Chemistry Class 10 ICSE Solutions — Selina Chapter 12

These chemistry class 10 ICSE solutions explain Selina Chapter 12, Organic Chemistry, section by section. The chapter defines organic compounds, the unique nature of carbon, hydrocarbons, functional groups, and naming rules. It covers alkanes, alkenes, alkynes, alcohols, and carboxylic acids with preparations, reactions, and tests. The explanations follow the book’s order, with formulas and worked patterns.

Download Organic Chemistry Class 10 ICSE Chemistry Chapter 12 PDF

Download the Organic Chemistry Class 10 ICSE Chemistry chapter PDF from the Selina Concise textbook to revise offline or print for practice. It is the full chapter as printed in the book — 36 pages, 53 sections, 92 questions.

What is inside Chapter 12

36 pages · 53 sections · 2 worked examples · 92 exercise questions

• Intext Questions: 22 questions
• EXERCISE 12A: 11 questions
• EXERCISE 12B: 16 questions
• EXERCISE 12C: 14 questions
• EXERCISE 12D: 8 questions
• EXERCISE 12F: 12 questions
• MISCELLANEOUS: 9 questions

12.1 INTRODUCTION

Organic chemistry began as the chemistry of living things. Early chemists believed sugar, starch, and acetic acid came only from plants and animals. They called these organic compounds. Substances from minerals, like common salt, were inorganic. Scientists thought a “vital force” of nature produced organic compounds.

That theory collapsed in 1828. Friedrich Wohler heated ammonium cyanate and obtained urea. This was an organic compound made from an inorganic salt.

\mathrm{NH_4CNO} \xrightarrow{\text{heat}} \mathrm{CO(NH_2)_2}

Wohler’s work proved organic compounds could be made in the laboratory. Kolbe prepared acetic acid from its elements in 1845. Berthelot synthesised methane in 1856. Organic chemistry became the chemistry of carbon compounds. You can explore the rest of the book at the Class 10 Chemistry solutions hub.

12.2 ORGANIC COMPOUNDS

Organic compounds all contain carbon. The definition excludes oxides of carbon, metallic carbonates, and related compounds like cyanides and carbides. The text lists seven sources:

1. Plants: sugar, starch, cellulose, drugs
2. Animals: urea, proteins, fats
3. Coal: benzene, toluene, dyes, drugs, perfumes
4. Petroleum: gasoline, petrol, naphtha
5. Fermentation: ethyl alcohol, acetic acid
6. Wood: methyl alcohol, acetone
7. Synthetic methods: laboratory synthesis

Organic and inorganic compounds differ in ten ways. Organic compounds always contain carbon, rarely dissolve in water, and dissolve in organic solvents. They have low melting and boiling points, burn easily, and form covalent bonds. They are non-electrolytes, show isomerism, and react slowly. Inorganic compounds mostly form ionic bonds and react fast.

These differences matter in daily life. Soaps, perfumes, food, fuels, medicines, explosives, dyes, and insecticides are all organic compounds.

12.3 UNIQUE NATURE OF CARBON ATOMS

Carbon is unique because of two properties: tetravalency and catenation.

Tetravalency means carbon shares four electrons to form four covalent bonds. Carbon has atomic number 6 and electronic configuration 2, 4. It can neither lose nor gain four electrons easily. So it shares them.

Catenation is the self-linking of carbon atoms. Carbon atoms join through covalent bonds to form straight, branched, and cyclic chains. The carbon-carbon bond is strong, so chains can be very long. Molecules with up to seventy carbon atoms are known.

Carbon forms single, double, and triple bonds. It bonds with oxygen and nitrogen too. This variety creates more than five million organic compounds. That is why a separate branch of chemistry exists. Organic chemistry is the chemistry of hydrocarbons and their derivatives.

12.4 TYPES OF ORGANIC COMPOUNDS

The chapter classifies organic compounds by family. Table 12.1 maps each family to its identifying group. This table is the roadmap for the whole chapter.

Return to this table while reading later sections. Each family appears again with its own preparation and reactions.

