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These are complete exercise solutions for Samacheer Kalvi 9th Science Chapter — Carbon and Its Compounds. Solve each question yourself first, then verify your method and answer below. Pay special attention to balanced equations, nomenclature rules, and reaction types — these are the most frequently tested areas.
Key Concepts (Quick Recap)
- Catenation: Carbon's ability to form bonds with other carbon atoms, producing long chains, branched chains, and rings. No other element forms as many compounds.
- Tetravalency: Carbon has 4 valence electrons (2,4 configuration), so it forms 4 covalent bonds.
- Covalent bonding: Sharing of electron pairs between atoms. Types: single bond (C–C), double bond (C=C), triple bond (C≡C).
- Allotropes of carbon: Different physical forms of the same element — Diamond (hardest, 3D network, each C bonded to 4 others), Graphite (soft, layered, each C bonded to 3 others, conducts electricity), Fullerene (C₆₀, buckminsterfullerene, cage structure).
- Hydrocarbons: Compounds of carbon and hydrogen only. Saturated (alkanes: CnH2n+2, single bonds), Unsaturated (alkenes: CnH2n, double bonds; alkynes: CnH2n-2, triple bonds).
- Homologous series: A series of organic compounds with the same functional group, each member differing by –CH₂– from the next. Same chemical properties, gradually changing physical properties.
- IUPAC nomenclature: Systematic naming — identify the longest carbon chain, number from the end nearest the functional group, name the functional group as suffix/prefix.
- Ethanol (C₂H₅OH): Active ingredient in alcoholic drinks, used as solvent and fuel. Reactions: oxidation → ethanoic acid; dehydration → ethene; reaction with sodium → sodium ethoxide + H₂.
- Ethanoic acid (CH₃COOH): Also called acetic acid; vinegar is 5–8% ethanoic acid. Reacts with alcohols to form esters (esterification). Reacts with bases to form acetate salts.
- Soaps and detergents: Soaps = Na/K salts of fatty acids; detergents = Na salts of sulphonic acids. Cleansing action: hydrophobic tail attaches to dirt, hydrophilic head attaches to water → micelle formation → emulsification.
I. Choose the Correct Answer
Q1. The number of valence electrons in carbon is: (a) 2 (b) 4 ✓ (c) 6 (d) 8
Answer(b) ✓ — Carbon has atomic number 6; electronic configuration 2,4. The 4 valence electrons allow carbon to form 4 covalent bonds (tetravalency).
Q2. The property of carbon to form bonds with other carbon atoms is called: (a) Tetravalency (b) Isomerism (c) Catenation ✓ (d) Allotropy
Answer(c) ✓ — Catenation is the self-linking property of carbon atoms, enabling chains, branches, and rings.
Q3. The allotrope of carbon that conducts electricity is: (a) Diamond (b) Graphite ✓ (c) Fullerene (d) Charcoal
Answer(b) ✓ — In graphite, each carbon is bonded to only 3 other carbons, leaving one delocalised electron per atom that enables electrical conductivity.
Q4. The general formula of alkanes is: (a) CnH2n (b) CnH2n+2 ✓ (c) CnH2n-2 (d) CnHn
Answer(b) ✓ — Alkanes are saturated hydrocarbons with single bonds only. Example: Methane CH₄ (n=1, 2×1+2=4H).
Q5. The IUPAC name of ethylene (C₂H₄) is: (a) Ethane (b) Ethene ✓ (c) Ethyne (d) Ethanol
Answer(b) ✓ — C₂H₄ has a double bond (C=C), so it is an alkene. The suffix for alkenes is "-ene." Two carbons → "eth" + "ene" = ethene.
Q6. The functional group present in ethanol is: (a) Carboxyl (–COOH) (b) Aldehyde (–CHO) (c) Hydroxyl (–OH) ✓ (d) Ketone (–C=O)
Answer(c) ✓ — Ethanol (C₂H₅OH) contains the hydroxyl (–OH) functional group, making it an alcohol.
