Important Questions: Carbon and its compounds | Science Class 10 PDF Download

Ans: Two key properties that allow carbon to form a vast number of compounds are:

• Tetravalency: Carbon can form four covalent bonds.
• Catenation: Carbon atoms can bond with each other to form long chains.

Ans: As carbon has four valence electrons and it can neither loose nor gain four electrons thus, it attains noble gas configuration only by sharing of electrons. Thus, it forms covalent compounds. The existence of large number of compounds is due to some unique properties of carbon which are:
(i) Catenation: Carbon atoms possess an unique property to link together to form very long chains. This property is referred to as catenation.
A large number of carbon atoms can join together to form straight chains, branched chains and rings as shown below:

(ii) Tetravalency: Due to small size and presence of four valence electrons, a carbon atom can form multiple bonds with some other carbon atoms as well as with other atoms like oxygen, nitrogen etc., This increases the variety of compounds formed by it and hence the number of compounds is tremendously increased.
Due to small size, the nucleus of carbon atom can hold its shared pairs of electrons strongly. As a result, the bonds that carbon forms with most of the other elements such as hydrogen, oxygen, nitrogen, etc. are very strong there by making these compounds exceptionally stable.

Ans: Carbon forms compounds mainly by covalent bonding.Carbon primarily forms covalent compounds due to its unique electronic structure:

• Carbon has four electrons in its outer shell, making it challenging to gain or lose electrons to achieve a stable octet.
• To attain a stable configuration, carbon shares its valence electrons with other atoms, either with other carbon atoms or different elements.

This ability to share electrons leads to the formation of a vast number of compounds, including:

• Catenation: Carbon can bond with itself, creating long chains, branched structures, or rings.
• Strong bonds: The small size of carbon allows for strong interactions with shared electrons, resulting in stable compounds.

Overall, carbon's unique properties enable it to form a diverse range of stable covalent compounds.

Ans:  Carbon is unique among group 14 elements due to its ability to form a vast number of compounds, estimated at around three million. This is primarily because of two key properties:

• Catenation: Carbon can bond with itself, creating long chains, branched structures, or rings. This allows for the formation of large molecules.
• Bond Strength: Carbon forms strong bonds with other elements, including single, double, and triple bonds. This stability is due to its small size, which allows the nucleus to hold onto shared electrons effectively.

In contrast, other elements in this group, like silicon, can form fewer and less stable compounds. Their bond energies decrease as you move down the group, leading to limited catenation.

Ans: Carbon exhibits the property of catenation to the maximum extent due to several key factors:

• Carbon can form strong bonds with other carbon atoms, creating large and complex molecules.
• This ability allows for various structures, including long chains, branched chains, and rings.
• Carbon's small size contributes to the strength of the carbon-carbon bonds, making them stable.
• With a valency of four, carbon can bond with up to four other atoms, enhancing its versatility.

Other elements, like silicon, show limited catenation, forming fewer and more reactive compounds.

Ans: Catenation is the ability of an atom to bond with other atoms of the same element. This property is notably exhibited by both carbon and silicon, but carbon demonstrates it to a far greater extent.

• Carbon can form long chains, branched structures, or rings due to its strong carbon-carbon bonds.
• Carbon has a valency of four, allowing it to bond with up to four other atoms, including other carbon atoms.
• This results in a vast number of stable compounds, as the bonds formed are strong and do not easily break.
• In contrast, silicon can form chains of only seven or eight atoms with hydrogen, but these compounds are highly reactive.

Overall, the unique ability of carbon to catenate leads to an extensive variety of compounds, making it essential for life and numerous applications.

Ans: Carbon cannot form C⁴⁺ cations because removal of 4 electrons from a carbon atom would require a large amount of energy and it cannot form C^4- anion because it would be difficult for the nucleus with 6 protons to hold 10 electrons.
Thus it shares electrons to form covalent compounds.

Ans: (a)
In a homologous series of carbon compounds, like alkanes or alkynes, each member differs from the next by a specific group of atoms (—CH₂), which is explained in statement (A). As the size of the molecules increases (more carbon and hydrogen atoms), their melting and boiling points also generally increase, as stated in (B). Thus, both (A) and (B) are correct, making option (a) the right answer.

