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← Science — all chaptersCarbon and its Compounds
Start here: why this chapter matters
In simple wordsYour food, clothes, medicines, plastics, fuels — and you yourself — are built mostly from compounds of one element: carbon. This chapter explains the trick that lets carbon make millions of compounds (it shares electrons and links up with itself), how chemists sort and name that huge family, and the everyday chemistry of fuels, alcohol, vinegar, soap and detergents.
This is the fourth chapter of NCERT's Class 10 Science textbook (pages 58–78 of the 2026-27 reprint; the contents page and the PDF agree). It has five parts — bonding in carbon: the covalent bond (4.1, with a box on allotropes), the versatile nature of carbon (4.2: saturated and unsaturated compounds, chains, branches and rings, functional groups, homologous series, nomenclature), chemical properties (4.3: combustion, oxidation, addition, substitution), ethanol and ethanoic acid (4.4) and soaps and detergents (4.5) — with 12 activities, Figures 4.1–4.13, Tables 4.1–4.4, two ‘Do You Know?’ boxes, five ‘More to Know!’ boxes, five in-text question boxes, a summary, 15 exercise questions and a group activity. Every equation here is checked in the source, and so is every electron-dot structure: each atom's own electrons and its full shell are counted by the code.
- Neighbouring chapters: Metals and Non-metals (Chapter 3) explained ionic bonds by electron transfer — this chapter sets covalent sharing against it; Acids, Bases and Salts (Chapter 2) gave carbonate + acid → CO2 and the lime-water test, reused for ethanoic acid; Chemical Reactions and Equations (Chapter 1) gave oxidation and rancidity. Our Environment (Chapter 13) and the Class 10 geography chapter Minerals and Energy Resources take up fossil fuels, biogas and CNG.
- Prelims — science: UPSC has asked about detergents in hard water (1988, 2002, 2003), hard water (1990, 2000), unsaturated fats (2003), trans fats (2011), the order of alkanes by molecular weight (2001), methanol in hooch tragedies (1996), gasohol (1982), ethanol from sugarcane (2009), vinegar (1993), acetylene in welding (1998), diamond as an element (1992) and methane in gobar gas (1979, 1980, 1982).
- GS3 — energy, environment, health: ethanol blending and biofuels, CNG and biogas, fossil fuels and pollution from sulphur and nitrogen in them, hydrogenated fats and diet, detergents and water quality.
| Word | What it means here |
|---|---|
| Covalent bond | A pair of electrons shared between two atoms; one shared pair = single bond, two = double, three = triple |
| Allotropes | Different forms of one element, differing in how the atoms are bonded (diamond, graphite, fullerenes) |
| Catenation | An element's ability to bond to itself in long chains, branches and rings — carbon's speciality |
| Tetravalency | Valency four: carbon forms four bonds |
| Saturated / unsaturated | Only single C–C bonds / at least one C=C or C≡C |
| Hydrocarbon | A compound of carbon and hydrogen only: alkanes, alkenes, alkynes |
| Structural isomers | Same molecular formula, different structure |
| Functional group | An atom or group (–OH, –CHO, >C=O, –COOH, –Cl) that gives a compound its characteristic chemistry |
| Homologous series | A family with the same functional group in which successive members differ by –CH2– |
| Ester / saponification | Sweet-smelling product of an acid and an alcohol / turning an ester back into alcohol and a sodium salt with NaOH — the soap-making reaction |
| Micelle | A cluster of soap molecules with the oil-loving tails inside and the water-loving heads outside |
Reading NCERT's chapter opening and Activity 4.1 (page 58)A small drawing of a lattice of linked balls (the diamond network), a QR code, and Activity 4.1: list ten things used since morning and sort them into a three-column table — things made of metal, things made of glass/clay, others. The text then notes that most ‘others’ are carbon compounds, asks how to test this (burn it; the CO2 turns lime water milky — Ca(OH)2 + CO2 → CaCO3 + H2O), and gives the scarcity numbers: only 0.02% of the earth's crust is carbon and the atmosphere has 0.03% carbon dioxide. The point: little carbon, enormous importance.
Common confusions
- Ionic vs covalent: ionic = electrons transferred (metal + non-metal, Chapter 3); covalent = electrons shared (mostly between non-metals, this chapter).
- Organic chemistry is the chemistry of carbon compounds — but carbides, oxides of carbon, carbonates and hydrogencarbonates are traditionally left out of it (NCERT's box, page 63).
- NCERT's 0.03% CO2 is an old figure; the concentration is now above 0.04% (about 420 parts per million — Beyond NCERT). Quote 0.03% only in an NCERT-based question.
At a glance — the six ideas to carry away
- Covalent bond = a shared pair of electrons. Carbon (2, 4) cannot easily gain or lose four electrons, so it shares. H2, Cl2: single; O2: double; N2: triple; CH4: four single bonds. Covalent compounds have low melting and boiling points and are poor conductors.
- Allotropes: diamond (each C bonded to 4 — rigid, hardest), graphite (each C bonded to 3 in hexagonal layers — slippery, conducts), fullerenes (C-60, football-shaped). Same chemistry, different structure.
- Catenation + tetravalency → chains, branches, rings. Saturated (only single C–C bonds, alkanes CnH2n+2) vs unsaturated (C=C alkenes CnH2n; C≡C alkynes CnH2n−2). Structural isomers: same formula, different structure (butane has two).
- Functional groups (–Cl/–Br, –OH, –CHO, >C=O, –COOH) decide chemistry; a homologous series differs by –CH2– (14 u). Names: stem + suffix (-ol, -al, -one, -oic acid, -ene, -yne) or prefix (chloro-, bromo-): propanone, propanal, propanoic acid.
- Reactions: combustion (saturated → clean blue flame; unsaturated → sooty yellow); oxidation of ethanol to ethanoic acid by alkaline KMnO4 or acidified K2Cr2O7; addition of H2 with Ni/Pd (hydrogenation of oils); substitution of H by Cl in sunlight.
- Ethanol: + Na → sodium ethoxide + H2; + hot conc. H2SO4 at 443 K → ethene. Ethanoic acid: vinegar is 5–8%; freezes at 290 K (glacial); weak acid; + ethanol → ester (sweet smell); ester + NaOH → soap-making reaction (saponification). Soap micelles trap oily dirt; hard water gives scum; detergents work in hard water.
1. Sharing electrons: the covalent bond
In simple wordsTwo neighbours each have half a ladder; neither can reach the roof alone, so they share — and both get up. Atoms do the same with electrons. Carbon needs four more electrons for a full shell; grabbing or giving away four is too hard, so it shares four pairs instead. The shared pair is the bond.
Carbon compounds behave differently from ionic ones. Table 4.1 shows low melting and boiling points — even acetic acid boils at only 391 K, below the lowest melting point (887 K) of the ionic compounds in the previous chapter — so the forces between molecules are weak; and since most are non-conductors, their bonding gives no ions. Carbon (atomic number 6; shells 2, 4) has four valence electrons. Gaining four (C4−) would leave six protons holding ten electrons; losing four (C4+) would take too much energy. So carbon shares. The shared electrons count for both atoms, and both reach a noble-gas configuration.
| Compound (Table 4.1) | Formula | Melting point (K) | Boiling point (K) |
|---|---|---|---|
| Acetic acid | CH3COOH | 290 | 391 |
| Chloroform | CHCl3 | 209 | 334 |
| Ethanol | CH3CH2OH | 156 | 351 |
| Methane | CH4 | 90 | 111 |
Hydrogen (one electron) shares with another hydrogen: one shared pair = a single covalent bond, H–H, and each atom reaches helium's 2. Chlorine (2, 8, 7; valency 1) forms Cl2 the same way. Oxygen (2, 6) needs two: two shared pairs = a double bond, O=O. Nitrogen (2, 5) needs three: a triple bond, N≡N. In methane, CH4 — a fuel and a major component of biogas and CNG — tetravalent carbon shares its four electrons with four hydrogens. Covalently bonded molecules have strong bonds within the molecule but weak forces between molecules: hence low melting and boiling points, and poor conduction because no charged particles form.
