Chapter 4: Carbon and Its Compounds

Internal Question and Answer :

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

Answer: The electron dot structure of carbon dioxide () :

      

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.)

Answer : The electron dot structure of sulphur () :

 

Internal Question and Answer :

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

Answer: The structural isomers for pentane () are:

(i) n-pentane (normal pentane):  

(ii) 2-methylbutane (isopentane):  

(iii) 2,2-dimethylpropane (neopentane):

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

Answer : The two properties of carbon that lead to the huge number of carbon compounds are:

(i) Catenation : Carbon has the unique ability to form bonds with other atoms of carbon, giving rise to large molecules. This property is called catenation.

(ii) Tetravalency : Carbon has a valency of four, it is capable of bonding with four other atoms of carbon or atoms of some other mono-valent element. This property is called tetravalency.

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

Answer: The molecular formula of cyclopentane is  .

The electron dot structure of cyclopentane:

        

4. Draw the structures for the following compounds.

(i) Ethanoic acid   (ii) Bromopentane*    (iii) Butanone    (iv) Hexanal.

*Are structural isomers possible for bromopentane?

Answer:  (i) Ethanoic acid (Acetic acid) → CH₃–COOH

        

(ii) Bromopentane (C₅H₁₁Br) → CH₃–CH₂–CH₂–CH₂–CH₂Br

        

(iii) Butanone → CH₃–CO–CH₂–CH₃

        

(iv) Hexanal → CH₃–CH₂–CH₂–CH₂–CH₂–CHO

    

Yes, structural isomers are possible. Bromopentane (C₅H₁₁Br) can have different structures depending on the position of the bromine atom and the carbon chain arrangement.

5. How would you name the following compounds ?

Answer:  (i) → Bromoethane     (ii) → Methanal      (iii) → Hexyne

Internal Question and Answer :

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

Answer: The conversion of ethanol to ethanoic acid is an oxidation reaction because oxygen is added to the ethanol molecule, and it results in the formation of a carboxylic acid (ethanoic acid). Oxidation involves gaining oxygen or losing hydrogen in a chemical reaction, which is precisely what happens in this conversion from ethanol to ethanoic acid.

 

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

Answer: A mixture of ethyne and oxygen is used for welding because it burns completely and produces a very hot flame. But a mixture of ethyne and air is not used because air does not contain enough oxygen for complete combustion. As a result, ethyne burns with a sooty flame and does not produce sufficient heat required for welding.

Internal Question and Answer :

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

Answer: The experimental tests to distinguish between an alcohol and a carboxylic acid are :

Test	Observation with Alcohol (e.g., Ethanol)	Observation with Carboxylic Acid (e.g., Ethanoic acid)
(a) Litmus Test	No change (Remains blue) or turns slightly blue (alcohols are neutral).	Turns Blue litmus to Red (acidic nature).
(b) Sodium Bicarbonate Test (NaHCO₃)	No brisk effervescence (no reaction).	Brisk effervescence (evolution of CO₂ gas) due to reaction: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

2. What are oxidising agents?

Answer: Oxidising agents are substances that add oxygen to another substance or remove hydrogen from it during a chemical reaction. For example, alkaline potassium permanganate and acidified potassium dichromate are common oxidising agents.

Internal Question and Answer :

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

Answer: No, detergent cannot be used to check whether water is hard because detergents work well in both hard and soft water and produce lather easily. Therefore, they do not help in identifying the hardness of water.

2. 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?

Answer:  Agitation is necessary because it helps the soap solution to penetrate the fibres of the clothes and loosens the dirt and oily particles. The movement produced by beating, scrubbing or washing machines helps remove the dirt trapped in the clothes, making them clean.

Exercises of Chapter 4: Carbon and Its Compounds

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

Answer: (b) 7 covalent bonds.

[ The molecular formula of ethane is C₂H₆.

 There are 6 C–H bonds and1 C–C bond.

Total covalent bonds = 6 + 1 = 7  ]

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

Answer:  (c) ketone.

[Butanone is a four-carbon compound containing the functional group ketone (–CO–).]

3. 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.

[ The blackened bottom is due to the deposition of soot formed during incomplete combustion of the fuel.]

4. Explain the nature of the covalent bond using the bond formation in  .
Solution:  A chemical bond which are formed by the sharing of an electron pair between two atoms are called covalent bond . Covalently bonded molecules are seen to have strong bonds within the molecule, but inter-molecular forces are small. Since the electrons are shared between atoms and no charged particles are formed, such covalent compounds are generally poor conductors of electricity.

