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📥 Download PDF (Free)Carbon and its Compounds is Chapter 4 of CBSE Class 10 Science. Carbon is a unique element - it can form bonds with other carbon atoms to create chains, branches, and rings, giving rise to an enormous number of compounds. This chapter covers the bonding in carbon, homologous series, nomenclature (IUPAC naming), chemical properties of carbon compounds, and important compounds like ethanol and ethanoic acid.
This is a high-weightage chapter for board exams - expect 5–8 marks. IUPAC nomenclature, properties of ethanol and ethanoic acid, and soap/detergent chemistry are the most frequently tested topics.
Key Concepts
1. Why is Carbon Special?
Carbon has atomic number 6 and electronic configuration 2, 4. It has 4 valence electrons - it needs to gain or lose 4 electrons to achieve a noble gas configuration. Since gaining or losing 4 electrons requires too much energy, carbon shares electrons to form covalent bonds.
Two special properties of carbon:
- Catenation: The ability of carbon atoms to bond with other carbon atoms to form long chains, branched chains, and rings. This is because the C–C bond is very strong (due to small atomic size).
- Tetravalency: Carbon has 4 valence electrons, so it can form 4 covalent bonds. This allows it to bond with many different elements (H, O, N, S, Cl, etc.) and with itself.
These two properties together explain why there are more carbon compounds (over 10 million) than compounds of all other elements combined.
2. Covalent Bonding
In a covalent bond, two atoms share one or more pairs of electrons to achieve a stable (noble gas) configuration.
Types of Covalent Bonds
| Type | Electron Pairs Shared | Example | Representation |
|---|---|---|---|
| Single bond | 1 pair (2 electrons) | H₂ (H–H), CH₄ | - |
| Double bond | 2 pairs (4 electrons) | O₂ (O=O), C₂H₄ | = |
| Triple bond | 3 pairs (6 electrons) | N₂ (N≡N), C₂H₂ | ≡ |
Properties of Covalent Compounds
- Low melting and boiling points (weak intermolecular forces)
- Generally poor conductors of electricity (no free ions or electrons)
- Usually insoluble in water, soluble in organic solvents
3. Hydrocarbons
Compounds made of only carbon and hydrogen are called hydrocarbons.
Types of Hydrocarbons
| Type | Bonds | General Formula | Examples | Key Feature |
|---|---|---|---|---|
| Saturated (Alkanes) | Only single bonds (C–C) | CₙH₂ₙ₊₂ | CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane) | Less reactive, burn cleanly |
| Unsaturated (Alkenes) | At least one double bond (C=C) | CₙH₂ₙ | C₂H₄ (ethene), C₃H₆ (propene) | More reactive, undergo addition reactions |
| Unsaturated (Alkynes) | At least one triple bond (C≡C) | CₙH₂ₙ₋₂ | C₂H₂ (ethyne/acetylene), C₃H₄ (propyne) | Most reactive among hydrocarbons |
4. Homologous Series
A homologous series is a family of organic compounds that have the same general formula and similar chemical properties, with each successive member differing by –CH₂– (14 atomic mass units).