⤵ Drawing an electron-dot structure (the method behind Figures 4.1–4.5)
Count valence electronsH 1 · C 4 · N 5 · O 6 · Cl 7
Work out how many each atom needsH needs 1 more (to 2); C 4, N 3, O 2, Cl 1 (to 8)
Share pairs to meet those needsone pair = single bond; O needs two (double); N needs three (triple)
Put the leftover electrons as lone pairse.g. O in water keeps two lone pairs
Checkevery H has 2 around it, every other atom 8
Reading NCERT's Figures 4.1–4.4 (pages 59–60)Pink overlapping circles stand for atoms; electrons are dots for one atom and crosses for the other. Figure 4.1: two H atoms, then the H2 molecule with its shared electrons in the overlap. Figure 4.2: the same, with the line notation H–H. Figure 4.3: two O atoms with six electrons each, then O2 with four electrons in the overlap and :Ö=Ö:. Figure 4.4: two N atoms, then N2 with six shared electrons and N≡N. The point: the number of shared pairs is the bond order — redrawn above in our own style.
Reading NCERT's Figure 4.5 — electron-dot structure for methane (page 61)A large carbon circle overlapped by four small blue hydrogen circles, each overlap holding one dot and one cross. The point: four single covalent bonds satisfy carbon's valency of four and give each hydrogen its duplet.
Common confusions
- Strong bonds, weak attractions: covalent bonds inside a molecule are strong; it is the forces between molecules that are weak — hence low melting points.
- O2 has a double bond; N2 a triple bond; Cl2, H2, F2 single bonds.
- Methane's electrons: carbon contributes 4, the hydrogens 4 — 8 around carbon, 2 around each H.
- ‘Covalent compounds don't conduct’ is a general rule — graphite (an element, not a compound) conducts.
Beyond NCERT
- In a polar covalent bond (such as O–H in water) the pair is shared unequally — the basis of water's ability to dissolve ionic salts (Chapter 3).
- Methane is also a potent greenhouse gas; rice paddies, cattle and wetlands release it (UPSC 2008, 2010 — tagged to environment chapters).
Why this matters for PrelimsMethane as the chief constituent of gobar gas / biogas was asked in 1979, 1980 and 1982; bonding type and conductivity are the basis of statement questions on covalent and ionic compounds.
2. Allotropes of carbon
In simple wordsThe same box of building blocks can make a solid fortress, a stack of thin sheets or a football. Carbon atoms do exactly that: bonded every way in 3-D they make diamond; bonded in flat sheets they make graphite; curled into a ball they make fullerene. Same atoms, same chemistry — very different things in your hand.
In diamond, each carbon is bonded to four others in a rigid three-dimensional structure — the hardest substance, with very high melting and boiling points. In graphite, each carbon is bonded to three others in the same plane, giving a hexagonal array; one of the bonds is a double bond, satisfying the valency; the hexagonal sheets lie in layers one above the other. Graphite is smooth and slippery and a very good conductor of electricity, unlike other non-metals. Their chemical properties are the same. Diamonds can be synthesised by subjecting pure carbon to very high pressure and temperature; synthetic diamonds are small but otherwise indistinguishable from natural ones. Fullerenes are another class: the first, C-60, has its atoms in the shape of a football and was named after the US architect Buckminster Fuller, whose geodesic domes it resembles.
Reading NCERT's ‘More to Know!’ — allotropes of carbon (page 61)Three ball-and-stick pictures — the structure of diamond (a 3-D lattice) with a diamond ring beside it, the structure of graphite (stacked hexagonal layers joined by dashed lines) with a photo of a boy writing with a pencil, and the structure of C-60, buckminsterfullerene (a cage of pentagons and hexagons). The point: bonding pattern, not the element, decides the properties — a jewel and a pencil lead are both pure carbon.
Reading NCERT's ‘Questions’ box (page 61)Two questions: the electron-dot structure of CO2 (two double bonds — drawn in Section 1) and of S8, a ring of eight sulphur atoms (each S bonded to two neighbours by single bonds and holding two lone pairs). Outlines in the in-text section.
Common confusions
- Graphite conducts electricity; diamond does not (diamond's non-conduction is Beyond NCERT but standard).
- Allotropes differ in physical properties; their chemical properties are the same — both burn to CO2.
- Diamond is an element (carbon), not a compound (UPSC 1992); ruby, sapphire and emerald are compounds.
- NCERT inconsistency: this box calls diamond ‘the hardest substance known’; Chapter 3 said ‘the hardest natural substance’. The safer exam line is hardest natural substance.
- Graphite's ‘one double bond’ is NCERT's simplification; in fact one electron per carbon is shared across the whole sheet, which is why it conducts (Beyond NCERT).
Beyond NCERT
- Graphite's layers are held only by weak forces, so they slide — graphite is a dry lubricant and pencil ‘lead’.
- Graphene is a single layer of graphite, one atom thick: very strong and an excellent conductor (UPSC 2012). The 2010 Nobel Prize in Physics went to Andre Geim and Konstantin Novoselov for it.
- C-60's cage has 12 pentagons and 20 hexagons (60 atoms, 90 bonds — checked against Euler's rule in this page's source). Its discovery in 1985 earned Curl, Kroto and Smalley the 1996 Nobel Prize in Chemistry.
- Carbon nanotubes are rolled-up graphene sheets.
Why this matters for PrelimsAllotropes, graphite's conductivity and the new carbon materials (graphene, nanotubes, fullerenes) are recurring science-and-technology items.
3. Why carbon is so versatile: catenation, tetravalency, saturated and unsaturated
In simple wordsCarbon is like a Lego brick with four studs that also loves clicking onto other carbon bricks. With four connection points and an endless supply of partners, you can build straight chains, branched trees, rings — and use single, double or triple links. That is why chemists know millions of carbon compounds.
The number of carbon compounds with known formulae runs into millions — more than all other elements' compounds together. Two reasons: (i) catenation — carbon bonds with other carbon atoms to form long chains, branched chains or rings, linked by single, double or triple bonds; no other element does this to the same extent (silicon's hydrogen compounds reach chains of only seven or eight atoms, and are very reactive); the C–C bond is very strong and stable. (ii) Tetravalency — four bonds, to carbon or to oxygen, hydrogen, nitrogen, sulphur, chlorine and others. Carbon's bonds to most elements are very strong, partly because carbon is small, so its nucleus holds the shared pairs tightly. Compounds with only single C–C bonds are saturated; those with double or triple C–C bonds are unsaturated and more reactive.
Building a structure step by step (Figure 4.6): for ethane, C2H6, link the carbons by a single bond, then satisfy each carbon's remaining three valencies with hydrogens. For ethene, C2H4, after the single bond one valency per carbon is left over — which is satisfied only by making the C–C bond double. Ethyne, C2H2, needs a triple bond.
⤵ Working out a hydrocarbon's structure (Figures 4.6–4.7)
Step 1 — link the carbonsC–C with a single bond
Step 2 — add hydrogensfill each carbon's remaining valencies (C₂H₆: three H on each carbon)
All valencies satisfied?yes → saturated (ethane)
One valency left on each C?make the C–C double (ethene); two left → triple (ethyne)
Step 3 — checkevery C has four bonds, every H one
Reading NCERT's ‘More to Know!’ — organic compounds (page 63)Tetravalency and catenation together give a huge number of compounds. They were first extracted from natural sources, and it was thought a ‘vital force’ in living systems was needed to make them; Friedrich Wöhler disproved this in 1828 by preparing urea from ammonium cyanate (NH4CNO —[heat]→ CO(NH2)2 — equation Beyond NCERT). Carbides, oxides of carbon, carbonates and hydrogencarbonates are still not counted as organic. The point: organic compounds obey ordinary chemistry and can be made in a laboratory.
Reading NCERT's Figure 4.6 (a–c) and Figure 4.7 (pages 63–64)4.6(a): ‘C—C, Step 1’. 4.6(b): the ethane structure with three H on each carbon, ‘Step 2’. 4.6(c): overlapping-circle electron-dot structure of ethane — two pink carbon circles sharing a pair, each with three blue hydrogens. Beside them, the ethene steps: C—C; then each carbon with two H and one free valency; then C=C. Figure 4.7: ethene's electron-dot picture with four electrons in the carbon–carbon overlap. The point: a leftover valency becomes an extra bond between the carbons.
Common confusions
- Saturated = only C–C single bonds (UPSC 2003: ‘unsaturated fats have only single bonds’ is false).
- Unsaturated compounds are more reactive, not less.
- Ethane has 7 covalent bonds (6 C–H + 1 C–C) — exercise 1 (b); ethene 5 bonds but 6 shared pairs.
- Catenation is carbon–carbon bonding; tetravalency is the four bonds. The two together explain the numbers.
Beyond NCERT
- Silicon sits below carbon in the periodic table but its Si–Si bonds are weaker, which is why silicon chemistry is dominated by Si–O (sand, silicates) rather than long Si–Si chains.
- Urea, Wöhler's compound, is today made industrially from ammonia and carbon dioxide (UPSC 1987).
