Direct Board Fact — Abundance of Carbon: Despite its immense importance, the amount of carbon present in the earth's crust and atmosphere is extremely meager:
• Earth's crust: Contains only 0.02% carbon in the form of minerals (like carbonates, hydrogen carbonates, coal, and petroleum).
• Atmosphere: Contains only 0.03% carbon dioxide ($CO_2$).
Yet, all living structures (plants, animals, human bodies) and everyday materials (food, clothing, medicines, fuels, books, plastics) are based on carbon!
How to Confirm the Presence of Carbon in a Material:
When any carbon-containing organic compound is burnt in air, it produces Carbon Dioxide ($CO_2$) gas.
Passing the evolved gas through clear lime water turns the lime water milky, confirming carbon.
Incomplete combustion of organic materials leaves behind a black char of carbon.
Carbon at the centre of everyday life
Board Keywords: The millions of carbon compounds found in nature arise from two unique properties of carbon: tetravalency and catenation.
02
Electronic Configuration of Carbon & Why it Cannot Form $C^{4+}$ or $C^{4-}$
Direct Board Question (Classic 3-Marker):"Carbon has atomic number 6. Why does it not form ionic bonds by gaining 4 electrons ($C^{4-}$) or losing 4 electrons ($C^{4+}$)?"
Atomic Profile: Atomic Number $Z = 6$. Electronic Configuration: K shell = 2, L shell = 4 (2,4). To attain stable noble gas configuration (Octet like Neon 2,8 or Duplet like Helium 2):
Why Carbon cannot gain 4 electrons ($C^{4-}$):
If carbon were to gain 4 electrons, it would form a $C^{4-}$ anion.
It would be extremely difficult for a small nucleus containing only 6 protons to hold onto 10 electrons (6 original + 4 extra) due to massive inter-electronic repulsions.
Hence, formation of $C^{4-}$ is energetically impossible.
Why Carbon cannot lose 4 electrons ($C^{4+}$):
If carbon were to lose 4 electrons, it would form a $C^{4+}$ cation.
Removing 4 successive electrons requires an enormous amount of ionisation energy to overcome strong nuclear attraction.
This would leave behind a carbon cation with 6 protons holding just 2 electrons, which is highly unstable.
Direct Board Model Conclusion: Carbon overcomes this problem by sharing its 4 valence electrons with other carbon atoms or with atoms of other elements (like H, O, N, Cl). The bonds formed by sharing of electrons are called covalent bonds.
03
Covalent Bond — Nature & Types (Single, Double & Triple)
Direct Board Definition: A chemical bond formed by the mutual sharing of one or more pairs of valence electrons between two atoms is called a Covalent Bond.
Three Classes of Covalent Bonds:
Bond Type
Shared Electron Pairs
Symbol
Standard Board Example
Single Covalent Bond
1 pair (2 electrons)
—
Hydrogen molecule ($H_2$): $H \cdot + \cdot H \rightarrow H—H$
Board Exam Rule: The shared pair of electrons belongs to the valence shells of both participating atoms and leads to both attaining an octet/duplet noble gas configuration.
04
Step-by-Step Electron Dot Structures ($H_2, O_2, N_2, H_2O, NH_3$)
How to Draw Electron Dot Structures in Board Exams:
1. Write the valence electrons as dots or crosses around each atom.
2. Place shared pairs between the bonding atoms.
3. Enclose with intersecting circles showing 2 electrons for H (duplet) and 8 electrons for other atoms (octet).
High-Frequency Board Molecules:
1. Hydrogen Molecule ($H_2$): Hydrogen has atomic number 1 (K=1). Two H atoms share one pair:
H• + •H → H : H → H − H
2. Chlorine Molecule ($Cl_2$): Atomic number 17 (2,8,7). Two Cl atoms share one electron pair:
:Cl• + •Cl: → :Cl : Cl: → Cl − Cl
3. Oxygen Molecule ($O_2$): Atomic number 8 (2,6). Two O atoms share two pairs (Double Bond):
:O: + :O: → :O :: O: → O = O
4. Nitrogen Molecule ($N_2$): Atomic number 7 (2,5). Two N atoms share three pairs (Triple Bond):
:N••• + •••N: → :N ::: N: → N ≡ N
5. Water Molecule ($H_2O$): Oxygen shares one electron each with two Hydrogen atoms (leaves 2 lone pairs on O):
H : O : H → H − O − H (Angle shaped)
6. Ammonia Molecule ($NH_3$): Nitrogen shares one electron each with three Hydrogen atoms (leaves 1 lone pair on N):
H : N(:) : H with bottom H → Trigonal Pyramidal shape
05
Methane ($CH_4$) — Tetravalency of Carbon & Gas Properties
Direct Board Definition — Methane ($CH_4$): Methane is the simplest hydrocarbon and the first member of the alkane family. It is widely used as a fuel and is the major component of Compressed Natural Gas (CNG) and Biogas (up to 75%).
Structure of Methane: Carbon has 4 valence electrons and hydrogen has 1. Carbon shares each of its 4 electrons with 4 separate hydrogen atoms, forming 4 single C—H covalent bonds:
H − C(H)2 − H | 4 single covalent bonds around central carbon
Methane ($CH_4$) Electron Sharing
06
Why Covalent Compounds Have Low Melting/Boiling Points & Poor Conductivity
Direct 3-Mark Board Reasoning Question:"State two characteristic properties of covalent compounds and explain the reasons behind them."
Property
Observation
Scientific Board Reason
1. Low Melting & Boiling Points
Methane melts at $90\text{ K}$, Ethanol boils at $351\text{ K}$
Although the covalent bonds within molecules are strong, the intermolecular forces of attraction between separate molecules are very weak. A small amount of thermal energy easily overcomes them.
2. Poor Electrical Conductivity
Do NOT conduct electricity in solid, liquid, or aqueous form
In covalent compounds, electrons are shared between atoms and no free ions or free electrons are formed. Conduction requires mobile charge carriers.
Comparison Trap: Ionic compounds have high MP/BP because of strong electrostatic attraction between ions throughout the lattice. Covalent compounds have low MP/BP because intermolecular forces are weak van der Waals forces.
07
Allotropes of Carbon: Diamond, Graphite & Fullerenes ($C_{60}$)
Definition — Allotropy: The phenomenon in which an element exists in two or more different physical forms having identical chemical properties but vastly different physical properties due to different atomic arrangements.
Comparative Analysis of the Three Allotropes:
Property
Diamond
Graphite
Fullerene ($C_{60}$)
Structure & Bonding
Each carbon is bonded to 4 other carbon atoms in a rigid 3-dimensional tetrahedral network.
Each carbon is bonded to 3 other carbon atoms in the same plane, forming hexagonal arrays stacked in parallel layers held by weak van der Waals forces.
Consists of 60 carbon atoms arranged in interlocking hexagons (20) and pentagons (12) resembling a geodesic dome or soccer ball (Buckminsterfullerene).
Hardness / Texture
Hardest naturally occurring substance known.
Soft, smooth, and slippery to touch (layers slide over each other).
Dark solid at room temperature.
Electrical Conductivity
Bad conductor (no free electrons; all 4 valence electrons are tied in bonds).
Good conductor of electricity (each carbon has one free/delocalised valence electron).
Lubricant for heavy machinery, pencil leads, electrodes in dry cells.
Nanotechnology, drug delivery, lubricants.
Synthetic Diamonds: Diamonds can be synthesised by subjecting pure carbon to extremely high pressure and temperature. These synthetic diamonds are small but otherwise indistinguishable from natural diamonds.
08
Versatile Nature of Carbon — Catenation & Tetravalency
Direct Board Question:"Why does carbon form an exceptionally large number of compounds (millions of compounds)?"
The Two Unique Factors:
1. Catenation: The unique ability of carbon to form strong, stable covalent bonds with other carbon atoms, giving rise to long straight chains, branched chains, and closed rings of varying sizes.
• Why is C—C bond so exceptionally strong? Carbon has a very small atomic size. The small size enables the nucleus to hold onto the shared pairs of electrons firmly. (Silicon also shows catenation up to 7-8 atoms, but Si—Si bonds are weak and very reactive).
2. Tetravalency: Since carbon has a valency of 4, it is capable of bonding with 4 other atoms of carbon or atoms of other monovalent elements (H, Cl), divalent elements (O, S), and trivalent elements (N).
Board Exam Summary:Catenation + Tetravalency + Small atomic size = Unmatched stability and diversity of carbon compounds.
09
Saturated vs Unsaturated Carbon Compounds (Alkanes vs Alkenes/Alkynes)
Direct Board Definitions:
• Saturated Carbon Compounds: Compounds of carbon where all carbon atoms are linked together only by single covalent bonds (C—C). They are relatively unreactive.
• Unsaturated Carbon Compounds: Compounds of carbon containing at least one double bond ($C=C$) or triple bond ($C \equiv C$) between two carbon atoms. They are much more reactive.
