🧪 Chemistry

📌 0. Exam Pattern & 2025 Analysis (2 min read)

• MP Police SI Prelims has 10 Science questions: Physics 4 + Chemistry 3 + Biology 3. So Chemistry = 3 questions in every shift.
• Analysis of the 36 Chemistry questions in the official 2025 paper (16–21 Jan 2026, 12 shifts):
  • Atomic structure + Periodic table — the most (~14 questions): amu, isotopes, valency, quantum numbers, spectral series, electronegativity, dipole moment.
  • Polymers (~6): Teflon, nylon, polycarbonate, elastomer, starch plastic, wool-silk.
  • Soap / micelle (~3), Surface chemistry & kinetics (~5), Biomolecules & drugs (~4), d-block (KMnO₄) (~2), organic naming (~3).
• ⚠️ The level is not limited to Class 10 — many questions came from NCERT Class 11-12 concepts. That is why every topic on this page also has "Advanced (11-12) quick facts".

📌 1. Matter, Mixtures & Colloids (6 min read)

1.1 States of Matter

• Anything that occupies space and has mass = Matter. Three common states: Solid — fixed shape & volume; Liquid — fixed volume, no fixed shape; Gas — neither shape nor volume (most compressible).
• Plasma = 4th state — very hot, ionised gas. Found in the Sun/stars and inside neon signs.
• Bose-Einstein Condensate (BEC) = 5th state — at very low temperature. Named after Indian scientist Satyendra Nath Bose and Einstein.
• Attraction between particles: Solid > Liquid > Gas. Diffusion is fastest in gases.

1.2 Change of State

• Evaporation increases with: temperature ↑, surface area ↑, wind ↑, humidity ↓.
• Pressure cooker: higher pressure raises the boiling point of water → food cooks faster. On mountains air pressure is low → water boils below 100°C → food takes longer to cook.
• Adding salt to water raises the boiling point and lowers the freezing point (that is why salt is used to melt ice on roads).

1.3 Physical vs Chemical Change

1.4 Pure Substances & Mixtures

• Element: only one kind of atom — Fe, O₂, Na. Compound: elements chemically combined in a fixed ratio — H₂O, NaCl, CO₂ (properties differ from those of its elements).
• Mixture: no fixed ratio; components keep their own properties.
  • Homogeneous: uniform throughout — salt solution, air, alloys (brass), soda water.
  • Heterogeneous: — sand + water, oil + water, cornflakes-in-milk.
• Solution — smallest particles, no Tyndall effect. Suspension — large particles that settle down (muddy water, chalk-water). Colloid — intermediate size, particles do not settle, show the Tyndall effect (scattering of light — sunbeams coming through trees in a forest).

1.5 Types of Colloids (learn the examples)

• Breaking an emulsion (Demulsification): by heating, freezing or Centrifugation. A substance that keeps an emulsion stable = emulsifier (e.g. soap, protein).

1.6 Separation Techniques

📌 2. Atomic Structure (8 min read) — most questions in 2025

2.1 Sub-atomic Particles & their Discoverers

2.2 Atomic Models

• Dalton (1808): atom is indivisible — later proved wrong.
• Thomson model: "Plum pudding / watermelon model" — a positive sphere with electrons embedded in it.
• Rutherford's α-particle scattering experiment (Gold foil experiment): α-particles (+ve) were fired at a thin gold foil:
  • Most passed straight through → most of the atom is empty space.
  • Some were deflected through large angles → there is a small, heavy, positive nucleus at the centre that deflects α-particles by repulsion.
  • Very few (NCERT: about 1 in 12000) bounced straight back.
  • Drawback: revolving electrons should lose energy and fall into the nucleus — it could not explain stability.
• Bohr model (1913): electrons revolve in fixed orbits/shells K, L, M, N; each shell has a fixed energy. Max electrons in a shell = 2n² (K=2, L=8, M=18, N=32).

2.3 Atomic Number, Mass Number, Isotopes (Z, A, Isotopes)

• Atomic number, Z = number of protons (= electrons in a neutral atom). Concept: Henry Moseley (1913).
• Mass number, A = protons + neutrons. Neutrons = A − Z.
• Isotopes: same Z, different A → different neutrons. Chemical properties are the same (because electrons are the same). Hydrogen has 3: Protium ¹H (0 neutron), Deuterium ²H / D (1 neutron — heavy water D₂O), Tritium ³H (2 neutrons, radioactive). Carbon: C-12, C-13, C-14 (C-14 radioactive — carbon dating). Chlorine: Cl-35, Cl-37.
• Isobars: same A, different Z — ⁴⁰Ar and ⁴⁰Ca. Isotones: same number of neutrons.
• ⚠️ Fluorine has only one stable isotope (F-19) in nature (monoisotopic element); so in a "which has no isotopes" type question, the answer is Fluorine (Hydrogen, Carbon and Chlorine all have several isotopes).

