CBSE Class 10 Science Carbon and its compound Notes

CHAPTER - 4

CARBON AND ITS COMPOUNDS

CARBON: INTRODUCTION

Carbon is the fourth most abundant element in the universe by mass. It is also the second most abundant element in the human body after oxygen. It is the 15th most common element in the Earth's crust. Carbon was discovered in prehistory and was known to the ancients. They used to manufacture charcoal by burning organic material.

Carbon is a non-metal. It belongs to group 14 (earlier called group IV A) of the modern periodic table. The elements of this group have four electrons in the valence shell.

Atomic Number: 6
Electronic configuration: 2, 4
Valence electrons: 4
Property: Non-metal

Compounds having carbon atoms among their components are known as carbon compounds. Earlier, carbon compounds could only be obtained from living sources; hence they are also known as organic compounds.

BONDING IN CARBON: COVALENT BOND

A bond formed by sharing of electrons is called a covalent bond. Two or more atoms share electrons to make their configuration stable. In this type of bond, all the atoms have equal rights over the shared electrons. Compounds formed because of covalent bonds are called COVALENT COMPOUNDS.

FORMATION OF COVALENT BONDS

1. A covalent bond is the chemical bond formed through the sharing of electrons between two non-metal atoms.
2. Compounds which have covalent bonds are called covalent compounds.
3. Examples of covalent compounds or molecules are chlorine (Cl₂), carbon dioxide (CO₂), ammonia (NH₃), water (H₂O), and tetrachloromethane (carbon tetrachloride, CCl₄).
4. During the formation of covalent molecules, each non-metal atom provides one, two or three electrons to be shared with other atoms. The bond formed is called a covalent bond.
5. Through this process, each non-metal atom in covalent molecules achieves a stable electron arrangement.
6. The type of covalent bond formed in a covalent compound depends on the number of electron pairs shared between non-metal atoms.

Single covalent bond: one pair of electrons is shared. Examples of single covalent compounds are chlorine gas (Cl₂), hydrogen chloride (HCl), water (H₂O), methane (CH₄), ammonia (NH₃) and tetrachloromethane (CCl₄). Single covalent bonds can also be formed between different non-metal atoms.

Formation of hydrogen chloride (HCl):
Valence electron of hydrogen = 1
Atomic number of chlorine = 17
Electronic configuration of chlorine: 2, 8, 7
Electrons in the outermost orbit of chlorine = 7
Valence electron of chlorine = 7

H (1 valence electron) shares one pair of electrons with Cl (7 valence electrons), so both atoms get a stable arrangement. H–Cl (HCl).

Formation of hydrogen molecule (H₂):
Atomic number of H = 1
Electronic configuration of H = 1
Valence electron of H = 1

Hydrogen forms a duet to obtain a stable configuration. This configuration is similar to helium (a noble gas). Since hydrogen has one electron in its valence shell, it requires one more electron to form a duet. So, in the formation of a hydrogen molecule, one electron from each hydrogen atom is shared.

Formation of chlorine molecule (Cl₂):
Valence electron of chlorine = 7. Each chlorine atom needs one more electron to complete its octet, so one pair of electrons is shared between the two chlorine atoms.

Formation of water (H₂O):
Valence electron of hydrogen = 1
Atomic number of oxygen = 8
Electronic configuration of oxygen = 2, 6
Valence electron of oxygen = 6

Formation of methane (CH₄):
Valence electron of carbon = 4
Valence electron of hydrogen = 1

DOUBLE COVALENT BOND

1. A double covalent bond is the type of covalent bond formed through the sharing of two pairs of electrons between two non-metal atoms.
2. Examples of molecules which have double covalent bonds are oxygen (O₂) and carbon dioxide (CO₂).
3. During the formation of a double bond, each non-metal atom contributes two electrons to be shared, so that each atom achieves a stable electron arrangement.

Formation of oxygen molecule (O₂):
Valence electron of oxygen = 6. Each oxygen atom needs two more electrons, so two pairs of electrons are shared. In total, four electrons are shared, two from each oxygen atom, and a double covalent bond is formed.

Formation of carbon dioxide (CO₂):
Valence electron of carbon = 4
Valence electron of oxygen = 6
In carbon dioxide, two double covalent bonds are formed.

TRIPLE COVALENT BOND

1. A triple covalent bond is the type of covalent bond formed through the sharing of three pairs of electrons between two non-metal atoms.
2. An example of a molecule which has a triple covalent bond is the nitrogen molecule (N₂).

Formation of nitrogen (N₂):
Atomic number of nitrogen = 7
Electronic configuration of nitrogen = 2, 5
Valence electron of nitrogen = 5
In the formation of nitrogen, three electrons are shared by each nitrogen atom. Thus one triple bond is formed because six electrons are shared in total.

Formation of ethyne (C₂H₂) and ethene (C₂H₄): the same sharing principle applies. Ethene has one double bond between the carbon atoms, and ethyne has one triple bond.

Properties of covalent bonds:

1. Intermolecular forces are small.
2. Covalent bonds are weaker than ionic bonds. As a result, covalent compounds have low melting and boiling points.
3. Covalent compounds are poor conductors of electricity because no charged particles are formed in a covalent bond.

Since carbon compounds are formed by covalent bonding, carbon compounds generally have low melting and boiling points and are poor conductors of electricity.

ALLOTROPY

Allotropy is the property by which an element can exist in more than one form that are physically different but chemically similar.

Allotropes of carbon

Carbon exists in three allotropic forms: crystalline forms (diamond and graphite), amorphous forms (coke and charcoal) and fullerene.

Diamond: Each carbon atom is bonded to four other carbon atoms, forming a rigid three-dimensional structure. This accounts for its hardness and rigidity.

General properties of diamond:
1. It is a colourless transparent substance with extraordinary brilliance due to its high refractive index.
2. It is quite heavy.
3. It is extremely hard (the hardest natural substance known).
4. It does not conduct electricity because of the absence of free electrons.
5. It has high thermal conductivity and a high melting point.
6. It burns on strong heating to form carbon dioxide.

Diamond: each carbon atom is bonded to four other carbon atoms by covalent bonds, forming a rigid three-dimensional structure.

Graphite: Each carbon atom is bonded to three other carbon atoms in the same plane, giving hexagonal layers. The layers are held together by weak van der Waals forces, which account for its softness.