Comparative Table:
Category
Bonding Type
General Formula
Flame Colour on Burning
Bromine Water Test
Saturated (Alkanes)
Only C—C single bonds
$C_nH_{2n+2}$
Clean blue, non-sooty flame
Does NOT decolourise reddish-brown bromine water
Unsaturated (Alkenes)
Contains $C=C$ double bond
$C_nH_{2n}$
Yellow, sooty flame (high % of carbon)
Decolourises bromine water rapidly
Unsaturated (Alkynes)
Contains $C \equiv C$ triple bond
$C_nH_{2n-2}$
Luminous smoky / sooty flame
Decolourises bromine water rapidly
10
Hydrocarbons & Nomenclature Rules for Alkanes, Alkenes, Alkynes
Hydrocarbons: Compounds containing only hydrogen and carbon.
Direct Board Definition — Structural Isomers: Compounds that possess the same molecular formula but different structural arrangements of atoms are called structural isomers, and the phenomenon is called structural isomerism.
The Classic Example — Isomers of Butane ($C_4H_{10}$):
1. n-Butane (Straight Chain):
CH3 − CH2 − CH2 − CH3 [n-Butane]
2. Isobutane / 2-Methylpropane (Branched Chain):
CH3 − CH(CH3) − CH3 [Isobutane / 2-Methylpropane]
Cyclic / Ring Carbon Compounds:
Cyclohexane ($C_6H_{12}$): Saturated ring compound containing 6 carbon atoms in a ring with single bonds.
Ring of 6 CH2 groups linked by single C−C bonds
Benzene ($C_6H_6$): Unsaturated cyclic compound containing alternate single and double bonds:
Hexagonal ring with alternate single and double bonds (CH=CH−CH=CH−CH=CH)
Board Fact: Methane ($CH_4$), Ethane ($C_2H_6$), and Propane ($C_3H_8$) do NOT show isomerism because branching is only possible from 4 or more carbon atoms!
12
Functional Groups — Heteroatoms & Families of Organic Compounds
Direct Board Definition: An atom or group of atoms (often containing heteroatoms like O, Cl, Br, N) that replaces hydrogen in a carbon chain and determines the characteristic chemical properties of the organic compound, irrespective of the length and nature of the carbon chain.
Standard Board Functional Groups Table:
Heteroatom
Class of Compound
Formula of Functional Group
Prefix / Suffix
Example ($C_3$ compound)
Halogen (Cl / Br)
Haloalkanes
—Cl, —Br
Prefix: chloro- / bromo-
Chloropropane ($CH_3CH_2CH_2Cl$)
Oxygen
Alcohols
—OH
Suffix: -ol
Propanol ($CH_3CH_2CH_2OH$)
Oxygen
Aldehydes
—CHO (—C(=O)H)
Suffix: -al
Propanal ($CH_3CH_2CHO$)
Oxygen
Ketones
>C=O (non-terminal)
Suffix: -one
Propanone ($CH_3COCH_3$, Acetone)
Oxygen
Carboxylic Acids
—COOH (—C(=O)OH)
Suffix: -oic acid
Propanoic acid ($CH_3CH_2COOH$)
Board Caution: In aldehydes (—CHO) and carboxylic acids (—COOH), the functional group is always at the terminal end of the chain. In ketones (>C=O), the carbonyl carbon must be attached to two other carbon atoms (never terminal; smallest ketone is Propanone with 3 carbons).
13
Homologous Series — Definition & 4 Distinct Characteristics
Direct Board Definition: A series of organic compounds having the same functional group, where members can be represented by the same general formula, and any two successive members differ by a —$CH_2$— unit.
4 Cardinal Characteristics of Any Homologous Series (3-Mark Board Question):
1. Molecular Formula Difference: Any two consecutive members differ by one carbon and two hydrogen atoms (—$CH_2$— unit).
2. Molecular Mass Difference: Any two consecutive members differ by 14 atomic mass units (14 u) [$C=12, H=1 \times 2 = 14$].
3. Chemical Properties: All members show very similar chemical properties because they possess the identical functional group.
4. Gradation in Physical Properties: As molecular mass increases along the series, there is a gradual increase in melting points, boiling points, and densities due to increasing van der Waals forces. Solubility in water typically decreases as the hydrocarbon chain lengthens.
IUPAC Nomenclature of Carbon Compounds (4-Step System)
The 4-Step Rule for Naming Any Carbon Compound: Step 1: Identify the number of carbon atoms in the longest continuous chain (gives the root word: meth-, eth-, prop-, but-, pent-). Step 2: Identify whether the chain is saturated (single bond → -ane) or unsaturated (double bond → -ene, triple bond → -yne). Step 3: Identify the functional group and select the appropriate prefix or suffix. Step 4: If suffix begins with a vowel (a, e, i, o, u), drop the terminal 'e' of the alkane name before adding the suffix!
Combustion Reactions & Flame Characteristics (Blue vs Yellow Sooty)
Direct Board Concept — Combustion: All allotropic forms of carbon and carbon compounds burn in excess oxygen/air to release $CO_2$, water vapour ($H_2O$), and huge amounts of heat and light.
Blue Clean Flame vs Yellow Sooty Flame (Classic 2-Mark Question):
Saturated Hydrocarbons (Alkanes): Burn with a clean, non-sooty blue flame because of complete combustion in sufficient air supply.
Unsaturated Hydrocarbons (Alkenes & Alkynes): Burn with a yellow, smoky, sooty flame because they have a higher percentage of carbon, leading to unburnt carbon particles (soot) that glow yellow.
Why do cooking gas stove burners give a blue flame? The burners have small air inlets (holes) that supply sufficient oxygen for complete combustion. If the air holes get blocked, incomplete combustion occurs and the bottom of the cooking vessel gets blackened by unburnt carbon soot.
16
Oxidation Reaction — Conversion of Ethanol to Ethanoic Acid
Direct Board Definition — Oxidising Agents: Substances that are capable of adding oxygen to or removing hydrogen from other substances are called oxidising agents.
Conversion of Ethanol into Ethanoic Acid: Alcohols can be easily oxidised to carboxylic acids by heating in the presence of strong oxidising agents:
Colour Change Observation: When alkaline $KMnO_4$ is added dropwise to warm ethanol, its purple/pink colour discharges initially. On adding excess, the purple colour persists when all ethanol is fully oxidised.
17
Addition Reaction & Industrial Hydrogenation of Vegetable Oils
Direct Board Definition: A chemical reaction in which unsaturated hydrocarbons add atoms or groups of atoms across double or triple bonds in the presence of a catalyst to form saturated compounds.
Essential Board Applications & Health Note (Frequent 2-Mark Board Question):
Application: Used industrially to convert liquid vegetable oils (unsaturated, containing double bonds) into solid vegetable ghee/vanaspati (saturated fats).
Health Advisory: Vegetable oils contain healthy unsaturated fatty acids which do not raise blood cholesterol. Animal fats and vanaspati ghee contain saturated fatty acids which are harmful to cardiovascular health. Hence, vegetable oils should be chosen for cooking.
Chemical Test for Unsaturation: Add bromine water (reddish-brown) to the substance. Unsaturated hydrocarbons (alkenes/alkynes/vegetable oils) decolourise bromine water rapidly, whereas saturated hydrocarbons (alkanes/animal fats) do not.
18
Substitution Reaction of Alkanes (Chlorination of Methane in Sunlight)
Direct Board Definition: A reaction in which one atom or group of atoms in a molecule is directly replaced (substituted) by another atom or group without changing the remaining structure.
Chlorination of Methane in the Presence of Sunlight: Saturated hydrocarbons are inert to most reagents, but react rapidly with chlorine in the presence of sunlight (photochemical substitution):
Board Exam Reason: Saturated hydrocarbons undergo substitution reactions (instead of addition) because all their four valencies are already satisfied by single covalent bonds; there are no double or triple bonds to open up.
19
Ethanol ($C_2H_5OH$) — Properties, Reaction with Sodium & Health Hazards
Physical Properties of Ethanol: Colourless liquid with a pleasant smell and burning taste; boiling point $78^\circ\text{C}$ ($351\text{ K}$); soluble in water in all proportions; neutral to litmus.
1. Reaction with Sodium Metal (Gas Evolution Test):
When a small piece of sodium is dropped into absolute ethanol, effervescence occurs and Hydrogen gas ($H_2 \uparrow$) is liberated, forming Sodium Ethoxide:
Test for $H_2$ Gas: The evolved gas burns with a characteristic 'POP' sound.
Commercial Uses & Health Aspects:
Uses: Excellent solvent for paints, varnishes, tinctures of iodine, cough syrups, and tonics. Used as biofuel (power alcohol: petrol + 10% ethanol).
Denatured Alcohol: To prevent misuse of industrial ethanol for drinking, it is made poisonous by adding small amounts of Methanol ($CH_3OH$), pyridine, and copper sulphate dye (blue colour).