Ans: A homologous series is a group of compounds that share the same functional group, with each member differing by a specific number of CH₂ units. For example, the molecular formulas of two consecutive members of the aldehyde series are:

• CH₃CHO (Ethanal)
• C₂H₅CHO (Butanal)

Ans: The first two members of homologous series having functional group —Br are :

• CH₃Br
• C₂H₅Br

Ans: (a) Three homologue of alcohol are the following:CH₃OH, CH₃CH₂OH, CH₃CH₂CH₂OH
Third homologue of alcohol is CH₃CH₂CH₂OH
(b) Three homologue of aldehyde are the following:
HCHO, CH₃CHO, CH₃CH,CHO
Third homologue of aldehyde is CH₃CH₂CHO.

Ans: C₄H₆ (or C_nH_2n-2, n = 4) i.e., alkyne series.
The first member of alkyne series is ethyne (C₂H₂) HC ≡ CH.

Ans: The molecular formula of first two members of homologous series having - Cl functional group are CH₃CI and CH₃CH₂CI.

Ans: The molecular formula of first two members of homologous series having - OH functional group are CH₃OH and CH₃CH₂OH.

Ans: Homologous series of alkenes have general formula, C_nH_2n whose first member is ethene.
2^nd member of homologous series of alkenes is C₃H₆ i.e., propene.
3^rd member of homologous series of alkenes is C₄H₈ i.e., butene.

Ans: Methane, CH₄ is an alkane. Alkanes have general formula, C_nH_2n+2.
2^nd member of homologous series of alkanes is C₂H₆ i.e., ethane.
3^rd member of homologous series of alkanes is C₃H₈ i.e., propane.

Ans: 

These are called structural isomers as they have the same molecular formula i.e., C₄H₁₀ but different structures. As the molecular formula is C₄H₁₀, common name of this compound is butane. The alkyne of four carbon atoms is butyne. Its structure i is as follows:
The molecular formula of butyne is C₄H₆.

Ans: A cyclic unsaturated carbon compound is a compound that contains a carbon ring with one or more double or triple bonds between the carbon atoms. An example of a cyclic unsaturated carbon compound is benzene, which has a ring of six carbon atoms with alternating double bonds.

Ans: The compound 'X' is likely an unsaturated compound (such as an alkene or alkyne) that burns in air, producing a yellow, sooty flame with significant smoke.

Ans: (A) S is a saturated hydrocarbon.
(B) Q is an organic acid.
Structural formula:

Ans: Structure

Name: Cyclopentane
Number of single bonds: 15
Structure: C₅H₁₀​​​​​

Ans: (b)

Six single bonds between Carbons and Hydrogens, three single bonds between six Carbons and Hydrogens ( alternatively) . Three double bonds present between carbons ( alternatively in hexagonal ring). So a total of nine single bonds, three double bonds are present in benzene ring.

Ans: (c)
Structure (I) is cyclohexane, a saturated cyclic hydrocarbon with single bonds between all the carbon atoms.
Structure (II) is benzene, an unsaturated cyclic hydrocarbon with alternating double bonds, giving it aromatic stability.
Structure (III) is cyclohexene, an unsaturated cyclic hydrocarbon with one double bond.
Thus, (I) is a saturated cyclic hydrocarbon, and (II) and (III) are unsaturated cyclic hydrocarbons, making (c) the correct answer.

Ans: (i) Molecular formula of benzene is C₆H₆.

(ii) Number of single bonds = 9
Number of double bonds = 3
(iii) Alkynes are the unsaturated hydrocarbons that contain at least one triple bond between two carbon atoms (—C ≡ C—)

Ans: (i)  A commercially important carbon compound with the functional group —OH is Ethanol with the molecular formula C₂H₅OH.
(ii)

NOTE: Name of the product for each reaction is given in bold letters under the reaction.

Ans: (a) The first two members of homologous series having functional group —Br. are CH₃Br, C₂H₅Br
(b) (i) Aldehyde (ii) Ketone
(c) When a 5% alkaline potassium permanganate solution is added drop by drop to warm ethanol in a test tube, the following is observed:

The purple colour of potassium permanganate fades, indicating a reaction.

The role of KMnO₄ in this reaction is as an oxidising agent.

OR
(c) When ethanol is heated at 443 K with excess concentrated H₂SO₄, the compound formed is Ethene. 