General properties of graphite:
1. It is a greyish black opaque substance.
2. It is lighter than diamond and feels soft and slippery to touch.
3. It is a good conductor of electricity (due to the presence of free electrons) but a bad conductor of heat.
4. It burns on strong heating to form carbon dioxide.

Graphite: hexagonal layers of carbon atoms joined by covalent bonds; the layers are held together by weak binding forces.

Fullerenes: Fullerenes form another type of carbon allotrope. The first one identified contained 60 carbon atoms (C-60) arranged in the shape of a football. Since this shape looks like the geodesic dome designed by the US architect Buckminster Fuller, it is named buckminsterfullerene.

General properties of fullerenes:
1. These are dark solids at room temperature.
2. They are neither too hard nor too soft.
3. They are the purest allotropic forms of carbon because of the absence of free valencies or surface bonds.
4. On burning, they produce only carbon dioxide gas.

VERSATILE NATURE OF CARBON

Initially, compounds of carbon could only be obtained from living sources, and there was no way of synthesizing them. Hence, carbon compounds are also known as organic compounds. Carbon forms a very large number of compounds. So far, the formulae of about 3 million carbon compounds are known.

Reasons for the large number of carbon compounds:

1. Carbon can bond with other carbon atoms. This property is called CATENATION. Because of catenation, carbon can form long chains by bonding with other carbon atoms. Carbon can make single, double and triple bonds through catenation.
2. Carbon can form branched chains as well as straight chains through catenation.
3. Because of its valency of four, carbon can bond with four other carbon atoms or with atoms of other monovalent elements. It also forms compounds with oxygen, nitrogen, sulphur, hydrogen and many other elements. These compounds have specific properties that depend on the element other than carbon present in the molecule.
4. The bonds that carbon forms with other elements are very strong, which makes these compounds very stable. The main reason is the small size of carbon. As a result, the shared pair of electrons is held tightly by the nucleus.

ORGANIC COMPOUNDS

The compounds of carbon, except its oxides, carbonates and hydrogen carbonate salts, are known as organic compounds. These compounds were initially extracted from natural substances, and it was believed that a vital force was necessary for their synthesis (vital force theory).

HYDROCARBONS

Compounds formed from hydrogen and carbon only are known as hydrocarbons. They are regarded as the parent organic compounds, and all other organic compounds are considered to be derived from them by replacing one or more hydrogen atoms with other atoms or groups of atoms.

Hydrocarbons are divided into classes as shown below.

• Hydrocarbons
  • Aliphatic or open chain compounds
    • Saturated: alkanes (paraffins), C_nH_2n+2
    • Unsaturated
      • Alkenes (olefins), C_nH_2n
      • Alkynes, C_nH_2n-2
  • Cyclic or closed chain compounds
    • Alicyclic
    • Aromatic

ALIPHATIC HYDROCARBONS

The word aliphatic is derived from the Greek word aleiphar, meaning fat. Aliphatic hydrocarbons were so named because they were derived from fats and oils. They can be acyclic compounds, which are straight-chain or branched-chain compounds, or cyclic compounds, which have rings of carbon atoms.

AROMATIC HYDROCARBONS

The word aromatic is derived from the word aroma, meaning fragrance. Aromatic compounds have a characteristic smell. Structurally, they include benzene and its derivatives.

Aliphatic hydrocarbons are divided into two categories: saturated hydrocarbons and unsaturated hydrocarbons. In saturated hydrocarbons, carbon atoms are linked to each other by single bonds. In unsaturated hydrocarbons, multiple bonds (double or triple bonds) are present between carbon atoms.

SATURATED HYDROCARBONS: ALKANES

General formula = C_nH_2n+2. Suffix: ane.

Alkanes are organic compounds which contain a carbon-carbon single bond. They were earlier named paraffins (Latin: little affinity) because of their low chemical reactivity. According to the IUPAC system, they are named alkanes (the suffix "ane" is added to the root word).

UNSATURATED HYDROCARBONS

These are hydrocarbons which contain carbon-to-carbon double bonds or carbon-to-carbon triple bonds in their molecules. They are further classified into two types: alkenes and alkynes.

i) Alkenes: General formula C_nH_2n. Suffix: ene.

Hydrocarbons containing at least one carbon-to-carbon double bond are called alkenes. They were previously called olefins (Greek: olefiant, oil-forming), because the lower gaseous members of the family form oily products when treated with chlorine.

In the IUPAC system, the name of an alkene is derived by replacing the suffix "ane" of the corresponding alkane with "ene". In higher alkenes, the position of the double bond is indicated by assigning numbers 1, 2, 3, 4 and so on to the carbon atoms in the molecule.

For example: CH₃–CH₃ is ethane and H₂C=CH₂ is ethene.

Alkene	Common name	IUPAC name
CH2=CH2	Ethylene	Ethene
CH3CH=CH2	Propylene	Propene
CH3CH2–CH=CH2	α-Butylene	But-1-ene
CH3CH=CHCH3	β-Butylene	But-2-ene

ii) Alkynes: General formula C_nH_2n-2. Suffix: yne.

Hydrocarbons containing a carbon-to-carbon triple bond are called alkynes. Alkynes are named in the same way as alkenes, by replacing the suffix "ane" of the alkane with "yne". In higher members, the position of the triple bond is indicated by numbers 1, 2, 3, 4 and so on.

Alkyne	Common name	IUPAC name
HC≡CH	Acetylene	Ethyne
H3C–C≡CH	Methyl acetylene	Propyne
H3C–C≡C–CH3	Dimethyl acetylene	But-2-yne
H3C–CH2–C≡CH	Ethyl acetylene	But-1-yne

HOMOLOGOUS SERIES

A homologous series is a group or class of organic compounds having similar structure and similar chemical properties, in which successive compounds differ by a CH₂ group.

Characteristics of a homologous series:
1. Each member differs from the preceding or succeeding member by a common difference of CH₂ and by a molecular mass of 14 amu (amu = atomic mass unit).
2. All members contain the same elements and the same functional group.
3. All members have the same general molecular formula. Alkane = C_nH_2n+2; Alkene = C_nH_2n; Alkyne = C_nH_2n-2.
4. The members show a regular gradation in their physical properties with increasing molecular mass.
5. The chemical properties of the members are similar.
6. All members can be prepared by the same general method.