Harmful Effects of Methanol: Methanol is oxidised in the liver to methanal (formaldehyde), which coagulates cellular protoplasm and attacks the optic nerve causing permanent blindness and death.
20
Dehydration of Ethanol to Ethene (Role of Conc. $H_2SO_4$ at $443\text{ K}$)
Direct Board Reaction (High-Frequency 2-Marker): Heating ethanol with excess concentrated sulphuric acid at $443\text{ K}$ ($170^\circ\text{C}$) results in the removal of a water molecule to form an alkene (Ethene).
The Role of Concentrated Sulphuric Acid ($H_2SO_4$):
Concentrated $H_2SO_4$ acts as a powerful Dehydrating Agent.
A dehydrating agent is a chemical substance that removes the elements of water ($H$ and $OH$) from a compound.
Crucial Board Condition: Both the temperature ($443\text{ K}$) and excess conc. $H_2SO_4$ must be explicitly written in the board exam to get full marks.
Direct Board Terms:
• Vinegar: A 5% to 8% solution of ethanoic acid in water is called vinegar and is widely used as a food preservative in pickles.
• Glacial Acetic Acid: The melting point of pure ethanoic acid is $290\text{ K}$ ($17^\circ\text{C}$). In cold winter climates, it freezes into ice-like crystals, hence named Glacial Acetic Acid.
Why Ethanoic Acid is a Weak Acid (Comparison with Mineral Acids):
Ethanoic acid ($CH_3COOH$) undergoes only partial ionisation in aqueous solution to yield relatively fewer hydronium ions ($H_3O^+$).
Mineral acids like Hydrochloric acid ($HCl$) ionise completely in water.
Hence, ethanoic acid has a higher pH (~3) and is much weaker than $HCl$ (pH ~1).
22
Reactions of Ethanoic Acid with Bases, Carbonates & Bicarbonates
Distinguishing Test between Alcohol and Carboxylic Acid: Only carboxylic acids react with carbonates/bicarbonates to produce effervescence of $CO_2$ gas. Alcohols do NOT react!
Observation for Board Practical Exams: Brisk effervescence of a colourless, odourless gas is observed. On passing through lime water, lime water turns milky due to insoluble $CaCO_3$, confirming the gas is Carbon Dioxide ($CO_2$).
23
Esterification Reaction (Preparation of Sweet-Smelling Esters)
Direct Board Definition — Esterification: The chemical reaction between a carboxylic acid and an alcohol in the presence of a few drops of concentrated sulphuric acid (as catalyst) to form a sweet, fruity smelling compound called an Ester.
Sensory Observation: On pouring the contents into a beaker of water and smelling, a sweet, pleasant fruity smell is detected.
Uses of Esters: Because of their sweet fragrance, esters are extensively used in making perfumes, deodorants, synthetic flavouring agents for ice creams, candies, and cold drinks.
24
Saponification Reaction (Alkaline Hydrolysis of Esters & Soap Formation)
Direct Board Definition — Saponification: The reaction of an ester with a strong alkali (like Sodium Hydroxide $NaOH$) to yield the original alcohol and the sodium salt of the carboxylic acid (Soap).
Why is it called Saponification? The word is derived from the Latin sapo (soap). It is called saponification because this alkaline hydrolysis reaction is used in the commercial manufacture of soaps from natural fats and oils (which are glyceryl esters of long-chain fatty acids like stearic, palmitic, and oleic acids).
Industrial Soap Preparation: Fat / Oil + $NaOH$ → Soap + Glycerol.
25
Structure of a Soap Molecule (Hydrophilic Head & Hydrophobic Tail)
Direct Board Definition — Soap: Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids) having 15 to 18 carbon atoms (e.g. Sodium Stearate $C_{17}H_{35}COO^-Na^+$).
The Two Chemically Opposing Ends:
Part of Soap Molecule
Chemical Nature
Solubility Behaviour
1. Hydrophobic Tail
Long non-polar hydrocarbon chain (e.g. $-C_{17}H_{35}$)
Water-repelling (hydrophobic); insoluble in water, but readily dissolves in oils, grease, and dirt.
2. Hydrophilic Head
Short polar ionic end ($-COO^-Na^+$)
Water-attracting (hydrophilic); soluble in water, but insoluble in oil and hydrocarbons.
Structure of Soap Molecule
26
Cleansing Action of Soap & Mechanism of Micelle Formation
Direct 5-Mark Board Question:"Explain the cleansing action of soaps with the help of a labelled diagram of micelle formation. Why does soap not form micelles in ethanol?"
Step-by-Step Cleansing Mechanism:
Step 1 (Orientation): Most dirt on clothes is oily in nature and does not dissolve in water. When soap is added to water, the hydrophobic hydrocarbon tails dissolve in the oil droplet, while the hydrophilic ionic heads project outwards into the surrounding water.
Step 2 (Micelle Formation): A radially symmetric spherical cluster of soap molecules called a Micelle is formed. The oily dirt is trapped securely at the hydrophobic interior core of the micelle.
Step 3 (Emulsification): The negatively charged ionic heads ($-COO^-$) pointing outward on the surface of the micelle repel each other, preventing the micelles from coalescing into larger droplets or settling down. An emulsion is formed.
Step 4 (Rinsing): Upon mechanical agitation or washing, the suspended micelles carrying the trapped oily dirt are rinsed away by water, leaving the fabric clean.
Why do micelles NOT form in Ethanol? Ethanol is a non-polar/organic solvent. In ethanol, the hydrocarbon tail of soap is soluble in the solvent, so soap dissolves uniformly as individual molecules and does not need to cluster into micelles!
27
Soaps vs Synthetic Detergents — Hard Water Action & Scum Formation
Direct Board Question:"Why are detergents preferred over soaps for washing clothes with hard water?"
A large amount of soap is wasted in precipitating the scum before any lather is formed.
Highly Effective. The charged sulphonate ends ($-SO_3^-Na^+$) do not form insoluble precipitates with $Ca^{2+}$ and $Mg^{2+}$ ions in hard water. They lather freely even in hard water.
Biodegradability
100% Biodegradable (easily broken down by aquatic bacteria).
Branched-chain detergents are non-biodegradable and cause water pollution (modern straight-chain ones are biodegradable).
1. Ethane, with molecular formula C₂H₆ has: (a) 6 covalent bonds (b) 7 (c) 8 (d)
9.
ANSWER(b) 7 covalent bonds. There are
six C—H bonds and one C—C bond.
2. Butanone is a four-carbon compound with which functional group?
ANSWERKetone. The suffix “-one”
identifies a ketone.
3. While cooking, if the bottom of the vessel is getting blackened on the
outside, what does it mean?
ANSWERThe fuel is not burning
completely. Incomplete combustion produces soot; insufficient air can cause this.
4. Explain the nature of the covalent bond using bond formation in CH₃Cl.
ANSWERCarbon shares electrons with three
hydrogen atoms and one chlorine atom. These shared electron pairs form covalent bonds. Sharing
allows the outer shells of the bonded atoms to attain stable configurations. No ions are formed by
simple electron transfer.
In a full electron-dot drawing, show the relevant shared and lone electron pairs.
6. What is a homologous series? Explain with an example.
ANSWERA family of compounds having the same
functional group and similar chemical properties, with successive members differing by a —CH₂— unit.
Example: methanol, ethanol, propanol and butanol.
7. How can ethanol and ethanoic acid be differentiated on the basis of physical
and chemical properties?
ANSWEREthanol is an alcohol; ethanoic acid is
a carboxylic acid and is acidic. Ethanoic acid reacts with sodium hydrogencarbonate/carbonate to
release CO₂, while ethanol does not give this characteristic reaction. Ethanoic acid also has a
characteristic acidic behaviour; ethanol is not a carboxylic acid.
8. Why does micelle formation take place when soap is added to water? Will a
micelle be formed in ethanol also?
ANSWERSoap has a hydrophilic ionic end and a
hydrophobic hydrocarbon end. In water, the tails cluster inward around oily material while the ionic
ends face water, producing a micelle. The behaviour depends on the solvent, so the usual water-based
micelle arrangement should not simply be assumed for ethanol.
9. Why are carbon and its compounds used as fuels for most applications?
ANSWERMost carbon compounds release a large
amount of heat and light on burning, making them useful sources of energy. Many commonly used fuels
are carbon or carbon compounds.
10. Explain formation of scum when hard water is treated with soap.
ANSWERHard water contains calcium and
magnesium salts. Ca²⁺/Mg²⁺ react with soap and form insoluble salts, producing a white/curdy
precipitate called scum. This reduces the effectiveness of soap.
11. What change will you observe if you test soap with litmus paper (red and
blue)?
ANSWERSoap solution is alkaline. It turns
red litmus blue; blue litmus does not show a corresponding change.