Why this matters for PrelimsSaturated vs unsaturated is the chemistry behind fats and oils questions (2003, 2011); catenation and tetravalency are the two ‘reasons’ an explanation question will want.
4. Chains, branches, rings and isomers
In simple wordsFour beads can be strung in a straight line, or three in a line with one hanging off the side — same beads, different necklaces. Molecules with the same formula but a different arrangement are isomers. Carbon chains can also close into rings, like a bracelet.
Chains can contain many carbons; Table 4.2 lists the first six saturated hydrocarbons, from methane to hexane. For butane, C4H10, four carbons can form two skeletons — a straight chain and a branched one; both have the same formula. Compounds with an identical molecular formula but different structures are structural isomers. Carbon atoms can also form rings: cyclohexane, C6H12, is saturated; benzene, C6H6, a ring with alternating double bonds, is unsaturated. Straight, branched and cyclic compounds can all be saturated or unsaturated. Compounds of only carbon and hydrogen are hydrocarbons: saturated ones are alkanes; those with one or more double bonds alkenes; with one or more triple bonds alkynes.
| No. of C | Alkane (Table 4.2) | Formula CnH2n+2 | Alkene CnH2n (our column) | Alkyne CnH2n−2 (our column) |
|---|---|---|---|---|
| 1 | Methane | CH4 | — | — |
| 2 | Ethane | C2H6 | C2H4 (ethene) | C2H2 (ethyne) |
| 3 | Propane | C3H8 | C3H6 (propene) | C3H4 (propyne) |
| 4 | Butane | C4H10 | C4H8 (butene) | C4H6 (butyne) |
| 5 | Pentane | C5H12 | C5H10 (pentene) | C5H8 (pentyne) |
| 6 | Hexane | C6H14 | C6H12 (hexene) | C6H10 (hexyne) |
Reading NCERT's Table 4.2 and Figures 4.8–4.10 (pages 64–65)Table 4.2: number of carbons, name, formula and full structure for methane, ethane, propane, butane, pentane and hexane — every structure a straight chain with H above, below and at the ends. Figure 4.8(a): two skeletons, C—C—C—C and a C—C with two C branching from the second carbon; 4.8(b): the two complete C4H10 molecules. Figure 4.9: cyclohexane as a six-carbon ring skeleton and as the complete molecule with twelve H. Figure 4.10: benzene, a six-carbon ring with three alternating double bonds and one H on each carbon. The point: same atoms, different connections — formula alone does not identify a compound.
Common confusions
- Cyclohexane (C6H12) is saturated even though it has the alkene formula pattern CnH2n; benzene (C6H6) is unsaturated. A general formula alone cannot tell a ring from a double bond.
- Isomers have the same molecular formula — not merely the same atoms in a different ratio.
- Methane, ethane and propane have no isomers; butane has 2, pentane 3 (hexane 5 — Beyond NCERT).
- In increasing molecular weight: methane < ethane < propane < butane (UPSC 2001) — each step adds CH2, 14 u.
Beyond NCERT
- Systematic names of the branched isomers: 2-methylpropane (isobutane) and, for pentane, 2-methylbutane and 2,2-dimethylpropane.
- LPG is mainly butane and propane; CNG and biogas are mainly methane (biogas also has much carbon dioxide).
- Benzene is a known carcinogen; exposure comes from vehicle exhaust, tobacco smoke and some solvents (UPSC 2020, tagged to environment).
Why this matters for PrelimsOrdering alkanes by molecular weight (2001) and recognising which fuels contain which hydrocarbons are this section's exam uses.
5. Functional groups, homologous series and names
In simple wordsA family surname tells you a lot whatever the first name. In carbon chemistry the ‘surname’ is the functional group — –OH, –COOH and so on — and it decides how the compound behaves, whatever the length of its chain. Members of one family differ by one CH2 link at a time, like rungs on a ladder; their names follow simple rules.
Carbon also bonds with halogens, oxygen, nitrogen and sulphur, which can replace hydrogen in a chain while carbon's valency stays satisfied; such an element is a heteroatom. Heteroatoms and the groups containing them give specific properties regardless of chain length — they are functional groups (Table 4.3): halo (–Cl, –Br), alcohol (–OH), aldehyde (–CHO), ketone (>C=O within the chain) and carboxylic acid (–COOH). A series in which the same functional group replaces hydrogen on chains of increasing length is a homologous series: CH3OH, C2H5OH, C3H7OH and C4H9OH behave alike. Successive members differ by –CH2–, i.e. 14 u (C 12 u, H 1 u). Alkenes run C2H4, C3H6, C4H8, C5H10 — general formula CnH2n; alkanes are CnH2n+2 and alkynes CnH2n−2. Along a series, melting and boiling points and solubility change gradually with molecular mass; chemical properties stay similar.
| Series (Activity 4.2, up to four carbons) | Members — names built by NCERT's rules, formulae checked |
|---|---|
| Halo alkane (chloro) | chloromethane CH3Cl · chloroethane C2H5Cl · chloropropane C3H7Cl · chlorobutane C4H9Cl |
| Alcohol | methanol CH3OH · ethanol C2H5OH · propanol C3H7OH · butanol C4H9OH |
| Aldehyde | methanal HCHO · ethanal CH3CHO · propanal C2H5CHO · butanal C3H7CHO |
| Ketone | propanone CH3COCH3 · butanone CH3COC2H5 |
| Carboxylic acid | methanoic acid HCOOH · ethanoic acid CH3COOH · propanoic acid C2H5COOH · butanoic acid C3H7COOH |
Naming (Section 4.2.5, Table 4.4): (i) count the carbons — three gives ‘propane’; (ii) show the functional group by a prefix (chloro-, bromo-) or a suffix (-ol, -al, -one, -oic acid); (iii) if the suffix begins with a vowel, drop the final ‘e’ of the chain name: propane − e + one = propanone; (iv) for an unsaturated chain, ‘ane’ becomes ‘ene’ or ‘yne’: propene, propyne.
⤵ Naming a carbon compound (rules i–iv, page 67)
Count the carbons1 meth · 2 eth · 3 prop · 4 but · 5 pent · 6 hex → e.g. propane
Identify the functional grouphalogen → prefix; –OH, –CHO, >C=O, –COOH → suffix
Prefixchloro + propane = chloropropane
Suffix starting with a voweldrop the ‘e’: propan + ol / al / one / oic acid
Double or triple C–C bond?-ane → -ene (propene) or -yne (propyne)
Reading NCERT's Table 4.3 and Activity 4.2 (pages 66–67)Table 4.3 has three columns — heteroatom, class of compounds, formula of functional group: Cl/Br — halo-alkane — –Cl, –Br (substitutes for hydrogen); oxygen — alcohol –OH, aldehyde (a carbon with H and =O), ketone (a carbon with =O in the chain), carboxylic acid (a carbon with =O and –OH). Activity 4.2 asks for the formula and mass differences between successive alcohols (14 u each — checked), whether they form a homologous series, and the series up to four carbons for the other groups (table above). The point: a family is defined by its functional group and a CH2 step.
Reading NCERT's Table 4.4 and the ‘Questions’ box (pages 68–69)Table 4.4 lists class, prefix/suffix and example: halo alkane — prefix chloro/bromo — chloropropane, bromopropane; alcohol -ol propanol; aldehyde -al propanal; ketone -one propanone; carboxylic acid -oic acid propanoic acid; alkenes -ene propene; alkynes -yne propyne. The Questions box asks: how many isomers of pentane; the two properties behind carbon's many compounds; cyclopentane's formula and dot structure; structures of ethanoic acid, bromopentane (with isomers?), butanone and hexanal; and names for CH3CH2Br, H–CHO and a six-carbon chain ending in C≡C–H. The point: practise both directions — name → structure and structure → name (outlines in the in-text section).
Common confusions
- -al = aldehyde, -ol = alcohol, -one = ketone. Propanal and propanol differ by one letter and a whole functional group.
- Butanone is a ketone (exercise 2 — (c)); ketones need at least three carbons.
- Successive members differ by CH2 = 14 u, not by ‘one carbon’ alone.
- Physical properties change gradually along a series; chemical properties stay similar.
- NCERT's names (propanol, butene, hexyne) are simplified; full IUPAC names give the position — propan-1-ol, but-1-ene, hex-1-yne (Beyond NCERT).
Beyond NCERT
- Common names: methanal = formaldehyde (its water solution is formalin), propanone = acetone (nail-polish remover), methanoic acid = formic acid (ant sting), ethanoic acid = acetic acid.
- Higher classes add amines (–NH2), esters and ethers as further functional groups.
