Carbon and Its Compounds — Class 10 Chemistry NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 10 Chemistry chapter "Carbon and Its Compounds" — 10 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 10 Chemistry chapter "Carbon and Its Compounds" — 10 important questions with detailed answers for CBSE bo…
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NCERT Solutions for Class 10 Chemistry — Carbon and Its Compounds. Step-by-step answers to all 10 textbook questions from this chapter, written for CBSE board preparation and free to use.
Carbon and Its Compounds — All 10 Questions Solved
Q1. What is covalent bonding? Explain how it differs from ionic bonding with examples.
Covalent Bonding:
Covalent bonding is the type of chemical bond formed by the sharing of electrons between two atoms. Both atoms contribute one or more electrons to the shared pair. The electrons are shared between the nuclei of both atoms.
Example: Cl2 molecule
Each chlorine atom has 7 valence electrons. Both atoms share one pair of electrons to achieve the stable octet configuration.
Cl-Cl (single covalent bond)
Difference between Covalent and Ionic Bonding:
1. Electron Transfer vs. Sharing:
Covalent Bonding: Electrons are shared between atoms
Ionic Bonding: Electrons are transferred from one atom to another
2. Types of Atoms Involved:
Covalent Bonding: Usually forms between non-metals or non-metal atoms
Ionic Bonding: Forms between a metal and a non-metal
3. Physical State:
Covalent Bonding: Compounds are usually gaseous or liquid at room temperature (few exceptions)
Ionic Bonding: Compounds are solid at room temperature
4. Melting Point:
Covalent Bonding: Low melting and boiling points (usually below 300°C)
Ionic Bonding: High melting and boiling points (usually above 300°C)
5. Electrical Conductivity:
Covalent Bonding: Do not conduct electricity in solid or liquid state
Ionic Bonding: Conduct electricity in molten or aqueous solution
6. Solubility:
Covalent Bonding: Generally insoluble in water, soluble in non-polar solvents
Ionic Bonding: Generally soluble in water, insoluble in non-polar solvents
Example Comparison:
Covalent: H2O (water) - low melting point (0°C), non-conducting
Ionic: NaCl (salt) - high melting point (801°C), conducts when molten or dissolved
Q2. What are allotropes of carbon? Describe the structure and properties of diamond, graphite, and buckminsterfullerene (C60).
Allotropes are different forms of the same element in the same physical state. Carbon exhibits allotropism due to its unique atomic structure and ability to form strong covalent bonds.
Diamond:
Structure: Each carbon atom is bonded to four other carbon atoms in a tetrahedral arrangement. Forms a three-dimensional network of very strong C-C covalent bonds throughout the crystal.
Properties:
- Hardest natural substance on Earth
- Transparent and colorless
- Does not conduct electricity (electrons are localized in bonds)
- Very high melting point (3823°C)
- Insoluble in water and organic solvents
- Used in cutting tools, drills, jewelry
Graphite:
Structure: Carbon atoms arranged in layers. Each carbon atom in a layer is bonded to three other carbon atoms in a planar hexagonal arrangement. Layers are held by weak van der Waals forces.
Properties:
- Soft and slippery (because layers can slide over each other)
- Black and opaque
- Conducts electricity well (has free electrons between layers)
- Melting point: 3652°C
- Insoluble in water but reacts with some oxidizing agents
- Used as a lubricant, in pencils, in electrodes, in crucibles
Buckminsterfullerene (C60):
Structure: 60 carbon atoms arranged in a spherical cage structure, resembling a soccer ball. Each carbon atom bonded to three others in a fullerene structure.
Properties:
- Formed by vaporizing graphite in a laser or electric arc
- Black solid
- Soluble in organic solvents like benzene
- Does not conduct electricity in pure form (but can when doped)
- Used in nanotechnology, medical research, superconductors
Comparison Table:
Diamond - Hardness: Very hard, Conductivity: Non-conductor, Melting point: 3823°C
Graphite - Hardness: Soft, Conductivity: Conductor, Melting point: 3652°C
C60 - Hardness: Hard, Conductivity: Non-conductor (pure), Applications: Nanotech
Q3. Explain catenation and its significance in carbon chemistry. Why is carbon unique in this property?
Catenation:
Catenation is the ability of an element to form covalent bonds with atoms of the same element, forming long chains or rings. Carbon exhibits exceptional catenation ability.