12.5 HYDROCARBONS

Hydrocarbons contain only carbon and hydrogen. They split into aliphatic open-chain and cyclic closed-chain compounds.

Aliphatic hydrocarbons divide into saturated and unsaturated types. Alkanes are saturated with general formula \mathrm{C_nH_{2n+2}}. All four valencies of carbon are satisfied by single bonds. Alkenes have one double bond, \mathrm{C_nH_{2n}}. Alkynes have a triple bond, \mathrm{C_nH_{2n-2}}.

Saturated compounds undergo substitution reactions. One atom replaces another. Methane reacts with chlorine: \mathrm{CH_4 + Cl_2 \rightarrow CH_3Cl + HCl}. Unsaturated compounds undergo addition reactions. Atoms add across the double bond: \mathrm{C_2H_4 + Br_2 \rightarrow C_2H_4Br_2}.

Cyclic hydrocarbons with three or more carbons are carbocyclic compounds. The benzene ring is special: six carbons with single and double bonds in alternate positions. These aromatic compounds often smell pleasant.

A quick check: classify \mathrm{C_3H_6}. It fits \mathrm{C_nH_{2n}}, so it is an alkene.

12.6 ALKYL GROUP

An alkyl group forms when an alkane loses one hydrogen atom.

\mathrm{C_nH_{2n+2} \xrightarrow{-\mathrm{H}} -C_nH_{2n+1}}

Methane loses hydrogen to give the methyl group: \mathrm{CH_4 \xrightarrow{-\mathrm{H}} -CH_3}. The name changes by replacing the suffix ane with yl. Methane becomes methyl, ethane becomes ethyl.

• Methyl: -\mathrm{CH_3}
• Ethyl: -\mathrm{C_2H_5}
• Propyl: -\mathrm{C_3H_7}
• Butyl: -\mathrm{C_4H_9}

Each alkyl group has one unpaired electron. The free valency is shown as a line with no symbol attached. Original pattern: remove hydrogen from propane \mathrm{C_3H_8} to get the propyl group \mathrm{C_3H_7}.

12.7 FUNCTIONAL GROUP

A functional group is an atom or group of atoms that defines a family’s structure and properties. Alkanes have no functional group. Alkenes use the double bond \gt\mathrm{C}{=}\mathrm{C}\lt. Alkynes use the triple bond -\mathrm{C}{\equiv}\mathrm{C}-.

Functional groups set behaviour in three ways. The same group gives similar chemical properties. Different groups give different properties. Each group forms its own homologous series.

12.9 ISOMERS

Isomers have the same molecular formula but different structural formulas. Isomerism comes from Greek: iso means same, meros means parts. Butane and isobutane both have formula \mathrm{C_4H_{10}}, yet their structures differ.

Chain isomerism arises when carbon atoms arrange in straight or branched chains. Pentane \mathrm{C_5H_{12}} shows this. Position isomerism arises when a substituent or functional group sits at different positions. But-1-ene and but-2-ene are position isomers; the double bond moves from carbon 1 to carbon 2.

The textbook marks functional isomerism and metamerism as not in syllabus. Ethanol \mathrm{CH_3CH_2OH} and dimethyl ether \mathrm{CH_3-O-CH_3} are functional isomers. Confirm this with your teacher before studying them.

Isomer numbers grow with carbon count. Butane has 2 isomers, hexane has 5, and \mathrm{C_{21}H_{44}} has about a million.

12.10 HOMOLOGOUS SERIES

A homologous series is a group of compounds with similar structure and chemical properties. Successive members differ by a \mathrm{CH_2} group. Methane plus \mathrm{CH_2} gives ethane: \mathrm{CH_4 + CH_2 \rightarrow C_2H_6}.

• Members differ by one \mathrm{CH_2} group and 14 a.m.u.
• All members share one general formula.
• Physical properties change gradually as molecular mass rises.
• Chemical properties are similar, so methane and ethane both chlorinate.
• The same general preparation works for all members.

The alkyl group decides physical properties. The functional group decides chemical reactivity.