12. What is hydrogenation? What is its industrial application?
ANSWERHydrogenation is addition of hydrogen to
an unsaturated compound in the presence of a catalyst such as nickel or palladium. It is used
industrially in the hydrogenation of vegetable oils.
13. Which hydrocarbons undergo addition reactions: C₂H₆, C₃H₈, C₃H₆, C₂H₂ and
CH₄?
ANSWERC₃H₆ and C₂H₂. They are
unsaturated hydrocarbons and contain multiple bonds.
14. Give a test to differentiate saturated and unsaturated hydrocarbons.
ANSWERUse bromine water as a test for
unsaturation: an unsaturated hydrocarbon decolourises bromine water under the standard test
conditions, whereas a saturated hydrocarbon does not readily do so.
15. Explain the mechanism of the cleaning action of soaps.
ANSWERSoap has hydrophobic hydrocarbon tails
and hydrophilic ionic ends. The tail interacts with oily dirt, while the ionic end interacts with
water. Soap molecules arrange into micelles with dirt in the centre, suspend/emulsify it in water,
and allow the dirt to be rinsed away.
NCERT writing tip: For structure/reaction questions, do not stop at the final
formula. Show the structure/equation and add the key condition or reason where the question asks
“explain”.
PYQ Practice
Board-style chapter questions • answers reveal on demand
1 MARK
Why does carbon form covalent bonds?
Give the reason in one precise sentence.
Answer Answer: Carbon has four valence electrons and cannot easily gain or lose four
electrons, so it completes its outer shell mainly by sharing electrons.
2 MARKS
State and explain the two properties responsible for carbon's versatility.
Use the exact keywords.
Answer Answer: Tetravalency and catenation. Carbon forms four covalent bonds and can
form strong C—C chains, branches and rings.
2 MARKS
Differentiate saturated and unsaturated carbon compounds.
Answer Answer: Saturated compounds have only single C—C bonds; unsaturated compounds
have C=C or C≡C bonds.
3 MARKS
Why do diamond and graphite have different physical properties though both are carbon?
Answer Answer: Their carbon atoms are bonded differently. Diamond has a rigid
three-dimensional structure with each carbon bonded to four others; graphite has layered hexagonal
arrays with each carbon bonded to three others. Hence their physical properties differ.
3 MARKS
Explain homologous series with an example and state the CH₂ relation.
Answer Answer: A homologous series contains compounds with the same functional group
and similar chemical properties. Successive members differ by CH₂. Example: CH₃OH, C₂H₅OH, C₃H₇OH.
3 MARKS
Write the reaction of ethanol with sodium and identify the gas evolved.
Answer
2Na + 2CH₃CH₂OH → 2CH₃CH₂O⁻Na⁺ + H₂
Hydrogen gas is evolved; sodium ethoxide is
formed.
3 MARKS
What happens when ethanol is heated with concentrated H₂SO₄ at 443 K?
Answer
CH₃CH₂OH → CH₂=CH₂ + H₂O
Concentrated H₂SO₄ acts as a dehydrating agent and
removes water.
3 MARKS
Explain addition and substitution reactions with one example each.
Answer Addition: unsaturated compound adds hydrogen, e.g. C₂H₄ + H₂ → C₂H₆ in
presence of Ni/Pd. Substitution: one atom/group replaces another, e.g. CH₄ + Cl₂ → CH₃Cl +
HCl in sunlight.
5 MARKS
Explain the cleansing action of soap with a labelled micelle diagram.
Answer Answer structure: (1) oil does not dissolve in water; (2) soap has
hydrophobic tail + hydrophilic ionic head; (3) tails point toward oil; (4) heads face water; (5)
micelles suspend/emulsify dirt so it can be rinsed away.
Competency-Based Questions
CBSE Board practice items with step-by-step solutions
Write down electron shell configuration of carbon in order to predict formulae of carbon compounds and illustrate the structure of molecules of carbon compounds with chain, branched & ring structure
CBQ 1 • Electronic Configuration of Carbon
What is the correct ground-state electronic shell configuration of carbon (atomic number $Z=6$)?
(a) $2, 4$
(b) $2, 8$
(c) $2, 2, 4$
(d) $2, 4, 4$
Correct Answer: Option (a)
$$2, 4$$ (K-shell: 2, L-shell: 4)
Carbon has atomic number 6. Two electrons occupy the inner K shell and the remaining four electrons occupy the outer valence L shell ($2, 4$).
CBQ 2 • Tetravalency and Covalent Bonding
The electronic configuration of a carbon atom is $2, 4$. How many covalent bonds can one carbon atom form with other atoms to achieve a stable octet?
(a) 1
(b) 2
(c) 4
(d) 6
Correct Answer: Option (c)
4 covalent bonds (Tetravalency)
Carbon has four valence electrons. Because gaining four electrons (to form $\text{C}^{4-}$) or losing four electrons (to form $\text{C}^{4+}$) is energetically unfavorable, carbon shares four pairs of electrons with other atoms, forming 4 covalent bonds.
Topic Focus
Draw structures of carbon compounds in order to classify them as saturated or unsaturated
CBQ 3 • Identification of Unsaturated Compounds
Which of these compounds contains carbon–carbon multiple bonds and is classified as an unsaturated hydrocarbon?
(a) Cyclobutane (ring structure with single bonds, $\text{C}_4\text{H}_8$)
Unsaturated hydrocarbons are compounds of carbon and hydrogen that contain at least one double bond (alkenes, $\text{C}=\text{C}$) or triple bond (alkynes, $\text{C}\equiv\text{C}$) between adjacent carbon atoms. Ethene contains a $\text{C}=\text{C}$ double bond and decolourises bromine water.
CBQ 4 • Classification of Acetic Acid
Acetic acid has the chemical structure: $$\text{CH}_3\text{COOH} \quad \left(\text{H}–\underset{\text{H}}{\overset{\text{H}}{\text{C}}}–\overset{\text{O}}{\overset{\parallel}{\text{C}}}–\text{OH}\right)$$ Why is the carbon chain of acetic acid classified as saturated?
(a) Because there is only a single covalent bond connecting the carbon atoms ($\text{C}–\text{C}$)
(b) Because there is a double bond between the carbon and oxygen atoms
(c) Because there are three single bonds between carbon and hydrogen atoms
(d) Because it contains a hydroxyl group
Correct Answer: Option (a)
Because there is a single bond between the carbon atoms.
In organic chemistry, saturation refers specifically to the bonding between carbon atoms in the chain. Even though the carboxylic functional group contains a carbon–oxygen double bond ($\text{C}=\text{O}$), the carbon skeleton possesses only $\text{C}–\text{C}$ single bonds, making the alkyl backbone saturated.
Topic Focus
Draw structures of carbon compounds and show types of bonds (single/ double/ triple) in order to classify them as alkanes/ alkenes/ alkynes
CBQ 5 • Recognition of Alkenes
Which of these carbon compounds represents an alkene?
Alkenes are unsaturated aliphatic hydrocarbons containing one or more carbon–carbon double bonds ($\text{C}=\text{C}$) with general molecular formula $\text{C}_n\text{H}_{2n}$. Ethene ($\text{C}_2\text{H}_4$) is the simplest alkene.
CBQ 6 • Classification of Alkynes
Consider the following four hydrocarbon compounds: • (A) $\text{H}–\text{C}\equiv\text{C}–\text{H}$ (Ethyne) • (B) $\text{H}_2\text{C}=\text{CH}_2$ (Ethene) • (C) $\text{H}_3\text{C}–\text{CH}_3$ (Ethane) • (D) $\text{H}_3\text{C}–\text{C}\equiv\text{C}–\text{H}$ (Propyne) Which of these compounds are classified as alkynes?
(a) Only (A)
(b) Only (B)
(c) Both (A) and (D)
(d) Both (B) and (C)
Correct Answer: Option (c)
Both (A) and (D)
Alkynes are unsaturated hydrocarbons that contain a carbon–carbon triple bond ($\text{C}\equiv\text{C}$) with general formula $\text{C}_n\text{H}_{2n-2}$. Both ethyne ($\text{H}–\text{C}\equiv\text{C}–\text{H}$) and propyne ($\text{CH}_3–\text{C}\equiv\text{CH}$) contain a $\text{C}\equiv\text{C}$ triple bond.
Topic Focus
Draw structures of carbon compounds with functional groups, in order to predict their properties due to functional groups and type of bonding present
CBQ 7 • Identification of Ketone Group
Identify the functional group present in acetone (propanone): $$\text{H}_3\text{C}–\overset{\text{O}}{\overset{\parallel}{\text{C}}}–\text{CH}_3$$
(a) Alcohol ($-\text{OH}$)
(b) Aldehyde ($-\text{CHO}$)
(c) Carboxylic acid ($-\text{COOH}$)
(d) Ketone ($>\text{C}=\text{O}$)
Correct Answer: Option (d)
Ketone ($>\text{C}=\text{O}$)
A carbonyl group ($>\text{C}=\text{O}$) bonded to two alkyl carbon groups within the interior of a carbon chain is a ketone. Propanone is the simplest ketone.