Why this matters for PrelimsUPSC seldom asks naming directly, but common-name ↔ chemical-name pairs (formalin, acetone, formic acid, vinegar) recur, and ‘homologous series’ is a standard definition.
6. Chemical properties: combustion, oxidation, addition, substitution
In simple wordsCarbon compounds do four main things. They burn (a cooking flame), they can be oxidised more gently (wine souring to vinegar), the unsaturated ones can add atoms across a double bond (liquid oil turning into solid ghee-like fat), and the saturated ones can swap a hydrogen for another atom (methane becoming chloromethane in sunlight).
Combustion. Carbon in all its allotropic forms burns in oxygen to give carbon dioxide with heat and light: C + O2 → CO2 + heat and light. NCERT prints the next two unbalanced and asks you to balance them: CH4 + 2O2 → CO2 + 2H2O + heat and light; CH3CH2OH + 3O2 → 2CO2 + 3H2O + heat and light. In Activity 4.3 (teacher's help) naphthalene, camphor and alcohol are burnt on a spatula: saturated hydrocarbons generally give a clean flame; unsaturated compounds a yellow flame with black smoke, leaving soot on a metal plate. In Activity 4.4, closing a burner's air hole gives a yellow sooty flame; opening it gives a blue one. Limited air causes incomplete combustion even of saturated hydrocarbons. Gas and kerosene stoves have air inlets so an oxygen-rich mixture burns with a clean blue flame; blackened vessel bottoms mean blocked air holes and wasted fuel. Coal and petroleum contain some nitrogen and sulphur, whose oxides are major pollutants.
⤵ Blue flame or sooty flame? (Activities 4.3–4.4)
Light the burner and adjust the air holecontrol how much oxygen mixes with the fuel
Air hole open — plenty of oxygencomplete combustion: blue, clean flame; CH₄ + 2O₂ → CO₂ + 2H₂O
Air hole closed — little oxygenincomplete combustion: yellow, sooty flame; soot (carbon) and CO form — 2CH₄ + 3O₂ → 2CO + 4H₂O (Beyond NCERT)
Unsaturated fuels (naphthalene, camphor)sooty even in air — black deposit on the metal plate
At homeblackened vessel bottoms → blocked air holes → wasted fuel
Reading NCERT's Activities 4.3–4.4 and the ‘Do You Know?’ on flames (pages 69–70)No figures for the activities. The box: a flame forms only when gaseous substances burn — charcoal in an angithi just glows, while wood first burns with a flame as its volatile parts vaporise. A luminous flame comes from heated particles glowing, and each element gives a characteristic colour — try a copper wire in a gas flame. It ends by asking what makes a candle flame yellow: glowing soot (unburnt carbon particles), the answer its own hint points to. The point: flame colour reports how completely a fuel is burning.
Reading NCERT's ‘More to Know!’ — formation of coal and petroleum (page 70)Coal formed from trees, ferns and other plants buried millions of years ago and pressed under earth and rock; oil and gas from tiny sea plants and animals that sank, were covered by silt and attacked by bacteria under high pressure; the oil and gas seeped into porous rock and were trapped like water in a sponge. It asks why they are called fossil fuels — they are the remains of ancient living things. The point: these fuels are stored ancient sunlight, finite and not renewable on a human time-scale.
Oxidation. In Activity 4.5, about 3 mL of ethanol is warmed in a water bath and 5% alkaline potassium permanganate is added drop by drop: at first its purple colour disappears (it is used up oxidising the ethanol); once all the ethanol is oxidised, excess permanganate stays purple. Alcohols are oxidised to carboxylic acids by alkaline KMnO4 + heat or acidified K2Cr2O7 + heat — CH3CH2OH → CH3COOH (NCERT's scheme; overall, as if with oxygen: CH3CH2OH + O2 → CH3COOH + H2O, Beyond NCERT). Substances that add oxygen to others are oxidising agents.
⤵ Ethanol to ethanoic acid (Activity 4.5)
Warm about 3 mL ethanol in a water bathgently — ethanol is flammable
Add 5% alkaline KMnO₄ drop by dropthe purple colour disappears: permanganate is being used up
Ethanol is oxidisedCH₃CH₂OH → CH₃COOH (oxygen added, hydrogen removed)
Keep addingonce no ethanol is left, the purple colour stays — excess oxidising agent
Addition. Unsaturated hydrocarbons add hydrogen in the presence of catalysts such as palladium or nickel to give saturated ones — R2C=CR2 + H2 → R2CH–CHR2; e.g. C2H4 + H2 —[Ni]→ C2H6 (example Beyond NCERT). A catalyst makes a reaction occur or proceed at a different rate without itself being affected. This is used in the hydrogenation of vegetable oils with a nickel catalyst. Vegetable oils generally have long unsaturated chains; animal fats saturated ones. Animal fats generally contain saturated fatty acids, said to be harmful for health; NCERT advises choosing oils with unsaturated fatty acids for cooking.
⤵ Hydrogenation of a vegetable oil (page 71)
Vegetable oillong unsaturated chains (C=C) — liquid
+ hydrogen, nickel catalystH₂ adds across each C=C: R₂C=CR₂ → R₂CH–CHR₂
Saturated fatsolid or semi-solid, e.g. vanaspati ghee (product name Beyond NCERT)
Substitution. Saturated hydrocarbons are fairly unreactive, but in sunlight chlorine reacts with them very fast, replacing hydrogen atoms one by one — one atom or group takes the place of another: CH4 + Cl2 —[sunlight]→ CH3Cl + HCl. With higher alkanes many products form. Continuing the replacement (Beyond NCERT): CH3Cl + Cl2 —[sunlight]→ CH2Cl2 + HCl; CH2Cl2 + Cl2 —[sunlight]→ CHCl3 + HCl; CHCl3 + Cl2 —[sunlight]→ CCl4 + HCl.
Reading NCERT's oxidation and hydrogenation schemes and the ‘Questions’ box (page 71)Two schemes without figures: ethanol → ethanoic acid with ‘alkaline KMnO4 + heat’ or ‘acidified K2Cr2O7 + heat’ over the arrow; and a C=C carrying four R groups becoming a C–C with an added H on each carbon, ‘nickel catalyst, H2’ over the arrow. The Questions box asks why ethanol → ethanoic acid is an oxidation, and why welders burn ethyne with oxygen rather than air. The point: oxidation adds oxygen (and removes hydrogen); welding needs complete combustion for the highest temperature — 2C2H2 + 5O2 → 4CO2 + 2H2O (Beyond NCERT equation).
Common confusions
- Addition is for unsaturated compounds (C=C, C≡C); substitution for saturated ones. Of C2H6, C3H8, C3H6, C2H2, CH4, only C3H6 and C2H2 add (exercise 13).
- Hydrogenation needs a catalyst — nickel (or palladium), not iron.
- Saturated hydrocarbons give a clean blue flame in enough air; unsaturated ones a sooty yellow flame.
- NCERT wording slip: page 71 says ‘chlorine is added to hydrocarbons’ in what it then correctly calls a substitution. Write ‘chlorine replaces hydrogen’.
- KMnO4 and K2Cr2O7 are the oxidising agents; ethanol is the substance oxidised.
Beyond NCERT — fats and health
- Partial hydrogenation of oils creates trans fats, which raise ‘bad’ (LDL) cholesterol and the risk of heart disease; FSSAI regulations limit industrially produced trans fat in oils and fats to not more than 2%. ‘Trans-fat free’ labels therefore usually signal foods not made from partially hydrogenated oils (UPSC 2011).
- A standard lab test for unsaturation: bromine water (orange-brown) is decolourised by an alkene — C2H4 + Br2 → C2H4Br2 — but not by an alkane (exercise 14).
- Activated charcoal adsorbs impurities — used to decolourise sugar solution and in water filters (UPSC 1990).
Why this matters for Prelims and GS3Unsaturated fats being more reactive (2003), trans fats (2011) and acetylene in welding (1998) are direct; incomplete combustion, soot (black carbon) and the oxides of sulphur and nitrogen from fossil fuels are the chemistry behind GS3 air-pollution answers.
7. Ethanol
In simple wordsEthanol is the ‘alcohol’ in alcoholic drinks, the solvent in many medicines, a fuel that can be blended into petrol — and, in its cousin methanol, a deadly poison. It is a small molecule with a big social and economic footprint.