Q7. Vinegar is a dilute solution of: (a) Ethanol (b) Ethanoic acid ✓ (c) Acetone (d) Formaldehyde
Answer(b) ✓ — Vinegar contains 5–8% ethanoic acid (acetic acid, CH₃COOH) in water.
Q8. Soaps are sodium or potassium salts of: (a) Sulphonic acids (b) Long-chain fatty acids ✓ (c) Mineral acids (d) Amino acids
Answer(b) ✓ — Soaps are Na/K salts of long-chain carboxylic acids (fatty acids). Example: Sodium stearate (C₁₇H₃₅COONa).
Q9. Which of the following is an unsaturated hydrocarbon? (a) Methane (CH₄) (b) Ethane (C₂H₆) (c) Propene (C₃H₆) ✓ (d) Butane (C₄H₁₀)
Answer(c) ✓ — Propene (C₃H₆) has a double bond (C=C), so it is an alkene — an unsaturated hydrocarbon. The others are alkanes (single bonds only).
Q10. Esterification is the reaction between: (a) An acid and a base (b) An alcohol and a carboxylic acid ✓ (c) A metal and an acid (d) An alkene and hydrogen
Answer(b) ✓ — Esterification: Alcohol + Carboxylic acid → Ester + Water. Example: C₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O (catalyst: conc. H₂SO₄).
Q11. The hardest allotrope of carbon is: (a) Graphite (b) Charcoal (c) Diamond ✓ (d) Fullerene
Answer(c) ✓ — Diamond has a rigid 3D tetrahedral structure where each carbon is bonded to 4 others with strong covalent bonds, making it the hardest natural substance.
II. Fill in the Blanks
1. The self-linking property of carbon atoms is called catenation. 2. The valency of carbon is 4 (tetravalency). 3. The general formula of alkynes is CnH2n−2. 4. The functional group present in carboxylic acids is –COOH (carboxyl group). 5. The IUPAC name of acetylene (C₂H₂) is ethyne. 6. The process of converting unsaturated hydrocarbons to saturated ones by adding hydrogen is called hydrogenation. 7. The reaction of an alcohol with a carboxylic acid to form an ester is called esterification. 8. Soaps do not form lather in hard water because they react with calcium and magnesium salts to form insoluble scum. 9. The molecular formula of buckminsterfullerene is C₆₀. 10. Ethanol reacts with sodium metal to produce sodium ethoxide and hydrogen gas.
III. True or False
1. Carbon can form bonds with other carbon atoms.
AnswerTrue ✓ — This is the property of catenation.
2. Diamond conducts electricity.
AnswerFalse — Correction: Diamond does NOT conduct electricity because all 4 valence electrons are involved in covalent bonding with no free electrons. Graphite conducts electricity.
3. Alkanes are unsaturated hydrocarbons.
AnswerFalse — Correction: Alkanes are saturated hydrocarbons with only single bonds (C–C). Alkenes and alkynes are unsaturated.
4. The functional group of alcohols is –OH.
AnswerTrue ✓ — The hydroxyl group (–OH) characterises alcohols.
5. Soaps work effectively in hard water.
AnswerFalse — Correction: Soaps form insoluble scum (calcium/magnesium salts of fatty acids) in hard water. Detergents work well in hard water.
6. Ethanoic acid is also called acetic acid.
AnswerTrue ✓ — Acetic acid is the common name; ethanoic acid is the IUPAC name.
7. The first member of the alkene homologous series is ethene (C₂H₄).
AnswerTrue ✓ — Methane (CH₄) has no double bond; the smallest alkene needs at least 2 carbon atoms for C=C.
8. Combustion of hydrocarbons always produces carbon dioxide and water.
AnswerTrue ✓ — Complete combustion: CₓHᵧ + O₂ → CO₂ + H₂O. Incomplete combustion may produce CO or soot (C).
