What is a homologous series? Give an example.

A homologous series is a group of organic compounds that have the same functional group, same general formula, and show a gradual change in physical properties. Each successive member differs by a CH₂ unit and 14 in molecular mass. Example: Alkanes — methane (CH₄), ethane (C₂H₆), propane (C₃H₈)...

What is the difference between saturated and unsaturated hydrocarbons?

Saturated hydrocarbons (alkanes) have only single bonds between carbon atoms (e.g., CH₄, C₂H₆). Unsaturated hydrocarbons have one or more double bonds (alkenes, e.g., C₂H₄) or triple bonds (alkynes, e.g., C₂H₂) between carbon atoms.

What happens when ethanol is oxidised? What is used as oxidising agent?

When ethanol (C₂H₅OH) is oxidised using alkaline KMnO₄ or acidified K₂Cr₂O₇, it is converted to ethanoic acid (CH₃COOH — vinegar). The orange colour of K₂Cr₂O₇ turns green.

How do soaps clean dirt?

Soap molecules have two ends: a hydrophilic (water-loving) head and a hydrophobic (water-repelling/oil-loving) tail. The hydrophobic end dissolves in oil/grease; the hydrophilic end faces water. This forms micelles — spherical structures that trap grease inside. When rinsed, the entire micelle (with dirt inside) is washed away.

Why don't soaps work in hard water?

Hard water contains Ca²⁺ and Mg²⁺ ions. Soap (sodium salt of fatty acid) reacts with these ions to form insoluble calcium/magnesium salts called scum. This precipitate wastes soap and leaves a grayish residue on clothes. Detergents don't form scum with hard water, which is why they replaced soaps for laundry.

CBSE Class 10 Science Chapter 4 Notes — Carbon and its Compounds

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Why Carbon is Special — Covalent Bonding

Covalent Bonds

Carbon has 4 valence electrons. It needs 4 more to complete its octet. Instead of transferring electrons (ionic bond), carbon shares electrons with other atoms — forming covalent bonds.

A covalent bond is formed by the mutual sharing of one or more electron pairs between two non-metal atoms.

Bond type	Electron pairs shared	Example
Single bond	1 pair	H—H in H₂; C—H in methane
Double bond	2 pairs	C=C in ethylene (C₂H₄)
Triple bond	3 pairs	C≡C in acetylene (C₂H₂)

Why Carbon Forms Huge Numbers of Compounds

1. Catenation: Carbon atoms can bond with each other to form:

Long chains: C-C-C-C-C...
Branched chains
Closed rings (cyclic structures)

2. Tetravalency: Carbon has 4 valence electrons, so it can bond with up to 4 other atoms — C, H, O, N, S, halogens.

No other element shows catenation to the same extent as carbon → hence millions of organic compounds exist.

Hydrocarbons — Saturated and Unsaturated

Hydrocarbons = compounds of carbon and hydrogen only.

Saturated Hydrocarbons (Alkanes)

Have only single bonds (C-C)
General formula: CₙH₂ₙ₊₂
Called alkanes

Name	Formula	Structural Formula
Methane	CH₄	H₃C-H
Ethane	C₂H₆	H₃C-CH₃
Propane	C₃H₈	H₃C-CH₂-CH₃
Butane	C₄H₁₀	H₃C-CH₂-CH₂-CH₃

Unsaturated Hydrocarbons

Alkenes (one double bond):

General formula: CₙH₂ₙ
Examples: Ethene (C₂H₄), Propene (C₃H₆)

Alkynes (one triple bond):

General formula: CₙH₂ₙ₋₂
Examples: Ethyne (C₂H₂), Propyne (C₃H₄)

Functional Groups

A functional group is an atom or group of atoms that determines the chemical properties of an organic compound.