Original check: propane \mathrm{C_3H_8} has molar mass 44. Butane \mathrm{C_4H_{10}} has molar mass 58. The difference is 14, matching the \mathrm{CH_2} rule.

12.11 NOMENCLATURE

Nomenclature is the system of naming organic compounds. Two systems exist: trivial and IUPAC. Trivial names come from sources. Methane was marsh gas from marshy places. Acetic acid comes from Latin acetum, meaning vinegar. Methyl alcohol came from wood.

• Root word: from the longest carbon chain.
• Suffix: shows the bond type, ane, ene, or yne.
• Prefix: shows the substituent and its position.

Mnemonic for the first ten root words: Meth, Eth, Prop, But, Pent, Hex, Hept, Oct, Non, Dec. These match one to ten carbon atoms.

Seven rules govern naming. Select the longest continuous chain as the parent chain; it need not be straight. Treat branches as substituents. Number the chain so substituents get the smallest locants. Give the functional group the smallest number, and replace the e of the alkane. Name different substituents alphabetically. Write position and name before the parent. Use di, tri, tetra for repeated groups.

Original pattern: a six-carbon chain with a methyl on the third carbon is 3-methylhexane. Numbering from the other end gives 4-methylhexane, which is wrong.

12.12 WRITING STRUCTURAL FORMULA FROM IUPAC NAME

Five steps turn a name into a structure. Write the carbon skeleton from the root word. Number the chain from any end. Place the double or triple bond at the suffix position. Attach substituents at their stated carbons. Add hydrogen atoms so each carbon has four bonds.

Original example: 3-bromo-2-methylpent-1-ene.

Step 1: write five carbons: \mathrm{C-C-C-C-C}.

Step 2: number them 1 to 5.

Step 3: place the double bond between carbons 1 and 2: \mathrm{C{=}C-C-C-C}.

Step 4: attach bromo at carbon 3 and methyl at carbon 2.

Step 5: add hydrogens. The final condensed formula is \mathrm{CH_2{=}C(CH_3)-CH(Br)-CH_2-CH_3}.

Two errors trap students. A carbon with five bonds is impossible; carbon is tetravalent. A double bond shifted from its stated position changes the name. Check every carbon’s valency before finishing.

12.13 ALKANES

Alkanes are saturated hydrocarbons with only single bonds. The general formula is \mathrm{C_nH_{2n+2}}. They are called paraffins, from Latin parum and affinis, meaning little affinity. They are relatively unreactive.

Natural gas and petroleum are the principal sources. Natural gas is mostly methane. Alkanes burn in oxygen to give carbon dioxide and water. Methane combustion: \mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}.

Alkanes with more than three carbons show chain isomerism. Butane has two isomers: n-butane (butane) and isobutane (2-methylpropane). The label n means normal, a straight chain. Iso means at least one carbon attached to three others.

Pentane has three isomers: n-pentane (pentane), isopentane (2-methylbutane), and neopentane (2,2-dimethylpropane). Neo means at least one carbon attached to four others.

12.14.3 Laboratory preparation of methane

Methane is made from sodium ethanoate and soda lime. Soda lime is a mixture of sodium hydroxide and calcium oxide. The reactants are heated in a hard glass test tube.

\mathrm{CH_3COONa + NaOH \xrightarrow[katex display="true"]\text{300°C}{\text{CaO}} Na_2CO_3 + CH_4 \uparrow}[/katex]

The gas is collected by downward displacement of water. Methane is slightly soluble in water and lighter than air.

12.14.4 Laboratory preparation of ethane

Ethane is prepared the same way, using sodium propionate instead. Sodium propionate and soda lime are heated in a boiling tube.

\mathrm{C_2H_5COONa + NaOH \xrightarrow[katex display="true"]\text{300°C}{\text{CaO}} Na_2CO_3 + C_2H_6 \uparrow}[/katex]

The gas is collected by downward displacement of water. The pattern mirrors methane’s preparation. The sodium salt of the acid with one carbon less gives the alkane.