Significance of Catenation in Carbon Chemistry:
1. Formation of Diverse Structures:
Carbon atoms can link with each other in various ways:
- Straight chains: C-C-C-C-C
- Branched chains: Carbon atoms branching off from main chain
- Cyclic structures: Rings of carbon atoms
- These different arrangements lead to different compounds called isomers
2. Formation of Homologous Series:
Catenation allows formation of alkanes (CnH2n+2), alkenes, alkynes, and other series
Example: Methane (CH4), Ethane (C2H6), Propane (C3H8), Butane (C4H10)
Each member differs by CH2 unit
3. Organic Chemistry Foundation:
Catenation is the basis of all organic chemistry. Millions of carbon compounds exist due to this property
4. Complex Molecules:
Carbon catenation allows formation of complex molecules like proteins, carbohydrates, fats, and polymers
Why Carbon is Unique in Catenation:
1. Small Atomic Size:
Carbon has a small atomic radius, allowing it to form strong C-C bonds with other carbon atoms
2. High Bond Energy:
Carbon-carbon bonds (347 kJ/mol) are strong and stable
The overlap of orbitals is efficient due to similar size
3. Valency:
Carbon has a valency of 4 and forms 4 covalent bonds
This allows it to form stable chains without strain
4. Low Electronegativity:
Carbon's electronegativity is moderate, so C-C bonds are non-polar and stable
Electrons in C-C bonds are not pulled away easily
5. Absence of d-orbitals in valence shell:
Carbon cannot expand its octet, limiting bond angles and orientations
This ensures proper geometry and strong bonding
Comparison with Other Elements:
Silicon (below carbon) has larger atoms and weaker Si-Si bonds (226 kJ/mol)
Hydrogen and Oxygen show less catenation than carbon
This makes carbon unique in forming extensive chains and ring structures
Q4. What is a homologous series? Explain the characteristics of alkanes as a homologous series.
Homologous Series:
A homologous series is a group of organic compounds with similar chemical properties, where each successive member differs from the previous one by a CH2 unit. Members have the same general formula and are arranged in order of increasing molecular weight.
Characteristics of a Homologous Series:
1. General Formula:
All members have the same general formula
For alkanes: CnH2n+2 (where n = 1, 2, 3, ...)
2. Structural Similarity:
Each member contains one more carbon atom and two more hydrogen atoms than the previous member
They have the same functional group
3. Gradation in Properties:
Physical properties change gradually as molecular weight increases
Chemical properties remain similar
4. Similar Chemical Reactions:
All members undergo similar types of reactions
Alkanes as a Homologous Series:
Alkanes are hydrocarbons containing only carbon and hydrogen with single C-C bonds. They form a perfect homologous series.
General Formula: CnH2n+2
Examples of Alkanes:
Methane (CH4) - n=1
Ethane (C2H6) - n=2
Propane (C3H8) - n=3
Butane (C4H10) - n=4
Pentane (C5H12) - n=5
Hexane (C6H14) - n=6
Heptane (C7H16) - n=7
Octane (C8H18) - n=8
Each member differs by CH2 unit:
C2H6 differs from C3H8 by CH2
C3H8 differs from C4H10 by CH2
Gradation of Properties in Alkanes:
1. Physical State:
Methane to Butane: Gases (at room temperature)
Pentane to Heptane: Liquids
Octane and above: Solids
2. Boiling Point:
Methane: -161°C
Ethane: -89°C
Propane: -42°C
Butane: 0°C
Pentane: 36°C
Hexane: 69°C
Boiling point increases as we go up the series
3. Density:
Increases with increase in molecular weight
Methane: 0.55 g/cm3 (gas)
Hexane: 0.66 g/cm3 (liquid)
Octane: 0.70 g/cm3 (liquid)
4. Solubility in Water:
Decreases with increase in chain length
Methane is slightly soluble
Higher alkanes are insoluble in water
5. Chemical Properties:
All alkanes show similar reactions:
- Combustion reaction
- Substitution reaction with halogens
- No reaction with dilute acids, bases, or KMnO4
- Generally unreactive at room temperature
Q5. Explain IUPAC nomenclature for alkanes. Write the IUPAC names for the following: CH4, C2H6, C3H8, C4H10.
IUPAC Nomenclature for Alkanes:
IUPAC stands for International Union of Pure and Applied Chemistry. It provides systematic rules for naming organic compounds.
Rules for Naming Alkanes:
1. Identify the longest carbon chain in the molecule
This is called the parent chain. The name is based on the number of carbons in the longest chain
2. Number the carbon atoms in the parent chain
Number from the end that gives the lowest numbers to substituents
3. Identify and name substituents (alkyl groups)
Alkyl groups are named by replacing -ane with -yl
Methyl (CH3), Ethyl (C2H5), Propyl (C3H7), Butyl (C4H9)
4. Write the name as:
position number + name of substituent + parent chain name
Example: 2-methylpropane
Number of Carbons and Corresponding Names:
1 carbon: meth-
2 carbons: eth-
3 carbons: prop-
4 carbons: but-
5 carbons: pent-
6 carbons: hex-
7 carbons: hept-
8 carbons: oct-
Suffix for alkanes: -ane
IUPAC Names for Given Compounds:
1. CH4 (Methane)
Structure: A single carbon bonded to 4 hydrogens
Number of carbons in chain: 1
Parent name: Methane
No substituents
IUPAC Name: Methane
2. C2H6 (Ethane)
Structure: Two carbons bonded together, each with hydrogens
Number of carbons in chain: 2
Parent name: Ethane
No substituents
IUPAC Name: Ethane
3. C3H8 (Propane)
Structure: Three carbons in a chain, saturated with hydrogens
Number of carbons in chain: 3
Parent name: Propane
No substituents
IUPAC Name: Propane
4. C4H10 (Butane)
This compound can exist in two isomers:
Isomer 1: n-Butane (straight chain)
Structure: C-C-C-C (four carbons in straight line)
Number of carbons: 4
Parent name: Butane
No substituents
IUPAC Name: Butane or n-Butane
Isomer 2: Isobutane or 2-methylpropane (branched chain)
Structure:
C
|
C-C-C
Main chain: 3 carbons (parent: propane)
Substituent: 1 methyl group at position 2
IUPAC Name: 2-methylpropane
Note: The term 'butane' usually refers to the straight-chain isomer, while 'isobutane' or '2-methylpropane' refers to the branched isomer. Both have molecular formula C4H10 but different structures (isomers).