2.4 Atomic Mass Unit & Mole (amu & Mole)

• Atomic mass unit (amu / u) = 1/12 of the mass of one C-12 atom ≈ 1.66 × 10⁻²⁷ kg. Atomic mass is written in this unit (not in kg/gram).
• Mole: 1 mole = 6.022 × 10²³ particles (Avogadro number). Mole = SI base unit (amount of substance).
• Molar mass: H₂O = 2(1) + 16 = 18 g/mol; CO₂ = 12 + 2(16) = 44 g/mol; NaCl = 23 + 35.5 = 58.5 g/mol.

2.5 Valency

• Valency is decided by the electrons in the outermost shell (valence shell) — how many electrons an atom will give/take/share to complete 8 (octet).
• Outer electrons 1-4 → valency = the same number; 5-7 → valency = 8 − electrons. Examples: Na (2,8,1) → 1; Mg → 2; Al → 3; C → 4; N (2,5) → 3 (valency of N in NH₃ is 3); O (2,6) → 2; Cl (2,8,7) → 1; noble gases → 0.

2.6 Advanced (11-12) quick facts — Quantum numbers & Spectrum

• Rules: Aufbau (the lower-energy orbital fills first), Pauli (max 2 electrons in one orbital, with opposite spins), Hund (first one electron in each orbital, then pairing).
• Hydrogen spectrum series (the level at which the falling electron ends):
  Lyman → n = 1 (UV) | Balmer → n = 2 (visible) | Paschen → n = 3 (IR) | Brackett → n = 4 (IR) | Pfund → n = 5 (IR).
  Trick: "Lucky Boy Played Badminton Properly" = 1, 2, 3, 4, 5.

📌 3. Periodic Table & Chemical Bonding (7 min read)

3.1 History of the Periodic Table

• Döbereiner's Triads: groups of 3 elements each — Li, Na, K; Ca, Sr, Ba; Cl, Br, I (atomic mass of the middle one = average of the other two).
• Newlands' Octaves: every 8th element is like the first (like musical notes) — true only up to Calcium.
• Mendeleev (1869): arranged elements in order of atomic mass; 63 elements known at that time; left gaps and predicted new elements — eka-boron (Scandium), eka-aluminium (Gallium), eka-silicon (Germanium). "Father of Periodic Table".
• Modern Periodic Law — Henry Moseley (1913): properties of elements are a periodic function of their atomic number.
• Modern table: 18 groups (vertical) and 7 periods (horizontal); 118 elements today. Blocks: s, p, d, f. Lanthanides & Actinides (f-block) placed separately at the bottom.

3.2 Important Groups

• Metalloids: B, Si, Ge, As, Sb, Te — properties of both metals and non-metals (Si, Ge = semiconductors).

3.3 Periodic Trends

• Electronegativity = the ability to pull the shared electrons of a bond towards itself. Fluorine is the most electronegative (Pauling scale 4.0) → the most non-metallic element. Order: F > O > N ≈ Cl.
• Electron affinity = energy released on adding an electron. Highest for Chlorine (not Fluorine — because of its small size, F has electron-electron repulsion). The electron affinity of Nitrogen is almost zero — because its half-filled p-orbital (2p³) is extra stable. Noble gases also have almost zero/positive values.
• Highest ionisation energy: Helium.

3.4 Chemical Bonding & Polarity

• Ionic bond: transfer of electrons (metal → non-metal) — NaCl, MgO. High melting point, dissolve in water, conduct current when molten/dissolved.
• Covalent bond: sharing of electrons — H₂, Cl₂, CH₄, H₂O. Single (H–H), double (O=O), triple (N≡N).
• Hydrogen bond (H-bond): H + a highly electronegative atom (F, O, N) — because of it water has a high boiling point, ice floats on water, the two strands of DNA are held together, and the α-helix of proteins stays stable.
• Dipole moment (μ = q × d, unit Debye): measure of polarity. In symmetrical molecules the dipoles cancel → μ = 0 (non-polar): CO₂ (linear), BeCl₂, BF₃ (trigonal planar), SO₃, CH₄, CCl₄, XeF₄ (square planar).
  Polar (μ ≠ 0): H₂O (bent), NH₃ (pyramidal), HCl, CHCl₃, SF₄ (see-saw shape, 1 lone pair).
• Dipole moment of methyl halides: CH₃Cl > CH₃F > CH₃Br > CH₃I. (F is the most electronegative, but the C–F bond is short, so CH₃Cl is at the top.)