12.15.2 Chemical properties

Methane and ethane are saturated. They do not react with bases, oxidising agents, or reducing agents. They undergo substitution and combustion.

Substitution with halogens: methane and chlorine in diffused sunlight give chloromethane. \mathrm{CH_4 + Cl_2 \rightarrow CH_3Cl + HCl}. Successive products follow up to tetrachloromethane. Ethane gives products up to hexachloroethane.

Combustion: complete burning gives carbon dioxide and water with a bluish non-sooty flame. Insufficient air gives carbon monoxide and water. Still less oxygen leaves soot.

Cracking: heating in the absence of air breaks alkanes down. Methane at 1500°C gives ethyne and hydrogen: \mathrm{2CH_4 \xrightarrow{1500^\circ\mathrm{C}} HC{\equiv}CH + 3H_2}.

Catalytic oxidation: methane with oxygen over copper tubes at 475 K gives methanol. Over molybdenum oxide it gives formaldehyde. With a manganese compound it gives formic acid.

Slow combustion: acidified potassium dichromate or permanganate oxidises the alkane step by step. Alcohol forms first, then aldehyde, then carboxylic acid, then carbon dioxide and water.

12.16 ALKENES

Alkenes are unsaturated hydrocarbons with one double bond. The general formula is \mathrm{C_nH_{2n}}. They are called olefins because lower members form oily products with chlorine or bromine.

Ethene is the first member. It is a planar molecule with 120° bond angles. It occurs in natural gas and acts as a plant hormone for fruit ripening.

Preparation: dehydration of ethanol with concentrated sulphuric acid at 170°C gives ethene. \mathrm{C_2H_5OH \xrightarrow[katex display="true"]\text{170°C}{\text{conc. H}_2\mathrm{SO}_4} C_2H_4 + H_2O}[/katex]. Industrial dehydration uses alumina at 350°C. Dehydrohalogenation of ethyl chloride with alcoholic potassium hydroxide also gives ethene.

Alkenes undergo addition reactions. Hydrogenation over nickel at 200°C gives ethane. Bromine in carbon tetrachloride is decolourised. Hydration in the presence of acid gives ethanol. HCl addition gives chloroethane.

Original pattern: apply hydrogenation to propene. \mathrm{CH_3-CH{=}CH_2 + H_2 \xrightarrow{\text{Ni}} CH_3-CH_2-CH_3}.

Combustion gives a sooty flame, or a pale blue flame in excess air. Uses include polythene, fruit ripening, and oxy-ethylene welding.

12.17 ALKYNES

Alkynes are unsaturated hydrocarbons with a triple bond. The general formula is \mathrm{C_nH_{2n-2}}. Ethyne, or acetylene, is the first member and its molecule is linear.

Laboratory preparation: calcium carbide reacts with water. \mathrm{CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2 \uparrow}. The reaction is exothermic. The gas is collected by downward displacement of water. Impurities like phosphine and hydrogen sulphide are removed. Phosphine is absorbed by acidified copper sulphate solution.

Ethyne burns in excess air with a brilliant white flame, used in oxy-acetylene welding. Addition reactions occur in stages. Hydrogen converts ethyne to ethene, then to ethane. Bromine is decolourised.

Original pattern: apply halogenation to but-2-yne. \mathrm{CH_3-C{\equiv}C-CH_3 + Br_2 \rightarrow CH_3-C(Br){=}C(Br)-CH_3}.

Four chemical tests distinguish the three hydrocarbon families:

Alkanes give no change in all four tests. Only alkynes give the red and white precipitates.

12.18 ALCOHOLS

Alcohols are hydroxyl derivatives of alkanes. One or more hydrogen atoms are replaced by the OH group. The general formula is \mathrm{C_nH_{2n+1}OH}.

• Monohydric: one OH group, like methyl alcohol \mathrm{CH_3OH}.
• Dihydric: two OH groups, like glycol \mathrm{C_2H_4(OH)_2}.
• Trihydric: three OH groups, like glycerol, IUPAC 1,2,3-propanetriol.