9. In a homologous series, each member differs by –CH₂– from the next.
AnswerTrue ✓
10. Fullerene is an allotrope of carbon with a cage-like structure.
AnswerTrue ✓ — Buckminsterfullerene (C₆₀) has a football-shaped cage structure.
IV. Match the Following
Set 1:
| Column I | Column II |
|---|
| 1. Diamond | (c) Hardest natural substance |
| 2. Graphite | (e) Conducts electricity |
| 3. Fullerene (C₆₀) | (d) Cage-like structure |
| 4. Coke | (a) Reducing agent in metallurgy |
| 5. Charcoal | (b) Adsorbent and fuel |
Set 2:
| Column I | Column II |
|---|
| 1. Alkane | (e) CnH2n+2 (single bond) |
| 2. Alkene | (a) CnH2n (double bond) |
| 3. Alkyne | (c) CnH2n−2 (triple bond) |
| 4. Alcohol | (d) –OH (hydroxyl group) |
| 5. Carboxylic acid | (b) –COOH (carboxyl group) |
V. Short Answers
Q1. Why does carbon form a large number of compounds?
Answer
Carbon forms a vast number of compounds due to two unique properties: 1. Catenation: Carbon atoms can bond with each other to form long chains, branched chains, and rings. This self-linking property is strongest in carbon because the C–C bond is very strong. 2. Tetravalency: Carbon has 4 valence electrons, so it can form 4 covalent bonds with other atoms (H, O, N, S, Cl, etc.), creating a wide variety of structures.
Additionally, carbon forms isomers — compounds with the same molecular formula but different structures — further increasing the number of compounds.
Q2. Differentiate between diamond and graphite.
Answer
| Property | Diamond | Graphite |
|---|
| Structure | 3D tetrahedral, each C bonded to 4 others | 2D layered, each C bonded to 3 others |
| Bonding | All 4 valence electrons used in bonds | 3 electrons bonded; 1 delocalised |
| Hardness | Hardest natural substance | Soft, slippery |
| Electrical conductivity | Non-conductor | Good conductor (delocalised electrons) |
| Appearance | Transparent, brilliant | Black, opaque |
| Density | 3.5 g/cm³ | 2.2 g/cm³ |
| Uses | Cutting, drilling, jewellery | Pencils, electrodes, lubricants |
Q3. What is a homologous series? Give an example.
Answer
A homologous series is a series of organic compounds having the same functional group, in which each successive member differs from the previous one by a –CH₂– unit (14 amu).
Example — Alkanes: CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane), C₄H₁₀ (butane) — each differs by CH₂, and all have the general formula CnH2n+2.
Characteristics: Same functional group, same chemical properties, gradually changing physical properties (boiling point increases with chain length), same general formula.
Q4. What are hydrocarbons? Classify them with examples.
Answer
Hydrocarbons are compounds made of only carbon and hydrogen.
| Type | Bonding | General Formula | Examples |
|---|
| Alkanes (saturated) | Single bonds only (C–C) | CnH2n+2 | CH₄ (methane), C₂H₆ (ethane) |
| Alkenes (unsaturated) | One or more double bonds (C=C) | CnH2n | C₂H₄ (ethene), C₃H₆ (propene) |
| Alkynes (unsaturated) | One or more triple bonds (C≡C) | CnH2n−2 | C₂H₂ (ethyne), C₃H₄ (propyne) |
Q5. Write the IUPAC names and formulae of the first four alkanes.
Answer
1. Methane — CH₄ 2. Ethane — C₂H₆ 3. Propane — C₃H₈ 4. Butane — C₄H₁₀
Q6. What is esterification? Write the reaction.
Answer
Esterification is the reaction between an alcohol and a carboxylic acid in the presence of a catalyst (conc. H₂SO₄) to form an ester and water.
Reaction: C₂H₅OH + CH₃COOH →(conc. H₂SO₄, heat)→ CH₃COOC₂H₅ + H₂O Ethanol + Ethanoic acid → Ethyl ethanoate + Water
Esters are sweet-smelling compounds used in perfumes and flavourings.