📌 4. Chemical Reactions, Catalysts & Adsorption (7 min read)

4.1 Types of Reactions (NCERT Class 10)

• Oxidation: gain of oxygen / loss of hydrogen / loss of electrons. Reduction: loss of oxygen / gain of hydrogen / gain of electrons. A substance that gets reduced itself and oxidises another = oxidising agent.
• Corrosion: Rust (Rust = Fe₂O₃·xH₂O) on iron — it needs both air (O₂) and moisture (water). Silver turns black (silver sulphide), copper turns green (basic copper carbonate). Prevention: paint, oil/grease, Galvanisation (= coating of zinc), alloys (stainless steel).
• Rancidity: oily/ghee-rich food gets oxidised and smells bad. Prevention: antioxidants, airtight containers, nitrogen gas in chips packets, refrigerator.

4.2 Rate of Reaction & Catalyst

• Rate increases with: concentration ↑, temperature ↑, surface area ↑ (powder reacts faster), Catalyst.
• A Catalyst increases the rate but is not used up itself; it lowers the Activation energy (provides a new, easier path). Enzymes = biocatalysts.
• Rate-determining step: in a multi-step reaction, the slowest step — i.e. the one with the highest activation energy. It decides the speed of the whole reaction.

4.3 Advanced (12): Adsorption

• Adsorption = particles of one substance collecting only on the surface of another (charcoal adsorbs gases — gas mask). Absorption = throughout the inside (water in a sponge).

• Assumptions of the Langmuir adsorption isotherm: (1) all adsorption sites on the surface are identical (equivalent/homogeneous); (2) only one layer (monolayer); (3) no interaction between adsorbed molecules.

📌 5. Acids, Bases & Salts (8 min read)

5.1 Basics

• Neutralisation: Acid + Base → Salt + Water.
• Acid + metal → salt + H₂ gas (a "pop" sound when a burning matchstick is brought near). Acid + carbonate/bicarbonate → salt + CO₂ + water (CO₂ turns lime water milky).
• ⚠️ While diluting an acid, add acid to water, not water to acid (a lot of heat is released and it may splash).

5.2 Indicators

• Olfactory (smell) indicators: onion, vanilla, clove — their smell disappears in a base.

5.3 pH Scale

• pH = "potenz" (German: power) of H⁺; scale 0–14; given by S.P.L. Sørensen (1909). pH < 7 acidic, = 7 neutral (pure water), > 7 basic. When pH falls by 1, H⁺ concentration increases 10 times.

• Teeth: when the pH of the mouth falls below 5.5, enamel (calcium hydroxyapatite — the hardest substance in the body) starts dissolving → tooth decay. Toothpaste is basic.
• Acid rain: rain with pH less than 5.6 (due to SO₂, NOₓ).
• Acidity in the stomach: antacid — milk of magnesia Mg(OH)₂ or baking soda.
• Soil: quicklime (CaO) / slaked lime (Ca(OH)₂) / chalk is added to very acidic soil; organic matter (manure) to very basic soil.
• Insect sting: the sting of an ant/nettle contains methanoic (formic) acid → relief with a mild base like baking soda.

5.4 Natural Acids (must memorise)

5.5 Everyday Salts (NCERT Class 10)

📌 6. Metals, Non-metals, Ores & Alloys (8 min read)

6.1 Metals vs Non-metals (Properties)

6.2 Exceptions (asked most often)

• Mercury, Hg — a liquid metal at room temperature. Bromine (Br) — a liquid non-metal.
• Gallium, Caesium — melting point so low that they melt on the palm.
• Sodium, Potassium — so soft that they can be cut with a knife; they catch fire in air, so they are kept in kerosene. White phosphorus burns in air → kept in water.
• Iodine — a non-metal, yet shiny. Graphite — a non-metal, yet a conductor of electricity. Diamond — the hardest natural substance, very high melting point.
• Gold, Silver — the most malleable/ductile (1 g of gold gives a wire ~2 km long). Lead, Mercury — poor conductors of heat.
• Tungsten — the highest melting point among metals → bulb filament. Lithium — the lightest metal. Osmium — the densest metal.
• In the Earth's crust: the most abundant element is Oxygen, then Silicon; the most abundant metal is Aluminium, then Iron.
• Aluminium oxide (Al₂O₃), Zinc oxide (ZnO) = amphoteric — react with both acids and bases.

6.3 Reactivity Series

K > Na > Ca > Mg > Al > Zn > Fe > Pb > [H] > Cu > Hg > Ag > Au
Trick: "Kaise NahaCar Mgan Ali Znak Feke Pbar Hota Cuchh Hg Ag Au" (a Roman Hindi mnemonic; the bold letters give the symbols in order: K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Au).
• Metals above H release H₂ from dilute acids; Cu, Hg, Ag, Au do not.
• Aqua regia = 3 parts conc. HCl + 1 part conc. HNO₃ — dissolves even gold (Au) and platinum.

6.4 Metallurgy (short)

• Mineral = a compound found in nature; Ore = a mineral from which the metal can be extracted profitably. Useless soil/sand = gangue.
• Roasting: heating a sulphide ore in excess air. Calcination: heating a carbonate ore in limited/no air.
• Low reactivity (Hg, Cu) — by heating alone; medium (Zn, Fe, Pb) — reduction with carbon; very reactive (Na, Mg, Ca, Al) — by electrolysis.
• Thermite reaction: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + a lot of heat → joining railway tracks.
• Electrolytic refining: impure metal = anode, thin strip of pure metal = cathode.

6.5 Important Ores

📍 MP link: Malajkhand (Balaghat) — the country's largest open-cast copper mine; Panna — diamond mine (Majhgawan).

6.6 Alloys

• The electrical conductivity and melting point of an alloy are lower than those of the pure metal.
• 24 carat = pure gold (soft). 22 carat = 22 parts gold + 2 parts copper/silver (~91.6%).

6.7 Advanced (12): d-block — KMnO₄

• Potassium permanganate (KMnO₄, "lal dawa") — deep purple. The oxidation state of Mn is +7 (d⁰), so a d-d transition is not possible; the colour comes from charge transfer (oxygen → manganese).
• In the lab, peroxodisulphate (S₂O₈²⁻) is used to oxidise Mn²⁺ to MnO₄⁻.
• KMnO₄: strong oxidising agent, disinfectant for water/wells. K₂Cr₂O₇ — orange.
• Transition metals: coloured compounds, variable oxidation states, catalysts (Fe, Ni, Pt, V₂O₅).

📌 7. Carbon Compounds, Soaps & Detergents (8 min read)

7.1 Properties of Carbon & Allotropes

• Carbon has valency 4 (tetravalency); its property of forming long chains with itself = Catenation → hence lakhs of carbon compounds.
• Carbon forms covalent bonds (giving/taking 4 electrons is difficult).

Amorphous forms: coal, charcoal (wood charcoal), coke, lamp black.

7.2 Hydrocarbons

• Saturated hydrocarbons burn with a blue (clean) flame; unsaturated ones with a yellow, smoky flame.
• Benzene C₆H₆ — aromatic ring. Positions of two substituents: ortho = 1,2; meta = 1,3; para = 1,4. E.g. para-dichlorobenzene = 1,4-dichlorobenzene.

7.3 Functional Groups

• Esterification: acid + alcohol → ester + water (acid catalyst).
• Saponification: ester (oil/fat) + NaOH → soap + glycerol/alcohol.
• Advanced (12): Gabriel phthalimide synthesis gives only primary aliphatic amines (ethylamine, n-propylamine, benzylamine). Aromatic amines (aniline) and tert-butylamine cannot be made (aryl halides / tertiary halides do not undergo SN2 reaction; a tertiary halide undergoes elimination).

7.4 Soaps & Detergents

• Soap = sodium/potassium salts of long-chain fatty acids (Na → hard soap, K → soft/liquid soap).
• Detergent = long-chain sulphonate or ammonium salts. They work even in hard water; soap forms scum (precipitate) with hard water (Ca²⁺, Mg²⁺).
• Soap molecule: one end is ionic, hydrophilic (–COO⁻Na⁺) → towards water; the other is a long hydrocarbon tail, hydrophobic → towards oil/dirt.
• Micelle: in water, soap molecules form a sphere — tails inside (hold the oil/dirt), ionic ends outside towards the water. The dirt gets trapped in the middle of the micelle and is washed away with the water.
• Advanced (12): A micelle forms only above a certain concentration (CMC — Critical Micelle Concentration) and above a certain temperature (Krafft temperature, T_k). That is why soap cleans less well in very cold water. Needed for a micelle: enough concentration, a non-polar tail, a polar solvent (water) — low temperature is not needed.

📌 8. Common Chemicals, Fuels & Fertilizers (8 min read)

8.1 Common Names & Formulas

• 📍 MP link — Bhopal Gas Tragedy (2–3 Dec 1984): leak of methyl isocyanate (MIC) gas from the Union Carbide factory — one of the world's biggest industrial disasters.
• Tear gas — used by police for crowd control: today mainly CS and CN (chloroacetophenone) gas.
• Monosodium glutamate (MSG / Ajinomoto) — flavour enhancer; Sodium benzoate — food preservative.

8.2 Fuels

• Calorific value = heat released when 1 kg of fuel burns completely (kJ/kg). A good fuel: high calorific value, cheap, easily available, little smoke/pollution, moderate ignition temperature.

• LPG — mainly butane (+ propane); it is odourless, so ethyl mercaptan is added to detect leaks. CNG — mainly methane. Biogas — methane (NCERT: up to ~75%) + CO₂; in a gobar gas plant, made by anaerobic (without oxygen) bacteria.
• Water gas = CO + H₂; Producer gas = CO + N₂; Coal gas = H₂ + CH₄ + CO.
• Types of coal (in increasing order of carbon %): Peat < Lignite < Bituminous < Anthracite (best). The slow conversion of dead vegetation into coal over millions of years = carbonisation (NCERT Class 8).
• From coal: coke (almost pure carbon — used in making steel), coal tar (naphthalene balls, dyes, explosives, perfumes), coal gas.
• Petroleum ("black gold") → fractional distillation in a refinery → LPG, petrol, kerosene, diesel, lubricating oil, paraffin wax, bitumen (roads).
• Octane number — ability of petrol to prevent "knocking" (higher = better). For diesel, cetane number.
• Incomplete burning → CO (carbon monoxide) — poisonous, colourless and odourless; it binds with haemoglobin and stops it from carrying oxygen (an angithi/coal stove in a closed room is dangerous).
• Zones of a flame (NCERT 8): outermost — blue, hottest, complete combustion (used by goldsmiths); middle — yellow, luminous, partial combustion; innermost — black, unburnt wax vapour.
• Fire fighting: no water on electrical or oil fires — a CO₂ extinguisher is best (cuts off the oxygen supply and does not damage equipment).

8.3 Fertilizers

• Main plant nutrients: N, P, K (nitrogen, phosphorus, potassium).
• Urea — CO(NH₂)₂ — highest nitrogen (~46%). Made from ammonia (Haber process) + CO₂.
• DAP (Diammonium phosphate) — N + P; SSP (Single superphosphate) — P; MOP (Muriate of potash = KCl) — K; ammonium sulphate, ammonium nitrate, CAN.
• Biofertilizers: Rhizobium (fixes N₂ in the root nodules of pulses / legumes), Azotobacter, Anabaena / Nostoc (blue-green algae — paddy fields), Mycorrhiza (phosphorus).
• Organic manure improves soil structure; excessive use of chemical fertilizers pollutes soil and water.

📌 9. Polymers & Plastics (7 min read) — 6 questions in 2025!

9.1 Basics

• Polymer = a large molecule formed by repeated joining of small units (monomers). E.g. ethene → polythene.
• Natural: starch, cellulose (cotton/wood), protein, DNA, natural rubber (polyisoprene), silk (protein fibroin), wool (protein keratin).
• Semi-synthetic: Rayon ("artificial silk") — from wood pulp (cellulose); vulcanised rubber.
• Synthetic: nylon, polyester, PVC, polythene, Teflon, Bakelite, acrylic.