Original pattern: propanol \mathrm{C_3H_7OH} is the third member of the series. It fits \mathrm{C_nH_{2n+1}OH} with n equal to 3.

12.19 ETHANOL

Ethanol is made by synthesis since alcohols do not occur free in nature. Methanol comes from destructive distillation of wood. Ethanol comes from fermentation of sugar.

\mathrm{C_6H_{12}O_6 \xrightarrow[katex display="true"]\text{37°C}{\text{enzymes}} 2C_2H_5OH + 2CO_2}[/katex]

Ethanol is called spirit of wine and grain alcohol. It comes from starchy grain.

Laboratory preparation uses hydrolysis of alkyl halides. Hot aqueous alkali converts the halide to the alcohol. \mathrm{CH_3Br + KOH \xrightarrow{\text{boil}} CH_3OH + KBr}. Ethyl chloride gives ethanol the same way.

Industrial method: hydration of ethene. Concentrated sulphuric acid at 80°C and 30 atm gives ethyl hydrogen sulphate. Hydrolysis with boiling water gives ethanol. Alternatively, ethene with water at 300°C and 60 atm over phosphoric acid gives ethanol directly.

12.20 CARBOXYLIC ACIDS

Carboxylic acids carry the carboxyl group, -\mathrm{COOH}. This group joins a carbonyl and a hydroxyl group. Section 12.7 introduced it with the suffix oic acid. Ethanoic acid \mathrm{CH_3COOH} is the chapter’s example.

The carboxyl group makes these compounds acidic. They form a homologous series like other families. The next section studies acetic acid in detail.

12.21 ACETIC ACID CH₃COOH

Acetic acid is ethanoic acid, \mathrm{CH_3COOH}. It carries the carboxyl group from section 12.7. The chapter covers its preparation, properties, test, and uses.

Acetic acid reacts with sodium to release hydrogen. Concentrated sulphuric acid dehydrates it. Phosphorus halides convert it to acyl halides. The test for acetic acid and its uses are detailed in the book.

Vinegar is a dilute solution of acetic acid. The acid is also used to make other organic compounds.

What to practise from Chemistry Class 10 ICSE Solutions

This chapter has seven question blocks. The table maps each block to the sections it tests.

Start with the Intext Questions to check definitions. Then attempt each exercise after reading its sections. For the official Class 10 Chemistry syllabus, visit the official CISCE website. You can also browse the full Class 10 ICSE books collection.

Chemistry Class 10 ICSE Solutions Frequently Asked Questions

How do I name an organic compound using IUPAC rules in Class 10 ICSE Chemistry?

Find the longest carbon chain as the parent chain. Number it so substituents get the smallest locants. Add the suffix for the bond type and the prefix for substituents. Section 12.11 gives the seven rules with examples.

What is the difference between saturated and unsaturated hydrocarbons?

Saturated compounds have only single bonds and all valencies satisfied. Unsaturated compounds have double or triple bonds. Saturated compounds undergo substitution; unsaturated ones undergo addition.

Which chemical tests distinguish alkanes, alkenes and alkynes?

Bromine in carbon tetrachloride and alkaline permanganate decolourise for alkenes and alkynes but not alkanes. Ammoniacal cuprous chloride and ammoniacal silver nitrate give red and white precipitates only with alkynes.

What do the chemistry class 10 ICSE solutions on this page cover?

This page explains Selina Chapter 12, Organic Chemistry, section by section. It covers carbon’s unique nature, hydrocarbons, functional groups, naming rules, and each family’s preparations and reactions. A practice list maps every exercise block to its sections.

Why do butane and pentane have different numbers of isomers?

The number of isomers grows with carbon atoms. Butane’s four carbons arrange in two ways, giving two isomers. Pentane’s five carbons arrange in three ways, giving three isomers.

Reference: ICSE Class 10 Chemistry textbook material, chapter 12: Organic Chemistry — CISCE-aligned.

More from this book

• Chapter 3: Acids Bases and Salts
• Chapter 4: Analytical Chemistry
• Chapter 6: Electrolysis