Functional Group	Name	Example	Formula
-OH	Hydroxyl	Ethanol	C₂H₅OH
-CHO	Aldehyde	Ethanal	CH₃CHO
-COOH	Carboxyl (acid)	Ethanoic acid	CH₃COOH
-CO-	Ketone	Propanone (acetone)	CH₃COCH₃
-Cl, -Br	Halo group	Chloromethane	CH₃Cl

Homologous Series

A homologous series is a group of organic compounds with: 1. Same functional group 2. Same general formula 3. Similar chemical properties 4. Each successive member differs by -CH₂- (14 in molecular mass) 5. Gradual change in physical properties (melting point, boiling point increase with molecular mass)

Alcohol series (alkanols): CH₃OH, C₂H₅OH, C₃H₇OH, C₄H₉OH...

Nomenclature (IUPAC Names)

Number of C atoms	Prefix	Example alkane
1	Meth-	Methane
2	Eth-	Ethane
3	Prop-	Propane
4	But-	Butane
5	Pent-	Pentane

The suffix tells you the type:

-ane (alkane, single bond)
-ene (alkene, double bond)
-yne (alkyne, triple bond)
-ol (alcohol, -OH group)
-al (aldehyde, -CHO)
-oic acid (carboxylic acid, -COOH)

Chemical Properties of Carbon Compounds

1. Combustion

All carbon compounds burn in oxygen to produce CO₂ and H₂O:

CH₄ + 2O₂ → CO₂ + 2H₂O (complete combustion — blue flame) CH₄ + limited O₂ → C + H₂O (incomplete combustion — yellow/sooty flame)

Saturated hydrocarbons burn with a clean blue flame. Unsaturated hydrocarbons burn with a yellow sooty flame (more carbon → more soot).

2. Addition Reaction (Unsaturated compounds)

Unsaturated hydrocarbons add substances across the double/triple bond:

C₂H₄ + H₂ →(Ni catalyst, heat) C₂H₆ (hydrogenation — turning vegetable oil to vanaspati ghee) C₂H₄ + Cl₂ → C₂H₄Cl₂

Industrial hydrogenation: Vegetable oils (unsaturated) + H₂ → vegetable ghee/margarine (saturated fats)

3. Substitution Reaction (Saturated compounds)

In saturated hydrocarbons, one hydrogen atom is replaced by another atom in the presence of sunlight:

CH₄ + Cl₂ →(sunlight) CH₃Cl + HCl (chloromethane)

4. Oxidation

Alcohols can be oxidised to carboxylic acids using oxidising agents:

C₂H₅OH + [O] →(alkaline KMnO₄ or K₂Cr₂O₇) CH₃COOH (ethanol → ethanoic acid)

Ethanol (C₂H₅OH) — Properties and Uses

Property	Details
Appearance	Colourless liquid
Smell	Pleasant
Boiling point	78°C
Solubility	Completely miscible with water

Uses of Ethanol

Solvent in medicines, perfumes, varnishes
As a fuel (mixed with petrol — E10, E20)
In alcoholic beverages
As an antiseptic (70% ethanol solution)

Reactions of Ethanol

With sodium: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑ (hydrogen gas is produced — sodium ethoxide forms)

Oxidation (to ethanoic acid): C₂H₅OH + 2[O] → CH₃COOH + H₂O

Dehydration (to ethylene): C₂H₅OH →(H₂SO₄, 170°C) C₂H₄ + H₂O

Ethanoic Acid (CH₃COOH) — Properties and Uses

Common name: Acetic acid
A 5–8% solution is called vinegar
Melting point: 17°C — freezes in cold weather into a glass-like solid → called glacial acetic acid

Properties

Sour taste and pungent smell
Weak acid (only partially dissociates)
Turns blue litmus red
pH around 3 in dilute solution

Reactions of Ethanoic Acid

With sodium carbonate: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑

With sodium hydroxide (neutralisation): CH₃COOH + NaOH → CH₃COONa + H₂O

Esterification (reaction with ethanol): CH₃COOH + C₂H₅OH ⇌(conc. H₂SO₄) CH₃COOC₂H₅ + H₂O (ethyl ethanoate — an ester — has a sweet fruity smell; used in perfumes and artificial flavours)

Soaps and Detergents

What is Soap?