9.2 Thermoplastic vs Thermosetting

9.3 Important Polymers & Uses

• Elastomers: rubber-like polymers — they stretch when pulled and return to their original shape when released (weak van der Waals forces). Natural rubber, neoprene, Buna-S.
• Fibres: strong intermolecular forces (H-bonds) → high tensile strength (nylon, polyester, silk).
• Wool and silk = fibrous proteins — long thread-like structure, strong intermolecular forces, insoluble in water → can form fibres. Albumin (egg white) = globular protein — spherical, soluble in water → does not form fibres.

9.4 Biodegradable Polymers & the Plastic Problem

• Most plastics are non-biodegradable — do not decompose for years; release poisonous gases when burnt. 5R: Reduce, Reuse, Recycle, Recover, Refuse.
• Biodegradable polymers: PHBV, Nylon-2-Nylon-6, starch-based plastics.
• Limitation of starch plastic: starch absorbs water/moisture and its mechanical properties (mechanical strength) are weak.

📌 10. Biomolecules & Chemistry in Daily Life (7 min read)

10.1 Carbohydrates

• Reducing sugar — reduces Fehling's solution (blue → red Cu₂O precipitate) and Tollens' reagent (silver mirror); for this it needs a free aldehyde/ketone (anomeric carbon).
• Two monosaccharides join by a glycosidic linkage. In sucrose, C1 of glucose and C2 of fructose — both carbonyl carbons get locked in the bond (acetal/ketal linkage), so sucrose is not reducing.

10.2 Proteins, Enzymes & Nucleic Acids

• Protein = a chain of amino acids joined by peptide bonds. About ~20 amino acids in all.
  • Essential amino acids — the body cannot make them itself; they must come from food (valine, leucine, lysine, tryptophan, phenylalanine...). Non-essential — the body can make them. (Basis of classification = the body's ability to synthesise them.)
  • Fibrous proteins (keratin — hair/wool/nails, collagen, fibroin — silk, myosin) — insoluble in water. Globular (albumin, insulin, haemoglobin, enzymes) — soluble.
  • Structure: primary (sequence), secondary (α-helix, β-pleated sheet — stabilised by H-bonds), tertiary, quaternary. Denaturation: structure is destroyed by heat/acid — boiling an egg, curdling of milk.
• Enzymes = biocatalysts (mostly proteins); they lower activation energy; highly specific. According to NCERT, 6 classes — based on the type of reaction they catalyse: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.
  Examples: zymase (sugar → ethanol, yeast), invertase (sucrose → glucose + fructose), diastase (starch → maltose), pepsin (protein).
• Nucleic acids: DNA (deoxyribose; A, T, G, C) and RNA (ribose; A, U, G, C) — details on the Biology page.

10.3 Medicines & Everyday Chemicals

• Aspirin (the acetyl form of salicylic acid) — the acetyl group reduces gastric irritation while the pain-relieving/anti-inflammatory properties remain; it thins the blood (helps prevent heart attacks). Salicylic acid itself harms the stomach more.

📌 11. Noble Gases, Hydrogen, Air & Radioactivity (6 min read)

11.1 Noble / Inert Gases (Group 18)

• These gases are almost inert because their outer shell is completely filled (2 electrons in He, 8 in the others).

11.2 Hydrogen, Water & Air

• Hydrogen — the lightest element, the most abundant in the universe. Isotopes: protium, deuterium, tritium.
• Hardness of water: temporary — bicarbonates of Ca/Mg → removed by boiling; permanent — chlorides/sulphates of Ca/Mg → removed by washing soda or ion-exchange.
• Dry air: Nitrogen ~78%, Oxygen ~21%, Argon ~0.93%, CO₂ ~0.04%.
• Ozone (O₃) layer — in the stratosphere; absorbs the Sun's UV rays. CFCs (chlorofluorocarbons) — fridge/AC gases — destroy it. Montreal Protocol (1987).
• Greenhouse gases: CO₂, methane (CH₄), N₂O, CFCs, water vapour → global warming. Acid rain — SO₂ and NOₓ.

11.3 Radioactivity

• Discovery: Henri Becquerel (1896) — in uranium. Marie & Pierre Curie — discovered polonium and radium; Marie Curie — Nobel in two different subjects (Physics 1903, Chemistry 1911).
• SI unit: becquerel (Bq); old unit: curie (Ci).

• Half-life: the time in which half of the radioactive nuclei decay.
• Carbon dating — age of fossils/old wood from C-14 (half-life ~5730 years) (W.F. Libby).
• Uses: Cobalt-60 — cancer treatment; Iodine-131 — thyroid; Uranium-235 — fuel of nuclear reactors.