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NCERT Class 10 Science Carbon and its Compounds: Exercise Solutions

Step-by-step working for every question of End-of-chapter exercises in NCERT's Class 10 Science book, chapter “Carbon and its Compounds”, written by Shishya. Each question is in our own short words; open the NCERT chapter for the exact wording and figures.

1. Exercises Q1

   Ethane (C₂H₆) has (a) 6 (b) 7 (c) 8 (d) 9 covalent bonds.

   1. Ethane has the molecular formula C₂H₆.
   2. Each carbon atom forms four covalent bonds to satisfy its valency.
      why?

      In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

   3. There is one single covalent bond between the two carbon atoms.
      why?

      In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

   4. Each carbon atom forms three single covalent bonds with hydrogen atoms.
      why?

      In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

   5. Total bonds from carbon-hydrogen connections are 3 + 3 = 6.
   6. Total covalent bonds in ethane are 6 (C-H) + 1 (C-C) = 7.
      why?

      In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

   7. This matches option (b).

   Answer: (b) 7 covalent bonds

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2. Exercises Q2

   Butanone is a four-carbon compound with which functional group: (a) carboxylic acid (b) aldehyde (c) ketone (d) alcohol?

   1. The name butanone contains the suffix 'one'.
   2. In organic chemistry nomenclature, the suffix 'one' indicates the presence of a ketone functional group.
   3. A ketone has a carbonyl group (C=O) bonded to two carbon atoms.
   4. Therefore, butanone is a ketone.

   Answer: (c) ketone

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3. Exercises Q3

   If a cooking vessel blackens on the outside, it means (a) the food is not fully cooked (b) the fuel is not burning completely (c) the fuel is wet (d) the fuel is burning completely.

   1. When a cooking vessel blackens on the outside, it indicates the presence of soot.
   2. Soot is a black substance made of unburnt carbon particles.
   3. This happens when the fuel does not get enough oxygen to burn completely.
   4. Incomplete combustion leaves behind carbon particles that stick to the vessel.
   5. Therefore, the blackening shows that the fuel is not burning completely.

   Answer: (b) the fuel is not burning completely

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4. Exercises Q4

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

   1. Carbon has 4 valence electrons and needs 4 more to complete its octet.
   2. Chlorine has 7 valence electrons and needs 1 more to complete its octet.
   3. Hydrogen has 1 valence electron and needs 1 more to complete its duet.
   4. Carbon shares one electron with each of the three hydrogen atoms to form three C-H covalent bonds.
      why?

      In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

   5. Carbon shares one electron with the chlorine atom to form one C-Cl covalent bond.
      why?

      In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

   6. This sharing of electrons allows carbon to achieve a stable octet configuration.
      why?

      In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

   7. Simultaneously, each hydrogen achieves a stable duet and chlorine achieves a stable octet.
   8. The electrostatic attraction between the shared electron pairs and the nuclei holds the atoms together in CH3Cl.

   Answer: In CH3Cl, carbon forms covalent bonds by sharing its valence electrons with three hydrogen atoms and one chlorine atom, allowing all atoms to achieve stable electronic configurations.

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5. Exercises Q5

   Describe the electron dot structures of (a) ethanoic acid (b) H₂S (c) propanone (d) F₂.

   1. (a) Ethanoic acid (CH₃COOH) has 24 valence electrons. The carbon atoms are single-bonded to each other. The first carbon is bonded to three hydrogens. The second carbon is double-bonded to one oxygen and single-bonded to another oxygen. The second oxygen is bonded to a hydrogen. Each oxygen has two lone pairs.
   2. (b) H₂S has 8 valence electrons. The sulfur atom is in the center. It forms single bonds with two hydrogen atoms. Sulfur has two lone pairs of electrons remaining.
   3. (c) Propanone (CH₃COCH₃) has 24 valence electrons. The central carbon is double-bonded to an oxygen atom. This central carbon is also single-bonded to two other carbon atoms. Each of these outer carbons is bonded to three hydrogen atoms. The oxygen atom has two lone pairs.
   4. (d) F₂ has 14 valence electrons. The two fluorine atoms are joined by a single covalent bond. Each fluorine atom has three lone pairs of electrons.
      why?