Ethanol is a liquid at room temperature (boils at 351 K, Table 4.1), commonly called alcohol, the active ingredient of alcoholic drinks. As a good solvent it is used in tincture iodine, cough syrups and tonics; it is soluble in water in all proportions. Small quantities of dilute ethanol cause drunkenness — a practice NCERT notes is condemned yet socially widespread; even a small quantity of pure ethanol (absolute alcohol) can be lethal, and long-term consumption causes many health problems. With sodium (Activity 4.6, teacher's demonstration with a rice-grain-sized piece): hydrogen is evolved and sodium ethoxide forms — 2Na + 2CH3CH2OH → 2CH3CH2ONa + H2. Dehydration: heating with excess concentrated sulphuric acid at 443 K removes water and gives ethene — CH3CH2OH —[hot conc. H₂SO₄, 443 K]→ C2H4 + H2O (CH2=CH2); the acid acts as a dehydrating agent.
⤵ Two reactions of ethanol (Activity 4.6 and page 72)
+ sodium (absolute alcohol)bubbles of hydrogen: 2Na + 2CH₃CH₂OH → 2CH₃CH₂ONa + H₂
Test the gasburns with a pop — hydrogen; the other product is sodium ethoxide
+ excess hot conc. H₂SO₄, 443 Kthe acid removes water
Productethene, CH₂=CH₂ — an unsaturated hydrocarbon
Reading NCERT's ‘Do You Know?’ — how do alcohols affect living beings? (page 72)Large amounts of ethanol slow metabolism and depress the central nervous system — loss of coordination, mental confusion, drowsiness, lowered inhibitions and finally stupor, while the person may feel relaxed. Methanol is far worse: even small amounts can kill. In the liver it is oxidised to methanal, which reacts rapidly with cell components and coagulates the protoplasm, like an egg being cooked; it also damages the optic nerve, causing blindness. To stop industrial ethanol being drunk, poisons such as methanol are added and a blue dye marks it: denatured alcohol. The point: methanol is the killer in hooch tragedies (UPSC 1996).
Reading NCERT's ‘More to Know!’ — alcohol as a fuel (page 73)Sugarcane is among the most efficient converters of sunlight into chemical energy; its juice gives molasses, which is fermented to ethanol. Some countries use alcohol as a petrol additive because it burns cleanly to carbon dioxide and water in sufficient air. The point: ethanol is a renewable liquid fuel — the basis of gasohol (UPSC 1982) and ethanol blending.
Common confusions
- Methanol (CH3OH) causes blindness and death; ethanol (C2H5OH) is the drinking alcohol. Denatured alcohol is ethanol made poisonous with methanol and dyed blue.
- Ethanol + sodium gives hydrogen — like a metal with an acid — but ethanol is not an acid; it does not affect litmus.
- Dehydration of ethanol gives ethene (an alkene), not ethane.
- Absolute alcohol = pure ethanol; it is not ‘stronger drink’ — it is dangerous to drink.
Beyond NCERT — ethanol as fuel (GS3)
- Fermentation by yeast: C6H12O6 —[yeast]→ 2C2H5OH + 2CO2.
- Petrol mixed with ethanol was long called gasohol (UPSC 1982). India's National Policy on Biofuels, 2018 set a target of 20% ethanol blending in petrol (E20) by 2030; an amendment in 2022 brought the E20 target forward to 2025-26. Feedstocks allowed include sugarcane juice and molasses, damaged food grains, maize, cassava and rotten potatoes (UPSC 2020).
- Debates around blending — set out without a verdict: supporters point to lower oil imports, farmer incomes and cleaner exhaust; critics raise water use by sugarcane, diversion of food grains and engine compatibility and mileage in older vehicles.
Why this matters for Prelims and GS3Methanol poisoning (1996), gasohol (1982), ethanol from sugarcane (2009) and biofuel feedstocks (2020) make ethanol one of the most-asked organic compounds; ethanol blending is a live GS3 energy topic.
8. Ethanoic acid and esters
In simple wordsVinegar is mostly water with a little ethanoic acid — a weak acid that still does acid things: turns litmus red, neutralises bases, fizzes with baking soda. Mix it with alcohol and a pinch of strong acid, and you get a fruity smell: an ester, the kind of molecule behind perfumes and flavours.
Ethanoic acid — commonly acetic acid — belongs to the carboxylic acids. A 5–8% solution in water is vinegar, widely used to preserve pickles. Pure ethanoic acid melts at 290 K, so it often freezes in cold winters — hence glacial acetic acid. Unlike mineral acids such as HCl, which are completely ionised, carboxylic acids are weak acids (Activity 4.7: both turn blue litmus red, but universal indicator shows dilute acetic acid as less acidic). Esterification (Activity 4.8): 1 mL absolute ethanol, 1 mL glacial acetic acid and a few drops of concentrated sulphuric acid are warmed in a water bath for at least five minutes and poured into 20–50 mL of water — a sweet smell: CH3COOH + CH3CH2OH —[acid]→ CH3COOCH2CH3 + H2O (ethyl ethanoate; NCERT draws the arrow as reversible). Esters are used in perfumes and flavouring agents. With sodium hydroxide an ester gives back the alcohol and the sodium salt of the acid — saponification, because it is used to make soap: CH3COOC2H5 + NaOH → C2H5OH + CH3COONa. Soaps are sodium or potassium salts of long-chain carboxylic acids.
Like mineral acids, ethanoic acid reacts with a base: NaOH + CH3COOH → CH3COONa + H2O (sodium ethanoate, commonly sodium acetate). With carbonates and hydrogencarbonates (Activity 4.9) it gives a salt, carbon dioxide and water — the gas turns lime water milky: 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2; CH3COOH + NaHCO3 → CH3COONa + H2O + CO2.
⤵ Making an ester (Activity 4.8, Figure 4.11)
Mix in a test tube1 mL absolute ethanol + 1 mL glacial acetic acid + a few drops of conc. H₂SO₄
Warm in a water bath, at least 5 minutesthe acid catalyses the reaction
Pour into 20–50 mL of water and smella sweet, fruity smell — an ester
What formedCH₃COOH + CH₃CH₂OH —[acid]→ CH₃COOCH₂CH₃ + H₂O
⤵ Making soap at home or school (Group Activity II, page 78)
About 20 mL castor, cotton-seed, linseed or soyabean oilin a beaker
Add 30 mL of 20% sodium hydroxide; heat and stiruntil the mixture thickens — saponification of the fat
Add 5–10 g common salt; stir, coolsalt helps the soap separate (Beyond NCERT reason)
Cut and perfumea fat + alkali → soap + glycerol (Beyond NCERT equation: (C₁₇H₃₅COO)₃C₃H₅ + 3NaOH → 3C₁₇H₃₅COONa + C₃H₅(OH)₃)
Reading NCERT's Activities 4.7–4.9 and Figure 4.11 — formation of ester (page 73–74)Figure 4.11: a test tube containing the reaction mixture standing in a beaker of water on a wire gauze and tripod stand over a burner — a water bath, so the flammable mixture is heated gently. Activity 4.7 compares dilute acetic acid and dilute HCl with litmus and universal indicator; Activity 4.9 repeats the carbonate test of Chapter 2 with ethanoic acid. The point: ethanoic acid is a genuine but weak acid, and esters form only with gentle, indirect heating.
Reading NCERT's ‘Questions’ box (page 74)Two questions: how to distinguish an alcohol from a carboxylic acid by experiment (add sodium hydrogencarbonate — only the acid fizzes with CO2; or test with litmus), and what oxidising agents are. Outlines in the in-text section.
Common confusions
- Vinegar = 5–8% acetic (ethanoic) acid (UPSC 1993), not citric or oxalic acid.
- Glacial acetic acid is pure acetic acid, named because it freezes (at 290 K, about 17 °C) — not ‘acid from glaciers’.
- Esterification: acid + alcohol → ester + water (acid catalyst). Saponification: ester + NaOH → alcohol + sodium salt. Opposite directions.
- Ethanoic acid is weak because it ionises only partly — not because it is dilute.
- NCERT layout point: page 74 writes NaOH over the saponification arrow; it is a reactant, consumed in the reaction (the version without it does not balance — checked in the source).
Beyond NCERT
- Formic (methanoic) acid is a stronger acid than acetic acid; the 1998 UPSC assertion was true but ‘it is an organic acid’ was not the reason.
- Many fruit smells are esters (e.g. pentyl ethanoate smells of bananas); fats and oils are themselves esters of glycerol with long-chain fatty acids.
Why this matters for PrelimsVinegar (1993) and natural acids, glacial acetic acid, esters as flavours, and saponification as the soap-making reaction are standard statement material.