CBQ 8 • Alcohol Functional Group
Which functional group combines with an alkyl group to produce an alcohol?
(a) $-\text{CHO}$ (Aldehyde)
(b) $-\text{OH}$ (Hydroxyl)
(c) $-\text{COOH}$ (Carboxyl)
(d) $>\text{C}=\text{O}$ (Carbonyl)
Correct Answer: Option (b)
$$-\text{OH}\text{ (Hydroxyl group)}$$
When a hydrogen atom in an alkane chain is replaced by a hydroxyl group ($-OH$), an alcohol (general formula $R-\text{OH}$) is produced, such as methanol ($\text{CH}_3\text{OH}$) and ethanol ($\text{C}_2\text{H}_5\text{OH}$).
Topic Focus
Classify carbon compounds in homologous series in order to predict their properties
CBQ 9 • Identifying a Homologous Series
Which of the following sets of compounds constitutes a true homologous series?
$$\text{CH}_3\text{OH},\; \text{C}_2\text{H}_5\text{OH},\; \text{C}_3\text{H}_7\text{OH}$$ (Homologous series of alcohols)
A homologous series is a family of organic compounds having the same functional group and similar chemical properties, in which successive members differ by a constant $-\text{CH}_2-$ unit ($14\text{ u}$ in molecular mass).
CBQ 10 • Characteristics of Homologous Members
Why are the compounds $\text{CH}_3\text{OH}$, $\text{C}_2\text{H}_5\text{OH}$, $\text{C}_3\text{H}_7\text{OH}$, and $\text{C}_4\text{H}_9\text{OH}$ grouped together as a homologous series?
(a) Because of a random increase in the number of carbon atoms
(b) Because of an increase in the number of hydrogen atoms only
(c) Because the same functional group ($-OH$) substitutes for hydrogen along the carbon chain, conferring identical chemical characteristics
(d) Because they have identical boiling points
Correct Answer: Option (c)
Because of the presence of the same functional group substituting for hydrogen in a carbon chain.
The chemical properties of a carbon compound are determined primarily by its functional group. Since all members possess the $-OH$ group and consecutive members differ by $-\text{CH}_2-$, they share similar chemical properties while exhibiting a smooth gradation in physical properties.
Topic Focus
Identify the functional group, type of bonding, number of C atoms present in a carbon compound, in order to correctly name them
CBQ 11 • Root Word for 2 Carbon Atoms
A saturated aliphatic hydrocarbon contains two carbon atoms connected by a single covalent bond. What is the systematic IUPAC name of this compound?
(a) Butane
(b) Ethane
(c) Methane
(d) Propane
Correct Answer: Option (b)
Ethane ($\text{C}_2\text{H}_6$)
Root word for 2 carbon atoms is "eth-". Since it is a saturated hydrocarbon with carbon–carbon single bonds, the suffix is "-ane", yielding "ethane".
CBQ 12 • Nomenclature Basis of Ethane
The structural formula of ethane is: $$\text{H}–\underset{\text{H}}{\overset{\text{H}}{\text{C}}}–\underset{\text{H}}{\overset{\text{H}}{\text{C}}}–\text{H}$$ Which option scientifically explains why this molecule is named "ethane"?
(a) Presence of a functional group connected with a single bond
(b) The root word "eth-" indicates two carbon atoms and suffix "-ane" indicates only single bonds between carbon atoms
(c) It has eight total atoms in the molecule
(d) Six hydrogen atoms are connected to a single carbon atom
Correct Answer: Option (b)
Two carbon atoms ("eth-") with a carbon–carbon single bond ("-ane").
IUPAC naming derives from: (1) chain length: 2 carbons = "eth-"; (2) saturation: only $\text{C}–\text{C}$ single bonds = alkane suffix "-ane". Thus, the compound is ethane.
Topic Focus
Observe how carbon compounds burn in oxygen, in order to classify them as saturated or unsaturated
CBQ 13 • Complete Combustion of Methane
A student combusts methane ($\text{CH}_4$) in an excess of oxygen. What products and flame characteristics are observed?
(a) Water and carbon monoxide with a smoky yellow flame
(b) Carbon dioxide and hydrogen gas
(c) Carbon dioxide and water with liberation of heat and a clean blue flame
(d) Carbon dioxide and ozone
Correct Answer: Option (c)
$$\text{CO}_2 + 2\text{H}_2\text{O} + \text{Heat and Light}$$
Saturated hydrocarbons like methane undergo complete combustion in sufficient air/oxygen to yield carbon dioxide and water vapour, releasing substantial heat with a clean, non-sooty blue flame: $$\text{CH}_4(g) + 2\text{O}_2(g) \longrightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g) + \text{Heat and Light}$$
CBQ 14 • Sooty Flame and Unsaturation
When naphthalene (an aromatic compound) is ignited, it burns with a luminous yellow flame giving off thick black smoke and sooty deposits. What does this observation indicate about the nature of the hydrocarbon?
(a) It is saturated, because black smoke indicates complete combustion
(b) It is unsaturated, because its high carbon percentage leads to incomplete combustion and unburnt carbon particles (soot)
(c) It is saturated, because yellow flames indicate complete oxidation
(d) Any burning organic substance always gives a smoky flame
Unsaturated and aromatic compounds have a significantly higher carbon-to-hydrogen ratio than saturated alkanes. In atmospheric air, oxygen is insufficient to completely oxidise all carbon, resulting in unburnt carbon particles glowing yellow and escaping as black soot.
Topic Focus
Illustrate the chemical properties of carbon compounds (like combustion, oxidation, addition & substitution) along with balanced chemical reaction
CBQ 15 • Oxidation of Ethanol to Ethanoic Acid
Consider the conversion of ethanol into ethanoic acid: $$\text{CH}_3–\text{CH}_2\text{OH} \xrightarrow{\text{Alkaline }\text{KMnO}_4 + \Delta} \text{CH}_3\text{COOH}$$ Which reagent acts as the chemical oxidising agent that drives this reaction?
Alkaline potassium permanganate ($\text{KMnO}_4$) or acidified potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$).
Alkaline $\text{KMnO}_4$ supplies nascent oxygen $[\text{O}]$ that oxidises the primary alcohol group ($-\text{CH}_2\text{OH}$) to a carboxylic acid group ($-\text{COOH}$). Substances that add oxygen to starting materials are called oxidising agents.
CBQ 16 • Oxidation Product with Acidified Dichromate
Ethanol is heated with acidified potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4$): $$\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4 + \Delta} X$$ Which chemical formula represents product $X$?
(a) $\text{CH}_2\text{O}$ (Formaldehyde)
(b) $\text{CH}_3\text{CH}_3$ (Ethane)
(c) $\text{CH}_3\text{OCH}_3$ (Dimethyl ether)
(d) $\text{CH}_3\text{COOH}$ (Ethanoic acid)
Correct Answer: Option (d)
$$\text{CH}_3\text{COOH}\text{ (Ethanoic acid)}$$
Acidified $\text{K}_2\text{Cr}_2\text{O}_7$ is a powerful oxidising agent that turns from orange to green as chromium(VI) is reduced to chromium(III), cleanly oxidising ethanol into ethanoic acid: $$\text{CH}_3\text{CH}_2\text{OH} + 2[\text{O}] \longrightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O}$$
Topic Focus
Identify how carbon compounds react with hydrogen in the presence of nickel catalyst, in order to write a balanced chemical reaction
CBQ 17 • Hydrogenation of Vegetable Oils
An unsaturated alkene undergoes catalytic addition of hydrogen gas in the presence of nickel: $$R_2\text{C}=\text{C}R_2 + \text{H}_2 \xrightarrow{\text{Ni catalyst}} X$$ What is the structure of product $X$?
The carbon–carbon double bond opens up, and one hydrogen atom adds to each carbon atom across the bond in the presence of finely divided nickel or palladium catalyst. This reaction is commercially used in the hydrogenation of liquid vegetable oils into solid vegetable ghee (vanaspati).
CBQ 18 • Role of Catalyst in Hydrogenation
Ethene reacts with hydrogen to form ethane in the presence of a palladium catalyst: $$\text{H}_2\text{C}=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Pd}} \text{H}_3\text{C}–\text{CH}_3$$ What is the precise definition and function of the catalyst in this reaction?
(a) A substance that alters/accelerates the rate of a chemical reaction without undergoing any permanent chemical change itself
(b) A reactant that donates its own hydrogen atoms and is consumed
(c) An agent that converts single bonds into double bonds
(d) A substance that generates oxygen during the reaction
Correct Answer: Option (a)
A catalyst accelerates the reaction without itself being consumed.
Catalysts like nickel ($\text{Ni}$), palladium ($\text{Pd}$), or platinum ($\text{Pt}$) provide an active metal surface on which $\text{H}_2$ molecules adsorb and dissociate into atomic hydrogen, significantly lowering the activation energy for addition to the $\text{C}=\text{C}$ double bond.
