NCERT Solutions Class 10 Science Chapter 4 Carbon and its Compounds

NCERT Solutions Class 10 Science Chapter 4, "Carbon and its Compounds," provides step-by-step answers and comprehensive explanations designed to help students master organic chemistry fundamentals and score maximum marks in CBSE Class 10 Board examinations. This chapter forms the foundation for higher secondary chemistry, covering the unique bonding nature of carbon, homologous series, functional groups, structural isomerism, essential chemical reactions of ethanol and ethanoic acid, as well as the cleansing action of soaps and detergents. These verified solutions strictly follow the latest NCERT textbook syllabus and CBSE marking schemes.

Chapter Overview: Carbon and its Compounds

Carbon is a non-metal with atomic number 6 and an electronic configuration of 2, 4. To achieve noble gas stability, carbon does not gain four electrons (which would require overcoming huge inter-electronic repulsion in a small nucleus with 6 protons) or lose four electrons (which demands exceptionally high ionization enthalpy). Instead, carbon shares its valence electrons with other atoms, forming strong covalent bonds.

The vast diversity of millions of carbon compounds is attributed to two distinct properties:

• Catenation: The unique ability of carbon atoms to form direct, stable covalent bonds with other carbon atoms, producing long straight chains, branched chains, or cyclic ring structures.
• Tetravalency: Having four valence electrons, carbon can bond with up to four other atoms of carbon, hydrogen, oxygen, nitrogen, sulfur, or halogens.

Key areas emphasized in the CBSE Class 10 curriculum include:

1. Covalent Bonding & Electron Dot Structures: Lewis dot diagrams for single, double, and triple bonds (H₂, O₂, N₂, CH₄, CO₂, C₂H₄, C₂H₂).
2. Allotropes of Carbon: Structural differences and physical properties of Diamond, Graphite, and Buckminsterfullerene (C₆₀).
3. Saturated and Unsaturated Hydrocarbons: Alkanes (C_nH_2n+2), Alkenes (C_nH_2n), and Alkynes (C_nH_2n-2).
4. Functional Groups & Homologous Series: Alcohols (—OH), Aldehydes (—CHO), Ketones (>C=O), Carboxylic Acids (—COOH), and Halogens (—Cl, —Br).
5. Chemical Properties: Combustion, Oxidation, Addition (Hydrogenation), and Substitution reactions.
6. Commercial Compounds: Ethanol (C₂H₅OH) and Ethanoic Acid (CH₃COOH), including esterification, saponification, and reaction with carbonates.
7. Soaps and Detergents: Hydrophilic and hydrophobic ends, micelle formation, and scum formation in hard water.

Important Chemical Equations and Reactions

The following chemical equations are frequently tested in board examinations:

• Complete Combustion:
  CH₄ + 2O₂ → CO₂ + 2H₂O + Heat + Light
• Oxidation of Ethanol to Ethanoic Acid:
  CH₃CH₂OH + 2[O] (Alkaline KMnO₄ + Heat OR Acidified K₂Cr₂O₇ + Heat) → CH₃COOH + H₂O
• Addition Reaction (Industrial Hydrogenation of Vegetable Oils):
  R₂C=CR₂ + H₂ (Ni / Pd Catalyst, Heat) → R₂CH—CHR₂
• Substitution Reaction of Methane:
  CH₄ + Cl₂ (in presence of Sunlight) → CH₃Cl + HCl
• Esterification Reaction:
  CH₃COOH + C₂H₅OH (Conc. H₂SO₄ catalyst) ⇔ CH₃COOC₂H₅ (Ethyl ethanoate) + H₂O
• Saponification (Alkaline Hydrolysis of Esters):
  CH₃COOC₂H₅ + NaOH → CH₃COONa (Sodium ethanoate) + C₂H₅OH
• Dehydration of Ethanol:
  C₂H₅OH (Conc. H₂SO₄ at 443 K) → CH₂=CH₂ + H₂O
• Reaction of Ethanoic Acid with Sodium Hydrogencarbonate:
  CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑

NCERT In-Text Questions and Detailed Step-by-Step Solutions

In-Text Questions (Page 61)

Question 1: What would be the electron dot structure of carbon dioxide which has the formula CO₂?

Solution:

1. Given Formula: CO₂
2. Valence Electrons: Carbon (atomic number 6) has 4 valence electrons. Oxygen (atomic number 8) has 6 valence electrons.
3. Bond Formation: To attain an octet, the central carbon atom shares two pairs of electrons with each of the two oxygen atoms, forming two double covalent bonds (O=C=O).
4. Electron Dot Structure:
   Each oxygen atom retains 2 lone pairs (4 non-bonding electrons) and shares 4 bonding electrons with carbon:
   :Ö::C::Ö: or :Ö=C=Ö:

Final Answer: Carbon forms two double covalent bonds with two oxygen atoms sharing a total of 8 bonding electrons.

Question 2: What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur? (Hint: The eight atoms of sulphur are joined together in the form of a ring.)

Solution:

1. Given Formula: S₈
2. Valence Electrons: Sulfur (atomic number 16) has the electronic configuration 2, 8, 6. Each S atom needs 2 electrons to complete its octet.
3. Structure: Eight sulfur atoms are linked in a puckered, crown-shaped ring. Each sulfur atom forms a single covalent bond with two neighboring sulfur atoms and holds two lone pairs.
4. Electron Dot Representation: In the S₈ ring, each S atom shares one electron with the sulfur on its left and one with the sulfur on its right: (—S—S—S—S—S—S—S—S—).

Final Answer: The S₈ molecule exists as a crown-shaped octagonal ring where each sulfur atom shares 2 electrons (one with each adjacent neighbor) to complete its octet.