9. Soaps, micelles and detergents
In simple wordsOil and water refuse to mix. A soap molecule is a go-between with two ends: one loves water, the other loves oil. Many soap molecules surround an oily speck with their oil-loving tails stuck into it and their water-loving heads facing out — a little ball called a micelle — and the water carries the ball away. Hard water spoils the trick by turning soap into scum; detergents are built not to.
In Activity 4.10, two tubes of water each get a drop of cooking oil; tube B also gets soap. After shaking, the oil separates quickly in A but stays dispersed far longer in B — soap helps oil mix with water. Soap molecules are sodium or potassium salts of long-chain carboxylic acids. The ionic end interacts with water; the carbon chain interacts with oil. Soap molecules form micelles, with the chain ends towards the oil droplet and the ionic ends outside, making an emulsion; the micelle pulls the dirt into the water and clothes come clean.
Hard water (Activity 4.11): soap gives plenty of foam in distilled or rain water, but little foam and a white curdy precipitate in hard water from a tubewell or hand-pump. The scum is soap reacting with calcium and magnesium salts that make water hard, so more soap is needed. Detergents solve this: they are generally sodium salts of sulphonic acids or ammonium salts with chloride or bromide ions, with long hydrocarbon chains; their charged ends do not form insoluble precipitates with calcium and magnesium ions, so they stay effective in hard water (Activity 4.12). Detergents are used in shampoos and laundry products.
⤵ How soap cleans (Figure 4.13)
Oily dirt clings to the clothoil does not dissolve in water
Soap molecules gather at the dirthydrophobic tails dig into the oil; ionic heads stay in water
Agitation lifts the dirt offrubbing or stirring helps the micelle form and detach (in-text question)
The micelle floats awayion–ion repulsion keeps micelles apart as a colloid; rinsing removes them
⤵ Soap vs detergent in hard water (Activities 4.11–4.12)
Equal hard water in two tubescontains Ca²⁺ and Mg²⁺ salts (hydrogencarbonates, sulphates or chlorides)
Five drops of soaplittle foam; white curdy scum — insoluble calcium/magnesium soap
Five drops of detergentplenty of foam; no scum — charged ends form no precipitate
Conclusiondetergents work in hard water; soap is wasted as scum
Reading NCERT's Activity 4.10 and Figure 4.12 — formation of micelles (page 74)Figure 4.12: a ring of soap molecules drawn as balls on wavy tails, all tails meeting at a central oil droplet, with Na+ ions outside the heads. The point: the micelle's inside is oily, its outside is ionic — so the whole package dissolves in water.
Reading NCERT's ‘More to Know!’ — micelles, with Figure 4.13 (page 75)The box explains hydrophilic (water-loving) and hydrophobic (water-hating, hydrocarbon-loving) ends; at the water surface soap lines up with heads in the water and tails sticking out; inside water the tails cluster inward. Micelles stay in solution as a colloid, kept apart by ion–ion repulsion, and are big enough to scatter light — so soap solution looks cloudy. Figure 4.13, ‘effect of soap in cleaning’, shows a strip of cloth with a blob of grease, soap molecules approaching, surrounding and finally lifting the grease away as a sphere ringed with soap molecules; a labelled soap molecule shows the hydrophilic head and hydrophobic tail. The point: cleaning is physical capture, not chemical destruction, of dirt.
Reading NCERT's Activities 4.11–4.12 and the ‘Questions’ box (page 76)No figures. Activity 4.11 compares foam in distilled and hard water (with a note that hard water can be made by dissolving calcium or magnesium hydrogencarbonates, sulphates or chlorides); Activity 4.12 compares soap and detergent in hard water. The Questions box asks whether a detergent can test for hard water (no — it foams either way) and why clothes need agitation. The point: soap is the hard-water test; detergents are the hard-water remedy.
Common confusions
- Micelle in water: tails inside (in the oil), heads outside. In a hydrocarbon solvent the arrangement reverses.
- Hydrophilic = water-loving (the ionic head); hydrophobic = water-hating (the hydrocarbon tail).
- Detergents work in hard water because their calcium and magnesium salts are soluble (UPSC 2002) — not because they soften the water.
- Detergents lower the surface tension of water (UPSC 1988: ‘increase’ is false).
- You cannot test for hard water with a detergent; use soap.
- Soap is basic: it turns red litmus blue (exercise 11).
Beyond NCERT
- Hard water also forms scale in kettles and boilers, wasting fuel and risking damage — why hard water is unsuitable for boilers and for washing with soap (UPSC 2000); it is generally drinkable. Washing soda removes permanent hardness (Chapter 2).
- Washing powders contain builders and fillers: sodium sulphate keeps the powder dry and free-flowing; sodium silicate keeps it alkaline and protects machine parts (UPSC 2003).
- Some older detergents with branched chains were not biodegradable and caused foaming in rivers; phosphate builders feed algal blooms (eutrophication) — a water-quality issue for GS3.
Why this matters for Prelims and GS3Detergents in hard water (1988, 2002, 2003) and hard water itself (1990, 2000) are the chapter's most-asked Prelims items; detergent phosphates and non-biodegradable surfactants link to eutrophication in GS3.
NCERT's in-text questions and activities — short answers
Short outlines in page order, for questions in the running text, the activities and the five ‘Questions’ boxes.
Activity 4.1 (page 58): Sort ten items into metal, glass/clay and others. How can you test that the ‘others’ contain carbon?
Most ‘others’ (food, clothes, paper, plastic, medicines) are carbon compounds. Burn a sample: carbon gives carbon dioxide, which turns lime water milky — Ca(OH)2 + CO2 → CaCO3 + H2O.
Page 59: Carbon has atomic number 6. Its electron distribution and valence electrons?
K 2, L 4 — four valence electrons; carbon is tetravalent.
Page 60: Chlorine (atomic number 17) — configuration, valency and the dot structure of Cl₂?
2, 8, 7; valency 1. Two Cl atoms share one pair: a single bond, each Cl keeping three lone pairs (drawn in Section 1).
Page 60: Show the bonding in water. Single or double bonds?
Oxygen (6 valence electrons) shares one pair with each of two H atoms: two single bonds, and O keeps two lone pairs (drawn in Section 1).
Page 60: Nitrogen (atomic number 7) — configuration and combining capacity?
2, 5; combining capacity 3. In N2 each atom contributes three electrons: a triple bond.
Page 60: Draw the dot structure of NH₃. Single, double or triple bonds?
N shares one pair with each of three H atoms: three single bonds, and N keeps one lone pair; N has 8, each H 2 (drawn in Section 1).
Page 61, Q1: Electron-dot structure of CO₂.
Carbon shares two pairs with each oxygen: O=C=O, two double bonds; each O keeps two lone pairs (drawn in Section 1).
Page 61, Q2: Electron-dot structure of S₈ (a ring of eight sulphur atoms).
Eight S atoms in a ring, each joined to its two neighbours by single bonds and carrying two lone pairs — 2 + 4 = 6 own electrons, 8 around each (checked in the source).
Page 63: Draw the structure of propane, C₃H₈.
H3C–CH2–CH3: three carbons in a chain with single bonds; end carbons carry 3 H, the middle carbon 2 H — all single bonds, so propane is saturated (drawn in Section 5).
Page 64: Electron-dot structure of ethyne; how many bonds between the carbons?
Three — a triple bond; each carbon also shares one pair with an H (drawn in Section 3).
Page 65: Electron-dot structure for cyclohexane.
Six carbons in a ring, each sharing one pair with each neighbouring carbon and one pair with each of its two hydrogens — 6 C–C and 12 C–H single bonds.
Page 66: Difference between propane and butane? Mass differences between successive alkanes?
Each differs by –CH2–; mass difference 14 u (12 + 2 × 1) — checked for every pair in the source.
Page 67: Do the alkenes C₃H₆, C₄H₈, C₅H₁₀ differ by –CH₂–? General formulae?
Yes. Alkenes CnH2n (n ≥ 2); alkanes CnH2n+2; alkynes CnH2n−2 (n ≥ 2).
Activity 4.2 (page 67): Differences in formula and mass for CH₃OH/C₂H₅OH, C₂H₅OH/C₃H₇OH, C₃H₇OH/C₄H₉OH. A homologous series? Generate series for the other functional groups.
Each pair differs by CH2 and 14 u; same –OH group → yes, a homologous series. Other series up to four carbons are in the table in Section 5 (e.g. methanal, ethanal, propanal, butanal; propanone, butanone; methanoic to butanoic acid).
Page 68, Q1: How many structural isomers can you draw for pentane?
Three: pentane (straight chain), 2-methylbutane and 2,2-dimethylpropane — computed by enumerating skeletons in the source and drawn in Section 4 (names Beyond NCERT).