VI. Detailed Answers
Q1. Explain covalent bonding in carbon compounds with examples.
Answer
Carbon has 4 valence electrons (electronic configuration 2,4). It cannot lose 4 electrons (requires too much energy) or gain 4 electrons (nucleus cannot hold 10 electrons). Instead, carbon shares electrons with other atoms — this is covalent bonding.
Types of covalent bonds in carbon compounds: 1. Single bond (C–C): One pair of shared electrons. Example: Ethane (H₃C–CH₃) 2. Double bond (C=C): Two pairs of shared electrons. Example: Ethene (H₂C=CH₂) 3. Triple bond (C≡C): Three pairs of shared electrons. Example: Ethyne (HC≡CH)
Examples of covalent molecules:
- Methane (CH₄): C shares 1 electron with each of 4 H atoms → 4 single covalent bonds
- Carbon dioxide (CO₂): O=C=O — two double bonds
- Nitrogen (N₂): N≡N — one triple bond
Properties of covalent compounds:
- Low melting and boiling points (weak intermolecular forces)
- Generally insoluble in water, soluble in organic solvents
- Poor conductors of electricity (no free ions/electrons, except graphite)
Q2. Describe the allotropes of carbon in detail.
Answer
Allotropes are different physical forms of the same element. Carbon has three main allotropes:
1. Diamond:
- Each carbon atom is bonded to 4 other carbon atoms in a tetrahedral 3D network
- Hardest natural substance (used in cutting and drilling tools)
- Transparent, brilliant, does not conduct electricity
- High melting point (~3550°C)
- Density: 3.5 g/cm³
2. Graphite:
- Each carbon atom is bonded to 3 others in hexagonal layered sheets
- One electron per carbon is delocalised (free to move) → conducts electricity
- Soft and slippery — layers can slide over each other (used as lubricant and in pencil leads)
- Dark grey/black, opaque
- Density: 2.2 g/cm³
3. Fullerene (C₆₀):
- Discovered in 1985 by Kroto, Smalley, and Curl
- 60 carbon atoms arranged in a football-shaped cage (20 hexagons + 12 pentagons)
- Also called Buckminsterfullerene or buckyball
- Has applications in nanotechnology, medicine, and electronics
- Other fullerenes: C₇₀, C₇₆
Other forms of carbon: Coal, coke, charcoal, carbon black, lamp black — amorphous forms used as fuel, reducing agents, and adsorbents.
Q3. Explain the properties and reactions of ethanol.
Answer
Ethanol (C₂H₅OH):
- Common name: Ethyl alcohol
- Functional group: Hydroxyl (–OH)
- Colourless liquid with a pleasant smell, miscible with water in all proportions
- Active ingredient in alcoholic beverages (5–15%), also used as solvent and fuel
Chemical reactions of ethanol:
1. Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + Heat (Burns with a clean blue flame, used as fuel)
2. Oxidation (with alkaline KMnO₄ or acidified K₂Cr₂O₇): C₂H₅OH + 2[O] → CH₃COOH + H₂O Ethanol → Ethanoic acid (Orange K₂Cr₂O₇ turns green — confirms oxidation)
3. Reaction with sodium metal: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑ Sodium ethoxide + Hydrogen gas (Hydrogen gas burns with a pop sound)
4. Dehydration (with conc. H₂SO₄ at 170°C): C₂H₅OH →(conc. H₂SO₄, 170°C)→ CH₂=CH₂ + H₂O Ethanol → Ethene + Water (conc. H₂SO₄ acts as a dehydrating agent)
5. Esterification (with ethanoic acid): C₂H₅OH + CH₃COOH →(conc. H₂SO₄)→ CH₃COOC₂H₅ + H₂O Ethyl ethanoate (sweet-smelling ester)
Q4. Explain the cleansing action of soaps and compare soaps with detergents.