Topic Focus
Identify how carbon compounds react with chlorine in the presence of sunlight, in order to write a balanced chemical reaction
CBQ 19 • Photochemical Chlorination of Methane
Methane reacts with chlorine gas in the presence of diffuse sunlight: $$\text{CH}_4 + \text{Cl}_2 \xrightarrow{h\nu \text{ (sunlight)}} X + \text{HCl}$$ What is product $X$?
Alkanes are fairly unreactive (paraffins), but in the presence of UV light or sunlight, chlorine atoms replace hydrogen atoms one by one in a free-radical substitution reaction: $$\text{CH}_4 + \text{Cl}_2 \xrightarrow{h\nu} \text{CH}_3\text{Cl} + \text{HCl}$$
CBQ 20 • Substitution Mechanism
In the reaction: $\text{CHCl}_3 + \text{Cl}_2 \xrightarrow{h\nu} \text{CCl}_4 + \text{HCl}$, what type of chemical reaction occurs?
(a) Addition of hydrogen
(b) Dehydration reaction
(c) Substitution reaction, where a chlorine atom replaces a hydrogen atom from the carbon skeleton
(d) Neutralisation reaction
Correct Answer: Option (c)
Substitution reaction (replacement of hydrogen by chlorine).
Reactions in which one type of atom or a group of atoms takes the place of another atom or group of atoms on a carbon chain without changing the saturation of the chain are called substitution reactions.
Topic Focus
Perform physical and chemical tests in order to distinguish between Ethanol & Ethanoic acid based on their properties (reaction with other substances)
Pure ethanoic acid freezes into ice-like crystals during winter at $290\text{ K}$ ($17^\circ\text{C}$), giving it the name "glacial acetic acid". When ethanoic acid is warmed with ethanol in the presence of a few drops of concentrated $\text{H}_2\text{SO}_4$, what characteristic product is formed?
(a) Ethyl ethanoate ($\text{CH}_3\text{COOC}_2\text{H}_5$), a sweet-smelling ester, and water
Carboxylic acids react with alcohols in the presence of an acid catalyst to produce sweet, fruity-smelling esters through esterification: $$\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{Conc. }\text{H}_2\text{SO}_4} \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}$$ Esters are used in perfumes and flavoring agents.
Topic Focus
Describe the process of micelle formation in order to understand how soaps work
CBQ 22 • Orientation of Soap Molecule & Cleansing Mechanism
A soap molecule has two chemically distinct ends: an ionic carboxylate head ($-\text{COO}^-\text{Na}^+$) and a long non-polar hydrocarbon tail. How does soap interact with oily dirt in water to clean clothes?
(a) The ionic head dissolves in oily dirt while the tail dissolves in water
(b) Both ends avoid water and dissolve in air
(c) The hydrophobic hydrocarbon tail embeds into the oily dirt droplet while the hydrophilic ionic head faces outwards into the water, forming spherical micelles that keep dirt suspended
(d) Soap breaks the dirt into volatile gases that evaporate
Correct Answer: Option (c)
The hydrophobic tail embeds in the oil droplet while the hydrophilic ionic head faces outward in water, forming micelles.
Oil droplets do not dissolve in water. Soap molecules form spherical clusters called micelles where the hydrophobic hydrocarbon tails point inward entrapping the oily grease, and the ionic heads ($-\text{COO}^-\text{Na}^+$) project outwards into the water. Ionic repulsion between the charged micelle surfaces prevents them from coalescing into larger droplets, keeping the grease emulsified in water so it is easily rinsed away.
SECTION 2
Case Study & Contextual Questions
Experimental Context
Context for Questions 23 to 25: The diagrams illustrate electron sharing in diatomic molecules of four different elements: • Hydrogen ($\text{H}_2$: single covalent bond, 1 pair shared) • Carbon (forms networks / $\text{C}_2$ unstable in standard conditions) • Oxygen ($\text{O}_2$: double covalent bond, 2 pairs shared) • Nitrogen ($\text{N}_2$: triple covalent bond, 3 pairs shared)
CBQ 23 • SAS21S100401 • Covalent Bond Strength in Diatomic Molecules
Which diatomic molecule has the strongest covalent bond between its atoms?
A. Hydrogen ($\text{H}_2$)
B. Carbon
C. Nitrogen ($\text{N}_2$)
D. Oxygen ($\text{O}_2$)
Correct Answer: C (Nitrogen, $\text{N}_2$)
Nitrogen ($\text{N}_2$)
Nitrogen atoms share three pairs of valence electrons to form a triple covalent bond ($\text{N}\equiv\text{N}$). A triple covalent bond has a remarkably high bond dissociation enthalpy ($945\text{ kJ/mol}$), making it much stronger than single ($\text{H}–\text{H}$) or double ($\text{O}=\text{O}$) bonds.
Experimental Context
Context (continued): Atoms in molecules of $\text{H}_2, \text{O}_2, \text{N}_2$, and $\text{CH}_4$ achieve noble gas electron configurations by sharing electron pairs.
CBQ 24 • SAS21S100402 • Nature of Bonding via Electron Sharing
What is the scientific term for the chemical bond formed by the mutual sharing of electron pairs between two atoms?
A. Ionic bond
B. Covalent bond
C. Metallic bond
D. Hydrogen bond
Correct Answer: B (Covalent bond)
Covalent bond
A chemical bond formed by the mutual sharing of valence electrons between atoms of the same or different elements so that each atom attains a stable valence shell is defined as a covalent bond.
Experimental Context
Context (continued): In a nitrogen molecule ($\text{N}_2$), each nitrogen atom shares three valence electrons.
CBQ 25 • SAS21S100403 • Symbolic Representation of Triple Bond
Which of the following is the standard structural formula representing the chemical bonding between the atoms of a nitrogen molecule?
Three shared pairs of electrons between two nitrogen atoms are symbolized by three horizontal parallel lines representing a triple covalent bond.
CBQ 26 • SAS21S100404 • Fundamental Properties of Carbon Compounds
Evaluate which of the following statements about carbon compounds are correct (Yes) or incorrect (No): 1. They are good conductors of electricity. 2. They exist in either saturated or unsaturated forms. 3. They generally have lower boiling points than ionic compounds.
A. 1: Yes, 2: Yes, 3: No
B. 1: No, 2: Yes, 3: Yes
C. 1: Yes, 2: No, 3: Yes
D. 1: No, 2: No, 3: Yes
Correct Answer: B (1: No, 2: Yes, 3: Yes)
1: No | 2: Yes | 3: Yes
• Statement 1 (No): Carbon compounds are covalent and do not form free mobile ions or delocalised electrons (except graphite), making them poor conductors of electricity. • Statement 2 (Yes): They exist as saturated (only single $\text{C}–\text{C}$ bonds) or unsaturated (double/triple bonds) compounds. • Statement 3 (Yes): Intermolecular forces in covalent carbon compounds are weak van der Waals interactions, resulting in significantly lower melting and boiling points compared to ionic lattices.
Experimental Context
Structural Context: Consider an incomplete three-carbon skeleton: $\text{H}_3\text{C}–\dot{\text{C}}–\text{CH}_3$, where the central carbon atom has two unpaired valence electrons available for bonding.
CBQ 27 • SAS21S100405 • Carbonyl Group Valency Requirements
How many oxygen atoms can combine with the central carbon atom to satisfy its tetravalency and form a stable neutral compound?
A. One
B. Two
C. Three
D. Four
Correct Answer: A (One oxygen atom)
One oxygen atom (yielding Propanone / Acetone, $\text{CH}_3–\text{CO}–\text{CH}_3$)
Oxygen has a valency of 2 (it requires 2 electrons to complete its octet). By forming a double covalent bond with the central carbon ($\text{C}=\text{O}$), exactly one oxygen atom satisfies the remaining two valencies of carbon, producing a stable ketone.
CBQ 28 • SAS21S100406 • Identification of Alkene Molecular Formula
Which of the following hydrocarbons contains a carbon–carbon double bond ($\text{C}=\text{C}$)?
A. $\text{CH}_4$ (Methane)
B. $\text{C}_2\text{H}_4$ (Ethene)
C. $\text{C}_3\text{H}_8$ (Propane)
D. $\text{C}_4\text{H}_{10}$ (Butane)
Correct Answer: B ($\text{C}_2\text{H}_4$)
$$\text{C}_2\text{H}_4\text{ (Ethene)}$$
Alkenes fit the general formula $\text{C}_n\text{H}_{2n}$. For $n=2$, the formula is $\text{C}_2\text{H}_4$ ($\text{H}_2\text{C}=\text{CH}_2$), which possesses a $\text{C}=\text{C}$ double bond. The other options ($\text{CH}_4, \text{C}_3\text{H}_8, \text{C}_4\text{H}_{10}$) fit the alkane formula $\text{C}_n\text{H}_{2n+2}$ containing only single bonds.