Q6. What are functional groups? Explain the structures and properties of hydroxyl (-OH) and carboxyl (-COOH) groups.
Functional Groups:
Functional groups are atoms or groups of atoms bonded in a specific way that give characteristic chemical properties to an organic molecule. They determine the type of reactions that the compound can undergo.
A functional group is the reactive part of an organic molecule. The same functional group in different molecules produces similar chemical reactions.
Examples of Functional Groups:
- Hydroxyl (-OH): Found in alcohols
- Carboxyl (-COOH): Found in carboxylic acids
- Aldehyde (-CHO): Found in aldehydes
- Ketone (C=O): Found in ketones
- Ester (-COO-): Found in esters
Hydroxyl Group (-OH):
Structure: An oxygen atom bonded to a hydrogen atom, and the oxygen is bonded to a carbon atom
R-O-H (where R is the rest of the molecule)
Characteristic Properties:
1. Polarity:
The O-H bond is polar due to high electronegativity of oxygen
This makes compounds with hydroxyl groups polar
2. Hydrogen Bonding:
Hydroxyl groups can form hydrogen bonds with water and other polar molecules
This increases solubility in water
3. Acidic Nature:
The hydrogen in the -OH group can be released as H+ ion
Makes the compound weakly acidic
4. Reactions:
Can undergo oxidation reactions
Can form esters with carboxylic acids
Can undergo esterification and saponification
Examples of Compounds with Hydroxyl Group:
Ethanol (CH3CH2OH) - Sweet smell, used in drinks and as fuel
Glycerol (CH2OH-CHOH-CH2OH) - Sweet taste, used in cosmetics
Carboxyl Group (-COOH):
Structure: A carbon atom double bonded to an oxygen and single bonded to an -OH group
R-COOH or R-CO-OH (where R is the rest of the molecule)
Alternative structure: Contains both a carbonyl (C=O) and a hydroxyl group
Characteristic Properties:
1. Acidic Nature:
Carboxylic acids are weak acids
The hydrogen of the -COOH group can be released as H+ ion
CH3COOH -> CH3COO- + H+
2. Ionization in Water:
Carboxylic acids partially ionize in water
The extent of ionization depends on the strength of the acid
Ethanoic acid: CH3COOH -> CH3COO- + H+ (partial ionization)
3. Reactions:
React with metals to produce hydrogen gas
2CH3COOH + Zn -> (CH3COO)2Zn + H2
React with bases (alkalis) to form salts and water:
CH3COOH + NaOH -> CH3COONa + H2O
React with alcohols to form esters:
CH3COOH + C2H5OH -> CH3COOC2H5 + H2O
This reaction is called esterification
4. Polarity and Hydrogen Bonding:
The -COOH group is polar and can form hydrogen bonds
Most carboxylic acids are soluble in water
5. Polymerization:
Multiple carboxyl groups can link together through condensation
Examples of Compounds with Carboxyl Group:
Acetic acid (Ethanoic acid): CH3COOH - Found in vinegar
Formic acid (Methanoic acid): HCOOH - Found in ant venom
Palmic acid: Long-chain carboxylic acid found in fats and oils
Comparison:
Hydroxyl Group: -OH, Weakly acidic, Found in alcohols
Carboxyl Group: -COOH, Strongly acidic (weak acid), Found in carboxylic acids
Q7. Explain the properties of ethanol (C2H5OH) and ethanoic acid (CH3COOH). Why does ethanoic acid show acidic properties?