Importance of homologous series:

1. It helps to predict the properties of members of the series that are yet to be prepared.
2. It gives a systematic study of the members.
3. The nature of any member can be predicted if the properties of the first member are known.

FUNCTIONAL GROUP

A functional group is an atom or group of atoms, or a reactive part, which is responsible for the characteristic properties of a compound. The chemical properties of organic compounds are determined by the functional group, while their physical properties are largely determined by the rest of the molecule.

CLASSIFICATION OF ORGANIC COMPOUNDS BASED ON FUNCTIONAL GROUP

1. ALCOHOLS: Alcohols contain the -OH group attached to an alkyl group. The general formula is R-OH, where R is an alkyl group. The IUPAC name is derived by replacing the "e" of alkane with "ol", giving alkanol.

Molecular formula	Common name	IUPAC name
CH3OH	Methyl alcohol	Methanol
CH3-CH2-OH	Ethyl alcohol	Ethanol
CH3-CH2-CH2-OH	n-Propyl alcohol	1-Propanol
CH3-CH(OH)-CH3	Isopropyl alcohol or secondary propyl alcohol	2-Propanol
CH3-CH2-CH2-CH2-OH	n-Butyl alcohol	1-Butanol
CH3-CH(CH3)-CH2-OH	Isobutyl alcohol	2-Methyl-1-propanol

2. ALDEHYDES: Aldehydes contain the -CHO group attached to an alkyl group or a hydrogen atom. The general formula is R-CHO. The IUPAC name is derived by replacing the "e" of alkane with "al", giving alkanal.

Molecular formula	Common name	IUPAC name
HCHO	Formaldehyde	Methanal
CH3-CHO	Acetaldehyde	Ethanal
CH3-CH2-CHO	Propionaldehyde	Propanal
CH3-CH2-CH2-CHO	Butyraldehyde	Butanal

3. KETONES: Ketones contain the carbonyl group (-CO-) attached to two alkyl groups. The general formula is R-CO-R'. The IUPAC name is derived by replacing the "e" of alkane with "one", giving alkanone.

Molecular formula	Common name	IUPAC name
CH3COCH3	Dimethyl ketone (Acetone)	Propanone
CH3COCH2CH3	Ethyl methyl ketone	Butanone
CH3CH2COCH2CH3	Diethyl ketone	3-Pentanone

4. CARBOXYLIC ACIDS: Carboxylic acids contain the -COOH group attached to a hydrogen atom or an alkyl group. The general formula is R-COOH. The IUPAC name is derived by replacing the "e" of alkane with "oic acid", giving alkanoic acid.

Molecular formula	Common name	IUPAC name
HCOOH	Formic acid	Methanoic acid
CH3-COOH	Acetic acid	Ethanoic acid
CH3-CH2-COOH	Propionic acid	Propanoic acid
CH3-CH2-CH2-COOH	n-Butyric acid	Butanoic acid

ISOMERISM

Carbon compounds with the same molecular formula can have different structures and therefore different properties. This phenomenon is called isomerism, and the compounds are called isomers.

For example, two arrangements are possible for butane (C₄H₁₀), an alkane with four carbon atoms. Such a pair of isomers is called chain isomers, and the isomerism is called chain isomerism. Chain isomers are compounds that have the same molecular formula but differ in the arrangement of the carbon chain.

Straight chain structure: CH₃–CH₂–CH₂–CH₃ (each carbon carries enough H atoms to have four bonds).
Branched chain structure: CH₃–CH(CH₃)–CH₃ (a central carbon bonded to three CH₃ groups and one H).

NOMENCLATURE OF CARBON COMPOUNDS

In general, the names of organic compounds are based on the name of the basic carbon chain, modified by a prefix (before the root) or a suffix (after the root) showing the functional group.

Steps to name an organic compound:
Step 1: Count the number of carbon atoms in the compound and write the root word for it. (Root words for up to 10 carbon atoms are tabulated below.)
Step 2: If the compound is saturated, add the suffix "ane" to the root word. If it is unsaturated, add "ene" for a double bond and "yne" for a triple bond.
Step 3: If a functional group is present, indicate it by adding its suffix.

For example, CH₃CH₂CH₃ has three carbon atoms, so the root word is "prop", and it has only single bonds, so the suffix is "ane". The name is propane. The compound CH₃CH=CH₂ is named propene, because the suffix "ene" is used for a double bond.

The prefixes "iso" and "neo" represent the presence of one or two carbon atoms respectively as a side chain.

If the functional group is named as a suffix, the final "e" of alkane (or alkene or alkyne) is replaced by the appropriate suffix.

If the functional group and substituents are not on the first carbon, their location is indicated by the digits 1, 2, 3 and so on.

INTEXT QUESTIONS PAGE NO. 68

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

Answer:
Pentane (C₅H₁₂) has a skeleton of five carbon atoms. It can exist as a straight chain and as two branched chains. Hence there are three structural isomers: n-pentane, isopentane (2-methylbutane) and neopentane (2,2-dimethylpropane).

n-Pentane: CH₃–CH₂–CH₂–CH₂–CH₃
2-Methylbutane: CH₃–CH(CH₃)–CH₂–CH₃
2,2-Dimethylpropane: C(CH₃)₄ (a central carbon bonded to four CH₃ groups)

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

Answer:
(i) Catenation: the ability of carbon atoms to form bonds with other carbon atoms.
(ii) Tetravalency: with a valency of four, carbon is capable of bonding with four other atoms.

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

Answer:
General formula of cycloalkanes = C_nH_2n. For cyclopentane, n = 5, so the formula is C₅H₁₀.

[Figure: Electron dot structure of cyclopentane]

Each line represents one shared pair of electrons: each C forms 2 C–C bonds and 2 C–H bonds.

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: CH₃COOH, i.e. H–C(H₂)–C(=O)–OH
(ii) Bromopentane: CH₃CH₂CH₂CH₂CH₂Br
(iii) Butanone: CH₃CH₂C(=O)CH₃
(iv) Hexanal: CH₃CH₂CH₂CH₂CH₂CHO

Yes, structural isomers of bromopentane are possible, for example 1-bromopentane (CH₃CH₂CH₂CH₂CH₂Br) and 2-bromopentane.