Characteristics:
- Same general formula (e.g., CₙH₂ₙ₊₂ for alkanes)
- Same functional group
- Similar chemical properties
- Gradual change in physical properties (melting point, boiling point increase with molecular size)
- Successive members differ by CH₂ (14 u)
Example - Alkane series: CH₄ (methane) → C₂H₆ (ethane) → C₃H₈ (propane) → C₄H₁₀ (butane) → C₅H₁₂ (pentane)
5. IUPAC Nomenclature (Naming Organic Compounds)
The name of an organic compound has three parts: Prefix + Root word + Suffix
Root Words (based on number of carbon atoms)
| Carbon Atoms | Root Word | Carbon Atoms | Root Word |
|---|---|---|---|
| 1 | Meth- | 6 | Hex- |
| 2 | Eth- | 7 | Hept- |
| 3 | Prop- | 8 | Oct- |
| 4 | But- | 9 | Non- |
| 5 | Pent- | 10 | Dec- |
Suffix (based on bond type)
| Type | Suffix | Example (2 carbons) |
|---|---|---|
| Alkane (single bond) | -ane | Ethane (C₂H₆) |
| Alkene (double bond) | -ene | Ethene (C₂H₄) |
| Alkyne (triple bond) | -yne | Ethyne (C₂H₂) |
Functional Groups and Their Suffixes/Prefixes
| Functional Group | Formula | Suffix/Prefix | Example |
|---|---|---|---|
| Alcohol (Hydroxyl) | –OH | -ol | Methanol (CH₃OH), Ethanol (C₂H₅OH) |
| Aldehyde | –CHO | -al | Methanal (HCHO), Ethanal (CH₃CHO) |
| Ketone | >C=O | -one | Propanone (CH₃COCH₃) - acetone |
| Carboxylic acid | –COOH | -oic acid | Methanoic acid (HCOOH), Ethanoic acid (CH₃COOH) |
| Halide (Chloro, Bromo) | –Cl, –Br | Chloro-, Bromo- | Chloromethane (CH₃Cl) |
6. Chemical Properties of Carbon Compounds
a) Combustion (Burning)
Carbon compounds burn in oxygen to produce CO₂, H₂O, and energy (heat and light).
- CH₄ + 2O₂ → CO₂ + 2H₂O + Energy
- C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + Energy
Saturated hydrocarbons (alkanes) burn with a clean blue flame (sufficient oxygen).
Unsaturated hydrocarbons (alkenes, alkynes) burn with a yellow, sooty flame (incomplete combustion - carbon particles glow yellow).
b) Oxidation
Controlled oxidation using oxidising agents can convert one functional group to another:
- Ethanol → Ethanoic acid (using alkaline KMnO₄ or acidified K₂Cr₂O₇)
- C₂H₅OH + [O] → CH₃COOH + H₂O
Alkaline KMnO₄ is used as an oxidising agent. The purple colour of KMnO₄ decolourises as it is used up.
c) Addition Reaction
Unsaturated hydrocarbons add hydrogen (H₂) across the double/triple bond in the presence of a catalyst (Ni or Pd) to form saturated compounds.
- C₂H₄ + H₂ → C₂H₆ (ethene → ethane) - with Ni catalyst
Hydrogenation of vegetable oils: Vegetable oils (unsaturated) + H₂ → Vanaspati ghee (saturated fat). This is why vanaspati ghee is harmful - it increases cholesterol.
d) Substitution Reaction
Saturated hydrocarbons (alkanes) undergo substitution - one atom is replaced by another.
- CH₄ + Cl₂ → CH₃Cl + HCl (in presence of sunlight)
- Methane → Chloromethane (one H replaced by Cl)
7. Important Carbon Compounds
A. Ethanol (C₂H₅OH) - Common Alcohol
Properties:
- Colourless liquid with a pleasant smell
- Boiling point: 78°C
- Miscible (fully soluble) in water
- Used in alcoholic drinks, medicines, as a solvent, and as a fuel
Reactions of Ethanol:
- With sodium: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑ (sodium ethoxide + hydrogen)
- Dehydration (with conc. H₂SO₄ at 170°C): C₂H₅OH → C₂H₄ + H₂O (ethanol → ethene). Conc. H₂SO₄ acts as a dehydrating agent.
- Oxidation (with alkaline KMnO₄): Ethanol → Ethanoic acid (acetic acid)
Effects of alcohol consumption:
- Ethanol is the alcohol in beverages - consumed in diluted form
- Slows nervous system, affects coordination, judgement, and liver
- Methanol (CH₃OH) is extremely poisonous - even 10 mL can cause blindness, 30 mL can cause death. It is sometimes found in illicit liquor.
B. Ethanoic Acid (CH₃COOH) - Acetic Acid
Properties:
- Colourless liquid with a pungent (vinegar-like) smell
- Boiling point: 118°C
- 5–8% solution in water = vinegar
- Pure ethanoic acid freezes at 16.6°C - below room temperature in winter. The frozen form looks like ice, so it is called glacial acetic acid
Reactions of Ethanoic Acid:
- With base (NaOH): CH₃COOH + NaOH → CH₃COONa + H₂O (sodium acetate + water - neutralisation)
- With carbonate/bicarbonate: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑ - this reaction produces brisk effervescence (CO₂ gas)
- CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑
- Esterification: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (in presence of conc. H₂SO₄ as catalyst). The product (ethyl ethanoate/ethyl acetate) is an ester - it has a sweet, fruity smell.
8. Soaps and Detergents
Esters and Saponification
Esters react with NaOH (a base) in a reaction called saponification to form soap + alcohol:
Ester + NaOH → Soap (sodium salt of fatty acid) + Alcohol
Actual soap-making: Animal fat/vegetable oil + NaOH → Soap + Glycerol
Structure of Soap
A soap molecule has two parts:
- Hydrophilic end (head): Ionic part (–COO⁻Na⁺) - dissolves in water
- Hydrophobic end (tail): Long hydrocarbon chain - dissolves in oil/grease
How Does Soap Clean?
- The hydrophobic (oil-loving) tails of soap molecules attach to the oil/grease on a dirty surface
- The hydrophilic (water-loving) heads remain in the water
- This forms a spherical cluster called a micelle - the oil is trapped inside, surrounded by soap molecules
- When rinsed with water, the micelles (with trapped dirt and oil) are washed away
Why Soap Doesn’t Work in Hard Water
Hard water contains dissolved calcium and magnesium salts. Soap reacts with Ca²⁺/Mg²⁺ ions to form an insoluble white precipitate (scum) - this wastes soap and doesn’t clean well.
2C₁₇H₃₅COONa + CaCl₂ → (C₁₇H₃₅COO)₂Ca↓ + 2NaCl
Detergents
Detergents (synthetic cleansing agents) work like soap but are effective in both hard and soft water. They don’t form scum with hard water because their calcium/magnesium salts are soluble.
| Feature | Soap | Detergent |
|---|---|---|
| Made from | Vegetable oil/animal fat + NaOH | Petroleum products |
| In hard water | Forms scum, poor cleaning | Works well, no scum |
| Biodegradable? | Yes | Some are not (environmental concern) |
Important Definitions
| Term | Definition |
|---|---|
| Covalent bond | Chemical bond formed by sharing of electron pairs between two atoms |
| Catenation | Ability of carbon to form bonds with other carbon atoms, creating chains, branches, and rings |
| Tetravalency | Carbon has 4 valence electrons and can form 4 covalent bonds |
| Hydrocarbon | Compound containing only carbon and hydrogen |
| Saturated hydrocarbon | Hydrocarbon with only single bonds between carbon atoms (alkanes) |
| Unsaturated hydrocarbon | Hydrocarbon with at least one double or triple bond (alkenes, alkynes) |
| Homologous series | Family of compounds with same general formula, differing by –CH₂– per member |
| Functional group | Atom or group of atoms that determines the chemical properties of a compound |
| Isomers | Compounds with the same molecular formula but different structural arrangements |
| Esterification | Reaction between a carboxylic acid and an alcohol to form an ester + water |
| Saponification | Hydrolysis of ester (or fat) with NaOH to form soap + glycerol |
| Micelle | Spherical cluster of soap molecules in water that traps oil/grease inside |
| Hard water | Water containing dissolved calcium and magnesium salts |
Solved Examples (NCERT-Based)
Example 1
Why does carbon form compounds mainly by covalent bonding?
Answer: Carbon has 4 valence electrons (electronic configuration 2, 4). To achieve a noble gas configuration, it would need to gain 4 electrons (too much energy for the nucleus to hold 10 electrons) or lose 4 electrons (too much energy needed to remove 4 electrons). Therefore, carbon shares electrons with other atoms to form covalent bonds - this is the most energy-efficient way to achieve stability.
Example 2
Why does ethene (C₂H₄) burn with a yellow, sooty flame while ethane (C₂H₆) burns with a clean blue flame?