Soap is the sodium salt or potassium salt of a long-chain fatty acid.

Hard soap: sodium salts (NaOH used) — used for laundry
Soft soap: potassium salts (KOH used) — used for skin/shaving

Making soap (saponification): Fat/Oil + NaOH →(boiling) Soap (sodium salt of fatty acid) + Glycerol

How Soap Cleans (Micelle Formation)

Soap molecules have two ends:

Hydrophilic head (ionic end, -COO⁻Na⁺) — attracted to water
Hydrophobic tail (long carbon chain) — repels water; dissolves in oil/grease

When soap is added to dirty water: 1. The hydrophobic tails of many soap molecules insert into the grease/oil droplet 2. The hydrophilic heads point outward toward water 3. This creates a micelle — a spherical structure with grease trapped inside 4. Micelles are washed away with water, carrying the dirt with them

Why Soaps Don't Work in Hard Water

Hard water contains Ca²⁺ and Mg²⁺ ions. Soap reacts with them to form insoluble precipitates (called scum): 2C₁₇H₃₅COONa + CaCl₂ → (C₁₇H₃₅COO)₂Ca↓ + 2NaCl

This wastes soap and leaves a grey residue.

Detergents vs Soaps

Feature	Soaps	Detergents
Made from	Natural fats/oils	Petrochemicals
Hard water	Forms scum (doesn't work well)	Works in hard water
Sea water	Doesn't work	Works
Biodegradable	Yes	Not always
Cost	Cheaper	Usually costlier

Synthetic detergents are sulphonate/sulphate salts of long chains. They don't react with Ca²⁺/Mg²⁺ so no scum forms.

Board Exam Tips

8–10 marks from this chapter — one of the most important chapters
Carbon compounds have a huge variety of questions: naming, writing reactions, explaining properties
Know all functional groups, their names, and one example compound for each
Esterification reaction with ethanol + ethanoic acid is always in exams — know products AND conditions (conc. H₂SO₄ catalyst, heat)
Soaps and detergents — micelle diagram is often asked as a 3-mark diagram question
"Why does soap not work in hard water?" — 2-mark question every year
Hydrogenation of vegetable oil — know the catalyst (Ni) and product (vanaspati)

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Intext Questions

Page 61, Q1: What would be the electron dot structure of carbon dioxide?

AnswerCarbon dioxide (CO₂): Carbon forms double bonds with each oxygen atom. O::C::O (each :: represents a double bond) Structural representation: O=C=O Carbon has 4 bonds (2 double bonds = 4 bond pairs), satisfying its valency of 4.

Page 65, Q1: How many structural isomers can you draw for pentane?

AnswerPentane (C₅H₁₂) has three structural isomers: 1. n-Pentane — straight chain: CH₃–CH₂–CH₂–CH₂–CH₃ 2. Isopentane (2-methylbutane) — CH₃–CH(CH₃)–CH₂–CH₃ (one branch at C2) 3. Neopentane (2,2-dimethylpropane) — central carbon with four methyl groups

Page 68, Q2: Would you be able to check if water is hard by using a detergent?

AnswerNo. Detergents (synthetic) work equally well in hard water and soft water because they do not form insoluble scum with calcium and magnesium salts. To check whether water is hard, use soap — soap forms a curdy precipitate (scum) with hard water, so lathering difficulty indicates hardness.

Chapter-End Exercise

Q1. Ethane, with molecular formula C₂H₆, has how many covalent bonds?

AnswerEthane (C₂H₆):

1 C–C bond (between the two carbon atoms)
6 C–H bonds (3 from each carbon)
Total: 7 covalent bonds

Q2. Butanone is a four-carbon compound with the functional group ketone (–CO–). Write its structural formula.