5. Draw the electron dot structures for
(a) Ethanoic acid.        (b)        (c) Propanone.    (d)

Answer: (a) The electron dot structures of ethanoic acid: 

      

(b)  The electron dot structures of :    

        

(c) The electron dot structures of propanone:   

    

(d) The electron dot structures of :

          
6. What is an homologous series? Explain with an example.
Solution: A series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is called a homologous series. An example of an homologous series is the alkanes.

Alkanes are a family of hydrocarbons with the general formula

Each member of this series has a carbon chain differs from the previous one by a  group .

Methane -  

Ethane -

Propane -  

Butane -

Pentane  -  

Hexane -

In this example, each alkane differs by a   group, and they share similar chemical properties due to their structural similarity.

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

Answer: The difference between ethanol and ethanoic acid based on their physical properties are :

(a) Physical Properties

Property	Ethanol (C₂H₅OH)	Ethanoic Acid (CH₃COOH)
Odor	Pleasant, alcoholic smell	Pungent, vinegar-like smell
Taste	Burning taste	Sour taste
pH	7 (neutral)	2–3 (acidic)
Reaction with litmus	No effect on blue/red litmus	Turns blue litmus red
State at room temperature	Liquid	Liquid (pure: glacial acetic acid solidifies below 17°C)

(b) Chemical Properties

Test	Ethanol	Ethanoic Acid
Sodium bicarbonate test (NaHCO₃)	No reaction (no effervescence)	Brisk effervescence due to CO₂ gas
Sodium metal test	Reacts slowly, produces H₂ gas	Reacts vigorously, produces H₂ gas
Esterification (with conc. H₂SO₄ + acetic acid/ethanol)	With ethanoic acid → fruity smell of ester	With ethanol → fruity smell of ester
Oxidation with K₂Cr₂O₇ + H₂SO₄	Oxidized to ethanoic acid (orange to green color change)	No further oxidation under mild conditions

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

Answer: Micelle formation takes place when soap is added to water because soap molecules have two ends : a hydrophilic (water-attracting) ionic end and a hydrophobic (water-repelling) hydrocarbon tail. In water, the hydrophobic tails come together and form a cluster with the tails inside and the ionic ends outside. This structure is called a micelle.

A micelle will not be formed in solvents like ethanol because both the hydrocarbon tail and the ionic end of soap are soluble in ethanol. Therefore, the soap molecules do not arrange themselves into micelles.

9. Why are carbon and its compounds used as fuels for most applications?
Answer: Carbon and its compounds are used as fuels for most applications because they burn in air to produce a large amount of heat energy. They have high calorific values and usually burn with a clean flame. Also, fuels like coal, petroleum, and natural gas are easily available, easy to store and convenient to transport and use.

10. Explain the formation of scum when hard water is treated with soap.
Answer: When soap is added to hard water, the calcium and magnesium salts present in hard water react with soap to form an insoluble substance called scum. This scum prevents the formation of lather and reduces the cleaning efficiency of soap. Therefore, more soap is required for washing in hard water.

11. What change will you observe if you test soap with litmus paper (red and blue)?
Answer: Soap is basic in nature. Therefore, when tested with litmus paper:

(i) Red litmus paper turns blue.

(ii) Blue litmus paper shows no change.

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

Answer: Hydrogenation is the chemical process of adding hydrogen (H₂) to an unsaturated organic compound (containing double or triple bonds) in the presence of a catalyst (Ni / Pd) to convert it into a saturated compound.

Industrial application:
It is used to convert vegetable oils into solid fats. This increases the melting point and makes the oil semisolid or solid at room temperature, suitable for cooking and baking.

13. Which of the following hydrocarbons undergo addition reactions:  , ,  ,   and  

Answer: The hydrocarbons that undergo addition reactions are C₃H₆ (propene) and C₂H₂ (ethyne).

14. Give a test that can be used to differentiate chemically between butter and cooking oil.
Answer: Butter is a saturated compound while cooking oil is an unsaturated compound. Add bromine water to both samples. Cooking oil decolourises bromine water due to the presence of unsaturated bonds, whereas butter does not decolourise it.

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

Answer: Soap molecules have a hydrophilic (water-loving) head and a hydrophobic (water-repelling) tail. The hydrophobic tail dissolves in oily dirt while the hydrophilic head remains in water. This forms micelles that trap oil droplets. When rinsed with water the micelles are washed away and dirt is removed.