      In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

   Answer: (a) CH₃COOH structure with C-C single bond, C=O double bond, C-O-H single bonds, and lone pairs on oxygens; (b) H₂S structure with S in center, two S-H bonds, and two lone pairs on S; (c) Propanone structure with central C=O, two C-C single bonds, C-H bonds, and lone pairs on O; (d) F₂ structure with F-F single bond and three lone pairs on each F.

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6. Exercises Q6

   What is a homologous series? Explain with an example.

   1. A homologous series is a group of organic compounds that share the same general formula and functional group.
   2. Members of the series differ from each other by a CH₂ group in their molecular structure.
   3. They show a gradual change in physical properties like boiling point and melting point as the molecular mass increases.
   4. Chemical properties remain similar because the functional group stays the same throughout the series.
   5. For example, alcohols form a homologous series with the general formula CₙH₂ₙ₊₁OH.
   6. The first few members are methanol (CH₃OH), ethanol (C₂H₅OH), and propanol (C₃H₇OH).
   7. Each member has one more carbon and two more hydrogen atoms than the previous one, fitting the CH₂ difference rule.

   Answer: A homologous series is a family of compounds with the same functional group and general formula, differing by CH₂ units, such as alcohols (CH₃OH, C₂H₅OH, C₃H₇OH).

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7. Exercises Q7

   How can ethanol and ethanoic acid be told apart by their physical and chemical properties?

   1. Ethanol is a liquid with a characteristic alcoholic smell, while ethanoic acid has a sharp vinegar-like smell.
   2. Ethanol has a boiling point of 351 K, whereas ethanoic acid has a higher boiling point of 391 K.
   3. Ethanol is a neutral compound and does not change the color of litmus paper.
   4. Ethanoic acid is acidic and turns blue litmus paper red.
   5. When sodium bicarbonate is added to ethanol, no reaction occurs and no gas is evolved.
   6. When sodium bicarbonate is added to ethanoic acid, brisk effervescence is observed due to the release of carbon dioxide gas.

   Answer: Ethanol is neutral with an alcoholic smell and no reaction with NaHCO3, while ethanoic acid is acidic with a vinegar smell and reacts with NaHCO3 to release CO2.

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8. Exercises Q8

   Why do micelles form when soap is added to water? Do they form in ethanol too?

   1. Soap molecules have a hydrophobic tail that repels water and a hydrophilic head that attracts water.
   2. In water, the hydrophobic tails cluster together to hide from the water molecules.
   3. The hydrophilic heads face outward towards the water, forming a spherical structure called a micelle.
   4. This arrangement allows the soap to dissolve in water and trap grease or dirt inside the micelle.
   5. Ethanol is a polar solvent that can interact with both the hydrophobic tails and hydrophilic heads of soap molecules.
   6. Because ethanol does not force the tails to hide from the solvent, soap molecules do not cluster into micelles in ethanol.

   Answer: Micelles form in water to hide hydrophobic tails; they do not form in ethanol.

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9. Exercises Q9

   Why are carbon and its compounds used as fuels for most purposes?

   1. Carbon and its compounds (wood, coal, petroleum products, LPG, CNG) burn in air (oxygen) to give carbon dioxide and water.
   2. This combustion releases a large amount of heat and light, so these fuels have a high calorific value.
   3. Most of them catch fire at a moderate ignition temperature and burn in a controlled way, so the heat can be used safely for cooking, transport and industry.
   4. They are also available in large amounts and are easy to store and transport (for example LPG cylinders and petrol).

   Answer: They burn in air releasing a lot of heat and light (high calorific value), burn in a controlled way at a moderate ignition temperature, and are widely available and easy to store and transport.

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10. Exercises Q10

    Explain how scum forms when hard water is treated with soap.