Experimental Context
Homologous Series Sequence: Consider the homologous series: $\text{C}_3\text{H}_4$ (propyne), $\text{C}_4\text{H}_6$ (butyne), $\text{C}_5\text{H}_8$ (pentyne).
CBQ 29 • SAS21S100407 • Alkyne Homologous Series Identification
Which of the following compounds belongs to this same homologous series?
The given series has the general formula of alkynes: $\text{C}_n\text{H}_{2n-2}$. For $n=2$, the formula is $\text{C}_2\text{H}_{2(2)-2} = \text{C}_2\text{H}_2$, the first member of the alkyne series.
CBQ 30 • SAS21S100408 • Combustion Products of Organic Compounds
What general scientific conclusions can be deduced from these four complete combustion reactions? 1. All carbon compounds release oxygen on combustion. 2. All carbon compounds release water on reacting with oxygen. 3. All carbon compounds produce carbon dioxide on reacting with oxygen.
A. 1: Yes, 2: Yes, 3: Yes
B. 1: No, 2: Yes, 3: Yes
C. 1: Yes, 2: No, 3: Yes
D. 1: No, 2: No, 3: Yes
Correct Answer: B (1: No, 2: Yes, 3: Yes)
1: No | 2: Yes | 3: Yes
• Statement 1 (No): Combustion consumes oxygen rather than releasing it. • Statements 2 & 3 (Yes): Combustion of hydrogen-containing carbon compounds oxidises the carbon to carbon dioxide ($\text{CO}_2$) and hydrogen to water vapour ($\text{H}_2\text{O}$), while liberating exothermic heat.
Experimental Context
Refer to the four combustion reactions in Question 30:
$\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}$ contains the hydroxyl functional group ($-OH$) attached to an alkyl chain, classifying it as a primary alcohol (propanol).
Experimental Context
Allotropes of Carbon: • Allotrope 1 (Graphite): Carbon atoms arranged in planar hexagonal sheets held by weak van der Waals forces. • Allotrope 2 (Diamond): Each carbon atom covalently bonded to four other carbon atoms in a rigid 3D tetrahedral network.
CBQ 32 • SAS21S100410 • Allotropes of Carbon: Diamond vs. Graphite
Which allotrope is harder, and what structural feature accounts for its extraordinary hardness?
A. Graphite, because layers can slide smoothly over one another
B. Diamond, because each carbon atom is tetrahedrally bonded to four other carbon atoms by strong covalent bonds in a rigid 3D lattice
C. Graphite, because it contains free delocalised electrons
D. Diamond, because it has an ionic crystal structure
Correct Answer: B (Diamond)
Diamond is the hardest natural substance known.
In diamond, every carbon atom uses all four valence electrons to form four strong, directional covalent $\sigma$-bonds with neighboring carbon atoms in a rigid, three-dimensional tetrahedral network. Breaking this lattice requires breaking an enormous number of covalent bonds simultaneously. In contrast, graphite consists of planar sheets held only by weak van der Waals forces, making it soft and slippery.
SECTION 3
CBSE Item Bank Questions
Item Data & Reference
Period 2 Elements: $\text{Li, Be, B, C, N, O, F, Ne}$
CBQ 33 • Science10NG4 — 1(a)(viii) • 1 mark
Give the symbol of the element from Period 2 which forms both saturated and unsaturated covalent compounds.
Symbol: $\text{C}$ (Carbon)
Carbon forms single bonds (saturated alkanes like ethane $\text{C}_2\text{H}_6$) as well as double and triple bonds (unsaturated alkenes $\text{C}_2\text{H}_4$ and alkynes $\text{C}_2\text{H}_2$) due to catenation and tetravalency.
Item Data & Reference
Period 2 Elements: $\text{Li, Be, B, C, N, O, F, Ne}$
CBQ 34 • Science10NG4 — 1(a)(ix) • 1 mark
Give the symbol of the element from Period 2 which forms highly flammable gaseous/liquid compounds with hydrogen.
Symbol: $\text{C}$ (Carbon)
Carbon combines with hydrogen to form hydrocarbons such as methane ($\text{CH}_4$, primary constituent of CNG) which are highly combustible fuels.
CBQ 35 • Science10GD4 — 1(a) • 2 marks
State the group number and period number of carbon in the modern Periodic Table, and justify your answer based on its electronic configuration.
Group: 14 | Period: 2
• Electronic configuration: $2, 4$ (atomic number 6). • Period: 2 (electrons occupy 2 energy levels: K and L). • Group: $10 + 4 = 14$ (it has 4 valence electrons in the outermost shell).
CBQ 36 • Science10GD4 — 1(b)(i) • 1 mark
Carbon reacts with the element having atomic number 8. Name the major compound formed during complete combustion.
Compound Name: Carbon dioxide ($\text{CO}_2$)
Atomic number 8 corresponds to Oxygen ($\text{O}$). When carbon reacts with oxygen, it forms carbon dioxide ($\text{CO}_2$) during complete combustion (or carbon monoxide $\text{CO}$ in limited oxygen).
CBQ 37 • Science10GD4 — 1(b)(ii) • 1 mark
Name the type of chemical bonding present in carbon dioxide ($\text{CO}_2$).
Both carbon and oxygen are non-metals. Carbon shares two pairs of electrons with each of the two oxygen atoms, forming two double covalent bonds: $\text{O}=\text{C}=\text{O}$.
CBQ 38 • Science10GD4 — 1(c)(i) • 1 mark
Alkanes have the general molecular formula $\text{C}_n\text{H}_{2n+2}$. Calculate the molecular formula of the alkane for $n=2$.
Substituting $n=2$ into $\text{C}_n\text{H}_{2n+2}$ gives $\text{C}_2\text{H}_{2(2)+2} = \mathbf{\text{C}_2\text{H}_6}$ (Ethane).
CBQ 39 • Science10GD4 — 1(c)(ii) • 1 mark
Draw and specify the complete structural formula of ethane ($n=2$).
Structural Formula: $\text{CH}_3–\text{CH}_3$
$$\text{H}–\underset{\text{H}}{\overset{\text{H}}{\text{C}}}–\underset{\text{H}}{\overset{\text{H}}{\text{C}}}–\text{H}$$ Each carbon atom forms four single covalent bonds: one $\text{C}–\text{C}$ bond and three $\text{C}–\text{H}$ bonds.
CBQ 40 • Science10GD4 — 1(d)(i) • 1 mark
Alkenes have the general molecular formula $\text{C}_n\text{H}_{2n}$. Calculate the molecular formula of the alkene for $n=2$.
$$\underset{\text{H}}{\overset{\text{H}}{\text{C}}}=\underset{\text{H}}{\overset{\text{H}}{\text{C}}}$$ A double covalent bond connects the two carbon atoms, and each carbon is bonded to two hydrogen atoms.
CBQ 42 • Science10GD4 — 1(e) • 4 marks
Alkenes like ethene are polymerised to make plastics (polyethene). Discuss why plastics are widely used in food packaging, and explain two severe environmental consequences when plastic wrappers are discarded indiscriminately.
Utility: Moisture-proof, lightweight, cheap, and hygienic. Environmental Impact: Non-biodegradable, causes soil/drain clogging, and releases toxic gases on incineration.
1. Advantages for food packaging: • Impermeable barrier against moisture, airborne microbes, and oxidation, keeping food fresh. • Chemically inert, non-toxic, lightweight, durable, and highly cost-effective. 2. Environmental hazards: • Non-biodegradable persistence: Synthetic polyalkene polymers resist microbial degradation and persist in ecosystems for centuries, choking drains, causing urban flooding, and harming terrestrial/aquatic wildlife. • Toxic emissions: Open incineration of plastic waste releases hazardous pollutants like dioxins, furans, and dense carbon particulates into the atmosphere.
CBQ 43 • Science10SG4 — 1(a) • 2 marks
Ethanol ($\text{C}_2\text{H}_5\text{OH}$) is an industrially vital carbon compound. State one characteristic physical property and one major commercial use of ethanol.
Property: Volatile liquid with boiling point $78^\circ\text{C}$ ($351\text{ K}$), soluble in water in all proportions. Use: Universal solvent, hand sanitiser/antiseptic, and ingredient in medicines like tincture of iodine and cough syrups.
• Property: Ethanol is a colourless, volatile liquid with a pleasant alcoholic odour, neutral pH, and complete miscibility with water. • Use: Widely used as a solvent in pharmaceutical formulations (cough syrups, tonics), an antiseptic in hand sanitisers, and as a clean-burning biofuel additive in petrol (power alcohol).
CBQ 44 • Science10SG4 — 1(b)(i) • 1 mark
Determine the total number of covalent bonds present in one molecule of ethanol ($\text{CH}_3–\text{CH}_2–\text{OH}$).