The role of concentrated H₂SO₄ is as a dehydrating agent. 
The chemical equation for this reaction is:

Ans: (a) Aldehyde (—CHO) group.
(b) C_nH_2nO
(c) Compound (i) is propanal, and compound (ii) is ethanal. They belong to the same homologous series where each successive compound differs from each other by a—CH₂ unit.
Other member of same homologous series:

Ans: (i) The organic compound X with two carbon atoms and the suffix -ol is Ethanol.

(ii) When X is heated with excess concentrated sulphuric acid at 443 K, it undergoes dehydration to form ethene. The reaction can be represented as:

Ans:

Ans: (i) Aldehyde: Propanal
(ii) Ketone: Propanone
​

Ans: (i) The process in which compounds of carbon react with oxygen to give carbon dioxide, water, heat and light, is known as combustion. Alkanes burn in air and release large amount of heat, therefore can be used as excellent fuels.
(ii) Oxidation is a process in which oxygen is added to a substance.
(iii) Hydrogenation means addition of hydrogen to an unsaturated compound.
(iv) When alcohol is added to carboxylic acid in the presence of acid catalyst then, a fruity smelling ester is formed. This process is called esterification.
(v) Esters react in the presence of an acid or a base to give the alcohol and carboxylic acid. This reaction is known as saponification because it is used in the preparation of soap.

Ans:

The products of the combustion reaction of a saturated hydrocarbon are:

• Carbon dioxide (CO₂)
• Water (H₂O)

Ans:

(i) Oxygen is added to ethanol to produce ethanoic acid. 

(ii) Alkaline potassium permanganate or Acidified potassium dichromate

(iii)

(iv) It is oxidation reaction while other is combustion reaction/ burning of ethanol is exothermic while other is endothermic.

Ans: (a) The first two members of homologous series having functional group —Br. are CH₃Br, C₂H₅Br
(b) (i) Aldehyde (ii) Ketone
(c) When a 5% alkaline potassium permanganate solution is added drop by drop to warm ethanol in a test tube, the following is observed:

The purple colour of potassium permanganate fades, indicating a reaction.

The role of KMnO₄ in this reaction is as an oxidising agent.

OR
(c) When ethanol is heated at 443 K with excess concentrated H₂SO₄, the compound formed is Ethene. 

The role of concentrated H₂SO₄ is as a dehydrating agent. 
The chemical equation for this reaction is:

Ans: (i) 5% solution of KMnO₄ is prepared by adding 5 g of KMnO₄ in 95 g of water.
(ii) Here alkaline KMnO₄ acts as an oxidising agent. It oxidises ethanol to ethanoic acid by donating nascent oxygen. If excess of KMnO₄ is added the purple colour will persist indicating no more alcohol is left and reaction stops.
(iii)

Ans: (a) (i) When ethanol is heated with cone. H₂SO₄ at 443 K, ethene is obtained due to dehydration of ethanol.
(ii) When 5 % alkaline KMn04 solution is added drop by drop to warm ethanol then it gets oxidised to ethanoic acid.
(b) Addition reactions: Those reactions in which atoms or group of atoms are simply added to a double or triple bond without the elimination of any atom or molecule, are known as addition reactions.
Substitution reactions: The reactions which involve the displacement or substitution of an atom or a group of atoms in an organic compound by another atom or group of atoms, are known as substitution reactions. Saturated hydrocarbons are fairly unreactive and inert in the presence of most of the reagents. However, in presence of sunlight, hydrocarbons undergo rapid substitution reactions. e.g.,

Ans: When a 5% alkaline potassium permanganate solution is added drop by drop to warm propanol, a chemical reaction occurs:

• The propanol is oxidised to propanoic acid.
• In this process, the alkaline potassium permanganate acts as the oxidising agent.

The chemical equation for this reaction is:

Ans: (A) Ethanol (C₂H₅OH) belongs to alcohol's homologous series. 
A- Ethanol (C₂H₅OH) 
B- Ethene (C₂H₄) 
C- Ethane (C₂H₆) 
(B) Dehydration occurs when ethanol is heated with concentrated sulphuric acid at 443 K, resulting in the formation of ethene Concentrated sulphuric acid serves as a dehydrating agent in this reaction.
(C) Compound C is ethane. When it undergoes combustion, it forms CO₂ and water.
(D) Hydrogenation reactions are used in the production of saturated vegetable ghee from unsaturated vegetable oils. 
(E) Sodium ethoxide is formed when ethanol (C₂H₅OH) reacts with sodium (Na). The reaction is as follows:

Ans: (i) Carboxylic acids react with alcohols in the presence of a little concentrated sulphuric acid to form pleasant smelling esters. This reaction is called esterification reaction.
(ii) Those reactions in which atoms or group of atoms are simply added to a double or triple bond without the elimination of any atom or molecule, are known as addition reactions.