    1. Soap is the sodium or potassium salt of a fatty acid, such as stearic acid.
    2. Hard water contains dissolved calcium and magnesium ions in the form of chlorides or sulphates.
    3. When soap is added to hard water, the sodium ions in the soap are replaced by calcium or magnesium ions.
    4. This chemical reaction produces calcium or magnesium salts of the fatty acid.
    5. These calcium and magnesium salts are insoluble in water.
    6. The insoluble precipitate that forms is called scum.
    7. Scum appears as a white or greyish deposit on the surface of the water or on washed clothes.

    Answer: Scum forms because the calcium and magnesium ions in hard water react with soap to form insoluble calcium or magnesium salts of fatty acids.

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11. Exercises Q11

    What change do you see when soap is tested with red and blue litmus?

    1. Soaps are salts of long-chain fatty acids and a strong base, usually sodium or potassium hydroxide.
    2. Because they are formed from a strong base, soap solutions are alkaline in nature.
    3. Alkaline substances turn red litmus paper blue.
    4. Alkaline substances do not change the colour of blue litmus paper.
    5. Therefore, soap turns red litmus blue and leaves blue litmus unchanged.

    Answer: Soap turns red litmus paper blue and does not change the colour of blue litmus paper.

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12. Exercises Q12

    What is hydrogenation? What is its industrial use?

    1. Hydrogenation is a chemical reaction where hydrogen gas is added to unsaturated hydrocarbons in the presence of a catalyst.
    2. This process converts liquid unsaturated fats into solid saturated fats.
    3. The catalyst used is typically finely divided nickel, palladium, or platinum.
    4. Industrially, this method is used to convert vegetable oils into edible fats like vanaspati ghee.
    5. The addition of hydrogen saturates the carbon-carbon double bonds in the oil molecules.
    6. This makes the product solid at room temperature and gives it a longer shelf life.
    7. The process is also used in the food industry to produce margarine and other solid fats from liquid oils.

    Answer: Hydrogenation is the addition of hydrogen to unsaturated hydrocarbons in the presence of a catalyst; its industrial use is to convert vegetable oils into solid edible fats like vanaspati ghee.

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13. Exercises Q13

    Which of C₂H₆, C₃H₈, C₃H₆, C₂H₂ and CH₄ undergo addition reactions?

    1. Addition reactions occur in unsaturated hydrocarbons that contain double or triple bonds.
    2. C₂H₆ and C₃H₈ are alkanes with only single bonds, so they are saturated and do not undergo addition reactions.
    3. CH₄ is a saturated alkane with single bonds, so it does not undergo addition reactions.
    4. C₃H₆ is an alkene with a carbon-carbon double bond, making it unsaturated and capable of addition reactions.
    5. C₂H₂ is an alkyne with a carbon-carbon triple bond, making it unsaturated and capable of addition reactions.

    Answer: C3H6 and C2H2 undergo addition reactions.

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14. Exercises Q14

    Give a test to tell saturated from unsaturated hydrocarbons.

    1. Take a small amount of the hydrocarbon sample in a test tube.
    2. Add a few drops of dilute bromine water to the sample and shake the mixture gently.
    3. Observe the colour of the bromine water after a short time.
    4. If the reddish-brown colour of bromine water disappears, the hydrocarbon is unsaturated.
    5. If the reddish-brown colour remains unchanged, the hydrocarbon is saturated.
    6. This happens because unsaturated hydrocarbons undergo addition reactions with bromine, while saturated ones do not.

    Answer: Unsaturated hydrocarbons decolourise bromine water, while saturated hydrocarbons do not.

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15. Exercises Q15

    Explain how soap cleans.

    1. Soap molecules have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail.
    2. The hydrophobic tails attach to the oily or greasy dirt on the surface.
    3. The hydrophilic heads face outward towards the surrounding water.
    4. This arrangement forms a structure called a micelle around the dirt particle.
    5. The micelle keeps the dirt suspended in the water instead of letting it settle back.
    6. When the water is rinsed, the micelles carrying the dirt are washed away.

    Answer: Soap cleans by forming micelles around dirt and oil, which are then washed away with water.

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