Ethanol (C2H5OH):
Structure: CH3-CH2-OH
Ethanol is the simplest alcohol with the hydroxyl functional group
Physical Properties:
1. Appearance: Clear, colorless liquid
2. Odor: Characteristic pungent odor
3. Boiling Point: 78.4°C (lower than water due to different intermolecular forces)
4. Melting Point: -114°C
5. Density: 0.79 g/cm3 (less dense than water, so floats on water)
6. Solubility: Completely miscible with water (mixes in all proportions)
Reason: Ethanol can form hydrogen bonds with water molecules due to the -OH group
Chemical Properties of Ethanol:
1. Combustion (Burning):
C2H5OH + 3O2 -> 2CO2 + 3H2O
Ethanol burns with a blue flame to produce carbon dioxide and water
Used as fuel in some vehicles (biofuel)
2. Oxidation:
Ethanol can be oxidized to acetaldehyde and then to acetic acid
C2H5OH + [O] -> CH3CHO + H2O (oxidation to aldehyde)
CH3CHO + [O] -> CH3COOH (oxidation to acetic acid)
When exposed to air, ethanol slowly oxidizes (turns sour)
3. Esterification:
Ethanol reacts with carboxylic acids to form esters
C2H5OH + CH3COOH -> CH3COOC2H5 + H2O
The reaction requires a catalyst (concentrated H2SO4) and heat
Esters have fruity odors
4. Reaction with Metals:
Ethanol reacts with active metals like sodium to produce hydrogen gas
2C2H5OH + 2Na -> 2C2H5ONa + H2
Sodium ethoxide (C2H5ONa) is formed
5. Reaction with Phosphorus Pentachloride:
C2H5OH + PCl5 -> C2H5Cl + POCl3 + HCl
The -OH group is replaced by chlorine
Uses of Ethanol:
- Alcoholic beverages
- Industrial solvent
- Biofuel
- Manufacturing other chemicals
Ethanoic Acid (CH3COOH):
Structure: CH3-COOH (or CH3-CO-OH)
Ethanoic acid is the simplest carboxylic acid with the carboxyl functional group
Commonly known as acetic acid, found in vinegar
Physical Properties:
1. Appearance: Clear, colorless liquid
2. Odor: Pungent vinegar-like smell
3. Boiling Point: 118°C (higher than ethanol due to strong hydrogen bonding)
4. Melting Point: 16.6°C (can form ice-like crystals in cold)
5. Density: 1.05 g/cm3 (slightly denser than water)
6. Solubility: Completely miscible with water
Reason: The -COOH group can form strong hydrogen bonds with water
Chemical Properties of Ethanoic Acid:
1. Acidic Nature:
Ethanoic acid is a weak acid. It partially ionizes in water:
CH3COOH ⇌ CH3COO- + H+
The equilibrium favors the undissociated form
Turns blue litmus paper red
pH around 2.4 (1% solution)
2. Reaction with Bases (Alkalis):
CH3COOH + NaOH -> CH3COONa + H2O
Ethanoic acid reacts with sodium hydroxide to form sodium ethanoate and water
The reaction is neutralization
CH3COOH + Ca(OH)2 -> (CH3COO)2Ca + H2O
Calcium ethanoate is formed
3. Reaction with Metals:
CH3COOH + Zn -> (CH3COO)2Zn + H2
Ethanoic acid reacts with active metals to produce hydrogen gas
The metal reacts with the acidic hydrogen
4. Reaction with Carbonates and Bicarbonates:
2CH3COOH + Na2CO3 -> 2CH3COONa + H2O + CO2
Ethanoic acid reacts with carbonates to produce CO2 gas, which causes effervescence
CH3COOH + NaHCO3 -> CH3COONa + H2O + CO2
5. Esterification:
CH3COOH + C2H5OH -> CH3COOC2H5 + H2O (Ethyl ethanoate)
CH3COOH + CH3OH -> CH3COOCH3 + H2O (Methyl ethanoate)
Ethanoic acid reacts with alcohols in the presence of concentrated H2SO4 catalyst and heat
Esters are formed (reversible reaction)
6. Oxidation:
Ethanoic acid cannot be easily oxidized further due to stability of carboxyl group
7. Esterification with Alcohols:
Produces pleasant-smelling esters
Ethyl ethanoate smells like fruit and is used in solvents and fragrances
Why Ethanoic Acid Shows Acidic Properties:
1. Ionization in Water:
The hydrogen of the -COOH group ionizes to release H+ ions
CH3COOH ⇌ CH3COO- + H+
These H+ ions make the solution acidic
2. Electron-Withdrawing Effect of Oxygen:
The two oxygen atoms in the carboxyl group are highly electronegative
They pull electron density away from the O-H bond
This weakens the O-H bond and makes it easier to release H+ ion
3. Stability of Conjugate Base:
When CH3COOH loses H+, it forms CH3COO- (acetate ion)
The negative charge on the acetate ion is stabilized by resonance
The charge is distributed between the two oxygen atoms
This stability favors ionization of the acid
4. Comparison with Ethanol:
Ethanol (C2H5OH) does not release H+ ions because:
- The -OH group in ethanol has only one oxygen
- The C-O bond is less polar than the C=O bond in carboxylic acids
- The alcohol is amphoteric but shows negligible acidity
- No resonance stabilization of the resulting alkoxide ion
Conclusion:
Ethanoic acid shows acidic properties because the -COOH group can ionize to release H+ ions. The presence of two oxygen atoms (C=O and O-H) and resonance stabilization of the conjugate base make it acidic, unlike ethanol which cannot ionize.
Q8. Explain esterification and saponification reactions. Write the chemical equations with examples.
Esterification:
Definition: Esterification is a chemical reaction between a carboxylic acid and an alcohol in the presence of a catalyst to form an ester and water.
