NCERT Solutions For Class 10 Chemistry: Carbon and Its Compounds

Intext Questions (Page 61)

Question 1: What would be the electron dot structure of carbon dioxide which has the formula $\text{CO}_2$?

Answer-
In carbon dioxide ($\text{CO}_2$), one carbon atom (with four valence electrons) shares electrons with two oxygen atoms (each with six valence electrons). To satisfy the octet rule, carbon shares two electron pairs with each oxygen atom, resulting in the formation of two double bonds.

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.)

Answer-
The molecule of sulphur ($\text{S}_8$) is made up of eight sulphur atoms joined together in the form of a ring. Since sulphur atoms must achieve an octet configuration, each sulphur atom shares one pair of electrons with its two adjacent sulphur atoms, forming single covalent bonds in the ring structure.

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Intext Questions (Page 68)

Question 1: How many structural isomers can you draw for pentane?

Answer-
Pentane has the molecular formula $\text{C}_5\text{H}_{12}$. It is possible to draw three structural isomers for pentane:

1. A straight chain structure (n-pentane).
2. A branched chain structure with one methyl group on the second carbon atom (isopentane or 2-methylbutane).
3. A branched chain structure with two methyl groups on the second carbon atom (neopentane or 2,2-dimethylpropane).

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

Answer-
The huge number of carbon compounds (estimated to be in millions) is due to two characteristic features of carbon:

1. Catenation: This is the unique ability of carbon to form bonds with other atoms of carbon, giving rise to large molecules. These can be long chains, branched chains, or rings, linked by single, double, or triple bonds. The carbon-carbon bond is very strong and stable.
2. Tetravalency: Carbon has a valency of four, meaning it is capable of bonding with four other atoms of carbon or atoms of other mono-valent elements (like hydrogen, oxygen, nitrogen, sulphur, or chlorine).

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

Answer-
Cyclopentane is a cyclic carbon compound.
The formula for cyclopentane is $\text{C}_5\text{H}_{10}$.
The electron dot structure would show five carbon atoms arranged in a ring structure, where each carbon atom is linked to its two neighbouring carbon atoms by a single covalent bond, and each carbon atom is also bonded to two hydrogen atoms by single covalent bonds, ensuring all atoms achieve noble gas configuration.

Question 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: This is a carboxylic acid. It has two carbon atoms. The structure consists of a $\text{CH}_3$ group bonded to a $\text{COOH}$ functional group. The $\text{COOH}$ group contains a carbon atom double-bonded to an oxygen atom and single-bonded to an $\text{OH}$ group.
(ii) Bromopentane: This is a haloalkane with five carbon atoms. The bromine atom ($\text{Br}$) replaces one hydrogen atom.
(iii) Butanone: This is a ketone with four carbon atoms. The ketone functional group ($\text{C}=\text{O}$) is present at the second carbon atom.
(iv) Hexanal: This is an aldehyde with six carbon atoms. The aldehyde functional group ($\text{CHO}$) is located at the end of the chain.

Yes, structural isomers are possible for bromopentane. Since the bromine atom can be attached to the first, second, or third carbon atom in the chain (1-bromopentane, 2-bromopentane, 3-bromopentane), and the carbon chain itself can be branched (e.g., bromomethylbutane isomers), several structural isomers exist.

Question 5: How would you name the following compounds?
(i) $\text{CH}_3—\text{CH}_2—\text{Br}$ (ii) (Structure/formula not explicitly shown) (iii) (Structure/formula not explicitly shown)

Answer-
(i) $\text{CH}_3—\text{CH}_2—\text{Br}$: This compound has two carbon atoms (ethane skeleton) and the functional group is bromo (a haloalkane prefix). The name is Bromoethane.
(ii) (Referring to the implied compound structure, which appears to be Ketone in the source list based on context): If the structure is $\text{CH}_3\text{COCH}_3$, it would be Propanone.
(iii) (Referring to the implied compound structure, which appears to be Carboxylic acid in the source list based on context): If the structure is $\text{CH}_3\text{CH}_2\text{COOH}$, it would be Propanoic acid.

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Intext Questions (Page 71)

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

Answer-
The conversion of ethanol ($\text{CH}_3\text{CH}_2\text{OH}$) to ethanoic acid ($\text{CH}_3\text{COOH}$) is considered an oxidation reaction because the reaction involves the addition of oxygen to the starting material, ethanol. This addition of oxygen is carried out by oxidising agents like alkaline potassium permanganate or acidified potassium dichromate.