Ans: (b)
Assertion (A): Esterification is a process in which a sweet-smelling substance is produced. This is true. Esterification is a chemical reaction between an acid (usually a carboxylic acid) and an alcohol, resulting in the formation of an ester, which typically has a fruity or sweet smell.
Reason (R): When esters react with sodium hydroxide, an alcohol and sodium salt of carboxylic acid are obtained. This is also true. This reaction is known as saponification, where an ester reacts with a strong base to produce an alcohol and the salt of a carboxylic acid.
Since both statements are true, but the reason does not explain the assertion (as esterification and saponification are different processes), the correct answer is (b) Both (A) and (R) are true, and (R) is not the correct explanation of (A).

  Structural isomers are compounds that have the same molecular formula but different structures. Two isomers of butane (C₄H₁₀) are:

Ans: (c)
The molecular formula C₄H₁₀ represents butane and its isomer isobutane (also called 2-methylpropane).
Structures (I) and (II)in the image represent these two structural isomers of C₄H₁₀:
(I) shows a straight chain structure (n-butane).
(II) shows a branched structure (isobutane or 2-methylpropane).
Thus, the correct answer is (c) (I) and (II).

Ans: The electron dot structure of ethe is as follows: 

Ans: (a) Formation of oxygen molecule:

(b) Formation of nitrogen molecule:

Ans: 

Ans: The molecules of soap are sodium or potassium salts of long-chain carboxylic acids. Detergents are generally ammonium or sulphonate salts of long chain carboxylic acids.
Difference between Soap and Detergent
Soaps:
(i) Soaps are sodium salts of long chain carboxylic acids.
(ii) The ionic group in soap is -COO^-Na⁺.
(iii) Soaps are not useful when water is hard.
Detergents
(i) Detergents are the odium salt of long chain benzene sulphonic acids.
(ii) The ionic groups in detergents is SO₃^-Na⁺ or SO₄^-Na⁺.
(iii) Detergent can be used for washing purpose even when water is hard
Cleansing action of soap can be described as follows:
The soap molecule is generally represented as RCOONa. In solution, it ionises to form RCOO^- and Na⁺. Each soap molecule has a polar head group (carboxylate ion, COO^- group) and a long nonpolar hydrocarbon tail (R group from long chain fatty acid). The polar head attracts the polar water molecule and is called hydrophilic end and the nonpolar tail attracts the water-insoluble oily or greasy dirt particles.

Ans: (a)
Soap forms lather easily in soft water, while hard water (which contains calcium and magnesium ions) reacts with soap to form scum, preventing the formation of lather.
In this case:
(i) Test tube P contains Na₂SO₄ (sodium sulfate), and Test tube Q contains K₂SO₄ (potassium sulfate). Both sodium and potassium salts do not contribute to water hardness, so they will allow soap to form lather.
(ii) Test tube R contains CaSO₄ (calcium sulfate), and Test tube S contains MgSO₄ (magnesium sulfate). Both calcium and magnesium contribute to water hardness, which prevents lather formation.
Therefore, a good amount of lather will be obtained in test tubes P and Q.

Ans: (d)
In this scenario:

• Sample C is kept at 95 ºC, which is close to boiling temperature. At this high temperature, the temporary hardness will be significantly reduced due to the decomposition of bicarbonates, allowing more lather to form when soap is added.
• Samples A (at room temperature) and B (at 50 ºC) will still retain more temporary hardness than sample C, and therefore will produce less lather.

Thus, sample C will give the maximum amount of lather, making the correct answer (d) C only.

Ans: A micelle is a tiny cluster of molecules with non-polar groups inside and hydrophilic groups outside. When soap is added to water:

• The hydrophobic tails of soap molecules avoid water, forming a cluster.
• The hydrophilic heads remain in contact with water, stabilising the structure.

If ethanol is used instead of water, micelles will not form because ethanol lacks the necessary polar characteristics.The formation of micelles helps clean oily spots on clothes by:

• Trapping oily dirt in the centre of the micelle.
• Allowing the dirt to be easily rinsed away due to the colloidal nature of micelles.

Thus, soap effectively removes grease and dirt from fabrics.