Isomers of bromopentane: (i) 1-bromopentane, CH₃CH₂CH₂CH₂CH₂Br; (ii) 2-bromopentane, CH₃CH₂CH₂CHBrCH₃; (iii) 3-bromopentane, CH₃CH₂CHBrCH₂CH₃.

5. How would you name the following compounds? (a) CH₃-CH₂-Br (b) H-CHO (c) CH₃CH₂CH₂CH₂C≡CH

Answer:
(a) Bromoethane (for two carbons, the root word is "eth").
(b) Methanal, also called formaldehyde (for one carbon, the root word is "meth").
(c) 1-hexyne (for six carbons, the root word is "hex", and for a triple bond the suffix is "yne").

The compounds in the question: (a) CH₃–CH₂–Br; (b) H–CHO (H–C(H)=O); (c) CH₃CH₂CH₂CH₂C≡CH.

CHEMICAL PROPERTIES OF CARBON COMPOUNDS

COMBUSTION

All carbon compounds burn in oxygen to yield carbon dioxide and water vapour. Heat and light are also released. This reaction is called combustion.

(i) C + O₂ → CO₂ + heat and light
(ii) CH₄ + O₂ → CO₂ + H₂O + heat and light
(iii) CH₃CH₂OH + O₂ → CO₂ + H₂O + heat and light

Once carbon and its compounds ignite, they keep burning without the need for additional energy. That is why these compounds are used as fuels.

Saturated hydrocarbons give a clean blue flame due to complete combustion. Unsaturated hydrocarbons give a yellow, sooty flame with lots of black smoke, because they do not undergo complete combustion.

OXIDATION

Oxidation is a process of gain of oxygen or loss of hydrogen. Substances which are capable of providing oxygen to other substances are called oxidising agents. For example, alkaline KMnO₄ and acidified K₂Cr₂O₇ can both act as oxidising agents.

CH₃CH₂OH —[acidic K₂Cr₂O₇ + Δ / alk. KMnO₄ + Δ]→ CH₃COOH + H₂O

ADDITION REACTION

A reaction in which a reagent adds completely to a substance without the removal of any small molecule is called an addition reaction. For example, the addition of hydrogen (in the presence of a catalyst such as palladium or nickel) to unsaturated hydrocarbons gives saturated hydrocarbons. This is called hydrogenation.

R₂C=CR₂ + H₂ —[Nickel catalyst]→ R₂CH–CHR₂ (i.e. R–CH(R)–CH(R)–R)

Hydrogenation of vegetable oils (unsaturated compounds) in the presence of a nickel catalyst gives vanaspati ghee (saturated compounds). This process is called hardening of oils.

Vegetable oils —[Ni catalyst, H₂, 200°C]→ Ghee

SUBSTITUTION REACTION

A reaction in which a reagent substitutes (replaces) an atom or a group of atoms in the reactant is called a substitution reaction. These are generally shown by saturated compounds and benzene.

Most saturated hydrocarbons are fairly inert and unreactive towards most reagents. So, sunlight is needed for their substitution reactions. When chlorine is added to hydrocarbons in the presence of sunlight, Cl replaces the H atoms one by one.

FUELS AND FLAMES

FUELS

Carbon compounds which store energy and burn with heat and light are called fuels. The released energy is used for cooking food, running machines in factories, and other purposes. In fuels, carbon can be in the free state, as in coal, coke and charcoal, or in the combined state, as in petrol, LPG, kerosene, petroleum and natural gas.

Fossil fuels were formed by the decomposition of the remains of prehistoric plants and animals buried under the earth long ago. Examples are coal, petroleum and natural gas.

COAL

Coal is a complex mixture of compounds of carbon, hydrogen and oxygen, with some free carbon along with traces of nitrogen and sulphur. It was formed by the decomposition of plants and trees buried under the earth millions of years ago.

PETROLEUM

Petroleum is a dark, viscous, foul-smelling oil, also known as rock oil or black gold. It was formed by the decomposition of the remains of extremely small plants and animals buried under the sea millions of years ago.

INTEXT QUESTIONS PAGE NO. 71

FLAME

A flame is the region where the combustion of gaseous substances takes place. Depending on the amount of oxygen available, flames are of two types:

(i) Blue or non-luminous flame: When the oxygen supply is sufficient, fuels burn completely and produce a blue flame. Since no light is produced, the flame is called non-luminous, for example the burning of LPG in a gas stove.
(ii) Yellow or luminous flame: When the air supply is insufficient, fuels burn incompletely and produce a yellow flame. The yellow colour is due to unburnt carbon particles. This flame produces light, so it is called luminous, for example the burning of wax vapour.

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

Answer:
The conversion of ethanol to ethanoic acid involves the addition of oxygen to ethanol, so it is an oxidation reaction.

CH₃CH₂OH + O₂ → CH₃C(=O)–OH + H₂O

2. A mixture of oxygen and ethyne is burnt for welding. Why is a mixture of ethyne and air not used?

Answer:
When ethyne is burnt in air, it gives a sooty flame due to incomplete combustion, caused by the limited supply of air. When ethyne is burnt with oxygen, it gives a clean flame at about 3000°C due to complete combustion. This oxy-acetylene flame is used for welding. Such a high temperature cannot be reached without mixing oxygen. This is why a mixture of ethyne and air is not used.

2CH≡CH + 5O₂ → 4CO₂ + 2H₂O + heat + light

SOME IMPORTANT CARBON COMPOUNDS: ETHANOL AND ETHANOIC ACID

Most fuels, medicines, paints, explosives, synthetic polymers, perfumes and detergents are basically organic compounds. Organic chemistry has made our life colourful and comfortable. Two commercially important compounds are ethanol and ethanoic acid.

ETHANOL (C₂H₅OH)

Ethanol, also called ethyl alcohol or simply alcohol, is one of the most important members of the alcohol family.