Question 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 ($\text{C}_2\text{H}_2$) and air is not used for welding because air contains only about 21% oxygen. Burning ethyne in air results in incomplete combustion, producing a sooty flame and insufficient heat. For welding, a very high temperature is required. Burning ethyne with pure oxygen provides a highly oxygen-rich mixture, ensuring complete combustion and producing a very hot, clean blue flame necessary for melting metals.

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Intext Questions (Page 74)

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

Answer-
A simple chemical test to distinguish between an alcohol (like ethanol) and a carboxylic acid (like ethanoic acid) is the reaction with carbonates or hydrogencarbonates.

1. Carboxylic Acid Test: Ethanoic acid (carboxylic acid) reacts with sodium carbonate or sodium hydrogencarbonate to give a salt, carbon dioxide ($\text{CO}_2$), and water. The evolution of $\text{CO}_2$ gas, which turns lime-water milky, confirms the presence of the acid.
2. Alcohol Test: Alcohols (like ethanol) do not react with carbonates or hydrogencarbonates, so no $\text{CO}_2$ gas will be evolved.

Another distinguishing chemical test is the Esterification reaction:

1. Carboxylic Acid + Alcohol: When the sample (acid) is warmed with a second alcohol (e.g., ethanol) in the presence of concentrated sulphuric acid, a sweet-smelling ester is formed.
2. Alcohol + Alcohol: No such ester is formed if the sample is an alcohol.

Question 2: What are oxidising agents?

Answer-
Oxidising agents are substances that are capable of adding oxygen to other compounds in a chemical reaction. They cause the oxidation of the starting material. Examples of common oxidising agents include alkaline potassium permanganate and acidified potassium dichromate, which are used to convert alcohols to carboxylic acids.

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Intext Questions (Page 76)

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

Answer-
No, you would not be able to check if water is hard by using a detergent. Hard water contains calcium and magnesium salts. Detergents are effective in hard water because their charged ends do not form insoluble precipitates with calcium and magnesium ions; thus, they will produce foam regardless of the presence of hardness salts. To check for hardness, soap must be used, as soap reacts with these ions to form a white curdy precipitate (scum).

Question 2: People use a variety of methods to wash clothes... Why is agitation necessary to get clean clothes?

Answer-
Agitation (beating, scrubbing, mixing) is necessary to get clean clothes because the soap molecules form structures called micelles around the oily dirt. Most dirt is oily, and oil does not dissolve in water. Agitation helps the soap micelles effectively pull out the dirt (oil) suspended within them, forming an emulsion. This mechanical energy ensures that the dirt, collected in the center of the micelle, is fully suspended in the solution as a colloid so that it can be easily rinsed away.

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Exercise Questions (Page 77-78)

Question 1: Ethane, with the molecular formula $\text{C}_2\text{H}_6$ has
(a) 6 covalent bonds.
(b) 7 covalent bonds.
(c) 8 covalent bonds.
(d) 9 covalent bonds.

Answer-
Ethane ($\text{C}_2\text{H}_6$) has two carbon atoms linked by a single covalent bond, and each carbon atom is linked to three hydrogen atoms by single covalent bonds. Therefore, there are one $\text{C}-\text{C}$ bond and six $\text{C}-\text{H}$ bonds, totaling 7 covalent bonds.
The correct option is (b) 7 covalent bonds.

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

Answer-
Butanone is named using the suffix "-one". This suffix indicates the presence of the ketone functional group.
The correct option is (c) ketone.

Question 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-
If the bottom of the cooking vessel is getting blackened, it means that the air holes are blocked or there is an insufficient supply of oxygen, resulting in the incomplete combustion of the fuel. The black soot deposited is unburnt carbon.
The correct option is (b) the fuel is not burning completely.

Question 4: Explain the nature of the covalent bond using the bond formation in $\text{CH}_3\text{Cl}$.

Answer-
The bonding in methyl chloride ($\text{CH}_3\text{Cl}$) involves covalent bonds. A covalent bond is formed by the sharing of an electron pair between two atoms, allowing both atoms to achieve a stable noble gas configuration.
In $\text{CH}_3\text{Cl}$, carbon is tetravalent, meaning it needs four electrons to complete its octet. It achieves this by sharing its four valence electrons with:

1. Three hydrogen atoms (each sharing one electron).
2. One chlorine atom (which needs one electron to complete its octet).
   All four bonds ($\text{C}-\text{H}$ and $\text{C}-\text{Cl}$) are single covalent bonds. Since the electrons are shared and no charged particles (ions) are formed, the compound $\text{CH}_3\text{Cl}$ is generally a poor conductor of electricity.

