CBSE Class X Science Chapter 4 Carbon and its Compounds Questions and Answers

Chapter 4: Carbon and its Compounds

Complete Class X Textbook Questions & Answers

In-Text Questions (Page 61)

Q1. What would be the electron dot structure of carbon dioxide which has the formula CO₂?

In carbon dioxide (CO₂), the carbon atom is at the centre and shares two pairs of valence electrons with each of the two oxygen atoms. This forms double covalent bonds between the carbon atom and each oxygen atom, allowing all three atoms to achieve a completely filled octet.

Figure 4.1a: Electron dot structure of Carbon Dioxide (CO₂)

Q2. 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.)

A sulphur molecule consists of eight sulphur atoms (S₈) arranged in a closed ring (crown shape). Each sulphur atom shares one electron with each of its two neighboring sulphur atoms to form single covalent bonds, completing its octet.

Figure 4.1b: Electron dot structure of an eight-membered Sulphur ring (S₈)

In-Text Questions (Page 68–69)

Q1. How many structural isomers can you draw for pentane?

Three structural isomers can be drawn for pentane (C₅H₁₂):

1. n-pentane: A straight chain of 5 carbon atoms.
2. Isopentane (2-methylbutane): A branched chain with 4 carbon atoms in the main chain and 1 methyl branch.
3. Neopentane (2,2-dimethylpropane): A branched chain with 3 carbon atoms in the main chain and 2 methyl branches on the central carbon.

Q2. What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?

The two characteristic properties of carbon are:

• Catenation: The unique ability of carbon to form strong, stable covalent bonds with other carbon atoms, giving rise to long chains, branched chains, or ring structures.
• Tetravalency: Carbon has four valence electrons, enabling it to bond with four other atoms of carbon or monovalent/polyvalent atoms of other elements (e.g., hydrogen, oxygen, nitrogen, sulphur, halogens).

Q3. What will be the formula and electron dot structure of cyclopentane?

The chemical formula of cyclopentane is C₅H₁₀. It consists of five carbon atoms linked together in a 5-membered ring with single bonds, where each carbon atom is also bonded to two hydrogen atoms.

Figure 4.2: Carbon skeleton and electron dot structure of Cyclopentane (C₅H₁₀)

Q4. Draw the structures for the following compounds: (i) Ethanoic acid (ii) Bromopentane* (iii) Butanone (iv) Hexanal. (*Are structural isomers possible for bromopentane?)

Structural formulas for the compounds:

• (i) Ethanoic acid (CH₃COOH): Contains a 2-carbon chain ending with a carboxylic acid group (–COOH).
• (ii) Bromopentane (C₅H₁₁Br): A 5-carbon alkane chain with a bromine atom attached.
• (iii) Butanone (CH₃COCH₂CH₃): A 4-carbon chain containing a ketone functional group (–C=O) at the second carbon.
• (iv) Hexanal (CH₃CH₂CH₂CH₂CH₂CHO): A 6-carbon chain ending with an aldehyde group (–CHO).

*Yes, structural isomers are possible for bromopentane. Position isomers exist based on the position of the bromine atom (e.g., 1-bromopentane, 2-bromopentane, 3-bromopentane) alongside chain isomers involving branched carbon chains.

Q5. How would you name the following compounds?

Names according to functional groups and chain length:

• (i) CH₃–CH₂–Br: Bromoethane (2-carbon chain with a bromine substituent).
• (ii) H–C=O (HCHO): Methanal (1-carbon aldehyde group).
• (iii) H₃C–CH₂–CH₂–CH₂–C≡CH: 1-Hexyne (6-carbon chain with a triple bond at the end).

In-Text Questions (Page 71)

Q1. Why is the conversion of ethanol to ethanoic acid an oxidation reaction?

The conversion of ethanol (CH₃CH₂OH) to ethanoic acid (CH₃COOH) is an oxidation reaction because oxygen is added to the ethanol molecule (and hydrogen is removed) in the presence of oxidising agents such as alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇).

Q2. A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?

Ethyne is an unsaturated hydrocarbon. Burning ethyne in air results in incomplete combustion due to limited oxygen supply, producing a yellow, sooty flame with low heat output. However, burning ethyne with pure oxygen ensures complete combustion, generating a very hot, clean flame suitable for melting metals during welding.

In-Text Questions (Page 74)

Q1. How would you distinguish experimentally between an alcohol and a carboxylic acid?

