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1. Carbon: The Basis of Carbon Compounds
Carbon is present in a very large number of compounds. Two important properties explain why carbon forms such a wide variety of compounds: tetravalency and catenation.
Tetravalency
Carbon has atomic number 6 and electronic configuration 2,4. It has four valence electrons. Losing or gaining four electrons is energetically difficult, so carbon generally forms covalent bonds by sharing electrons.
Catenation
Carbon atoms can bond strongly with one another to form long chains, branched chains and rings. This self-linking property is called catenation.
Why carbon does not usually form C⁴⁺ or C⁴⁻ ions
Formation of C⁴⁺ would require removal of four electrons, requiring a very large amount of energy. Formation of C⁴⁻ would require adding four electrons to a small nucleus, which is also unfavourable. Carbon therefore commonly completes its valency by sharing electrons.
2. Covalent Bonding
A covalent bond is formed by sharing of electrons between atoms. The shared electrons help the atoms attain stable outer shells.
Methane (CH₄)
Carbon shares one electron with each of four hydrogen atoms, forming four C–H single covalent bonds.
Ethane (C₂H₆)
Two carbon atoms share one pair of electrons with each other and each carbon shares electrons with three hydrogen atoms.
Ethene (C₂H₄)
Two carbon atoms share two pairs of electrons, forming a double bond.
Ethyne (C₂H₂)
Two carbon atoms share three pairs of electrons, forming a triple bond.
| Compound | Formula | Bond between carbon atoms |
|---|---|---|
| Methane | CH₄ | No C–C bond |
| Ethane | C₂H₆ | Single bond |
| Ethene | C₂H₄ | Double bond |
| Ethyne | C₂H₂ | Triple bond |
3. Properties of Covalent Compounds
- Generally have relatively low melting and boiling points compared with ionic compounds because of weak intermolecular forces.
- Generally do not conduct electricity because they do not contain free ions or free electrons for conduction.
- Many are insoluble or only slightly soluble in water and dissolve better in organic solvents.
4. Versatile Nature of Carbon
4.1 Saturated and unsaturated carbon compounds
Carbon compounds containing only single bonds between carbon atoms are saturated. Compounds containing carbon-carbon double or triple bonds are unsaturated.
Alkanes
Example: C₂H₆ (ethane)
Alkenes / Alkynes
Examples: C₂H₄, C₂H₂
4.2 Chains, branches and rings
Carbon can form straight chains, branched chains and cyclic structures. This structural variety contributes greatly to the number of carbon compounds.
4.3 Allotropes of carbon
Allotropes are different structural forms of the same element.
| Allotrope | Key feature | Property/use |
|---|---|---|
| Diamond | Each carbon strongly bonded in a three-dimensional network | Extremely hard; used for cutting and drilling. |
| Graphite | Layered structure; electrons can move within layers | Conducts electricity; used in electrodes and lubricants. |
| Fullerenes | Carbon atoms arranged in cage-like molecules | Important in modern carbon chemistry and nanotechnology. |
5. Hydrocarbon Structures
Hydrocarbons are compounds containing only carbon and hydrogen.
Alkanes
Saturated hydrocarbons containing only single bonds. General formula: CₙH₂ₙ₊₂.
Alkenes
Unsaturated hydrocarbons containing at least one carbon-carbon double bond. For open-chain compounds with one double bond, general formula is CₙH₂ₙ.
Alkynes
Unsaturated hydrocarbons containing at least one carbon-carbon triple bond. For open-chain compounds with one triple bond, general formula is CₙH₂ₙ₋₂.
Structural formula vs molecular formula
A molecular formula gives the number of each kind of atom. A structural formula shows how atoms are connected.
6. Nomenclature of Carbon Compounds
Systematic naming helps identify the structure of an organic compound.