AnswerButanone (methyl ethyl ketone): CH₃–CO–CH₂–CH₃ IUPAC name: Butan-2-one The carbonyl group (C=O) is on the second carbon.

Q3. Which of the following substances undergo addition reactions? Ethene, Propane, Ethyne, Benzene (not in new syllabus, ignore), Cyclohexane

AnswerEthene (CH₂=CH₂) and Ethyne (CH≡CH) undergo addition reactions because they have double and triple bonds respectively.

Example for ethene: CH₂=CH₂ + H₂ →(Ni catalyst, heat) CH₃–CH₃ (ethene → ethane)

Example for ethyne: CH≡CH + H₂ → CH₂=CH₂ → CH₃–CH₃

Propane and cyclohexane have only single bonds (saturated) and undergo substitution reactions, not addition reactions.

Q4. How can ethanol and ethanoic acid be distinguished using sodium carbonate?

Answer

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑

Add a small amount of sodium carbonate (Na₂CO₃) to both samples.
Ethanoic acid reacts with Na₂CO₃ to produce CO₂ gas with brisk effervescence:
Ethanol does NOT react with sodium carbonate — no gas is produced, no visible change.
The sample that shows effervescence (gas bubbles) is ethanoic acid; the one with no change is ethanol.

Q5. What is meant by saponification? Write the chemical equation.

AnswerSaponification is the process of making soap. When a fat or oil (ester) is heated with a strong base (NaOH or KOH), it undergoes hydrolysis to give glycerol and the sodium salt of a fatty acid (soap).

Fat/Oil + NaOH(aq) →(heat) Glycerol + Sodium salt of fatty acid (Soap)

Example: (C₁₇H₃₅COO)₃C₃H₅ + 3NaOH → C₃H₅(OH)₃ + 3C₁₇H₃₅COONa (Tristearin) + (Sodium hydroxide) → (Glycerol) + (Sodium stearate — soap)

Q6. Describe a test to distinguish between saturated and unsaturated hydrocarbons.

AnswerUse bromine water (Br₂ dissolved in water — orange-brown colour).

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

Add a few drops of bromine water to each hydrocarbon.
Unsaturated hydrocarbon (contains double or triple bond): bromine water is decolourised (becomes colourless) — addition reaction occurs.
Saturated hydrocarbon (only single bonds): bromine water remains orange-brown — no reaction (or only slow substitution in UV light).

Alternatively, use alkaline KMnO₄ (purple) — unsaturated compounds decolourise it.

Q7. What are the two properties of carbon that lead to the large number of carbon compounds?

Answer1. Catenation — Carbon atoms have the unique ability to form strong covalent bonds with other carbon atoms. These chains can be long, branched, or form rings. No other element shows this property to the same extent.

2. Tetravalency — Carbon has 4 valence electrons, so it can form 4 covalent bonds. It bonds with H, O, N, S, Cl, and other elements, creating enormous variety in the compounds formed.

Together, these two properties allow carbon to form millions of different compounds — organic chemistry.

Q8. Write the IUPAC names of: (a) CH₃OH (b) CH₃COOH (c) CH₃CHO (d) HCOOH

Answer(a) CH₃OH — Methanol (functional group: –OH, alcohol; parent chain: 1 carbon) (b) CH₃COOH — Ethanoic acid (functional group: –COOH; parent chain: 2 carbons) (c) CH₃CHO — Ethanal (functional group: –CHO, aldehyde; parent chain: 2 carbons) (d) HCOOH — Methanoic acid (formic acid; parent chain: 1 carbon with –COOH)

Frequently Asked Questions

Carbon forms a huge number of compounds because of two unique properties: (1) Catenation — carbon atoms can bond with each other to form long chains, branched chains, and rings; (2) Tetravalency — carbon has 4 valence electrons, allowing it to form 4 covalent bonds with other atoms (C, H, O, N, etc.).