Question 5: Draw the electron dot structures for
(a) ethanoic acid. (b) $\text{H}_2\text{S}$. (c) propanone. (d) $\text{F}_2$.

Answer-
Since direct drawing is not possible, the structure is described based on bond types:

(a) Ethanoic acid ($\text{CH}_3\text{COOH}$): The structure contains seven covalent bonds. There are three $\text{C}-\text{H}$ single bonds, one $\text{C}-\text{C}$ single bond, one $\text{C}=\text{O}$ double bond, one $\text{C}-\text{O}$ single bond, and one $\text{O}-\text{H}$ single bond.

(b) $\text{H}_2\text{S}$ (Hydrogen Sulphide): The sulphur atom has six valence electrons. It achieves stability by forming two single covalent bonds with two hydrogen atoms (each having one electron). The sulphur atom will have two lone pairs of electrons.

(c) Propanone ($\text{CH}_3\text{COCH}_3$): This molecule contains three carbon atoms. There are six $\text{C}-\text{H}$ single bonds, two $\text{C}-\text{C}$ single bonds, and one $\text{C}=\text{O}$ double bond (at the central carbon atom).

(d) $\text{F}_2$ (Fluorine): Fluorine has seven valence electrons (like chlorine). Two fluorine atoms share one pair of electrons to form a single covalent bond ($\text{F}-\text{F}$), allowing both atoms to attain a stable octet.

Question 6: What is an homologous series? Explain with an example.

Answer-
A homologous series is a series of compounds in which the same functional group substitutes for hydrogen in a carbon chain.
The defining features are:

1. The chemical properties of the compounds in the series remain similar, as they are determined solely by the functional group.
2. Successive members in the series differ by a $-\text{CH}_2-$ unit.
3. As the molecular mass increases, a gradation in physical properties is seen; for instance, melting and boiling points increase with increasing molecular mass.

Example: The homologous series of alcohols:
Methanol ($\text{CH}_3\text{OH}$), Ethanol ($\text{C}_2\text{H}_5\text{OH}$), Propanol ($\text{C}_3\text{H}_7\text{OH}$), and Butanol ($\text{C}_4\text{H}_9\text{OH}$). Each successive member differs by the $-\text{CH}_2-$ unit.

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

Answer-
Differentiation based on physical and chemical properties:

Basis of Differentiation	Ethanol (Alcohol)	Ethanoic Acid (Carboxylic Acid)
Physical Property (Acidity/pH)	It is neutral.	It is acidic in nature and reacts with blue litmus paper to turn it red.
Physical Property (Melting Point)	It is a liquid at room temperature. Its melting point is 156 K.	Its melting point is 290 K, meaning it often freezes in cold climates (glacial acetic acid).
Chemical Property (Reaction with $\text{Na}$)	Reacts with sodium to release hydrogen gas.	Also reacts with sodium, similar to mineral acids.
Chemical Property (Carbonate Test)	Does not react with carbonates or hydrogencarbonates.	Reacts vigorously with carbonates and hydrogencarbonates to evolve carbon dioxide ($\text{CO}_2$) gas.
Chemical Property (Esterification)	It acts as the alcohol component in esterification when reacted with an acid.	It acts as the acid component in esterification, reacting with alcohol to produce a sweet-smelling ester.

Question 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 in Water:
Micelle formation takes place because soap molecules have two ends with differing properties:

1. The ionic end (hydrophilic) that interacts with water.
2. The long hydrocarbon chain (hydrophobic) that interacts with oil/hydrocarbons.
   When soap is in water, the hydrophobic tails are insoluble in water. These molecules adopt a unique orientation where the tails move away from the water, gathering in the interior of the cluster, while the ionic, hydrophilic ends remain on the surface facing the water. This cluster formation is called a micelle.

Micelle Formation in Ethanol:
A micelle will generally not be formed in other solvents such as ethanol. Ethanol is an organic solvent (alcohol). Since the hydrophobic tail of the soap molecule interacts well with hydrocarbons and similar organic solvents like ethanol, the soap molecule does not need to form a closed cluster to keep the hydrocarbon tail away from the solvent.

Question 9: Why are carbon and its compounds used as fuels for most applications?