In-Text Questions (Page 68 & 69)

Question 3: How many structural isomers can you draw for pentane?

Solution:

Pentane has the molecular formula C₅H₁₂. It forms 3 structural isomers:

1. n-Pentane (Pentane): Straight five-carbon chain.
   CH₃—CH₂—CH₂—CH₂—CH₃
2. Isopentane (2-Methylbutane): Four-carbon parent chain with a methyl branch at C-2.
   CH₃—CH(CH₃)—CH₂—CH₃
3. Neopentane (2,2-Dimethylpropane): Three-carbon parent chain with two methyl branches at C-2.
   CH₃—C(CH₃)₂—CH₃

Final Answer: Exactly 3 structural isomers are possible for pentane.

Question 4: What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?

Solution:

1. Catenation: Carbon has the extraordinary property to link with other carbon atoms through strong covalent bonds (C—C bond energy is high due to small atomic size), forming chains of varying lengths and rings.
2. Tetravalency: Having a valency of 4, a single carbon atom can form bonds with 4 other monovalent atoms or multi-bond with oxygen, nitrogen, and sulfur.

Final Answer: Catenation and Tetravalency.

Question 5: What will be the formula and electron dot structure of cyclopentane?

Solution:

1. Molecular Formula: Cyclopentane contains 5 carbon atoms in a closed ring. General formula for cycloalkanes is C_nH_2n. Thus, the formula is C₅H₁₀.
2. Bonding: The 5 carbon atoms are arranged in a regular pentagon. Each carbon atom forms two single C—C covalent bonds with adjacent ring carbons and two single C—H covalent bonds with hydrogen atoms.
3. Total Covalent Bonds: 5 (C—C bonds) + 10 (C—H bonds) = 15 single covalent bonds (30 shared electrons).

Final Answer: Formula is C₅H₁₀ with 15 single covalent bonds in a closed 5-membered cyclic ring.

Question 6: Draw the structures for the following compounds: (i) Ethanoic acid, (ii) Bromopentane, (iii) Butanone, (iv) Hexanal. Are structural isomers possible for bromopentane?

Solution:

• (i) Ethanoic acid (CH₃COOH):
  CH₃—C(=O)—OH
• (ii) Bromopentane (C₅H₁₁Br):
  CH₃—CH₂—CH₂—CH₂—CH₂—Br (1-Bromopentane)
• (iii) Butanone (CH₃COCH₂CH₃):
  CH₃—C(=O)—CH₂—CH₃
• (iv) Hexanal (CH₃(CH₂)₄CHO):
  CH₃—CH₂—CH₂—CH₂—CH₂—CH=O

Isomers of Bromopentane: Yes, structural isomers are possible. By changing the position of the —Br atom and branching the carbon chain, we get positional and chain isomers such as 1-bromopentane, 2-bromopentane, 3-bromopentane, and 1-bromo-2-methylbutane.

Question 7: How would you name the following compounds?
(i) CH₃—CH₂—Br
(ii) H—CHO
(iii) CH₃—CH₂—CH₂—CH₂—C≡CH

Solution:

• (i) Two carbon chain with bromo prefix: Bromoethane.
• (ii) One carbon chain with aldehyde functional group (—CHO): Methanal (Formaldehyde).
• (iii) Six carbon chain with a terminal triple bond (alkyne) at C-1: Hex-1-yne (or 1-Hexyne).

In-Text Questions (Page 71 & 74)

Question 8: Why is the conversion of ethanol to ethanoic acid an oxidation reaction?

Solution:

The conversion is represented by the chemical equation:
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

This reaction is classified as an oxidation reaction because:

1. One oxygen atom is added to the ethanol molecule (from 1 oxygen in C₂H₆O to 2 oxygens in C₂H₄O₂).
2. Two hydrogen atoms are removed from the ethanol molecule (from 6 hydrogens to 4 hydrogens).
3. The reaction is carried out in the presence of strong oxidising agents like alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇).

Question 9: A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?

Solution:

Air contains only about 21% oxygen alongside 78% nitrogen. When ethyne (C₂H₂) is burnt in air, incomplete combustion occurs due to insufficient oxygen supply, producing a yellow, sooty flame with lower heat output. In contrast, burning ethyne in pure oxygen ensures complete combustion according to the equation:
2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O + High Heat

This oxy-acetylene flame reaches temperatures up to ~3000 °C, which is essential for melting and welding metals.

Question 10: How would you distinguish experimentally between an alcohol and a carboxylic acid?

Solution:

Test Method	Alcohol (e.g., Ethanol)	Carboxylic Acid (e.g., Ethanoic Acid)
1. Sodium Hydrogencarbonate Test (NaHCO3)	No brisk effervescence observed; no reaction.	Produces brisk effervescence with evolution of colourless CO2 gas which turns lime water milky.
2. Blue Litmus Paper Test	Neutral; no change in the colour of blue litmus paper.	Acidic; turns moist blue litmus paper red.

Question 11: What are oxidising agents?

Solution:

Oxidising agents are substances that supply oxygen or remove hydrogen in a chemical reaction, thereby causing the oxidation of other substances while getting reduced themselves. Examples in organic chemistry include alkaline KMnO₄ and acidified K₂Cr₂O₇.

In-Text Questions (Page 76)

Question 12: Would you be able to check if water is hard by using a detergent?

Solution:

No. Synthetic detergents are sodium salts of sulfonic acids or ammonium salts with chlorides/bromides. Their charged ends do not form insoluble precipitates with calcium (Ca²⁺) and magnesium (Mg²⁺) ions present in hard water. As a result, detergents lather easily and effectively in both soft water and hard water, making it impossible to detect water hardness solely through lather formation.

Question 13: People use a variety of methods to wash clothes. Usually after adding the soap, they 'beat' the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in