(1) Manufacture of ethanol from molasses

Molasses is a dark, syrupy liquid left after the crystallisation of sugar from concentrated sugarcane juice. Molasses still contains about 30% sucrose, which cannot be separated by crystallisation. It is converted into ethanol in the following steps:

(i) Dilution: Molasses is diluted with water to bring the sugar concentration down to about 8 to 10 percent.
(ii) Addition of ammonium salts: Molasses usually contains enough nitrogenous matter to feed the yeast during fermentation. If the nitrogen content is poor, it is fortified by adding ammonium sulphate or ammonium phosphate.
(iii) Addition of yeast: The solution from step (ii) is collected in large fermentation tanks and yeast is added. The mixture is kept at about 303 K for a few days. During this period, the enzymes invertase and zymase present in yeast convert sucrose into ethanol. The fermented liquid is called wash.
(iv) Distillation of wash: The fermented liquid, containing 15 to 18 percent alcohol and the rest water, is subjected to fractional distillation. The main fraction is an aqueous solution of ethanol containing 95.5% ethanol and 4.5% water, called rectified spirit. This is heated under reflux over quicklime for 5 to 6 hours and then allowed to stand for 12 hours. On distillation, pure alcohol (100%) is obtained. This is called absolute alcohol.

Reactions in step (iii):
C₁₂H₂₂O₁₁ + H₂O —[invertase]→ C₆H₁₂O₆ + C₆H₁₂O₆ (sucrose → glucose + fructose)
C₆H₁₂O₆ —[zymase]→ 2C₂H₅OH + 2CO₂↑ (glucose or fructose → ethanol)

FERMENTATION is the slow chemical change in an organic compound, brought about by enzymes, that leads to the formation of smaller molecules.

PROPERTIES OF ETHANOL

Physical properties:
(i) Ethanol is a clear liquid with a burning taste.
(ii) Its boiling point is 351 K, which is higher than that of the corresponding alkane.
(iii) It is completely miscible with water in all proportions.

Chemical properties:

(i) Dehydration
(a) Intramolecular dehydration: when ethanol is heated with excess concentrated H₂SO₄ at 443 K, water is removed from within a single molecule of ethanol, forming ethene.
(b) Intermolecular dehydration: when excess ethanol is heated with concentrated H₂SO₄ at 413 K, two molecules condense by losing one molecule of water to form ether.

(a) CH₃CH₂OH (ethanol) —[conc. H₂SO₄, 443 K]→ CH₂=CH₂ (ethene) + H₂O
(b) C₂H₅–OH + HO–C₂H₅ —[conc. H₂SO₄, 413 K]→ C₂H₅–O–C₂H₅ (diethyl ether) + H₂O

(ii) Reaction with sodium: ethanol reacts with sodium metal to form sodium ethoxide and hydrogen gas.

2C₂H₅OH + 2Na → 2C₂H₅ONa (sodium ethoxide) + H₂↑

(iii) Oxidation: ethanol is oxidised to ethanoic acid by alkaline KMnO₄ or acidified K₂Cr₂O₇. During this reaction, the orange colour of K₂Cr₂O₇ changes to green. Therefore, this reaction can be used to identify alcohols.

CH₃CH₂OH —[Oxidation]→ CH₃COOH (ethanoic acid) + H₂O

(iv) Esterification: ethanol reacts with ethanoic acid in the presence of concentrated H₂SO₄ (catalyst) to form ethyl ethanoate and water. The compound formed by the reaction of an alcohol with a carboxylic acid is called an ester, which has a fruity smell. The reaction is called esterification.

C₂H₅OH (ethanol) + CH₃COOH (ethanoic acid) —[conc. H₂SO₄]→ CH₃COOC₂H₅ (ethyl ethanoate) + H₂O

(v) Dehydrogenation: when ethanol vapour is passed over reduced copper at 573 K, it is dehydrogenated to acetaldehyde.

CH₃CH₂OH (ethanol) —[Cu, 573 K]→ CH₃CHO (acetaldehyde) + H₂

USES OF ETHANOL

1. As an anti-freeze in automobile radiators.
2. As a preservative for biological specimens.
3. As an antiseptic to sterilise wounds in hospitals.
4. As a solvent for drugs, oils, fats, perfumes, dyes and similar substances.
5. In the preparation of methylated spirit (95% ethanol and 5% methanol), rectified spirit (95.5% ethanol and 4.5% water), power alcohol (a mixture of petrol and ethanol) and denatured spirit (ethanol mixed with pyridine).
6. In cough and digestive syrups.

EVIL EFFECTS OF CONSUMING ALCOHOL

1. Ethanol slows down the metabolism of the body and depresses the central nervous system.
2. It causes mental depression and emotional disorders.
3. It causes health problems such as ulcers, high blood pressure, cancer, and brain and liver damage.
4. Nearly 40% of accidents are caused by drunk driving.
5. Methanol, unlike ethanol, can cause death even in very small quantities. Methanol is oxidised to methanal (formaldehyde) in the liver. Methanal reacts rapidly with the components of cells and causes the protoplasm to coagulate, in the same way an egg coagulates on cooking. Methanol also affects the optic nerve and causes blindness.

ETHANOIC ACID (CH₃COOH)

Ethanoic acid is most commonly known as acetic acid. It belongs to the group of acids called carboxylic acids. Acetic acid is present in many fruits, and the sour taste of fruits is due to this acid.

Preparation of ethanoic acid: Ethanol on oxidation in the presence of alkaline potassium permanganate or acidified potassium dichromate gives ethanoic acid.

CH₃CH₂OH (ethanol) —[Oxidation]→ CH₃COOH (ethanoic acid) + H₂O

Physical properties:
(i) Ethanoic acid is a colourless liquid with a sour taste.
(ii) It is miscible with water in all proportions.
(iii) Its boiling point (391 K) is higher than that of the corresponding alcohols, aldehydes and ketones.
(iv) On cooling, pure ethanoic acid freezes into ice-like flakes that look like glaciers. It is therefore called glacial acetic acid.

Chemical properties:

(i) Ethanoic acid is a weak acid, but it turns blue litmus red.
(ii) Reaction with metals: it reacts with metals such as Na, K and Zn to form metal ethanoate and hydrogen gas.
(iii) Reaction with carbonates and bicarbonates: it produces brisk effervescence due to the evolution of carbon dioxide.
(iv) Reaction with a base: it reacts with sodium hydroxide to form sodium ethanoate and water.
(v) Decarboxylation (removal of CO₂): when the sodium salt of ethanoic acid is heated with soda lime (a solid mixture of 3 parts NaOH and 1 part CaO), methane gas is formed.

2CH₃COOH + Zn → (CH₃COO)₂Zn + H₂↑
2CH₃COOH + 2Na → 2CH₃COONa + H₂↑
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂↑ + H₂O
CH₃COOH + NaHCO₃ → CH₃COONa + CO₂↑ + H₂O
CH₃COOH + NaOH → CH₃COONa + H₂O
CH₃COONa —[NaOH / CaO]→ CH₄↑ + Na₂CO₃

USES OF ETHANOIC ACID

1. For making vinegar, which is used as a preservative in food and fruit juices.
2. As a laboratory reagent.
3. For coagulating rubber from latex.
4. In the preparation of dyes, perfumes and medicines.

INTEXT QUESTIONS PAGE NO. 74

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

Answer:
Sodium bicarbonate test (NaHCO₃ test):
Alcohol + NaHCO₃ → no effervescence
Acid + NaHCO₃ → brisk effervescence
The sample that produces brisk effervescence with NaHCO₃, due to the release of CO₂, is a carboxylic acid.

2. What are oxidising agents?

Answer:
Substances which are capable of providing oxygen to other substances are called oxidising agents. For example, alkaline KMnO₄ and acidified K₂Cr₂O₇ can both act as oxidising agents.

SOAPS AND DETERGENTS

Most dirt is oily in nature, and oil does not dissolve in water. Soap molecules are sodium or potassium salts of long-chain carboxylic acids. The ionic end of soap dissolves in water, while the carbon chain dissolves in oil. Soap molecules form structures called micelles, in which the carbon chains point towards the oil droplet and the ionic ends face outwards. This forms an emulsion in water. The soap micelle helps to dissolve the dirt in water, so that clothes can be washed clean.

MICELLES

Soap molecules have two ends with different properties: one end is hydrophilic (it dissolves in water) and the other is hydrophobic (it dissolves in hydrocarbons). When soap is at the surface of water, its hydrophobic tail is not soluble in water, so the soap aligns along the surface with the ionic end in the water and the hydrocarbon tail sticking out of the water. Inside water, the molecules form clusters in which the hydrophobic tails are in the interior and the ionic ends are on the surface. This formation is called a micelle.

A soap micelle cleans because oily dirt collects in the centre of the micelle. The micelles stay in solution as a colloid and do not come together to precipitate, because of ion-ion repulsion. Thus the dirt suspended in the micelles is easily rinsed away. Soap micelles are large enough to scatter light, so a soap solution appears cloudy.

Soap molecule: a hydrophilic (ionic, Na⁺) end and a hydrophobic (hydrocarbon chain) end. In a micelle, the hydrophobic tails point inwards towards the oil droplet and the hydrophilic ionic ends (with Na⁺) face outwards into the water.

INTEXT QUESTIONS PAGE NO. 76

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

Answer:
No. Detergents are ammonium or sulphonate salts of long-chain carboxylic acids. Unlike soap, they do not react with calcium and magnesium ions in hard water to form scum. They give a good amount of lather in both soft and hard water. So detergents cannot be used to check whether water is hard.

2. Why is agitation necessary to get clean clothes?

Answer:
A soap molecule has a hydrophobic and a hydrophilic part. With their help, soap attaches to the grease or dirt particles and forms a cluster called a micelle. These micelles remain suspended as a colloid. To remove the micelles that trap the dirt, the clothes must be agitated.

EXERCISE QUESTIONS PAGE NO. 77 AND 78

1. 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. Ethane has 6 C-H bonds and 1 C-C bond, which is 7 covalent bonds in total.

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

Answer: (c) ketone. Butanone contains the carbonyl (-CO-) group, which is a ketone.

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 unburnt particles of the fuel present in the smoke blacken the outside of the vessel.

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

Answer:
Atomic numbers: C = 6, H = 1, Cl = 17
Electronic configuration: C = 2, 4; H = 1; Cl = 2, 8, 7
Carbon needs 4 electrons to complete its octet, each hydrogen needs 1 electron, and chlorine needs 1 electron. Carbon shares its 4 electrons, one with each of the 3 hydrogen atoms and one with the chlorine atom. It thus forms 4 covalent bonds.

H–C(H₂)–Cl: each line is a shared pair (C has 4 bonds; Cl keeps 3 lone pairs).

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

Answer:
[Figure: Electron dot structures of ethanoic acid, H₂S, propanone and F₂]

(a) Ethanoic acid: each line below is a shared pair of electrons.

(c) Propanone, CH₃COCH₃: each line is a shared pair of electrons.

(b) H₂S (H–S–H): S shares one electron with each H; S keeps two lone pairs.

(d) F₂ (F–F): one shared pair; each F has three lone pairs.

6. What is a homologous series? Explain with an example.

Answer:
A homologous series is a series of carbon compounds that have different numbers of carbon atoms but the same functional group. For example, the alkane series: CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane), C₄H₁₀ (butane), C₅H₁₂ (pentane). Each successive compound differs by a -CH₂ unit.

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

Answer:
I. Physical properties:
1. Smell: ethanoic acid has a pungent smell; ethanol has a pleasant smell.
2. Melting point: ethanol has a lower melting point (150 K) than ethanoic acid (290 K).
3. Physical state: ethanoic acid is solid (glacial acetic acid) in winter, but ethanol is always a liquid.
II. Chemical properties:
(i) Action with sodium hydrogen carbonate: ethanoic acid gives carbon dioxide with brisk effervescence. Ethanol does not react.
(ii) Action with caustic alkalies: ethanoic acid reacts with NaOH and KOH to form the corresponding salt and water. Ethanol does not react with either.

CH₃COOH + NaHCO₃ → CH₃COONa + CO₂↑ + H₂O
CH₃COOH + NaOH → CH₃COONa + H₂O
CH₃COOH + KOH → CH₃COOK + H₂O

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

Answer:
Soap is a sodium or potassium salt of long-chain fatty acids. It has one polar (hydrophilic) end and one non-polar (hydrophobic) end. When soap is added to water, the molecules cluster so that the non-polar ends are kept out of the water, inside the cluster, and the polar ends face the water on the surface. Dirt on clothes is organic and insoluble in water, so the hydrophobic ends attach to the dirt and trap it in the micelle. Micelles do not form in alcohol, because the alkyl chain of soap is soluble in alcohol.