Total Covalent Bonds: 8 covalent bonds
Detailed bond breakdown in $\text{C}_2\text{H}_5\text{OH}$: • Five $\text{C}–\text{H}$ single bonds = $5$ • One $\text{C}–\text{C}$ single bond = $1$ • One $\text{C}–\text{O}$ single bond = $1$ • One $\text{O}–\text{H}$ single bond = $1$ Total = $5 + 1 + 1 + 1 = \mathbf{8}$ single covalent bonds.
Ethanol can be converted to ethene by heating at $443\text{ K}$ with an acid: 1. Name the concentrated acid used for this conversion. 2. Why is this acid described as a "dehydrating agent"?
1. Acid: Concentrated Sulphuric Acid ($\text{Conc. }\text{H}_2\text{SO}_4$) 2. Role: It removes a molecule of water from the ethanol molecule.
1. Acid: Concentrated sulphuric acid ($\text{Conc. }\text{H}_2\text{SO}_4$) at $443\text{ K}$ ($170^\circ\text{C}$). 2. Dehydrating agent: In this reaction, concentrated $\text{H}_2\text{SO}_4$ chemically removes the elements of water (one $-OH$ from one carbon and one $-H$ from adjacent carbon) from ethanol: $$\text{CH}_3–\text{CH}_2\text{OH} \xrightarrow{\text{Conc. }\text{H}_2\text{SO}_4, \; 443\text{ K}} \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O}$$ Substances that extract water from compounds are called dehydrating agents.
CBQ 46 • Science10SG4 — 1(c)(i) & (ii) • 4 marks
In an experiment, a small piece of sodium metal is dropped into excess absolute ethanol. The gas produced is collected in a gas syringe over 60 seconds. 1. Identify the gas evolved and describe a confirmatory test for it. 2. Describe how the rate of gas evolution changes over time and suggest why.
1. Gas: Hydrogen gas ($\text{H}_2$); burns with a "pop" sound. 2. Rate: Gas evolution is rapid initially and progressively slows down as sodium surface area diminishes and is consumed.
1. Gas identity: Hydrogen gas ($\text{H}_2$). Equation: $$2\text{C}_2\text{H}_5\text{OH} + 2\text{Na} \longrightarrow 2\text{C}_2\text{H}_5\text{ONa (Sodium ethoxide)} + \text{H}_2(g)$$ Bringing a burning splinter near the mouth of the test tube results in the gas burning with a characteristic "pop" sound. 2. Rate progression: The rate of gas collection is fastest at the beginning because the sodium metal has fresh exposed surface area. Over time, as sodium reacts away and the surface area decreases, the reaction rate slows down and eventually ceases when all sodium is consumed.
CBQ 47 • Science10NB3 — 1(a)(i) • 3 marks
Ethanoic acid reacts with sodium metal to produce sodium ethanoate and a gas. Write the complete, balanced chemical equation including all state symbols.
Ethanoic acid is an acid and donates its ionisable carboxyl proton ($H^+$) to sodium, forming the soluble salt sodium ethanoate and liberating hydrogen gas with effervescence.
CBQ 48 • Science10NB3 — 1(a)(ii) • 1 mark
State one major household or commercial use of ethanoic acid.
Use: Food preservative and condiment in the form of vinegar ($5–8\%$ solution).
A $5–8\%$ dilute solution of ethanoic acid in water is called vinegar, which is extensively used as a preservative in pickles and food sauces, and as a flavoring agent.
Item Data & Reference
Experiment Setup: Saanvi dissolves different salts in distilled water to test lather formation with ordinary soap solution (sodium stearate): • Exp 1: $\text{NaCl}$ • Exp 2: $\text{CaCl}_2$ • Exp 3: $\text{MgCl}_2$ • Exp 4: $\text{KCl}$
CBQ 49 • Science10NB3 — 1(b) • 3 marks
In which experiments will Saanvi NOT observe the formation of rich lather (foam)? Explain the chemical reason.
No lather in: Experiments 2 ($\text{CaCl}_2$) and 3 ($\text{MgCl}_2$)
Saanvi will not observe lather in Experiment 2 ($\text{CaCl}_2$) and Experiment 3 ($\text{MgCl}_2$).
• Scientific Reason: Water containing dissolved calcium ($\text{Ca}^{2+}$) or magnesium ($\text{Mg}^{2+}$) salts is hard water. When soap (sodium salt of long-chain fatty acids) is added to hard water, the $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ ions displace $\text{Na}^+$ ions to form insoluble calcium/magnesium stearate precipitates called scum: $$2\text{C}_{17}\text{H}_{35}\text{COONa} + \text{Ca}^{2+} \longrightarrow (\text{C}_{17}\text{H}_{35}\text{COO})_2\text{Ca}\downarrow \text{ (Insoluble Scum)} + 2\text{Na}^+$$ Soap is consumed forming scum until all $\text{Ca}^{2+}/\text{Mg}^{2+}$ ions precipitate, preventing lather formation. • Conversely, $\text{NaCl}$ and $\text{KCl}$ do not cause hardness, and soap readily lathers in Exp 1 and Exp 4.
CBQ 50 • Activity Reflection: Ethanol vs Ethanoic Acid • 3 marks
Describe a chemical test using sodium carbonate ($\text{Na}_2\text{CO}_3$) or sodium hydrogen carbonate ($\text{NaHCO}_3$) to distinguish between a sample of ethanol and ethanoic acid. Write the balanced equation and observation.
Observation: Ethanoic acid gives brisk effervescence of $\text{CO}_2$; ethanol produces no reaction.
Chemical Differentiation:
• With Ethanoic Acid: Produces brisk effervescence of colourless, odourless carbon dioxide gas that turns lime water milky: $$2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \longrightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2\uparrow$$ $$\text{CH}_3\text{COOH} + \text{NaHCO}_3 \longrightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2\uparrow$$ • With Ethanol: No reaction or effervescence takes place because alcohols are far weaker acids and cannot decompose carbonates.
Rapid Revision
5–10 minute pre-exam recall
① Carbon Basics
Atomic number = 6; configuration = 2,4.
Four valence electrons → tetravalency.
Shares electrons → covalent bonds.
Covalent compounds generally have weak intermolecular forces and no ions.
② Why Carbon is Versatile
Tetravalency.
Catenation.
Strong C—C bonds.
Chains, branches, rings.
Single, double and triple C—C bonds.
③ Hydrocarbons
Hydrocarbon = carbon + hydrogen only.
Alkane = saturated.
Alkene = C=C.
Alkyne = C≡C.
Structural isomers = same formula, different structure.
④ Functional Groups & Naming
—OH → alcohol → -ol.
—CHO → aldehyde → -al.
>C=O → ketone → -one.
—COOH → carboxylic acid → -oic acid.
C=C → -ene; C≡C → -yne.
⑤ Homologous Series
Same functional group.
Similar chemical properties.
Successive members differ by CH₂.
Physical properties show gradual change with molecular mass.
⑥ Reaction Box
Combustion → CO₂ + H₂O + heat/light for complete combustion of hydrocarbons.
Oxidation → alcohol → carboxylic acid; KMnO₄/K₂Cr₂O₇ as oxidising agents.
Addition → unsaturated + H₂ → saturated.
Substitution → one atom/group replaces another.
⑦ Ethanol
2Na + 2C₂H₅OH → 2C₂H₅ONa + H₂.
443 K + excess conc. H₂SO₄ → ethene + water.
H₂SO₄ acts as dehydrating agent in the latter reaction.
⑧ Ethanoic Acid
Acetic acid; 5–8% aqueous solution = vinegar.
Weak carboxylic acid.
With NaOH → sodium ethanoate + water.
With Na₂CO₃/NaHCO₃ → salt + CO₂ + water.
CO₂ turns lime-water milky.
⑨ Soap & Detergent
Soap = sodium/potassium salts of long-chain carboxylic acids.
Hydrophilic head + hydrophobic tail.
Micelle traps oily dirt.
Hard water + soap → Ca/Mg scum.
Detergents remain effective in hard water.
🧠 Final memory chain: C = 4 → Catenation → Hydrocarbons → Functional
groups → Homologous series → Naming → COAS reactions → Ethanol/Ethanoic acid → Soap/Micelles.
Chapter Test
3 levels • 12 questions • instant score
Q1 • 1 mark
Carbon has how many valence electrons?
Q2 • 1 mark
Which bond is present in ethene?
Q3 • 1 mark
Successive members of a homologous series differ by:
Q4 • 1 mark
Butanone contains which functional group?
Q5 • 2 marks
Which pair can undergo addition reactions?
Q6 • 2 marks
A cooking vessel gets blackened at the bottom. What is the likely reason?
Q7 • 2 marks
Which is an oxidising agent mentioned in the chapter?
Q8 • 2 marks
Why does soap form scum in hard water?
Q9 • 3 marks
Which statement best explains carbon's huge variety of compounds?
Q10 • 3 marks
Ethanol at 443 K with excess concentrated H₂SO₄ gives: