Carbon and Its Compounds
Welcome to HSLC Guru! In this article, we present a complete English-medium guide to Class 10 Science Chapter 4 — Carbon and Its Compounds as per the ASSEB (Assam State School Education Board) syllabus. Carbon is one of the most fascinating elements in the periodic table because of its unique ability to form long chains, branched structures and rings, giving rise to millions of compounds. This chapter introduces you to covalent bonding, allotropes of carbon, hydrocarbons, functional groups, important reactions, and the chemistry of ethanol, ethanoic acid, soaps and detergents. The notes, textbook question answers, additional MCQs, fill in the blanks, true/false statements and glossary below are framed strictly for ASSEB students preparing for the HSLC examination.
Chapter Summary
Carbon is a non-metal with atomic number 6 and electronic configuration 2,4. To attain the stable noble gas configuration, carbon neither loses nor gains four electrons easily because both processes require huge energy. Instead, carbon shares its four valence electrons with other atoms to form covalent bonds. A covalent bond is formed by the mutual sharing of electrons between two atoms so that both can complete their octet (or duplet, in case of hydrogen). Covalent compounds generally have low melting and boiling points, are poor conductors of electricity and are usually insoluble in water but soluble in organic solvents like benzene, kerosene and ether. Examples of covalent molecules include H2, O2, N2, CH4, NH3 and H2O.
The two main features that make carbon form an enormous number of compounds are catenation (the self-linking property of carbon atoms to form long chains, branched chains and rings) and tetravalency (each carbon atom has four valence electrons and can form four covalent bonds with other atoms of carbon, hydrogen, oxygen, nitrogen, sulphur, halogens, etc.). Pure carbon exists in different physical forms called allotropes. The three crystalline allotropes are diamond (each carbon bonded to four others in a rigid 3-D tetrahedral network — the hardest natural substance), graphite (each carbon bonded to three others in hexagonal layers — soft, slippery and a good conductor of electricity) and fullerene (cage-like molecules such as C60, also called Buckminsterfullerene, which look like a football).
Hydrocarbons are compounds made up of only carbon and hydrogen. They are classified as saturated (alkanes — only single bonds, general formula CnH2n+2) and unsaturated (alkenes with C=C, formula CnH2n; and alkynes with C≡C, formula CnH2n−2). A series of compounds in which the same functional group substitutes hydrogen in a carbon chain and successive members differ by a –CH2 unit (mass 14 u) is called a homologous series. Members of a homologous series have similar chemical properties but show a gradation in physical properties. Functional groups such as halo (–X), alcohol (–OH), aldehyde (–CHO), ketone (>C=O), carboxylic acid (–COOH) and double/triple bonds determine the chemical behaviour of organic compounds. The IUPAC system of nomenclature uses the longest carbon chain as the parent name and adds prefixes/suffixes for substituents and functional groups (e.g., propan-1-ol, ethanoic acid, propanone).
Carbon compounds undergo important chemical reactions: combustion (burn in air/oxygen producing CO2, H2O, heat and light — saturated hydrocarbons usually give a clean blue flame, unsaturated ones give a sooty yellow flame), oxidation (alcohols are oxidised to carboxylic acids by alkaline KMnO4 or acidified K2Cr2O7), addition (unsaturated hydrocarbons add hydrogen in presence of nickel/palladium catalyst to form saturated ones — used in hydrogenation of vegetable oils to vanaspati) and substitution (saturated hydrocarbons react with chlorine in sunlight where hydrogen atoms are replaced one by one). Ethanol (CH3CH2OH) is a colourless liquid used in tinctures, cough syrups and as a solvent; it reacts with sodium to release H2 and dehydrates with hot conc. H2SO4 to ethene. Ethanoic acid (CH3COOH), a 5–8% aqueous solution of which is called vinegar, reacts with bases, carbonates and alcohols (esterification with conc. H2SO4 giving sweet-smelling esters). Soaps are sodium/potassium salts of long-chain carboxylic acids; they do not work well in hard water. Detergents are ammonium/sulphonate salts of long-chain hydrocarbons and work in both soft and hard water. In water, soap molecules form spherical aggregates called micelles in which the hydrophobic tails point inward trapping oil/dirt while the hydrophilic heads remain outside, allowing the dirt to be washed away.