Answer
Cleansing action of soap: A soap molecule has two ends with opposite properties: 1. Hydrophobic tail: Long hydrocarbon chain (water-repelling, oil-attracting) 2. Hyphilic head: Ionic end (–COONa) (water-attracting, oil-repelling)
Mechanism:
- When soap is added to dirty clothes in water, the hydrophobic tails attach to the oil/dirt particles while the hydrophilic heads remain in water.
- Many soap molecules surround a dirt particle, forming a micelle (spherical structure with tails inside and heads outside).
- Agitation breaks the dirt particles into smaller emulsified fragments.
- The micelles with trapped dirt are washed away with water.
Comparison of soaps and detergents:
| Property | Soaps | Detergents |
|---|
| Composition | Na/K salts of long-chain fatty acids | Na salts of sulphonic acids |
| Action in soft water | Good cleansing | Good cleansing |
| Action in hard water | Form insoluble scum (Ca/Mg salts) | No scum; effective cleaning |
| Biodegradability | Biodegradable | Some are non-biodegradable (cause pollution) |
| Examples | Sodium stearate, potassium palmitate | Sodium alkylbenzene sulphonate |
| Cleansing power | Less effective than detergents | More effective |
Why soaps do not work in hard water: Hard water contains Ca²⁺ and Mg²⁺ ions. Soap reacts with these ions to form insoluble calcium/magnesium salts (scum): Ca²⁺ + 2C₁₇H₃₅COONa → (C₁₇H₃₅COO)₂Ca↓ + 2Na⁺ This scum sticks to fabric and prevents effective cleaning. Detergents do not form such insoluble precipitates.
Common Mistakes
1. Confusing catenation and tetravalency — Catenation = carbon bonding with carbon (self-linking); tetravalency = carbon forming 4 bonds with other atoms. Both contribute to compound diversity, but they are distinct properties. 2. Writing methane as the first alkene — The first alkene is ethene (C₂H₄). Methane (CH₄) is an alkane. A C=C double bond requires at least 2 carbon atoms. 3. Forgetting catalysts in reactions — Esterification requires conc. H₂SO₄ as catalyst; dehydration of ethanol requires conc. H₂SO₄ at 170°C. Always mention conditions. 4. Balancing equations incorrectly — Always verify atom counts on both sides. Combustion: CₓHᵧ + O₂ → CO₂ + H₂O must balance C, H, and O. 5. Confusing allotropes and isotopes — Allotropes are different physical forms of the same element (diamond, graphite). Isotopes are atoms of the same element with different mass numbers (C-12, C-14). 6. Saying soaps do not clean at all in hard water — Soaps clean less effectively because they form scum; they still have some cleansing action but waste soap and leave deposits. 7. Confusing oxidation product of ethanol — Ethanol oxidises to ethanoic acid (CH₃COOH), NOT to ethanal (CH₃CHO) alone. In Class 9, the full oxidation product is ethanoic acid.
Exam Checklist
- [ ] Catenation and tetravalency definitions and significance
- [ ] Three allotropes of carbon — diamond, graphite, fullerene — structures and properties
- [ ] Hydrocarbon classification: alkanes, alkenes, alkynes — general formulas and examples
- [ ] Homologous series — definition, characteristics, and first 4 members of alkanes/alkenes
- [ ] IUPAC nomenclature rules for simple carbon compounds
- [ ] Balanced equations: combustion, oxidation, sodium reaction, dehydration, esterification
- [ ] Ethanol — physical properties and all 5 reactions with balanced equations
- [ ] Ethanoic acid — properties, esterification reaction, reaction with bases
- [ ] Soap vs detergent — composition, cleansing action (micelle), hard water behaviour
- [ ] Key differences: saturated vs unsaturated, diamond vs graphite, soap vs detergent
Internal Links
- [Carbon and Its Compounds Chapter Guide](/school/tamil-nadu-state-board/class/9/science/carbon-and-its-compounds)
- [All Class 9 Science Solutions](/school/tamil-nadu-state-board/class/9/science/solutions)
- [Class 9 Science Subject Guide](/school/tamil-nadu-state-board/class/9/science)
