogen · critical minerals & lithium · ethanol & biofuels · plastic pollution & green chemistry · battery chemistry & EVs · fertilizer & food chemistry. check for latest update or data

15-Minute Revision Box

Must-Remember Facts — Chemistry

Matter & atom:

States: solid, liquid, gas (+ plasma, BEC); sublimation = solid→gas (camphor, dry ice)
Electron (−, Thomson), proton (+, nucleus), neutron (0, Chadwick)
Atomic number = protons; isotopes differ in neutrons; C-14 = dating
Heavy water = D₂O; U-235 = fissile

Periodic table & bonding:

18 groups, 7 periods; Group 18 = noble gases, 17 = halogens, 1 = alkali metals
Mendeleev = father of periodic table
Ionic = transfer, covalent = share, metallic = free electrons
Ionic conducts in solution; metals conduct via free electrons

Acids, metals:

pH 0–14; 7 neutral, <7 acid, >7 base; blood ~7.4
Acid = H⁺ (sour, blue→red); base = OH⁻ (bitter, red→blue)
Mercury = only liquid metal; gold/platinum least reactive
Brass = Cu+Zn, bronze = Cu+Sn, steel = Fe+C, stainless = Fe+Cr+Ni

Organic & everyday:

Carbon = tetravalent, catenation; CNG = methane, LPG = butane/propane
Polymers: polythene, PVC, nylon, Teflon; plastic = non-biodegradable
CO₂ = fire extinguisher; N₂ = inert packaging; urea = N-fertilizer
Fission = split (reactor); fusion = join (Sun)

Highest-frequency themes: gases & uses · pH values · isotopes & dating · alloys · noble gases/halogens · polymers · fuels · fission vs fusion · green hydrogen & critical minerals.

Alloy	Composition
Steel	Fe + C
Stainless steel	Fe + Cr + Ni
Brass	Cu + Zn
Bronze	Cu + Sn
Solder	Pb + Sn

Gas	Use
CO₂	Fire extinguisher
N₂	Inert packaging
He	Balloons
Cl₂	Water treatment

Item	Value
Neutral pH	7
Electron	Thomson
Neutron	Chadwick
Periodic table	Mendeleev

Topic 6: Chemistry

On this page

Conceptual Clarity — How UPSC Tests Chemistry

1. Matter — States & Classification

2. Atomic Structure

3. The Periodic Table

4. Chemical Bonding

5. Acids, Bases & Salts (pH)

6. Metals, Non-Metals & Metallurgy

7. Organic Chemistry & Polymers

8. Everyday & Industrial Chemistry

9. Radioactivity & Nuclear Chemistry (Basics)

10. Current Affairs Link (2024–2026)

11. Prelims PYQs

12. Mains PYQs + Model Answers

Model Answer

Model Answer

Model Answer

Model Answer

15-Minute Revision Box

Must-Remember Facts — Chemistry