General Reaction:
R1-COOH + R2-OH -> R1-COO-R2 + H2O
(Carboxylic acid + Alcohol -> Ester + Water)
Conditions:
- A catalyst (usually concentrated H2SO4) is required
- Heat is required (usually around 140°C)
- The reaction is reversible and reaches equilibrium
Example 1: Formation of Ethyl Ethanoate
CH3COOH + C2H5OH -> CH3COOC2H5 + H2O
Ethanoic acid + Ethanol -> Ethyl ethanoate + Water
The ester formed has a fruity odor and is used in perfumes and solvents
Example 2: Formation of Methyl Ethanoate
CH3COOH + CH3OH -> CH3COOCH3 + H2O
Ethanoic acid + Methanol -> Methyl ethanoate + Water
Example 3: Formation of Propyl Ethanoate
CH3COOH + C3H7OH -> CH3COOC3H7 + H2O
Ethanoic acid + Propanol -> Propyl ethanoate + Water
Characteristics of Esters:
- Pleasant fruity odor
- Volatile liquids (low boiling points)
- Used in perfumes, flavoring agents, and solvents
- Used in paints and varnishes
Reverse Reaction - Hydrolysis of Esters:
Esters can be hydrolyzed (broken down) by acids, bases, or water under appropriate conditions
R1-COO-R2 + H2O -> R1-COOH + R2-OH (in acidic conditions)
R1-COO-R2 + NaOH -> R1-COONa + R2-OH (in basic conditions - saponification)
Saponification:
Definition: Saponification is the reaction of an ester with a strong base (usually NaOH or KOH) to produce a salt of carboxylic acid (soap) and an alcohol. It is the hydrolysis of fats and oils using alkalis.
General Reaction:
R1-COO-R2 + NaOH -> R1-COONa + R2-OH
(Ester + Sodium hydroxide -> Sodium salt of carboxylic acid + Alcohol)
The sodium salt of carboxylic acid formed is called soap (when the ester is a fat or oil)
Example 1: Saponification of Ethyl Ethanoate
CH3COOC2H5 + NaOH -> CH3COONa + C2H5OH
Ethyl ethanoate + Sodium hydroxide -> Sodium ethanoate + Ethanol
Example 2: Saponification of Fats/Oils (Soap Making)
Fats and oils are esters of glycerol with long-chain carboxylic acids (fatty acids)
Tristearin (a fat) + NaOH:
C3H5(OOCR)3 + 3NaOH -> C3H5(OH)3 + 3RCOONa
Fat + Sodium hydroxide -> Glycerol + Sodium salts of fatty acids (soap)
Specific Example - Saponification of Tallow (Beef Fat):
C3H5(OOCR)3 + 3NaOH -> Glycerol + 3RCOONa
Where R is typically C15H31 or C17H35
The products are glycerol and sodium stearate or sodium palmitate (common soaps)
Characteristics of Saponification:
- The reaction is irreversible (unlike esterification which is reversible)
- Heat is released (exothermic)
- Requires strong bases (KOH or NaOH)
- Used industrially for soap making
- The soap produced has cleaning properties
Soap Structure and Cleansing Action:
Soap (like sodium stearate) has structure: CH3(CH2)16-COONa
The molecule has two parts:
- Long hydrocarbon tail: Non-polar, hydrophobic (water-repelling)
- Carboxylate head: Polar, hydrophilic (water-attracting)
Cleansing Action:
When soap is added to water containing oil/dirt:
- The hydrophobic tail attaches to oil/dirt molecules
- The hydrophilic head remains in water
- Forms micelles (spherical structures with hydrophobic core and hydrophilic surface)
- Oil/dirt becomes suspended in water and can be washed away
Differences between Esterification and Saponification:
Esterification:
- Reaction between carboxylic acid and alcohol
- Requires acid catalyst (H2SO4)
- Reversible reaction
- Produces ester and water
- Example: CH3COOH + C2H5OH -> CH3COOC2H5 + H2O
Saponification:
- Reaction between ester and strong base (NaOH or KOH)
- No catalyst needed
- Irreversible reaction
- Produces salt of carboxylic acid and alcohol
- Example: CH3COOC2H5 + NaOH -> CH3COONa + C2H5OH
- Used for soap making when the ester is a fat or oil
Q9. What are soaps and detergents? Explain the difference between them and how they work for cleaning.
Soaps:
Definition: Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids). They are obtained by saponification of fats and oils.
Structure: A typical soap molecule (sodium stearate) has the structure:
CH3(CH2)16-COONa or R-COONa
Where R is a long hydrocarbon chain (typically C12 to C18 carbons)
Formation:
Fats and oils (esters) + Strong base (NaOH or KOH) -> Soap + Glycerol
Example: C3H5(OOCR)3 + 3NaOH -> 3RCOONa + C3H5(OH)3
Common Soaps:
- Sodium palmitate (from palm oil)
- Sodium stearate (from tallow/beef fat)
- Sodium oleate (from olive oil)
Detergents:
Definition: Detergents are synthetic compounds that have cleansing properties similar to soaps. They are usually sodium salts of sulfonated hydrocarbons or alkyl benzenes.