Basic carbon-chain prefixes
| Number of carbon atoms | Prefix | Example |
|---|---|---|
| 1 | Meth- | Methane |
| 2 | Eth- | Ethane |
| 3 | Prop- | Propane |
| 4 | But- | Butane |
| 5 | Pent- | Pentane |
| 6 | Hex- | Hexane |
| 7 | Hept- | Heptane |
| 8 | Oct- | Octane |
Common suffixes
- Single bond: -ane
- Double bond: -ene
- Triple bond: -yne
- Alcohol functional group: -ol
- Carboxylic acid: -oic acid
Functional groups
| Functional group | Representation | Class/example |
|---|---|---|
| Halogen | —Cl, —Br, etc. | Chloroethane |
| Alcohol | —OH | Ethanol |
| Aldehyde | —CHO | Ethanal |
| Ketone | >C=O | Propanone |
| Carboxylic acid | —COOH | Ethanoic acid |
Examples
7. Homologous Series
A homologous series is a group of organic compounds with the same functional group and similar chemical properties, where successive members differ by a —CH₂— unit and hence by 14 u in relative molecular mass.
| Alkane | Formula | Relative molecular mass |
|---|---|---|
| Methane | CH₄ | 16 |
| Ethane | C₂H₆ | 30 |
| Propane | C₃H₈ | 44 |
| Butane | C₄H₁₀ | 58 |
Members show a gradual change in physical properties such as melting and boiling points, while their chemical properties remain similar due to the same functional group.
8. Chemical Reactions of Carbon Compounds
8.1 Combustion
Carbon compounds generally burn in oxygen to produce carbon dioxide, water and heat/light when complete combustion occurs.
Unsaturated hydrocarbons may burn with a yellow, sooty flame when oxygen is insufficient.
8.2 Oxidation
Some carbon compounds are oxidised using oxidising agents such as alkaline KMnO₄ or acidified K₂Cr₂O₇. Ethanol can be oxidised to ethanoic acid.
8.3 Addition reaction
Unsaturated hydrocarbons can add hydrogen in the presence of a catalyst such as nickel to form saturated hydrocarbons.
Hydrogenation of vegetable oils is an important industrial application; it can convert unsaturated oils into more saturated products.
8.4 Substitution reaction
Saturated hydrocarbons can undergo substitution reactions, particularly in the presence of sunlight.
9. Ethanol (C₂H₅OH)
Ethanol is an alcohol with the functional group —OH. It is a colourless liquid and is miscible with water in all proportions.
Reaction with sodium
Hydrogen gas is evolved. Ethanol is much less acidic than ethanoic acid.
Oxidation
Dehydration
When ethanol is heated with concentrated sulphuric acid at about 443 K, it loses water to form ethene.
Uses and safety
Ethanol is used as a solvent, in medicines and perfumes, and as a fuel/additive in suitable contexts. Excessive consumption of alcoholic beverages is harmful to health and can lead to dependence and organ damage.
10. Ethanoic Acid (CH₃COOH)
Ethanoic acid is a carboxylic acid. A dilute aqueous solution is commonly called vinegar and is used in food preservation and flavouring.
Reaction with sodium
Reaction with sodium hydrogencarbonate
The brisk evolution of carbon dioxide is a useful test for carboxylic acids.
Reaction with sodium hydroxide
Esterification
Ethanoic acid reacts with ethanol in the presence of concentrated sulphuric acid to form an ester with a fruity smell.
The product is ethyl ethanoate.
Saponification
Esters react with a strong base such as NaOH to produce the corresponding alcohol and carboxylate salt. This alkaline hydrolysis is called saponification and is important in soap manufacture.
11. Soaps and Detergents
Soap
Soaps are generally sodium or potassium salts of long-chain carboxylic acids. They have a long hydrocarbon tail and an ionic/polar end.
Cleansing action of soap
The non-polar tail embeds in oily dirt while the ionic end interacts with water. Soap molecules form structures called micelles around grease, allowing dirt to be suspended in water and washed away.
Why soap does not work well in hard water
Hard water contains Ca²⁺ and Mg²⁺ ions. These react with soap to form insoluble salts called scum, reducing lather and cleansing efficiency.
Detergents
Detergents are generally synthetic cleansing agents whose structures allow them to work effectively even in hard water. They do not form the same insoluble scum with Ca²⁺ and Mg²⁺ under ordinary washing conditions.
| Soap | Detergent |
|---|---|
| Usually salts of long-chain fatty acids. | Synthetic surfactants with sulphate/sulphonate-type groups. |
| Forms scum in hard water. | Generally works better in hard water. |
| Many soaps are biodegradable. | Biodegradability depends on detergent structure. |
12. Quick Revision & Exam Focus
Carbon forms four covalent bonds.
Carbon bonds strongly with carbon atoms to form chains/rings.
Only single C–C bonds.