They can be distinguished using the following chemical tests:

• Sodium Hydrogen Carbonate Test: Add sodium hydrogencarbonate (NaHCO₃) or sodium carbonate (Na₂CO₃) solution. A carboxylic acid reacts to produce brisk effervescence due to the release of carbon dioxide gas (CO₂). Alcohols do not react with these carbonates.
• Litmus Test: A carboxylic acid turns blue litmus paper red because of its acidic nature, whereas an alcohol remains neutral and shows no color change on blue litmus.

Q2. What are oxidising agents?

Oxidising agents are substances that are capable of adding oxygen to other substances or removing hydrogen from them during a chemical reaction. Examples include alkaline potassium permanganate (KMnO₄) and acidified potassium dichromate (K₂Cr₂O₇).

In-Text Questions (Page 76)

Q1. Would you be able to check if water is hard by using a detergent?

No, because detergents form lathers/foams easily with both soft water and hard water. Unlike soaps, detergents do not form an insoluble precipitate (scum) when they react with the calcium and magnesium ions present in hard water. Therefore, detergents cannot be used to distinguish hard water from soft water.

Q2. 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 a washing machine. Why is agitation necessary to get clean clothes?

Soap molecules form spherical structures called micelles around oily dirt particles, trapping the dirt at the hydrophobic core while keeping the hydrophilic ionic ends facing outward in water. Physical agitation (beating, scrubbing, or spinning) is necessary to lift these emulsified dirt-containing micelles away from the fabric surfaces into the water so that they can be easily rinsed away.

End-of-Chapter Exercises (Page 77–78)

Q1. Ethane, with the molecular formula C₂H₆ has
(a) 6 covalent bonds.
(b) 7 covalent bonds.
(c) 8 covalent bonds.
(d) 9 covalent bonds.

Answer: (b) 7 covalent bonds.

Explanation: Ethane contains 1 carbon-carbon single covalent bond (C–C) and 6 carbon-hydrogen single covalent bonds (C–H), making a total of 7 covalent bonds.

Q2. Butanone is a four-carbon compound with the functional group
(a) carboxylic acid.
(b) aldehyde.
(c) ketone.
(d) alcohol.

Answer: (c) ketone.

Explanation: The suffix "-one" in Butanone indicates the presence of a ketone functional group (–C=O).

Q3. While cooking, if the bottom of the vessel is getting blackened on the outside, it means that
(a) the food is not cooked completely.
(b) the fuel is not burning completely.
(c) the fuel is wet.
(d) the fuel is burning completely.

Answer: (b) the fuel is not burning completely.

Explanation: Blackening of cooking vessels occurs due to soot deposition caused by incomplete combustion of fuel, usually when air holes are blocked and oxygen supply is insufficient.

Q4. Explain the nature of the covalent bond using the bond formation in CH₃Cl.

In chloromethane (CH₃Cl), carbon is the central tetravalent atom. Carbon shares one valence electron with each of the three hydrogen atoms to form three C–H single covalent bonds. It also shares its fourth valence electron with one chlorine atom (which has 7 valence electrons) to form a single C–Cl covalent bond. Through these 4 shared pairs of electrons, carbon achieves a stable octet, hydrogen attains a noble gas duplet, and chlorine completes its octet.

Q5. Draw the electron dot structures for (a) ethanoic acid, (b) H₂S, (c) propanone, (d) F₂.

Electron dot structures depict valence shell electron sharing:

• (a) Ethanoic acid (CH₃COOH): Carbon atoms share electrons with hydrogen atoms, each other, and oxygen atoms (including a double bond with one oxygen).
• (b) Hydrogen sulphide (H₂S): Sulfur shares one electron pair with each of the two hydrogen atoms.
• (c) Propanone (CH₃COCH₃): Central carbon shares a double bond (two pairs of electrons) with oxygen and single bonds with two methyl carbons.
• (d) Fluorine molecule (F₂): Two fluorine atoms share a single pair of valence electrons.

Figure 4.3: Electron Dot Structures for (a) CH₃COOH, (b) H₂S, (c) CH₃COCH₃, and (d) F₂

Q6. What is an homologous series? Explain with an example.

A homologous series is a series or family of organic compounds having the same functional group, similar chemical properties, and where successive members differ from each other by a –CH₂– group and a molecular mass of 14 u.

Example: The homologous series of alcohols:

• Methanol (CH₃OH)
• Ethanol (C₂H₅OH)
• Propanol (C₃H₇OH)
• Butanol (C₄H₉OH)

All members share the –OH functional group and show a gradual change in physical properties such as melting and boiling points.

Q7. How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?