Page 68, Q2: The two properties of carbon behind the huge number of carbon compounds?
Catenation and tetravalency.
Page 68, Q3: Formula and electron-dot structure of cyclopentane?
C5H10: a ring of five carbons, each sharing a pair with its two carbon neighbours and with two hydrogens — 5 C–C and 10 C–H single bonds.
Page 69, Q4: Draw (i) ethanoic acid (ii) bromopentane — are isomers possible? (iii) butanone (iv) hexanal.
(i) CH3–COOH (a carbon with =O and –OH). (ii) CH3CH2CH2CH2CH2Br — yes, isomers exist: Br on carbon 1, 2 or 3 of the chain, and on branched chains too. (iii) CH3–CO–CH2–CH3. (iv) CH3(CH2)4CHO.
Page 69, Q5: Name (i) CH₃–CH₂–Br (ii) H–CHO (iii) H₃C–CH₂–CH₂–CH₂–C≡C–H.
(i) bromoethane (ii) methanal (iii) hexyne (hex-1-yne) — six carbons, C6H10, matching CnH2n−2 (checked).
Page 69: Balance the combustion of methane and of ethanol.
CH4 + 2O2 → CO2 + 2H2O + heat and light
CH3CH2OH + 3O2 → 2CO2 + 3H2O + heat and light
CH3CH2OH + 3O2 → 2CO2 + 3H2O + heat and light
Activity 4.3 (page 69): Nature of flame and smoke when naphthalene, camphor and alcohol burn; deposits on a metal plate?
Naphthalene and camphor (unsaturated/rich in carbon) burn with a yellow, smoky flame and leave soot on the plate; alcohol burns with a nearly clean blue flame and little or no deposit.
Activity 4.4 (page 69): When do you get a yellow sooty flame and when a blue flame?
Yellow, sooty with the air hole closed (incomplete combustion); blue with it open (enough oxygen, complete combustion).
Page 70 (Do You Know?): What causes the yellow colour of a candle flame? Why are coal and petroleum fossil fuels?
Glowing particles of unburnt carbon (soot) from incomplete combustion. They formed from the remains of ancient organisms over millions of years.
Activity 4.5 (page 70): Does the colour of KMnO₄ persist at first? Why does it persist when excess is added?
At first the purple colour disappears — permanganate is used up oxidising ethanol. Once all the ethanol has been oxidised, extra permanganate has nothing to react with, so the colour stays.
Page 71, Q1: Why is ethanol → ethanoic acid an oxidation?
Oxygen is added (and hydrogen removed): CH3CH2OH → CH3COOH, using an oxidising agent such as alkaline KMnO4.
Page 71, Q2: Why is ethyne burnt with oxygen, not air, for welding?
In air, nitrogen dilutes the oxygen and combustion is incomplete — a sooty, cooler flame. Pure oxygen gives complete combustion and a very high temperature that melts metals: 2C2H2 + 5O2 → 4CO2 + 2H2O.
Activity 4.6 (page 72): What do you observe when sodium is dropped into ethanol? How will you test the gas?
Effervescence — hydrogen is evolved: 2Na + 2CH3CH2OH → 2CH3CH2ONa + H2. The gas burns with a pop. (Other substances giving hydrogen with metals: acids, Chapter 2.)
Activity 4.7 (page 73): Are both acids indicated by litmus? Does universal indicator show them equally strong?
Both turn blue litmus red; universal indicator shows dilute HCl at a lower pH (stronger) than dilute acetic acid — acetic acid is a weak acid.
Activity 4.8 (page 73): What do you smell?
A sweet, fruity smell — the ester ethyl ethanoate: CH3COOH + CH3CH2OH —[acid]→ CH3COOCH2CH3 + H2O.
Activity 4.9 (page 74): What happens with sodium carbonate and ethanoic acid? Can the gas be identified by lime water? Repeat with NaHCO₃.
Brisk effervescence; the gas turns lime water milky — CO2. 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2; with hydrogencarbonate CH3COOH + NaHCO3 → CH3COONa + H2O + CO2.
Page 74, Q1: How would you distinguish experimentally between an alcohol and a carboxylic acid?
Add sodium hydrogencarbonate: the acid fizzes (CO2 turns lime water milky); the alcohol does not. Or use blue litmus: only the acid turns it red.
Page 74, Q2: What are oxidising agents?
Substances that add oxygen to others (or remove hydrogen) — e.g. alkaline KMnO4, acidified K2Cr2O7.
Activity 4.10 (page 74): Do the oil and water layers separate? In which tube first?
Immediately after shaking, both look mixed; on standing, the oil separates first in tube A (no soap). In B, soap keeps the oil dispersed as an emulsion.
Page 75: Draw the micelle formed if soap is dissolved in a hydrocarbon.
The arrangement reverses: ionic heads point inwards (around any trace of water) and the hydrocarbon tails point outwards into the hydrocarbon (drawn in Section 9 — Beyond NCERT answer).
Activity 4.11 (page 76): Where is there more foam? Where a white curdy precipitate?
More foam in distilled (or rain) water; the curdy precipitate (scum) in hard water.
Activity 4.12 (page 76): Same foam with soap and detergent in hard water? Where is a curdy solid formed?
No — the detergent gives much more foam; the curdy solid forms with soap.
Page 76, Q1: Could you check whether water is hard by using a detergent?
No — detergents foam in both hard and soft water. Use soap: scum and poor lather show hardness.
Page 76, Q2: Why is agitation necessary to get clean clothes?
Beating, scrubbing or stirring helps soap molecules reach the oily dirt, loosens it from the fibres and lets micelles form and carry it away into the water.
NCERT exercise — answer outlines
Reading NCERT's Exercises and Group Activity (pages 77–78)Fifteen questions — three multiple-choice, then explanations, structures and tests — and a group activity: (I) build molecular models with a kit; (II) make soap from castor, cotton-seed, linseed or soyabean oil with 20% sodium hydroxide and common salt (flow in Section 8). No figures. NCERT's answers page (page 218) gives 1 (b), 2 (c), 3 (b).
1. Ethane, C₂H₆, has (a) 6 (b) 7 (c) 8 (d) 9 covalent bonds.
(b) 7 — six C–H bonds and one C–C bond (counted from the electron-dot data in the source).
2. Butanone is a four-carbon compound with the functional group (a) carboxylic acid (b) aldehyde (c) ketone (d) alcohol.
(c) ketone — the suffix ‘-one’.
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.
(b) — incomplete combustion deposits soot; the air holes need cleaning.
4. Explain the nature of the covalent bond using the bond formation in CH₃Cl.
Carbon (4 valence electrons) shares one pair with each of three H atoms and one pair with Cl (7 valence electrons). Four single covalent bonds: C reaches 8, each H 2, Cl 8 (with three lone pairs). No ions form, so the bonds are covalent — strong within the molecule, with weak forces between molecules (drawn in Section 3).
5. Draw electron-dot structures for (a) ethanoic acid (b) H₂S (c) propanone (d) F₂.
All four are drawn and checked in Section 3: (a) CH3–C(=O)–O–H, 8 shared pairs; (b) S with two single bonds to H and two lone pairs; (c) CH3–C(=O)–CH3, 10 shared pairs; (d) F–F, one shared pair, three lone pairs on each F.
6. What is a homologous series? Explain with an example.
A series of compounds with the same functional group in which successive members differ by –CH2– (14 u), with similar chemical properties and gradually changing physical properties. E.g. alcohols: CH3OH, C2H5OH, C3H7OH, C4H9OH; or alkanes CH4, C2H6, C3H8.
7. How can ethanol and ethanoic acid be differentiated by physical and chemical properties?
Physical: ethanol has a pleasant smell and stays liquid in winter (melts at 156 K); ethanoic acid smells of vinegar and freezes at 290 K. Chemical: ethanoic acid turns blue litmus red and fizzes with NaHCO3 (CO2); ethanol does neither. Both give H2 with sodium, so that test does not separate them.
8. Why does micelle formation take place when soap is added to water? Will a micelle form in ethanol?
The hydrocarbon tail is insoluble in water, so the tails cluster together away from water while the ionic heads face the water — a micelle. In ethanol, which dissolves the hydrocarbon part, soap dissolves as separate molecules, so micelles do not form in the same way.
9. Why are carbon and its compounds used as fuels for most applications?
They release a large amount of heat and light on burning, burn with a controllable flame, are widely available (coal, petroleum, natural gas, biomass) and easy to store and transport.