Answer-
Carbon and its compounds (like methane, alcohol, coal, petroleum) are major sources of fuels. They are used as fuels because when they burn in oxygen (combustion), they undergo oxidation reactions that release a large amount of heat and light energy. This energy release makes them highly effective and efficient sources of power for domestic and industrial applications.

Question 10: Explain the formation of scum when hard water is treated with soap.

Answer-
Hard water contains salts of calcium and magnesium. Soap molecules are sodium or potassium salts of long-chain carboxylic acids. When soap is added to hard water, the soap molecules react with the calcium ($\text{Ca}^{2+}$) and magnesium ($\text{Mg}^{2+}$) salts present in the hard water. This reaction forms an insoluble substance, which is seen as a white curdy precipitate or scum. The formation of scum leads to wasted soap, requiring the use of a larger amount of soap for cleaning.

Question 11: What change will you observe if you test soap with litmus paper (red and blue)?

Answer-
Soaps are sodium or potassium salts of long-chain carboxylic acids. They are derived from a strong base ($\text{NaOH}$ or $\text{KOH}$) and a weak acid (carboxylic acid). Therefore, soap solutions are generally basic in nature.
If you test soap solution with litmus paper, the following change will be observed:

1. Red litmus paper will turn blue.
2. Blue litmus paper will show no change.

Question 12: What is hydrogenation? What is its industrial application?

Answer-
Hydrogenation is an addition reaction in which unsaturated hydrocarbons (which contain double or triple bonds) add hydrogen gas in the presence of catalysts such as nickel or palladium. This reaction converts unsaturated hydrocarbons into saturated hydrocarbons.

The industrial application of hydrogenation is the hydrogenation of vegetable oils. Vegetable oils generally have long unsaturated carbon chains. This process uses a nickel catalyst to convert these unsaturated vegetable oils into solid fats (like vanaspati ghee), which have saturated carbon chains.

Question 13: Which of the following hydrocarbons undergo addition reactions: $\text{C}_2\text{H}_6$, $\text{C}_3\text{H}_8$, $\text{C}_3\text{H}_6$, $\text{C}_2\text{H}_2$ and $\text{CH}_4$.

Answer-
Addition reactions are characteristic of unsaturated hydrocarbons—those containing double bonds (alkenes) or triple bonds (alkynes). Saturated hydrocarbons (alkanes) are generally unreactive and inert in the presence of most reagents.

The compounds that undergo addition reactions are:

1. $\text{C}_3\text{H}_6$ (Propene, an alkene): It fits the general formula $\text{C}_n\text{H}_{2n}$ for alkenes.
2. $\text{C}_2\text{H}_2$ (Ethyne, an alkyne): It contains a triple bond.

The saturated compounds $\text{C}_2\text{H}_6$ (Ethane), $\text{C}_3\text{H}_8$ (Propane), and $\text{CH}_4$ (Methane) are alkanes and do not undergo addition reactions.

Question 14: Give a test that can be used to differentiate between saturated and unsaturated hydrocarbons.

Answer-
The test that can differentiate between saturated and unsaturated hydrocarbons is the observation of their burning characteristics or combustion.

1. Saturated Hydrocarbons (e.g., Methane, Propane): These generally give a clean flame (blue flame) when sufficient oxygen is available, indicating complete combustion.
2. Unsaturated Hydrocarbons (e.g., Ethene, Ethyne): These typically burn with a yellow flame that produces lots of black smoke. This black smoke results in a sooty deposit when burnt against a metal surface, due to incomplete combustion.

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

Answer-
The cleaning action of soap relies on the dual nature of the soap molecule, which consists of a long non-polar hydrocarbon chain (hydrophobic tail) and a charged ionic end (hydrophilic head).

1. Interaction with Dirt: Most dirt is oily in nature and is insoluble in water. The hydrophobic tail of the soap molecule interacts with the oil droplet (dirt).
2. Micelle Formation: Simultaneously, the hydrophilic ionic end interacts with the surrounding water. The soap molecules arrange themselves into a spherical structure called a micelle around the oily dirt. In this formation, the hydrocarbon tails are pointed inwards, encapsulating the dirt, while the ionic heads face outwards into the water.
3. Suspension and Removal: This structure forms an emulsion in water, where the oily dirt is collected in the center of the micelle. Since the outer surface of the micelle is charged (ionic), the micelles stay suspended in solution as a colloid and do not precipitate due to ion-ion repulsion. This allows the dirt, suspended within the micelles, to be easily rinsed away with the water.