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

Answer:
Carbon burns in oxygen (air) to form carbon dioxide and water, releasing a large amount of heat and light. Once ignited, carbon and its compounds keep burning without the need for additional energy. Hence they are used as fuels.

C + O₂ → CO₂ + heat + light

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

Answer:
Hard water contains salts of calcium and magnesium. When soap is added to hard water, the calcium and magnesium ions displace the sodium or potassium ions from the soap molecules. This forms an insoluble substance called scum, and a lot of soap is wasted in the process.

Ca²⁺ + 2RCOONa → (RCOO)₂Ca + 2Na⁺ (hard water + soap → calcium salt ppt.)
Mg²⁺ + 2RCOONa → (RCOO)₂Mg + 2Na⁺ (hard water + soap → magnesium salt ppt.)

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

Answer:
Soap is basic in nature, so it turns red litmus blue. Blue litmus remains blue.

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

Answer:
Hydrogenation is the addition of hydrogen to unsaturated hydrocarbons in the presence of palladium or nickel catalysts, which gives saturated hydrocarbons. Its industrial application is the hydrogenation of vegetable oils, which contain long chains of unsaturated carbon, to make vanaspati ghee.

R₂C=CR₂ + H₂ —[Nickel catalyst]→ R₂CH–CHR₂

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

Answer:
Unsaturated hydrocarbons containing a double or triple bond undergo addition reactions. So C₃H₆ and C₂H₂ undergo addition reactions.

Structures: C₂H₆: H₃C–CH₃; C₃H₆: CH₃–CH=CH₂; C₃H₈: CH₃–CH₂–CH₃; C₂H₂: H–C≡C–H; CH₄: H–C(H₂)–H.

14. Give a test that can be used to differentiate chemically between butter and cooking oil.

Answer:
Butter contains saturated compounds, while cooking oil contains unsaturated compounds. Unsaturated compounds decolourise alkaline KMnO₄. When a few drops of alkaline KMnO₄ are added to cooking oil, the pink colour disappears. With butter, the pink colour does not disappear.

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

Answer:
The dirt on clothes is organic and insoluble in water, so it cannot be removed by water alone. When soap dissolves in water, its hydrophobic ends attach to the dirt and lift it from the cloth. The soap molecules then arrange themselves in micelles that trap the dirt at the centre of the cluster. These micelles stay suspended in water, so the dirt is easily rinsed away.

 

CBSE Class 10 Science Chapter 4 Carbon and its compound Very Short Answer Type Questions 

Question. LPG (liquefied petroleum gas) is used as a fuel for cooking in homes. Why is LPG stored as a liquid?
Answer. LPG is stored as a liquid so that more fuel can be kept in a container. Liquids contain more particles per unit volume than gases. It is also easier to transport liquids than gases.

Question. Why carbon does not form the ionic compounds?
Answer. For formation of ionic compounds, carbon should either gain four electrons to form C^{4– i}ons or should lose four electrons to form C⁴⁺ ions but formation of both these ions is not possible due to high energy considerations.

Question. What are covalent compounds?
Answer. A covalent compound is a molecule formed by covalent bonds, in which the atoms share one or more pairs of valence electrons.

Question. Give different forms in which carbon occurs in nature.
Answer. Carbon occurs in free form e.g., graphite and diamond, in combined form like carbon dioxide, carbonates, etc.
In earth’s crust – 0.02% and in atmosphere – 0.03%.

Question. Write the molecular formula of first two members of homologous series having functional group —Cl.
Answer. The molecular formula of first two members of homologous series having –Cl functional group are CH₃Cl and CH₃CH₂Cl.

Question. Write the molecular formula of the 2nd and 3rd member of the homologous series whose first member is methane.
Answer. Methane, CH₄ is an alkane. Alkanes have general formula, C_nH_{2n + 2}. 2nd member of homologous series of alkanes is C₂H₆ i.e., ethane.
3rd member of homologous series of alkanes is C₃H₈ i.e., propane.

Question. Select saturated hydrocarbons from the following :
C₃H₆; C₅H₁₀; C₄H₁₀; C₆H₁₄; C₂H₄
Answer. Saturated hydrocarbons have general formula, C_nH_2n+2.Among the given compounds only C₄H₁₀ and C₆H₁₄ satisfy the above formula. Thus, these are saturated hydrocarbons.

Question. Write the name and formula of the 2nd member of homologous series having general formula C_nH_{2n – 2}.
Answer. General formula, C_nH_{2n + 2} belongs to alkyne series. The second member of this series is propyne i.e., (C₃H₄) or CH₃ — C ≡ CH.

Question. An organic compound burns with a sooty flame. Is it a saturated or an unsaturated compound?
Answer. It is an unsaturated compound. Unsaturated compounds have more percentage of carbon as compared to saturated compounds. Thus, unsaturated compounds undergo incomplete combustion in the presence of air producing a yellow flame with lots of black smoke.

 

CBSE Class 10 Science Chapter 4 Carbon and its compound Short Answer Type Questions 

Question. The molecular formulae of alkenes X and Y are CxH₈ and C₁₀Hy respectively. What are the relative molecular masses of X and Y?
Answer. The general formula of an alkene is C_nH_2n.
Thus, the molecular formula of X is C₄H₈, and the molecular formula of Y is C₁₀H₂₀.
Molecular mass of X = (4 × 12) + (8 × 1) = 56
Molecular mass of Y = (10 × 12) + (20 × 1) = 140

Question. What is homologous series of compounds?
List any two characteristics of homologous series.
Answer. A series of compounds having similar structures and similar chemical properties in which the successive member differs in their molecular formula by –CH₂ group is called homologous series.
Characteristics :
(a) All the members of homologous series have similar chemical properties.
(b) Any two consecutive members differ in their molecular formula by a –CH₂ group.

Question. A hydrocarbon molecule has 3 carbon atoms. Write down its molecular formula if it is an : (i) alkane (ii) alkene (iii) alkyne.
Answer. The number of carbon atoms in the molecule of this hydrocarbon is 3, that is, n = 3.
(i) The general formula of an alkane is C_nH_2n+2. On putting n = 3, we get C₃H_2×3+2 or C₃H₈ (propane).
(ii) For alkene having general formula C_nH_2n, we get,C₃H_{2 × 3} or C₃H₆ (propene).
(iii) For alkyne having general formula C_nH_2n–2 we get C₃H_{2 × 3 – 2} or C₃H₄ (propyne).

Question. Explain the nature of the covalent bond using the bond formation in CH₃Cl.
Answer. The molecule of chloromethane (CH₃Cl) consists of three elements i.e., carbon (Z = 6), hydrogen (Z = 1) and chlorine (Z = 17). Carbon atom has four valence electrons (2, 4) ; hydrogen has one (1) while chlorine has seven electrons in the valence shell (2, 8, 7). In order to complete its octet, carbon shares three valence electrons with three hydrogen atoms while one is shared with the electron of chlorine atom.

Question. State two properties of carbon which lead to a very large number of carbon compounds.
Answer. Carbon forms a large number of carbon compounds like long chains which may be straight or branched chains or ring of different sizes due to its tetravalency and unique property of catenation. Carbon due to its small size forms exceptionally stable compounds by forming strong bonds.

Question. Two non-metals A and B combine with each other by sharing of electrons to form compound C.
(a) What is the nature of compound C ?
(b) Will it dissolve in water or organic solvents?
(c) Will it be a good or bad conductor of electricity?
(d) Will it have high or low melting/boiling point ?
Answer. (a) Covalent
(b) Usually soluble in organic solvents
(c) Bad conductor
(d) Usually low melting / boiling point.

Question. (i) Which allotrope of carbon is used for making cores of lead pencils?
(ii) Which allotrope of carbon is used for making expensive jewellery?
(iii) Name an allotrope of carbon which contains both single and double bonds.
Answer. (i) Graphite (ii) Diamond (iii) Graphite

Question. What are covalent compounds? Why are they different from ionic compounds? List their three characteristic properties.
Answer. Covalent compounds are those compounds which are formed by sharing of valence electrons between the atoms e.g., Hydrogen molecule is formed by mutual sharing of electrons between two hydrogen atoms.
They are different from ionic compounds as ionic compounds are formed by the complete transfer of electrons from one atom to another e.g., NaCl is formed when one valence electron of sodium gets completely transferred to outer shell of chlorine atom.
The characteristic properties of covalent compounds are :
(i) They are generally insoluble or less soluble in water but soluble in organic solvents.
(ii) They have low melting and boiling points.
(iii) They do not conduct electricity as they do not contain ions.

Question. Why is homologous series of carbon compounds so called? Write the chemical formula of two consecutive members of any homologous series and state the part of these compounds that determines their (i) physical and (ii) chemical properties.
Answer. A homologous series is the family of organic compounds having the same functional group, similar chemical properties but the successive (adjacent) members of the series are differ by a CH₂ unit or 14 mass units.

Question. What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?
Answer. (i) Catenation : Carbon has the unique property of self linking which is known as catenation. In fact, any number of carbon atoms can be linked to one another by covalent bonds. This is on account of the stability of C — C bonds since the size of the carbon atom is quite small.
(ii) Linking of carbon with other atoms : Carbon is tetravalent in nature and can readily unite with atoms like hydrogen, oxygen, nitrogen, sulphur, etc. by electron sharing.

Question. What is a homologous series? Explain with an example.
Answer. A homologous series can be defined as a family of organic compounds having the same functional group, similar chemical properties and the successive members of which differ by a – CH₂ group or 14 mass units. For example, CH₃OH (methanol), CH₃CH₂OH (ethanol), CH₃CH₂CH₂OH (propanol), CH₃CH₂CH₂CH₂OH (butanol), etc. constitute a homologous series of alcohols. They have the same functional group, i.e., —OH (hydroxyl). Since they have the same functional group, they show similar chemical properties. The difference between any two successive members is a —CH2 group and 14 mass units. Their physical properties such as melting point and boiling point increase as the molecular mass increases. Their solubility in water, however, decreases with increase in molecular mass.

Question. (a) Why are most carbon compounds poor conductors of electricity?
(b) Write the name and structure of a saturated compound in which the carbon atoms are arranged in a ring. Give the number of single bonds present in this compound.
Answer. (a) Due to catenation, carbon forms covalent bonds with the constituent elements in the carbon compounds, hence it does not have mobile electrons and carbon compounds do not dissociate themselves into ions and hence, they are poor conductor of electricity.

Question. (a) What is a homologous series of compounds? List any two of its characteristics.
(b) What is the next higher homologue of C₃H₇OH? What is its formula and what is it called?
Answer. (a) A homologous series is the family of organic compounds having the same functional group, similar chemical properties but the successive (adjacent) members of the series are differ by a CH₂ unit or 14 mass units.
Two characteristics of homologous series are :
(i) The successive compounds of the homologous series differ by —CH₂ unit i.e. 14 mass units.
(ii) Each homologous series belongs to similar class of compounds which shows the same properties.
(b) Next higher homologue of C₃H₇OH is C₄H₉OH i.e., butanol.

Question. (a) Why two carbon atoms cannot be linked by more than three covalent bonds.
(b) Explain the following :
(i) Diamond is a covalent solid, yet has high melting point.
(ii) Graphite is a good conductor of electricity but diamond is not.
(iii) Diamond is used for making tools for cutting and drilling.
Answer. (a) Since the maximum angle strain is obtained when the two carbon atoms are linked by three covalent bonds, therefore, two carbon atoms cannot be linked to each other by more than three covalent bonds.
(b) (i) It is a giant molecule containing a large number of carbon-carbon single covalent bonds. To break these covalent bonds, a large amount of energy is needed and hence diamond has a high melting point.
(ii) Graphite has two-dimensional layered structure containing fused hexagonal rings of carbon atoms. Only three valence electrons are used for bond formation while the fourth electron is free to move.
Due to ability of this free electron to move, graphite is a good conductor of heat and electricity. Since in diamond, all the four electrons are involved in bond formation, therefore, diamond is a bad conductor of electricity.
(iii) Diamond is a good conductor of heat and is used for making cutting and drilling tools because the heat generated during cutting and drilling is easily absorbed by the network structure without overheating the diamond tool.

CBSE Class 10 Science Chapter 4 Carbon and its compound Case Based MCQs