Structure: Detergents have a similar two-part structure to soaps:
R-SO3Na (where R is a long hydrocarbon chain)
Or R-O-C6H4-SO3Na (aryl sulfonates)
Examples:
- Sodium alkyl sulfate (SDS) - commonly used in shampoos and detergents
- Sodium dodecyl benzene sulfonate - used in laundry detergents
Formation:
Detergents are synthesized chemically and do not come from natural fats and oils
They are manufactured from crude oil derivatives and other synthetic materials
How Soaps and Detergents Work for Cleaning:
Both soaps and detergents have similar molecular structure:
One end: Hydrophobic tail (water-repelling) - Long hydrocarbon chain
Other end: Hydrophilic head (water-loving) - Charged group (COO- for soap, SO3- for detergent)
Cleansing Mechanism:
1. In Water Alone (without soap or detergent):
Oil and water do not mix (immiscible) because oil is non-polar and water is polar
Dirt and grease cannot be removed by water alone
2. With Soap/Detergent Present:
When soap is added to water with dirt/oil:
Step 1: Soap molecules orient themselves at the oil-water interface
- Hydrophobic tails point toward the oil/dirt (non-polar)
- Hydrophilic heads point toward water (polar)
Step 2: Formation of Micelles
Multiple soap molecules arrange themselves around oil droplets
- The hydrophobic tails surround the oil/dirt
- The hydrophilic heads face outward toward the water
- This forms a spherical structure called a micelle
Step 3: Suspension in Water
The micelle with trapped oil/dirt becomes suspended in water
The negatively charged heads repel each other, preventing micelles from coalescing
The oil/dirt is now able to disperse in water and can be washed away
4. Washing Away
The emulsified oil/dirt is washed away with water
Differences between Soaps and Detergents:
Soaps:
1. Source: Derived from natural fats and oils
2. Chemical Composition: Sodium or potassium salts of long-chain fatty acids (R-COONa)
3. Ionizable Group: Carboxylate group (-COO-)
4. Hardness: Do not work well in hard water
5. Biodegradability: Biodegradable (easily broken down in nature)
6. pH: Work better in neutral to slightly alkaline solutions
7. Cost: Generally less expensive
8. Examples: Castile soap, bar soap, laundry soap
Detergents:
1. Source: Synthetic compounds made from crude oil or chemical synthesis
2. Chemical Composition: Usually sodium salts of sulfonated hydrocarbons or aryl sulfonates (R-SO3Na)
3. Ionizable Group: Sulfonate group (-SO3-) or sulfate group (-SO4-)
4. Hardness: Work well in hard water (hard water ions do not precipitate with detergents like they do with soaps)
5. Biodegradability: Some are biodegradable, some are not (older synthetic detergents were not biodegradable)
6. pH: Work well in a wide range of pH
7. Cost: More expensive than soaps
8. Examples: Laundry detergents, dishwashing liquids, shampoos
Why Detergents Work Better in Hard Water:
Hard water contains dissolved salts of calcium and magnesium (Ca2+, Mg2+)
With Soaps in Hard Water:
Ca2+ + 2(R-COO-) -> (R-COO-)2Ca (precipitate)
Mg2+ + 2(R-COO-) -> (R-COO-)2Mg (precipitate)
The soap precipitates and forms scum
Soap becomes ineffective
More soap is required
With Detergents in Hard Water:
Detergent ions do not form insoluble precipitates with Ca2+ and Mg2+
Detergents remain soluble and effective
Detergents continue to work as cleansing agents
Advantages of Detergents over Soaps:
1. Work in hard water without losing effectiveness
2. Work well in acidic conditions
3. More effective cleaning power
4. Do not form scum
5. Useful for cleaning various materials
Disadvantages of Detergents:
1. Some synthetic detergents are not biodegradable and pollute environment
2. Can cause algae bloom in water bodies
3. More expensive than soaps
4. Can irritate skin if not rinsed properly
Conclusion:
Both soaps and detergents are surfactants (surface-active agents) that reduce surface tension and help in cleansing. Soaps are natural products from fats and oils, while detergents are synthetic. Detergents are more versatile and work better in hard water, making them more commonly used in modern products. However, soaps remain valuable for specific applications and are more environmentally friendly.
Q10. What are micelles? Explain their structure and importance in the cleansing action of soaps and detergents.
Micelles:
Definition: Micelles are spherical aggregates of soap or detergent molecules arranged in a specific manner to enclose or surround oil/dirt molecules. They form spontaneously in aqueous solutions when soap or detergent concentration exceeds a certain level called the Critical Micelle Concentration (CMC).
Structure of Micelles:
A micelle consists of:
- Inner core: Non-polar region containing hydrophobic tails of soap/detergent molecules
- Outer surface: Hydrophilic heads in contact with water
Arrangement:
1. Multiple soap or detergent molecules (typically 50-100 molecules) arrange themselves together
2. The long hydrocarbon tails (hydrophobic, water-repelling) point inward toward the center
3. The charged head groups (hydrophilic, water-loving) point outward toward the aqueous solution
4. This creates a spherical or ellipsoidal structure
5. Non-polar oils and grease dissolve in the non-polar core
6. The charged heads on the surface interact with water molecules
Diagram of Micelle Structure:
Cross-section view:
Outside (water): H2O molecules
First layer: Charged heads of soap molecules (COO-, SO3-) facing water
Inner core: Long hydrocarbon tails pointing toward center
Center: Oil/dirt trapped in non-polar environment
Critical Micelle Concentration (CMC):
Definition: The minimum concentration of soap or detergent required for micelle formation
Below CMC:
- Individual soap molecules dissolved in water
- No micelles formed
At CMC:
- Molecules begin to aggregate
- First micelles start forming
Above CMC:
- Most excess molecules form micelles
- Additional micelles form as concentration increases
- Surface tension of solution remains constant
Importance of Micelles in Cleansing Action:
1. Solubilization of Oil and Grease:
Oils and greases are non-polar and do not dissolve in water (polar)
When micelles form, the non-polar oils dissolve in the non-polar core
This converts water-insoluble dirt into water-soluble micelles
The dirt can now be easily washed away with water
2. Stabilization of Emulsions:
Micelles prevent oil droplets from coalescing
The negatively charged head groups repel each other
This electrostatic repulsion keeps the oil droplets separated and suspended
Prevents the oil from settling back onto the fabric or surface
3. Prevention of Re-deposition:
Without micelles, washed-out oil can re-deposit on fabric
Micelles keep the oil suspended throughout the wash water
Ensures thorough rinsing removes all dirt
4. Increased Effective Concentration:
Micelles increase the effective surface area for cleaning
More soap molecules available for interaction with dirt
Improves cleaning efficiency
5. Handling of Different Types of Dirt:
Polar dirt and grease: Solubilized in micelle cores
Water-soluble dirt: Dispersed by ionic interaction with micelle heads
Mixed dirt: Both types can be handled simultaneously
Process of Micelle Formation and Cleaning:
Step 1: Micelle Formation
- Soap molecules accumulate at oil-water interface
- As more soap molecules added, they aggregate
- Once CMC reached, micelles form spontaneously
Step 2: Encapsulation of Oil/Dirt
- Oil/grease molecules dissolve in the non-polar core
- Hydrophobic tails completely surround the dirt
- Hydrophilic heads shield the dirt from water
Step 3: Suspension in Water
- Negatively charged heads point outward
- Repel each other (electrostatic repulsion)
- Prevents aggregation and coalescence
- Dirt remains uniformly dispersed in solution
Step 4: Rinsing
- Water carries the suspended micelles away
- Dirt never re-deposits because it remains encapsulated
- Multiple rinses ensure complete removal
Step 5: Dilution Effect
- As water is added during rinsing
- Micelles break apart
- The free space in water accommodates dispersed molecules
- Dirt is washed away permanently
Types of Micelles:
1. Normal Micelles (in water):
- Formed in aqueous solutions
- Hydrophobic tails inside, hydrophilic heads outside
- Used for cleaning applications
2. Reverse Micelles (in non-polar solvents):
- Formed in non-polar solvents
- Hydrophilic heads inside, hydrophobic tails outside
- Used in specialized applications
Factors Affecting Micelle Formation:
1. Temperature:
- Higher temperature favors micelle formation
- Increases molecular movement
- CMC decreases with increasing temperature
2. Concentration of Soap/Detergent:
- Must exceed CMC for micelle formation
- More concentrated solutions have more micelles
- Size of micelles may increase with concentration
3. Presence of Salts:
- Salts can increase or decrease CMC
- Affects the size and stability of micelles
4. pH:
- Affects ionization of head groups
- Influences electrostatic repulsion
- Changes in pH can alter micelle formation
5. Nature of the Molecule:
- Length of hydrocarbon chain affects CMC
- Longer chains have lower CMC
- Structure of head group influences micelle size
Advantages of Micelles in Detergency:
1. Enables cleaning of greasy/oily surfaces with water
2. Reduces interfacial tension between oil and water
3. Increases solubility of non-polar substances in water
4. Prevents re-deposition of dirt during washing
5. Makes emulsification and suspensions stable
6. Essential for effectiveness of soaps and detergents
7. Explains why detergents are superior to water alone
Conclusion:
Micelles are crucial structural units formed by soap and detergent molecules that enable them to trap and suspend oil/grease in water. Their formation is a spontaneous, thermodynamically favorable process that occurs above the CMC. The micelle structure, with hydrophobic interior and hydrophilic exterior, perfectly suited for capturing non-polar dirt and keeping it dispersed in water, makes micelles the key mechanism behind the cleansing action of soaps and detergents.
Carbon and Its Compounds — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. What type of bond is formed between carbon atoms in a hydrocarbon?
- A. Ionic bond
- B. Metallic bond
- C. Covalent bond
- D. Hydrogen bond
Answer: C. Covalent bond — Carbon shares its valence electrons with other carbon atoms or hydrogen atoms to form covalent bonds, achieving a stable electron configuration.
MCQ 2. How many valence electrons does a carbon atom have?
- A. 2
- B. 4
- C. 6
- D. 8
Answer: B. 4 — Carbon has an atomic number of 6, with its electronic configuration being 2, 4. Thus, it has 4 valence electrons.
MCQ 3. Which unique property of carbon allows it to form long chains, branched chains, and rings?
- A. Tetravalency
- B. Allotropy
- C. Catenation
- D. Isomerism
Answer: C. Catenation — Catenation is the self-linking property of carbon atoms through covalent bonds to form long chains, branches, and rings.
MCQ 4. Which allotrope of carbon is a good conductor of electricity?
- A. Diamond
- B. Graphite
- C. Fullerene
- D. Coke
Answer: B. Graphite — Graphite has delocalized electrons within its layers, which allows it to conduct electricity, unlike diamond where all electrons are fixed in covalent bonds.
MCQ 5. What is the general formula for alkanes?
- A. CnH2n
- B. CnH2n-2
- C. CnH2n+2
- D. CnHn
Answer: C. CnH2n+2 — Alkanes are saturated hydrocarbons containing only single bonds, and their general formula is CnH2n+2, where 'n' is the number of carbon atoms.
MCQ 6. The IUPAC name for CH3-CH2-CH3 is:
- A. Ethane
- B. Propene
- C. Propane
- D. Butane
Answer: C. Propane — The compound has three carbon atoms and is an alkane (all single bonds), so its IUPAC name is propane.
MCQ 7. Which of the following pairs represents structural isomers?
- A. Ethane and Ethene
- B. Propane and Butane
- C. n-Butane and Isobutane
- D. Methane and Methanol
Answer: C. n-Butane and Isobutane — Structural isomers have the same molecular formula but different structural arrangements of atoms. n-Butane and isobutane both have the molecular formula C4H10 but differ in their carbon chain structure.
MCQ 8. The functional group present in propanone is:
- A. Carboxylic acid
- B. Alcohol
- C. Aldehyde
- D. Ketone
Answer: D. Ketone — Propanone (CH3COCH3) belongs to the ketone family, characterized by the carbonyl group (-CO-) bonded to two alkyl groups.
MCQ 9. Which of the following is NOT a characteristic of a homologous series?
- A. All members have the same functional group.
- B. They show a gradual change in physical properties.
- C. Their chemical properties are identical.
- D. Consecutive members differ by a -CH2 unit.
Answer: C. Their chemical properties are identical. — While members of a homologous series have similar chemical properties due to the same functional group, their chemical reactivity can vary slightly with increasing carbon chain length. They are not identical.
MCQ 10. When ethanol reacts with sodium metal, the products formed are:
- A. Sodium ethoxide and water
- B. Sodium ethoxide and hydrogen gas
- C. Sodium hydroxide and ethane
- D. Sodium acetate and hydrogen gas
Answer: B. Sodium ethoxide and hydrogen gas — Ethanol reacts with active metals like sodium to produce sodium ethoxide and hydrogen gas, similar to the reaction of water with sodium.
MCQ 11. The functional group for alcohols is:
- A. -COOH
- B. -CHO
- C. -OH
- D. -CO-
Answer: C. -OH — The hydroxyl group (-OH) is the characteristic functional group for alcohols.
MCQ 12. Which of the following is an unsaturated hydrocarbon?
- A. Ethane
- B. Propane
- C. Butane
- D. Ethene
Answer: D. Ethene — Unsaturated hydrocarbons contain double or triple bonds between carbon atoms. Ethene (C2H4) has a carbon-carbon double bond, while ethane, propane, and butane are alkanes with only single bonds.
MCQ 13. Ethanol can be converted into ethanoic acid by which of the following reactions?
- A. Dehydration
- B. Hydrogenation
- C. Oxidation
- D. Substitution
Answer: C. Oxidation — Ethanol can be oxidized to ethanoic acid using oxidizing agents like alkaline KMnO4 or acidified K2Cr2O7.
MCQ 14. Which allotrope of carbon is the hardest known natural substance?
- A. Graphite
- B. Diamond
- C. Fullerene
- D. Coal
Answer: B. Diamond — Diamond has a rigid three-dimensional network structure, making it the hardest natural substance.
MCQ 15. The reaction between an alcohol and a carboxylic acid in the presence of an acid catalyst produces:
- A. Alkene and water
- B. Aldehyde and water
- C. Ester and water
- D. Ether and water
Answer: C. Ester and water — This reaction is known as esterification, where an alcohol and a carboxylic acid react to form an ester and water.
MCQ 16. The process of converting an ester back into an alcohol and a carboxylic acid (or its salt) using a base like NaOH is called:
- A. Esterification
- B. Hydrogenation
- C. Saponification
- D. Dehydration
Answer: C. Saponification — Saponification is the hydrolysis of an ester by a strong base, typically used in the production of soap (which are sodium or potassium salts of long-chain fatty acids).
MCQ 17. Which type of reaction is characteristic of unsaturated hydrocarbons?
- A. Substitution reaction
- B. Addition reaction
- C. Combustion reaction
- D. