BharatNotes Why this chapter matters for UPSC: This is Chapter 4 of NCERT's Class X Science (Reprint 2026-27). Carbon is scarce (the earth's crust has only 0.02 % carbon and the atmosphere 0.03 % carbon dioxide), yet it forms millions of compounds. The chapter explains why: carbon shares electrons instead of gaining or losing them, it bonds with itself into chains and rings, and it has four bonds to give. It then names those compounds, runs four kinds of reaction on them, and ends with two compounds you meet every day (ethanol and ethanoic acid) and with how soap cleans. Prelims general science draws on all of it: allotropes, hydrogenation, methanol poisoning and denatured alcohol, glacial acetic acid, micelles, hard water.
Contemporary hook: In July 2024 the Ministry of Environment, Forest and Climate Change restated that plastic carry bags thinner than 120 microns are prohibited from 31 December 2022. Polythene is made from ethene, a compound this chapter teaches, and the Beyond-the-textbook box below traces the line from the chemistry to the rule.
🧠 First Principles — Read This First
Carbon solves the octet problem by sharing. It has four valence electrons. Gaining four more would leave six protons holding ten electrons; losing four would cost too much energy. So carbon shares electron pairs, and a shared pair is a covalent bond. Covalent compounds are molecules held together strongly inside but weakly to each other, which is why they melt and boil at low temperatures and do not conduct electricity.
Two properties make carbon versatile. Catenation lets carbon atoms bond to each other in long chains, branched chains and rings, joined by single, double or triple bonds. Tetravalency lets each carbon bond to four other atoms, of carbon or of other elements. Add a functional group (an –OH, a –COOH, a halogen) and the compound takes that group's chemistry whatever the chain length. Everything else in the chapter, from naming to soap, follows from these ideas.
PART 1 — Quick Reference
Table 1: Bonding in carbon and what follows from it
| Idea | NCERT's statement |
|---|---|
| Why not ionic | C⁴⁻ is hard because it is "difficult for the nucleus with six protons to hold on to ten electrons"; C⁴⁺ would need "a large amount of energy to remove four electrons" |
| Covalent bond | A bond formed by sharing an electron pair between two atoms; both atoms reach the noble-gas configuration |
| Single, double, triple | H₂ has a single bond, O₂ a double bond (two shared pairs), N₂ a triple bond (three shared pairs) |
| Methane | CH₄: carbon shares its four valence electrons with four hydrogen atoms; widely used as a fuel and "a major component of bio-gas and Compressed Natural Gas (CNG)" |
| Properties | Strong bonds within the molecule, weak forces between molecules, so low melting and boiling points; no charged particles, so generally poor conductors of electricity |
Source: NCERT, Science Class X, ch. 4, Reprint 2026-27, section 4.1 (pp. 58-61).
NCERT Table 4.1, melting and boiling points (K): acetic acid (CH₃COOH) 290 and 391; chloroform (CHCl₃) 209 and 334; ethanol (CH₃CH₂OH) 156 and 351; methane (CH₄) 90 and 111.
Source: NCERT, Science Class X, ch. 4, Reprint 2026-27, Table 4.1 (p. 59).
Table 2: Allotropes of carbon
| Form | How the carbon atoms are bonded | Property NCERT gives |
|---|---|---|
| Diamond | Each carbon atom bonded to four others: a rigid three-dimensional structure | "Diamond is the hardest substance known" |
| Graphite | Each carbon atom bonded to three others in the same plane, a hexagonal array; one of these bonds is a double bond; the arrays lie in layers | Smooth and slippery; "a very good conductor of electricity", unlike other non-metals |
| Fullerenes | C-60, the first identified, has its atoms arranged like a football | Named after the US architect Buckminster Fuller, whose geodesic dome it resembled |
| Synthetic diamond | Pure carbon under very high pressure and temperature | Small, but "otherwise indistinguishable from natural diamonds" |
Diamond and graphite have very different physical properties "even though their chemical properties are the same".
Source: NCERT, Science Class X, ch. 4, Reprint 2026-27, "Allotropes of carbon" (pp. 61).
Table 3: Hydrocarbons, functional groups and names
| Term | Meaning, with NCERT's examples |
|---|---|
| Saturated compounds | Only single bonds between carbon atoms; "normally not very reactive". Alkanes: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀, pentane C₅H₁₂, hexane C₆H₁₄ (Table 4.2) |
| Unsaturated compounds | Double or triple bonds between carbon atoms; "more reactive than the saturated carbon compounds". Alkenes (double bond): ethene C₂H₄. Alkynes (triple bond): ethyne C₂H₂ |
| Hydrocarbons | Compounds of carbon and hydrogen only |
| Structural isomers | Same molecular formula, different structures: butane C₄H₁₀ has two carbon skeletons |
| Rings | Cyclohexane C₆H₁₂ (saturated ring); benzene C₆H₆ |
| Heteroatom | An element that replaces hydrogen in a hydrocarbon chain: halogens, oxygen, nitrogen, sulphur |
| Functional group | A heteroatom or group that confers specific properties "regardless of the length and nature of the carbon chain" (Table 4.3: halo –Cl, –Br; alcohol –OH; aldehyde; ketone; carboxylic acid) |
| Homologous series | Compounds in which the same functional group substitutes for hydrogen in a carbon chain; successive members differ by a –CH₂– unit |
Naming (Table 4.4), on a three-carbon chain: chloropropane, bromopropane (prefix chloro-, bromo-); propanol (-ol, alcohol); propanal (-al, aldehyde); propanone (-one, ketone); propanoic acid (-oic acid, carboxylic acid); propene (-ene, double bond); propyne (-yne, triple bond).
Source: NCERT, Science Class X, ch. 4, Reprint 2026-27, sections 4.2.1-4.2.5, Tables 4.2-4.4 (pp. 63-68).
Table 4: Four reactions of carbon compounds
| Reaction | What happens | NCERT's example or use |
|---|---|---|
| Combustion | Carbon and most carbon compounds burn in oxygen, giving out heat and light | C + O₂ → CO₂; methane and ethanol burn to CO₂ and H₂O (NCERT asks you to balance these two) |
| Oxidation | Oxidising agents add oxygen: alcohols become carboxylic acids | Ethanol → ethanoic acid with alkaline potassium permanganate or acidified potassium dichromate |
| Addition | Unsaturated hydrocarbons add hydrogen in the presence of palladium or nickel catalysts, giving saturated hydrocarbons | Hydrogenation of vegetable oils with a nickel catalyst |
| Substitution | In sunlight, chlorine replaces the hydrogen atoms of a saturated hydrocarbon one by one | CH₄ + Cl₂ → CH₃Cl + HCl (in the presence of sunlight) |
Source: NCERT, Science Class X, ch. 4, Reprint 2026-27, sections 4.3.1-4.3.4 (pp. 69-71).
Table 5: Ethanol and ethanoic acid
| Ethanol (CH₃CH₂OH) | Ethanoic acid (CH₃COOH) | |
|---|---|---|
| Common name | Alcohol | Acetic acid; a carboxylic acid |
| State and melting point | Liquid at room temperature (m.p. 156 K) | m.p. 290 K, so it often freezes in cold winters: "glacial acetic acid" |
| Everyday form | Active ingredient of alcoholic drinks; solvent in tincture iodine, cough syrups and many tonics; mixes with water in all proportions | Vinegar = a 5-8 % solution in water, a preservative in pickles |
| Reaction 1 | With sodium: 2Na + 2CH₃CH₂OH → 2CH₃CH₂O⁻Na⁺ (sodium ethoxide) + H₂ | With ethanol, acid catalyst: an ester (esterification) |
| Reaction 2 | Heated at 443 K with excess concentrated H₂SO₄: dehydrated to ethene, CH₂=CH₂ + H₂O | With NaOH: sodium ethanoate (sodium acetate) and water |
| Reaction 3 | Oxidised to ethanoic acid | With Na₂CO₃ or NaHCO₃: sodium acetate, water and CO₂ |
| Hazard | Absolute alcohol can be lethal even in small quantity; long-term use harms health | A weak acid, unlike HCl, which is completely ionised |
Source: NCERT, Science Class X, ch. 4, Reprint 2026-27, sections 4.4.1-4.4.2 (pp. 72-74).
Table 6: Soaps and detergents
| Soap | Detergent | |
|---|---|---|
| What it is | Sodium or potassium salt of a long-chain carboxylic acid | "generally sodium salts of sulphonic acids or ammonium salts with chlorides or bromides ions"; also a long hydrocarbon chain |
| How it cleans | Ionic end in water, carbon chain in the oil: micelles hold the oily dirt in the centre | Same two-ended action |
| In hard water | Reacts with calcium and magnesium salts to form scum; more soap is needed | Charged ends form no insoluble precipitate with Ca²⁺ and Mg²⁺, so it stays effective |
| Uses | Washing | Shampoos and products for cleaning clothes |
Source: NCERT, Science Class X, ch. 4, Reprint 2026-27, section 4.5 (pp. 74-76).
Table 7: NCERT lines to read with care
| NCERT says | The record |
|---|---|
| Combustion equations (ii) CH₄ + O₂ → CO₂ + H₂O and (iii) CH₃CH₂OH + O₂ → CO₂ + H₂O, followed by "Balance the latter two reactions like you learnt in the first Chapter" (p. 69) | They are printed unbalanced on purpose. Balanced: CH₄ + 2O₂ → CO₂ + 2H₂O; CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O. |
| "Diamond is the hardest substance known" (ch. 4, p. 61) | Chapter 3 of the same book says diamond is "the hardest natural substance known". Prefer the Chapter 3 wording in an answer. |
| Exercise 13 lists C₃H₆ among hydrocarbons to test for addition reactions | Read C₃H₆ as propene, an alkene (CnH₂n). NCERT's own cyclohexane (C₆H₁₂) shows that a saturated ring can share the alkene formula, so a formula alone does not prove a double bond. |
| Section 4.3.3 teaches hydrogenation of vegetable oils as an industrial use, then says "Oils containing unsaturated fatty acids should be chosen for cooking" (p. 71) | Both are NCERT's lines. The chemistry point for an exam: hydrogenation turns unsaturated chains into saturated ones. |
Source: NCERT, Science Class X, Reprint 2026-27, ch. 4 pp. 61, 69, 71, 78; ch. 3 "Physical properties".
PART 2 — Concepts & Narrative
Bonding in carbon: the covalent bond (4.1)
Carbon compounds are mostly poor conductors and have low melting and boiling points compared with the ionic compounds of Chapter 3. Both facts point to the same cause: the bonding produces no ions, and the forces between molecules are weak.
Carbon's atomic number is 6, so it has four electrons in its outer shell. It could gain four to make C⁴⁻, but six protons would struggle to hold ten electrons. It could lose four to make C⁴⁺, but that needs a large amount of energy. Instead it shares. NCERT builds up to carbon through simpler molecules: hydrogen (H₂, one shared pair, a single bond), oxygen (O₂, two shared pairs, a double bond) and nitrogen (N₂, three shared pairs, a triple bond). Methane, CH₄, follows: carbon is tetravalent, so it shares one electron pair with each of four hydrogen atoms.
Covalent bond: a bond "formed by the sharing of an electron pair between two atoms". The shared electrons belong to the outer shells of both atoms, so both reach the noble-gas configuration.
Allotropes of carbon. Diamond and graphite are both pure carbon; the difference lies in how the atoms are bonded (Table 2). Each diamond atom bonds to four others in a rigid three-dimensional structure, so diamond is very hard. Each graphite atom bonds to three others in a flat hexagonal sheet, one of the three bonds being a double bond, and the sheets lie in layers, so graphite is smooth and slippery and conducts electricity. Diamonds can be made by subjecting pure carbon to very high pressure and temperature. Fullerenes are a third class; the first identified, C-60, is shaped like a football and was named after Buckminster Fuller.
Same atoms, different properties. Diamond and graphite have the same chemical properties because they are the same element. Their physical properties differ because their structures differ. This is the cleanest example in school chemistry that structure, not composition alone, decides how a material behaves.
The versatile nature of carbon (4.2)
The number of carbon compounds whose formulae are known "was recently estimated to be in millions", more than the compounds of all other elements put together. NCERT gives two reasons.
- Catenation. Carbon bonds with other carbon atoms into long chains, branched chains and rings. No other element does this to the same extent: silicon forms hydrogen compounds with chains of "upto seven or eight atoms, but these compounds are very reactive". "The carbon-carbon bond is very strong and hence stable."
- Tetravalency. Carbon can bond with four other atoms: carbon, or monovalent elements, and also oxygen, hydrogen, nitrogen, sulphur and chlorine. The bonds are strong. "One reason for the formation of strong bonds by carbon is its small size": the nucleus holds the shared pairs tightly, while bigger atoms form weaker bonds.
Organic compounds and the "vital force". Carbon compounds were first extracted from living things, and it was thought they could be made only inside a living system, by a "vital force". "Friedrich Wöhler disproved this in 1828 by preparing urea from ammonium cyanate." The name organic chemistry stayed. It covers carbon compounds except carbides, oxides of carbon, and carbonate and hydrogencarbonate salts.
Saturated and unsaturated compounds (4.2.1)
NCERT builds structures in steps: join the carbon atoms with single bonds, then fill the remaining valencies with hydrogen. For ethane, C₂H₆, each carbon has three valencies left and takes three hydrogens. When single bonds satisfy every valency, as in ethane and propane, the compound is saturated. Ethene, C₂H₄, has one valency per carbon left over after the hydrogens, so the two carbons share a double bond. Ethyne, C₂H₂, needs a triple bond. Compounds with double or triple bonds are unsaturated and more reactive.
Chains, branches and rings (4.2.2)
The alkanes run methane, ethane, propane, butane, pentane, hexane (Table 4.2). Butane can be drawn with two different skeletons, a straight chain and a branched one, each with the formula C₄H₁₀.
Structural isomers: "compounds with identical molecular formula but different structures". Butane's two skeletons are the chapter's example.
Carbon atoms can also form rings: cyclohexane, C₆H₁₂, is a saturated ring, and benzene, C₆H₆, is a ring with double bonds. Straight, branched and ring compounds may each be saturated or unsaturated. Saturated hydrocarbons are alkanes; those with one or more double bonds are alkenes; those with one or more triple bonds are alkynes.
Functional groups and the homologous series (4.2.3-4.2.4)
When a halogen, oxygen, nitrogen or sulphur replaces hydrogen in a chain, it is a heteroatom. Heteroatoms and the groups containing them are functional groups, because they confer specific properties whatever the chain. Table 4.3 lists halo- (–Cl, –Br), alcohol (–OH), aldehyde, ketone and carboxylic acid.
Homologous series: a series of compounds "in which the same functional group substitutes for hydrogen in a carbon chain". CH₃OH, C₂H₅OH, C₃H₇OH and C₄H₉OH have very similar chemical properties. Successive members differ by a –CH₂– unit; with NCERT's atomic masses (C 12 u, H 1 u) that is 14 u. Alkenes follow CnH₂n (n = 2, 3, 4...). From Table 4.2, alkanes follow CnH₂n₊₂.
Physical properties change in steps along a series: melting and boiling points rise with molecular mass, and solubility changes similarly. "But the chemical properties, which are determined solely by the functional group, remain similar in a homologous series."
Naming carbon compounds (4.2.5)
- Count the carbons: three carbons give the stem propane.
- Show the functional group by a prefix or a suffix (Table 4.4).
- If the suffix begins with a vowel, drop the final "e" of the stem: propane − e + one = propanone.
- For an unsaturated chain, replace "ane" with "ene" (double bond) or "yne" (triple bond): propene, propyne.
Chemical properties (4.3)
Combustion (4.3.1). "Carbon, in all its allotropic forms, burns in oxygen to give carbon dioxide along with the release of heat and light." In Activities 4.3 and 4.4, saturated hydrocarbons generally give a clean flame and unsaturated compounds a yellow flame with lots of black smoke, which leaves soot on a metal plate. Limiting the air supply makes even saturated hydrocarbons burn incompletely with a sooty flame. A gas or kerosene stove has air inlets so that an oxygen-rich mixture gives a clean blue flame. "If you observe the bottoms of cooking vessels getting blackened, it means that the air holes are blocked and fuel is getting wasted." Coal and petroleum contain some nitrogen and sulphur, and burning them forms oxides of sulphur and nitrogen, "major pollutants in the environment".
Flame or glow. Charcoal in an angithi glows red without a flame because "a flame is only produced when gaseous substances burn". Wood or charcoal flames at first only while its volatile substances vapourise and burn. A luminous flame is seen when atoms of a gas are heated and glow, and each element gives its own colour. NCERT asks you to explain a candle's yellow flame: incomplete combustion gives soot, which is carbon.
How coal and petroleum formed. Coal is the remains of trees, ferns and other plants of millions of years ago, pressed under layers of earth and rock. Oil and gas are the remains of tiny sea plants and animals, buried in silt and turned to oil and gas by bacteria under high pressure; they seeped into porous rock. Both are fossil fuels because both are the remains of ancient life.
Oxidation (4.3.2). Combustion is complete oxidation. Alcohols can also be oxidised to carboxylic acids. In Activity 4.5, a 5 % solution of alkaline potassium permanganate is added drop by drop to warm ethanol: at first its colour disappears, as it oxidises the ethanol, and later drops keep their colour. Substances that add oxygen to others are oxidising agents; alkaline potassium permanganate and acidified potassium dichromate are two.
Addition (4.3.3). Unsaturated hydrocarbons add hydrogen in the presence of catalysts such as palladium or nickel to give saturated hydrocarbons. Vegetable oils generally have long unsaturated carbon chains, and animal fats saturated ones; hydrogenation of vegetable oils uses a nickel catalyst.
Catalyst: NCERT's definition is "substances that cause a reaction to occur or proceed at a different rate without the reaction itself being affected". Nickel in hydrogenation is the chapter's example.
Substitution (4.3.4). Saturated hydrocarbons are inert towards most reagents, but "in the presence of sunlight, chlorine is added to hydrocarbons in a very fast reaction". Chlorine replaces hydrogen atoms one by one: CH₄ + Cl₂ → CH₃Cl + HCl. With the higher alkanes, several products usually form.
Ethanol (4.4.1)
Ethanol is a liquid at room temperature, mixes with water in all proportions, and is a good solvent, which is why it is used in tincture iodine, cough syrups and many tonics. Small quantities of dilute ethanol cause drunkenness, but "intake of even a small quantity of pure ethanol (called absolute alcohol) can be lethal", and long-term drinking causes many health problems. Its two reactions in the chapter are with sodium (hydrogen gas and sodium ethoxide) and dehydration by excess hot concentrated sulphuric acid at 443 K, which removes water and gives ethene.
Methanol and denatured alcohol. Large amounts of ethanol slow metabolism and depress the central nervous system. Methanol is far worse: "Unlike ethanol, intake of methanol in very small quantities can cause death." The liver oxidises it to methanal, which reacts rapidly with cell components. "It coagulates the protoplasm, in much the same way an egg is coagulated by cooking." Methanol also damages the optic nerve and causes blindness. Industrial ethanol is made unfit to drink by adding poisons such as methanol and a blue dye: this is denatured alcohol.
Ethanoic acid (4.4.2)
Ethanoic acid (acetic acid) is a carboxylic acid. A 5-8 % solution in water is vinegar, a preservative in pickles. Pure ethanoic acid melts at 290 K and so often freezes in cold winters, hence the name glacial acetic acid. It is acidic, but "unlike mineral acids like HCl, which are completely ionised, carboxylic acids are weak acids" (Activity 4.7 compares the two with universal indicator).
Esterification and saponification. Ethanoic acid and absolute ethanol, warmed with a few drops of concentrated sulphuric acid (the acid catalyst), give an ester: CH₃COOH + CH₃CH₂OH → CH₃COOCH₂CH₃ + H₂O. Esters are generally sweet-smelling and are used in perfumes and as flavouring agents. Treated with sodium hydroxide, the ester splits back into the alcohol and the sodium salt of the carboxylic acid: CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa. This is saponification, so called because it is used to make soap.
With bases and carbonates. NaOH + CH₃COOH → CH₃COONa + H₂O. With carbonates and hydrogencarbonates it gives a salt, carbon dioxide and water: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂, and CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂ (Activity 4.9 tests the gas with lime-water).
Alcohol as a fuel. NCERT's box: "Sugarcane plants are one of the most efficient convertors of sunlight into chemical energy." Sugarcane juice gives molasses, which is fermented to ethanol. Some countries add alcohol to petrol because it is a "cleaner fuel which gives rise to only carbon dioxide and water on burning in sufficient air (oxygen)". Note the condition: in sufficient air.
Soaps and detergents (4.5)
Most dirt is oily and oil does not dissolve in water. A soap molecule has two ends: an ionic end that interacts with water and a hydrocarbon tail that interacts with oil.
Micelle: a cluster of soap molecules in water with the hydrophobic tails inside and the ionic ends on the surface. Oily dirt collects in the centre and is rinsed away. "The micelles stay in solution as a colloid and will not come together to precipitate because of ion-ion repulsion." "The soap micelles are large enough to scatter light", which is why soap solution looks cloudy.
Hard water. In Activity 4.11, soap gives more foam in distilled or rain water and a white curdy precipitate in hard water. The calcium and magnesium salts that make water hard react with soap to form scum, so more soap is needed. Detergents (Table 6) do not form insoluble precipitates with calcium and magnesium ions, so they work in hard water; for the same reason a detergent cannot tell you whether water is hard. The group activity makes soap by heating oil with 20 % sodium hydroxide solution and then adding common salt.
Did the 2020-21 edition differ?
No. This chapter's sections, boxes, activities and exercises read the same in the 2020-21 edition and in Reprint 2026-27.
Source: NCERT, Science Class X, 2020-21 edition (whole-book zip, Wayback Machine capture of 9 October 2021), compared with Reprint 2026-27.
Beyond the textbook: from ethene to the carry-bag rules
The chemistry. This chapter's ethene is the building block of a familiar plastic. NCERT's older Class VIII Science book put it simply: "Polythene (Poly+ethene) is an example of a plastic. It is used for making commonly used polythene bags." Chapter 13 of this book explains why such bags last: "many human-made materials like plastics will not be broken down by the action of bacteria or other saprophytes", and under ordinary conditions "these persist for a long time". Such substances are non-biodegradable.
The rules. The Ministry of Environment, Forest and Climate Change notified the Plastic Waste Management Amendment Rules, 2021, on 12 August 2021. As PIB summarised them in December 2022, they prohibit "plastic carry bags having thickness less than seventy-five microns with effect from 30th September 2021, and having thickness less than thickness of one hundred and twenty microns with effect from the 31st December, 2022". From 1 July 2022 they also ban identified single-use plastic items: ear buds with plastic sticks, plastic sticks for balloons, plastic flags, candy and ice-cream sticks, polystyrene (Thermocol) for decoration, plates, cups, glasses, cutlery, straws, trays, wrapping films around sweet boxes, invitation cards, cigarette packets, plastic or PVC banners under 100 microns, and stirrers. A PIB release of July 2024 restated the 120-micron rule and added that non-woven plastic carry bags under 60 GSM (grams per square metre) have been prohibited since 30 September 2021.
Source: NCERT, Science Class VIII (older edition), ch. "Synthetic Fibres and Plastics"; NCERT, Science Class X, Reprint 2026-27, ch. 13 (p. 214); PIB (MoEFCC), "Ban on Single Use Plastics", 12 December 2022, Release ID 1882855; PIB (MoEFCC), "Compulsory ban on polythene bags", 25 July 2024, Release ID 2036730.
PART 3 — UPSC Integration
Cross-paper relevance
- Prelims (general science) — covalent bonds, allotropes, catenation, saturated and unsaturated compounds, isomers, functional groups and suffixes, hydrogenation, substitution in sunlight, methanol and denatured alcohol, vinegar and glacial acetic acid, esters, micelles, soap and hard water.
- GS3 (Science and technology) — fuels and complete combustion; catalysts; hydrogenation of oils; why detergents work in hard water.
- GS3 (Environment) — oxides of sulphur and nitrogen from burning coal and petroleum; non-biodegradable plastics and the carry-bag rules.
- GS2 (Health) — the chemistry of methanol poisoning: methanal formed in the liver, and blindness from damage to the optic nerve.
Past questions on these themes: No past question in the bank is set directly on this chapter.
Frames for Mains Answers
1. Why carbon is everywhere. Start from scarcity (0.02 % of the crust), then explain abundance of compounds: covalent sharing, catenation, tetravalency, and strong bonds from carbon's small size. Close with Wöhler (1828), who showed organic compounds can be made outside living things.
2. Incomplete combustion is waste and pollution. A sooty flame and a blackened vessel mean blocked air holes and wasted fuel; coal and petroleum also release oxides of sulphur and nitrogen. Better burners and cleaner fuels address both.
3. Ethanol as a fuel. NCERT's case is that sugarcane converts sunlight efficiently and ethanol burns to carbon dioxide and water in sufficient air. Any blending target or figure you add must carry its date and source.
4. Plastics. Link the chemistry (ethene to polythene; non-biodegradable) to the rule (carry bags under 120 microns prohibited from 31 December 2022; single-use items from 1 July 2022).
Exam Strategy
Prelims fact-traps:
- C⁴⁻ is unlikely because six protons cannot hold ten electrons; C⁴⁺ needs too much energy. Carbon shares instead.
- H₂ single, O₂ double, N₂ triple bond. Methane is a major component of biogas and CNG.
- Diamond: four bonds per atom, 3-D, hardest. Graphite: three bonds per atom, hexagonal layers, conducts. C-60 is named after Buckminster Fuller.
- Silicon chains reach only seven or eight atoms and are very reactive.
- Wöhler, 1828, urea from ammonium cyanate.
- Saturated = single bonds only, alkanes; unsaturated = double (alkenes) or triple (alkynes) bonds; unsaturated are more reactive.
- Cyclohexane C₆H₁₂ is saturated; benzene C₆H₆.
- Homologues differ by –CH₂– (14 u); chemical properties similar, physical properties graded.
- Suffixes: -ol alcohol, -al aldehyde, -one ketone, -oic acid carboxylic acid, -ene, -yne.
- Oxidising agents: alkaline KMnO₄, acidified K₂Cr₂O₇. Hydrogenation catalyst: nickel (or palladium). Chlorination of methane needs sunlight.
- Ethanol dehydrates to ethene at 443 K with excess concentrated H₂SO₄. Ethanol + Na gives H₂ and sodium ethoxide.
- Methanol → methanal in the liver; blindness via the optic nerve. Denatured alcohol is coloured blue.
- Vinegar 5-8 % acetic acid; glacial acetic acid freezes (m.p. 290 K). Carboxylic acids are weak acids.
- Ester + NaOH → alcohol + sodium salt = saponification.
- Soap scum forms with calcium and magnesium; detergents work in hard water. Soap solution is cloudy because micelles scatter light.
- Carry bags: 75 microns from 30 September 2021; 120 microns from 31 December 2022.
Mains: NCERT's structure-and-bonding logic is the explanation layer for any question on fuels, plastics or materials. Add dated figures only from a named source.
Practice Questions
Questions 1-3 are the NCERT exercise MCQs. Practice (UPSC-pattern, not past papers): questions 4-10.
1. Ethane, with the molecular formula C₂H₆, has
(a) 6 covalent bonds.
(b) 7 covalent bonds.
(c) 8 covalent bonds.
(d) 9 covalent bonds.
Answer: (b). Six C–H bonds and one C–C bond (NCERT Answers, p. 218).
2. Butanone is a four-carbon compound with the functional group
(a) carboxylic acid.
(b) aldehyde.
(c) ketone.
(d) alcohol.
Answer: (c). The suffix -one marks a ketone (Table 4.4).
3. While cooking, if the bottom of the vessel is getting blackened on the outside, it means that
(a) the food is not cooked completely.
(b) the fuel is not burning completely.
(c) the fuel is wet.
(d) the fuel is burning completely.
Answer: (b). The air holes are blocked, combustion is incomplete, and fuel is wasted.
4. Consider the following statements about the allotropes of carbon:
1. In diamond, each carbon atom is bonded to four other carbon atoms.
2. Graphite is a very good conductor of electricity.
3. Synthetic diamonds differ chemically from natural diamonds.
Which of the statements given above are correct?
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3
Answer: (b). NCERT says synthetic diamonds are small but otherwise indistinguishable from natural ones.
5. With reference to the hydrogenation of vegetable oils, consider the following statements:
1. It is an addition reaction.
2. Nickel is used as a catalyst.
3. It converts saturated carbon chains into unsaturated ones.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3
Answer: (a). Hydrogen is added to unsaturated chains, making them saturated.
6. Consider the following statements:
1. Methanol is oxidised to methanal in the liver.
2. Methanol can cause blindness by affecting the optic nerve.
3. Denatured alcohol is ethanol made unfit to drink by adding poisonous substances such as methanol.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3
Answer: (d). All three are in NCERT's box on how alcohols affect living beings.
7. Consider the following statements:
1. Soaps are sodium or potassium salts of long-chain carboxylic acids.
2. Detergents form scum with calcium and magnesium ions in hard water.
3. Soap solution appears cloudy because micelles scatter light.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 only
(c) 1 and 3 only
(d) 1, 2 and 3
Answer: (c). Soaps, not detergents, form scum in hard water.
8. Consider the following statements about ethanoic acid:
1. Vinegar is a 5-8 % solution of it in water.
2. It is called glacial acetic acid because its pure form often freezes in cold winters.
3. Like hydrochloric acid, it is completely ionised in water.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3
Answer: (a). Carboxylic acids are weak acids.
9. Under the Plastic Waste Management Amendment Rules, 2021, plastic carry bags thinner than which thickness have been prohibited from 31 December 2022?
(a) 50 microns
(b) 75 microns
(c) 100 microns
(d) 120 microns
Answer: (d). The 75-micron limit applied from 30 September 2021.