Textbook Question and Answers
1 Mark Questions
Q1. What is the atomic number and electronic configuration of carbon?
Answer: The atomic number of carbon is 6 and its electronic configuration is 2,4.
Q2. Why is the bond formed between carbon atoms a covalent bond?
Answer: Because each carbon atom has four valence electrons and shares them with other atoms to complete its octet rather than losing or gaining electrons.
Q3. Define catenation.
Answer: The self-linking property of carbon atoms through strong covalent bonds to form long straight chains, branched chains and closed rings is called catenation.
Q4. What is meant by tetravalency of carbon?
Answer: Carbon has four electrons in its outermost shell and can form four covalent bonds with other atoms; this property is called tetravalency.
Q5. Name the three main allotropes of carbon.
Answer: Diamond, graphite and fullerene (e.g., Buckminsterfullerene C60).
Q6. Write the general formula of alkanes, alkenes and alkynes.
Answer: Alkane — CnH2n+2; Alkene — CnH2n; Alkyne — CnH2n−2.
Q7. What is a homologous series?
Answer: A series of organic compounds having the same general formula and same functional group, in which successive members differ by a –CH2 unit (14 u) and show similar chemical properties.
Q8. What is the IUPAC name of CH3COOH?
Answer: Ethanoic acid (common name — acetic acid).
Q9. Name the gas evolved when sodium metal reacts with ethanol.
Answer: Hydrogen gas (H2).
Q10. Why does an unsaturated hydrocarbon burn with a sooty flame?
Answer: Because of incomplete combustion due to the higher percentage of carbon, which produces unburnt carbon particles seen as soot.
2–3 Mark Questions
Q1. Differentiate between saturated and unsaturated hydrocarbons.
Answer: Saturated hydrocarbons (alkanes) contain only single bonds between carbon atoms, are less reactive, and burn with a clean blue flame. Unsaturated hydrocarbons contain at least one double bond (alkenes) or triple bond (alkynes) between carbon atoms, are more reactive (undergo addition reactions), and burn with a sooty yellow flame.
Q2. Why is diamond hard while graphite is soft and slippery, although both are made of carbon?
Answer: In diamond, each carbon is covalently bonded to four other carbon atoms in a rigid three-dimensional tetrahedral network, making it extremely hard. In graphite, each carbon is bonded to only three other carbons forming flat hexagonal layers held together by weak van der Waals forces. These layers slide over one another easily, making graphite soft and slippery.
Q3. What is an esterification reaction? Give one example.
Answer: The reaction between a carboxylic acid and an alcohol in the presence of a few drops of concentrated H2SO4 to form a sweet-smelling ester is called esterification. Example: CH3COOH + C2H5OH → CH3COOC2H5 (ethyl ethanoate) + H2O.
Q4. Explain how soap removes dirt from a piece of cloth using the concept of micelles.
Answer: A soap molecule has a long hydrophobic hydrocarbon tail and a hydrophilic ionic head (–COO–Na+). When soap is added to water containing greasy dirt, the hydrophobic tails attach themselves to oil/dirt particles while the hydrophilic heads point outwards into water, forming spherical structures called micelles. The dirt gets trapped inside the micelle and is washed away with water during rinsing.
Q5. What happens when ethanoic acid reacts with sodium hydrogen carbonate? Write the equation.
Answer: Ethanoic acid reacts with sodium hydrogen carbonate to give sodium ethanoate, water and carbon dioxide gas (which is liberated with brisk effervescence).
CH3COOH + NaHCO3 → CH3COONa + H2O + CO2↑
Q6. Why is the conversion of ethanol to ethanoic acid called an oxidation reaction?
Answer: When ethanol is heated with alkaline KMnO4 or acidified K2Cr2O7, it loses hydrogen and gains oxygen to form ethanoic acid. Since the addition of oxygen (or removal of hydrogen) takes place, the reaction is termed an oxidation reaction. The KMnO4/K2Cr2O7 acts as the oxidising agent.
5–6 Mark Questions
Q1. Explain the formation of a covalent bond in a methane (CH4) molecule. Why are covalent compounds poor conductors of electricity?
Answer: A carbon atom has four valence electrons (2,4) and needs four more to complete its octet. A hydrogen atom has one electron and needs one more to attain the helium configuration (duplet). In methane, one carbon atom shares each of its four electrons with one electron of four hydrogen atoms, forming four C–H single covalent bonds. Each shared pair of electrons constitutes a single bond. After sharing, carbon has 8 electrons in its valence shell and each hydrogen has 2, completing their stable configurations. Covalent compounds do not contain free ions or free electrons; the bonded electrons are localised between the two atoms. Since electricity is conducted only when charged particles move freely, covalent compounds are generally poor conductors of electricity.
Q2. Give the structures and IUPAC names of the first four members of the alkane series. Why do members of a homologous series show similar chemical properties?
Answer: The first four alkanes are: (i) Methane CH4, (ii) Ethane CH3–CH3 (C2H6), (iii) Propane CH3–CH2–CH3 (C3H8), (iv) Butane CH3–CH2–CH2–CH3 (C4H10). Each successive member differs from the previous one by a –CH2 unit (mass 14 u). Since all members have the same general formula (CnH2n+2) and the same type of bonding (only C–C and C–H single bonds), the kind of chemical reactions they undergo (such as combustion and substitution) is the same, although physical properties like melting point, boiling point and density change gradually with increasing molecular mass.
Q3. Describe the chemical properties of ethanol with proper equations.
Answer: (i) Reaction with sodium: 2CH3CH2OH + 2Na → 2CH3CH2ONa (sodium ethoxide) + H2↑. (ii) Dehydration with hot concentrated H2SO4 at 443 K: CH3CH2OH → CH2=CH2 + H2O. The H2SO4 acts as a dehydrating agent. (iii) Oxidation: with alkaline KMnO4 or acidified K2Cr2O7, ethanol is oxidised to ethanoic acid: CH3CH2OH + 2[O] → CH3COOH + H2O. (iv) Combustion: CH3CH2OH + 3O2 → 2CO2 + 3H2O + heat.
Q4. Distinguish between soaps and detergents. Why does soap not work well in hard water?
Answer: Soaps are sodium or potassium salts of long-chain fatty (carboxylic) acids; they are biodegradable and effective only in soft water. Detergents are ammonium or sulphonate salts of long-chain hydrocarbons; they work in both soft and hard water but many are non-biodegradable. Hard water contains calcium and magnesium ions. When soap is added to hard water, these ions react with soap molecules to form an insoluble curdy precipitate called scum (calcium/magnesium stearate). This wastes a lot of soap and leaves a sticky deposit on clothes, so soap does not lather well in hard water. Detergents do not form such precipitates with Ca2+ and Mg2+ ions, so they clean effectively even in hard water.
Q5. What are addition and substitution reactions? Explain with one example each.
Answer: Addition reaction: An unsaturated hydrocarbon (containing C=C or C≡C) reacts with another atom or molecule to add it across the multiple bond, giving a saturated product. Example — hydrogenation of ethene: CH2=CH2 + H2 → CH3–CH3 (in presence of Ni/Pd catalyst). This reaction is industrially used to convert vegetable oils into vanaspati ghee. Substitution reaction: In a saturated hydrocarbon (alkane), one or more hydrogen atoms are replaced (substituted) by another atom/group such as a halogen. Example — chlorination of methane in the presence of sunlight: CH4 + Cl2 → CH3Cl + HCl. Further substitution gives CH2Cl2, CHCl3 and CCl4.
Additional Multiple Choice Questions (MCQ)
Q1. The number of covalent bonds formed by a carbon atom is:
(a) 1 (b) 2 (c) 3 (d) 4
Answer: (d) 4
Q2. Which of the following is the hardest natural substance?
(a) Graphite (b) Diamond (c) Fullerene (d) Coke
Answer: (b) Diamond
Q3. The general formula of alkynes is:
(a) CnH2n (b) CnH2n+2 (c) CnH2n−2 (d) CnHn
Answer: (c) CnH2n−2
Q4. The functional group present in ethanoic acid is:
(a) –OH (b) –CHO (c) –COOH (d) >C=O
Answer: (c) –COOH
Q5. Which of the following is an unsaturated hydrocarbon?