10. Explain the formation of scum when hard water is treated with soap.
Hard water contains calcium and magnesium salts; soap reacts with them to form insoluble calcium and magnesium salts of the fatty acid — the curdy white scum: 2C17H35COO− + Ca2+ → (C17H35COO)2Ca (Beyond NCERT equation). Soap is wasted and lather is poor.
11. What change will you observe if you test soap with litmus paper (red and blue)?
Soap solution is basic: red litmus turns blue; blue litmus stays blue.
12. What is hydrogenation? What is its industrial application?
Addition of hydrogen to an unsaturated compound in the presence of a catalyst (nickel or palladium). Industrially, vegetable oils are hydrogenated with nickel to make solid fats (vanaspati ghee — Beyond NCERT name).
13. Which of C₂H₆, C₃H₈, C₃H₆, C₂H₂ and CH₄ undergo addition reactions?
C3H6 (propene, CnH2n) and C2H2 (ethyne, CnH2n−2) — the unsaturated ones. C2H6, C3H8, CH4 fit CnH2n+2 and are saturated.
14. Give a test to differentiate between saturated and unsaturated hydrocarbons.
Burning (NCERT): unsaturated ones give a yellow sooty flame, saturated ones a clean flame. Beyond NCERT: bromine water is decolourised by unsaturated hydrocarbons (C2H4 + Br2 → C2H4Br2) but not by saturated ones; alkaline KMnO4 is also decolourised.
15. Explain the mechanism of the cleaning action of soaps.
Soap molecules have an ionic, hydrophilic head and a hydrocarbon, hydrophobic tail. In water the tails dig into oily dirt and the heads face outward, forming micelles with the dirt trapped inside. Agitation detaches the dirt; ion–ion repulsion keeps micelles dispersed as an emulsion/colloid, and rinsing washes them away (Section 9).
What to remember
Numbers
Carbon in nature (NCERT)earth's crust 0.02% · atmosphere 0.03% CO₂ (now above 0.04% — Beyond NCERT)
Carbonatomic number 6 · shells 2, 4 · four valence electrons · tetravalent
Table 4.1 (K, mp/bp)acetic acid 290/391 · chloroform 209/334 · ethanol 156/351 · methane 90/111
Bond ordersH₂, Cl₂, F₂ single · O₂ double · N₂ triple · CO₂ two double
Ethane7 covalent bonds (exercise 1)
General formulaealkanes CₙH₂ₙ₊₂ · alkenes CₙH₂ₙ · alkynes CₙH₂ₙ₋₂
Homologous step–CH₂– = 14 u
Isomersbutane 2 · pentane 3 (hexane 5 — Beyond NCERT)
Ethanol dehydrationexcess hot conc. H₂SO₄ at 443 K → ethene
Vinegar5–8% ethanoic (acetic) acid · pure acid melts at 290 K (glacial)
Activity 4.5about 3 mL ethanol + 5% alkaline KMnO₄
Activity 4.81 mL ethanol + 1 mL glacial acetic acid + drops of conc. H₂SO₄; water bath ≥ 5 min; pour into 20–50 mL water
Soap making20 mL oil + 30 mL 20% NaOH, then 5–10 g salt
Wöhlerurea from ammonium cyanate, 1828
Chapter sizepages 58–78 · 12 activities · Figures 4.1–4.13 · Tables 4.1–4.4 · 15 exercises
One-liners
- A covalent bond is a shared pair of electrons; carbon shares because gaining or losing four electrons is too hard.
- Covalent compounds: strong bonds within molecules, weak forces between them — low melting points, poor conductors.
- Diamond: each C bonded to four, hardest; graphite: each C bonded to three in layers, slippery, conducts; C-60 is a football-shaped fullerene.
- Catenation + tetravalency explain the millions of carbon compounds.
- Saturated: only C–C single bonds (alkanes); unsaturated: C=C or C≡C (alkenes, alkynes), more reactive.
- Structural isomers share a molecular formula but differ in structure.
- Functional groups decide chemistry; a homologous series steps by –CH₂–.
- -ol alcohol · -al aldehyde · -one ketone · -oic acid carboxylic acid · -ene/-yne unsaturated · chloro-/bromo- halo.
- Oxidising agents (alkaline KMnO₄, acidified K₂Cr₂O₇) turn ethanol into ethanoic acid.
- Hydrogenation: unsaturated + H₂ with Ni/Pd → saturated (vegetable oil → solid fat).
- Substitution: CH₄ + Cl₂ in sunlight → CH₃Cl + HCl.
- Methanol poisons (blindness); denatured alcohol is ethanol made unfit to drink.
- Ester = acid + alcohol (sweet smell); saponification = ester + NaOH → alcohol + soap-type salt.
- Micelle: tails in the oil, heads in the water; detergents work in hard water, soaps form scum.
Traps to avoid
- Ionic = transfer of electrons; covalent = sharing.
- Graphite conducts electricity; diamond does not — same element, same chemistry.
- Cyclohexane C₆H₁₂ is saturated though it fits CₙH₂ₙ; benzene C₆H₆ is unsaturated.
- Unsaturated fats are MORE reactive; saturated ones have only single bonds.
- Addition for unsaturated compounds, substitution for saturated ones.
- Propanal (aldehyde) ≠ propanol (alcohol) ≠ propanone (ketone).
- Methanol (not ethanol) causes blindness in hooch tragedies.
- Ethanol + Na gives H₂ but ethanol is not an acid; ethanoic acid fizzes with NaHCO₃, ethanol does not.
- Detergents work in hard water because their Ca/Mg salts are soluble; they lower (not raise) surface tension.
- A detergent cannot test for hard water — use soap.
- NCERT slips: ‘chlorine is added’ in a substitution; diamond ‘hardest substance known’ (Ch. 3: hardest natural); NaOH written over the saponification arrow; 0.03% CO₂ is an old figure.
How to strategise
Prelims — what UPSC asks
- Soaps, detergents and hard water lead: detergents remove dirt but do not raise surface tension (1988); detergents lather in hard water because their Ca/Mg salts are soluble (2002); sodium sulphate and silicate in washing powder (2003); hard water defined by Ca/Mg salts (1990); hard water unsuitable for soap washing and boilers (2000).
- Fuels and alcohols: methane in gobar gas (1979, 1980, 1982); gasohol (1982); methanol in liquor tragedies (1996); ethanol from sugarcane (2009); acetylene in welding (1998); alkanes ordered by molecular weight (2001).
- Fats and acids: unsaturated fats more reactive, not ‘only single bonds’ (2003); trans-fat-free claims and hydrogenated oils (2011); vinegar is acetic acid (1993). Allotropes: diamond is an element (1992).
- Likely next: graphene and fullerenes, micelles and emulsions, esters and saponification, denatured alcohol, E20 ethanol blending and biofuel feedstocks, CNG vs LPG composition.
- Answer-key note (2011, trans fats): the stored key (c) ‘1 and 3’ is the intended answer and stands. Statement 1 is loose — trans fats come from partial hydrogenation, and fully hydrogenated fats contain little — while ruminant animal fats carry small natural amounts of trans fat, which is why statement 2 is excluded. Treat this as a slightly flawed question rather than a wrong key.
- Answer-key note (2000, hard water): the stored key (b) ‘washing with soap and boilers’ stands; hard water is generally safe to drink and usable for irrigation in most cases, so statements I and IV are rightly excluded.
Mains — where it appears and how to answer
- No GS Mains question in the bank so far is built on this chapter's core content. It supplies the chemistry for GS3 answers on ethanol blending and biofuels (feedstocks, food-vs-fuel and water debates — attributed, no verdict), clean cooking fuels (LPG, CNG, biogas), air pollution from incomplete combustion and from sulphur and nitrogen in fossil fuels, trans fats and nutrition, and detergents and eutrophication.
- Answer frame: chemistry in one line (a balanced equation such as fermentation or complete combustion) → why it matters (energy security, health, environment) → trade-offs, each attributed → policy instrument (blending mandates, standards, labelling).
- A small sketch — the micelle, or a blue-vs-sooty flame flow — fits in a 150-word answer.
Self-check
Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.
1. How many covalent bonds does a molecule of ethane (C₂H₆) have?
2. The bond between the two atoms in a nitrogen molecule is:
3. Which allotrope of carbon conducts electricity?
4. Which of these will undergo an addition reaction with hydrogen?
5. The correct name of CH₃–CO–CH₃ is:
6. Heating ethanol at 443 K with excess concentrated sulphuric acid gives:
7. Detergents remain effective in hard water because:
8. In a soap micelle in water, the hydrocarbon tails point:
UPSC questions from this chapter
Open any question to attempt it in the Question bank — your attempt is saved to your progress.