10. Carbon makes up only 0.02 % of the earth's crust, yet it forms millions of compounds. Explain why, and show how incomplete combustion of carbon fuels links household fuel waste to air pollution. (150 words)
NCERT exercises (where the answers are on this page): 4 Table 1 (in CH₃Cl, carbon shares one electron pair with each of three hydrogen atoms and one chlorine atom); 6 the homologous-series box; 7 Table 5; 8 the micelle box; 9 "Combustion"; 10 "Hard water"; 11 soap is the salt of a strong base and a weak carboxylic acid, so its solution is basic and turns red litmus blue (Chapter 2: salts of a strong base and a weak acid are basic); 12 "Addition"; 13 the unsaturated C₃H₆ (as propene) and C₂H₂ (Table 7); 14 the flame test in "Combustion" (clean flame for saturated, yellow sooty flame for unsaturated); 15 the micelle box.
📦 Revision Capsule
Hard Facts
- Earth's crust: 0.02 % carbon; atmosphere: 0.03 % carbon dioxide.
- H₂ single, O₂ double, N₂ triple bond; CH₄ four single bonds.
- Diamond 3-D (four bonds per atom); graphite hexagonal layers (three bonds per atom), conducts; C-60 fullerene.
- Wöhler, 1828: urea from ammonium cyanate.
- CH₄ + 2O₂ → CO₂ + 2H₂O; CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O.
- CH₄ + Cl₂ → CH₃Cl + HCl (sunlight).
- 2Na + 2CH₃CH₂OH → 2CH₃CH₂O⁻Na⁺ + H₂; ethanol at 443 K with excess hot concentrated H₂SO₄ → ethene + water.
- CH₃COOH + CH₃CH₂OH → ester + H₂O (acid catalyst); ester + NaOH → alcohol + sodium salt.
- 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂.
Core Concepts
- Carbon shares electrons because gaining or losing four is too hard.
- Catenation and tetravalency explain the number of carbon compounds; small size explains bond strength.
- A functional group fixes the chemistry; chain length grades the physical properties.
- Unsaturated compounds are more reactive and undergo addition; saturated ones undergo substitution.
- Soap micelles trap oily dirt; hard-water ions turn soap into scum.
Confused Pairs
- Saturated (single bonds) vs unsaturated (double or triple).
- Alkene (CnH₂n, double bond) vs alkyne (triple bond).
- Structural isomers (same formula) vs homologues (differ by –CH₂–).
- Addition (unsaturated + H₂) vs substitution (saturated + Cl₂ in sunlight).
- Catalyst (unchanged; nickel) vs oxidising agent (adds oxygen; KMnO₄, K₂Cr₂O₇).
- Ethanol (solvent, drinks) vs methanol (poison, blindness).
- Esterification (acid + alcohol) vs saponification (ester + NaOH).
- Soap (scum in hard water) vs detergent (works in hard water).
- 75 microns (30 Sep 2021) vs 120 microns (31 Dec 2022).
Data Points
- Table 4.1 (K): acetic acid 290/391; chloroform 209/334; ethanol 156/351; methane 90/111.
- Silicon chains: up to seven or eight atoms.
- Dehydration of ethanol: 443 K. Vinegar: 5-8 % acetic acid. Activity 4.5: 5 % alkaline KMnO₄.
- Soap-making activity: 20 mL oil, 30 mL of 20 % NaOH, 5-10 g common salt.
- Single-use plastic items banned from 1 July 2022; non-woven carry bags under 60 GSM prohibited from 30 September 2021.
PYQ Pattern
- No past question in the bank matches this chapter directly.
Sources
- NCERT, Science, Textbook for Class X, ch. 4 "Carbon and its Compounds", Reprint 2026-27 — ncert.nic.in PDF.
- NCERT, Science, Class X, ch. 13 "Our Environment", Reprint 2026-27 — ncert.nic.in PDF.
- NCERT, Science, Class X, Answers, Reprint 2026-27 — ncert.nic.in PDF.
- NCERT, Science, Class X, 2020-21 edition (whole-book zip), as archived on 9 October 2021 — Wayback Machine.
- NCERT, Science, Textbook for Class VIII (older edition), ch. "Synthetic Fibres and Plastics" (print citation).
- PIB (Ministry of Environment, Forest and Climate Change), "Ban on Single Use Plastics", 12 December 2022, Release ID 1882855 — pib.gov.in.
- PIB (Ministry of Environment, Forest and Climate Change), "Compulsory ban on polythene bags", 25 July 2024, Release ID 2036730 — pib.gov.in.