Property	Ethanol (CH3CH2OH)	Ethanoic Acid (CH3COOH)
Smell (Physical)	Has a pleasant, characteristic spirit smell.	Has a sharp, pungent vinegar-like smell.
Melting Point (Physical)	Low melting point (156 K); remains liquid in winter.	Melting point is 290 K; freezes easily in winter (glacial acetic acid).
Litmus Test (Chemical)	Neutral; does not change the color of blue or red litmus paper.	Acidic; turns blue litmus paper red.
Reaction with NaHCO3 (Chemical)	Does not react with sodium hydrogencarbonate.	Reacts with NaHCO3 to give brisk effervescence of CO2 gas.

Q8. Why does micelle formation take place when soap is added to water? Will a micelle be formed in other solvents such as ethanol also?

Soap molecules consist of two distinct parts: a hydrophilic (water-attracting) ionic end and a hydrophobic (water-repelling) long hydrocarbon chain end. When added to water, the hydrophobic tails aggregate inward to minimize contact with water, while the hydrophilic ionic heads face outward toward the aqueous environment, forming spherical clusters called micelles.

No, micelles will not be formed in ethanol. This is because the hydrocarbon tail of soap is soluble in organic solvents like ethanol, so the orientation needed to form clusters does not occur in non-water solvents.

Q9. Why are carbon and its compounds used as fuels for most applications?

Carbon and its compounds are widely used as fuels because:

• They undergo exothermic combustion in oxygen, releasing a large amount of heat and light energy per unit mass.
• Saturated hydrocarbons burn with a clean, non-sooty blue flame under optimum oxygen conditions, leaving little to no unburnt residues.
• They have optimal ignition temperatures and high heat values.

Q10. Explain the formation of scum when hard water is treated with soap.

Hard water contains dissolved salts of calcium (Ca²⁺) and magnesium (Mg²⁺) ions. When soap (sodium or potassium salt of long-chain fatty acids) is added to hard water, it reacts with these soluble calcium and magnesium ions to form an insoluble, sticky white precipitate called scum. This consumes a large amount of soap before lather can form.

Q11. What change will you observe if you test soap with litmus paper (red and blue)?

When soap solution is tested with litmus paper:

• Red litmus paper turns blue.
• Blue litmus paper remains blue.

This demonstrates that soap solution is basic (alkaline) in nature, as it is formed by the reaction of a weak fatty acid with a strong base (such as NaOH).

Q12. What is hydrogenation? What is its industrial application?

Hydrogenation is an addition reaction in which unsaturated hydrocarbons (alkenes or alkynes) add hydrogen atoms across double or triple carbon-carbon bonds in the presence of catalysts such as nickel (Ni) or palladium (Pd) to form saturated hydrocarbons.

Industrial Application: Hydrogenation is used industrially to convert liquid vegetable oils (which have long unsaturated carbon chains) into solid vegetable ghee/vanaspati fats using a nickel catalyst.

Q13. Which of the following hydrocarbons undergo addition reactions: C₂H₆, C₃H₈, C₃H₆, C₂H₂ and CH₄.

Addition reactions are characteristic of unsaturated hydrocarbons (alkenes and alkynes containing double or triple bonds).

• C₃H₆ (Propene) and C₂H₂ (Ethyne) will undergo addition reactions.

(C₂H₆, C₃H₈, and CH₄ are saturated alkanes and do not undergo addition reactions.)

Q14. Give a test that can be used to differentiate between saturated and unsaturated hydrocarbons.

Bromine Water Test:

• Pass the hydrocarbon through bromine water (which has a reddish-brown color).
• An unsaturated hydrocarbon rapidly decolourises the reddish-brown bromine water due to an addition reaction.
• A saturated hydrocarbon does not decolourise bromine water under normal conditions because it is unreactive to addition reactions.

Alternative Test: Combustion in air can also differentiate them; saturated hydrocarbons burn with a clean flame, whereas unsaturated hydrocarbons burn with a yellow, sooty flame.

Q15. Explain the mechanism of the cleaning action of soaps.

The mechanism of cleaning action of soap relies on its dual molecular structure:

1. Structure of Soap: A soap molecule has a long non-polar hydrophobic hydrocarbon tail (attracted to oil/dirt) and a polar hydrophilic ionic head (attracted to water).
2. Micelle Formation: When soap is mixed with dirty clothes in water, the hydrophobic tails attach themselves to oily dirt particles, while the hydrophilic ionic heads stick out into the surrounding water. These aggregate to form spherical clusters called micelles.
3. Emulsification and Removal: The oily dirt is trapped at the center of the micelle. Ion-ion repulsion prevents the micelles from precipitating together. Upon rinsing and agitation, the suspended micelles carrying the dirt are washed away, leaving the fabric clean.

Figure 4.4: Formation of Micelle around an oily dirt droplet during cleaning action of soap