Answer: Ethene is an unsaturated hydrocarbon (contains a C=C double bond) with a higher percentage of carbon relative to hydrogen. During combustion, there isn’t enough oxygen to completely burn all the carbon - unburnt carbon particles glow yellow and deposit as soot. Ethane is saturated (only single bonds) with a lower carbon-to-hydrogen ratio, so it burns completely in air, producing a clean blue flame.
Example 3
How would you distinguish between ethanol and ethanoic acid without tasting them?
Answer: Add a pinch of sodium hydrogen carbonate (NaHCO₃) to each. Ethanoic acid (a carboxylic acid) will react with NaHCO₃ and produce brisk effervescence due to CO₂ gas: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑. Ethanol will NOT react with NaHCO₃ - no bubbles will appear. This is a simple and reliable chemical test.
Example 4
Why is the conversion of vegetable oil to vanaspati ghee considered unhealthy?
Answer: Vegetable oils are unsaturated fats (contain C=C double bonds) - they are generally healthier. During hydrogenation (addition of H₂ using Ni catalyst), the double bonds are converted to single bonds, turning the oil into a saturated fat (vanaspati ghee). Saturated fats increase LDL cholesterol, which can lead to heart disease and blocked arteries. Additionally, the process may create trans fats, which are particularly harmful.
Important Questions for Board Exams
1-Mark Questions
- What is the general formula of alkenes?
- Name the functional group present in ethanol.
- What is the IUPAC name of CH₃COOH?
- Why does diamond have a high melting point even though it is a covalent compound?
- What is glacial acetic acid?
2-Mark Questions
- What is a homologous series? Give two characteristics.
- Write the chemical equation for the reaction of ethanol with sodium.
- Differentiate between saturated and unsaturated hydrocarbons with examples.
- What is hydrogenation? What is its industrial application?
- Why does soap form scum in hard water?
3-Mark Questions
- Explain why carbon forms compounds mainly by covalent bonding. What are catenation and tetravalency?
- Write the names and structural formulas of the first three members of the alkane homologous series.
- What is esterification? Write the reaction with equation. How is this reaction reversed?
- Describe the cleansing action of soap with a diagram of micelle formation.
- List three chemical properties of ethanoic acid with equations.
5-Mark Questions
- What are the chemical properties of carbon compounds? Explain combustion, oxidation, addition, and substitution reactions with examples.
- Compare soaps and detergents. Explain how soap cleans dirty clothes with a diagram.
Quick Revision Points
- Carbon: atomic number 6, valence electrons 4, forms covalent bonds by sharing
- Catenation = carbon chains/rings; Tetravalency = 4 bonds possible
- Alkanes (CₙH₂ₙ₊₂): single bonds, saturated; Alkenes (CₙH₂ₙ): double bond; Alkynes (CₙH₂ₙ₋₂): triple bond
- Root words: meth(1), eth(2), prop(3), but(4), pent(5)
- Functional groups: –OH (alcohol, -ol), –CHO (aldehyde, -al), –COOH (acid, -oic acid), >C=O (ketone, -one)
- Homologous series: same general formula, differ by CH₂, similar chemical properties
- Saturated → clean blue flame; Unsaturated → yellow sooty flame
- Ethanol + Na → Sodium ethoxide + H₂; Ethanol + conc. H₂SO₄ (170°C) → Ethene + H₂O
- Ethanoic acid + NaHCO₃ → brisk effervescence (CO₂) - key test for carboxylic acids
- Esterification: Acid + Alcohol → Ester + Water (sweet fruity smell)
- Saponification: Ester/Fat + NaOH → Soap + Glycerol
- Soap molecule: hydrophobic tail (in oil) + hydrophilic head (in water) → forms micelle
- Soap fails in hard water (scum forms); Detergents work in both hard and soft water
- Hydrogenation: Vegetable oil + H₂ (Ni catalyst) → Vanaspati ghee (unhealthy saturated fat)
Previous Chapter: Chapter 3 - Metals and Non-metals
Next Chapter: Chapter 5 - Life Processes
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- Chemical Reactions and Equations Class 10 Notes
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- Metals and Non-metals Class 10 Notes
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