Contains C=C or C≡C bonds.
Same functional group; successive members differ by CH₂.
Carbon compound + O₂ → CO₂ + H₂O + heat.
Unsaturated compound adds atoms/groups across a multiple bond.
An atom/group replaces another, common in saturated hydrocarbons.
C₂H₅OH; alcohol functional group.
CH₃COOH; carboxylic acid functional group.
Forms micelles; less effective in hard water.
Generally effective in hard water.
Must-learn equations
13. Practice Questions
- Why does carbon form covalent compounds?
- Explain tetravalency and catenation.
- Why does carbon not usually form C⁴⁺ or C⁴⁻ ions?
- What is a covalent bond?
- Draw/describe the bonding in methane.
- Differentiate single, double and triple covalent bonds.
- Why do covalent compounds generally have low melting and boiling points?
- Why are most covalent compounds poor conductors of electricity?
- What are allotropes? Name three allotropes of carbon.
- Differentiate saturated and unsaturated hydrocarbons.
- Write the general formulae of alkanes, alkenes and alkynes.
- What is a homologous series?
- What is a functional group?
- Name CH₃CH₂OH and CH₃COOH.
- Give the structural formula of ethene and ethyne.
- What happens when carbon compounds undergo complete combustion?
- Write the combustion equation of methane.
- What is an oxidation reaction of ethanol?
- What is an addition reaction? Give an example.
- What is a substitution reaction? Give an example using methane.
- What happens when ethanol reacts with sodium?
- What happens when ethanol is heated with concentrated sulphuric acid?
- Write two uses of ethanol.
- What is ethanoic acid? Give its formula.
- Write the reaction of ethanoic acid with sodium hydrogencarbonate.
- What is esterification?
- What is saponification?
- Explain the cleansing action of soap.
- Why is soap ineffective in hard water?
- Differentiate soaps and detergents.
14. CBSE PYQs & PYQ-Based Practice
The questions below are presented as PYQ-based/adapted CBSE exam practice; wording may be paraphrased rather than reproduced verbatim. CBSE's competency-based Class 10 Science material includes Carbon and Its Compounds questions on saturated/unsaturated compounds, bonding and homologous series. citeturn0search21 CBSE has also published the official Class X 2026–27 Science sample paper and marking scheme. citeturn0search0
- Valency: Why does carbon form covalent bonds instead of C⁴⁺ or C⁴⁻ ions?
- Structure: Draw the electron-dot/structural representation of methane and explain the bonds.
- Hydrocarbon: A compound has formula C₆H₁₂. State whether it can be saturated or unsaturated and justify with a suitable structure.
- Bonding: Which of CH₄, C₂H₄, C₃H₈ and C₄H₁₀ contains a double bond?
- Homologous series: Given C₃H₄, C₄H₆, C₅H₈, identify another member of the same series and explain the common difference.
- Nomenclature: Name compounds such as CH₃CH₂OH, CH₃COOH, CH₃CHO and CH₃COCH₃.
- Reaction: Write the equation for hydrogenation of ethene and state the catalyst.
- Substitution: Write the reaction of methane with chlorine in sunlight.
- Combustion: Write the balanced combustion equation of ethanol.
- Oxidation: How can ethanol be converted to ethanoic acid? Name a suitable oxidising agent.
- Ethanol: What happens when ethanol reacts with sodium? Write the equation and identify the gas evolved.
- Dehydration: What happens when ethanol is heated with concentrated H₂SO₄ at about 443 K?
- Ethanoic acid: Write its reaction with sodium hydrogencarbonate and identify the gas evolved.
- Esterification: Write the reaction between ethanol and ethanoic acid. Name the product and state the role of concentrated sulphuric acid.
- Saponification: What is saponification and why is it important in soap manufacture?
- Soap action: Explain how a soap molecule removes oily dirt using micelle formation.
- Hard water: Why does soap form scum with hard water? Name the ions responsible.
- Detergents: Why are detergents generally preferred over soaps for washing in hard water?
- Case-based: A student is given C₂H₄ and C₂H₆. Design a chemical test to distinguish the saturated and unsaturated compound.
- Long answer: Explain the versatile nature of carbon using tetravalency, catenation, multiple bonding, chain formation and functional groups.
15. Interactive Quiz
20 important MCQs from the complete chapter. Answer all questions and check your score instantly.