(a) C2H6 (b) C3H8 (c) C2H4 (d) CH4
Answer: (c) C2H4
Q6. Vinegar is a dilute aqueous solution of:
(a) Methanoic acid (b) Ethanoic acid (c) Citric acid (d) Sulphuric acid
Answer: (b) Ethanoic acid
Q7. The pleasant smell of an ester is destroyed when treated with:
(a) NaOH (b) HCl (c) Water (d) Alcohol
Answer: (a) NaOH (saponification)
Q8. Buckminsterfullerene contains how many carbon atoms?
(a) 12 (b) 24 (c) 60 (d) 100
Answer: (c) 60
Q9. Which gas is produced when ethanoic acid reacts with sodium carbonate?
(a) H2 (b) O2 (c) CO2 (d) Cl2
Answer: (c) CO2
Q10. The process of converting vegetable oils into vanaspati ghee is called:
(a) Hydrogenation (b) Halogenation (c) Esterification (d) Saponification
Answer: (a) Hydrogenation
Fill in the Blanks
Q1. The self-linking property of carbon atoms is called __________.
Answer: catenation
Q2. The IUPAC name of acetic acid is __________.
Answer: ethanoic acid
Q3. Successive members of a homologous series differ by __________ unit.
Answer: –CH2 (14 u)
Q4. Soap molecules form spherical aggregates in water known as __________.
Answer: micelles
Q5. Each carbon atom in graphite is bonded to __________ other carbon atoms.
Answer: three
True / False
Q1. Diamond is a good conductor of electricity.
Answer: False (graphite conducts electricity, not diamond).
Q2. Ethanol reacts with sodium metal to release hydrogen gas.
Answer: True
Q3. Saturated hydrocarbons undergo addition reactions readily.
Answer: False (unsaturated hydrocarbons undergo addition reactions; saturated ones undergo substitution).
Q4. Detergents work effectively in hard water.
Answer: True
Q5. Methane burns with a sooty yellow flame.
Answer: False (methane is saturated and burns with a clean blue flame).
Glossary
| Term | Meaning |
|---|---|
| Covalent bond | A chemical bond formed by mutual sharing of electrons between two atoms. |
| Catenation | Self-linking of carbon atoms to form long chains, branched chains and rings. |
| Tetravalency | Property of carbon to form four covalent bonds with other atoms. |
| Allotropes | Different physical forms of the same element, e.g., diamond, graphite and fullerene. |
| Hydrocarbon | An organic compound made of only carbon and hydrogen atoms. |
| Saturated hydrocarbon | Hydrocarbon containing only single carbon–carbon bonds (alkane). |
| Unsaturated hydrocarbon | Hydrocarbon containing at least one C=C or C≡C bond (alkene/alkyne). |
| Homologous series | A family of organic compounds with the same general formula and same functional group, successive members differing by –CH2. |
| Functional group | An atom or group of atoms that determines the chemical properties of an organic compound. |
| Isomers | Compounds having the same molecular formula but different structural formulae. |
| Combustion | The reaction of a substance with oxygen producing heat and light. |
| Addition reaction | Reaction in which atoms add across a double or triple bond of an unsaturated compound. |
| Substitution reaction | Reaction in which an atom/group replaces another atom of a saturated compound. |
| Esterification | Reaction of a carboxylic acid with an alcohol in presence of conc. H2SO4 giving an ester. |
| Saponification | Hydrolysis of an ester (or fat) by an alkali to form soap and alcohol/glycerol. |
| Soap | Sodium/potassium salt of long-chain carboxylic acid used as a cleansing agent. |
| Detergent | Ammonium/sulphonate salt of long-chain hydrocarbons that cleans even in hard water. |
| Micelle | Spherical aggregate of soap/detergent molecules formed in water with hydrophobic tails inside and hydrophilic heads outside. |
| Vinegar | 5–8% aqueous solution of ethanoic acid used as a food preservative. |
| Denatured alcohol | Ethanol made unfit for drinking by adding poisonous substances like methanol or pyridine. |
Stay connected with HSLC Guru for more chapter-wise notes, ASSEB question answers and exam-oriented study materials for Class 10 Science. Best wishes for your HSLC examination!