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.
On this page
- 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.
2. Atomic Structure
| 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.
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 |
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 |
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.
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).
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.
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 |
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).
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
| 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.
Q: Isotopes of an element have the same number of —
Answer: (a) Isotopes have the same atomic number (protons) but different mass number (neutrons), e.g. C-12 and C-14.
Q: A solution with pH value 2 is —
Answer: (a) On the 0–14 pH scale, values below 7 are acidic; pH 2 is strongly acidic (7 = neutral, above 7 = basic).
Q: Which gas is commonly used in fire extinguishers?
Answer: (c) CO₂ is denser than air & does not support combustion, so it smothers fires.
Q: Brass is an alloy of —
Answer: (b) Brass = copper + zinc; bronze = copper + tin; steel = iron + carbon.
Q: Which is the only metal that is liquid at room temperature?
Answer: (b) Mercury is the only metal liquid at room temperature; bromine is the only liquid non-metal.
Q: Carbon dating to find the age of fossils uses which isotope?
Answer: (b) The radioactive isotope C-14, with a known half-life, is used for carbon dating of once-living material.
Q: The elements of Group 18 of the periodic table are called —
Answer: (c) Group 18 are the noble/inert gases (He, Ne, Ar...) with a full outer shell, hence very unreactive.
Q: The main component of natural gas (CNG) is —
Answer: (a) Natural gas / CNG is mainly methane (CH₄); LPG is mainly butane & propane.
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.
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.
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.
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.
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.
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
- 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
- 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
- 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
- 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)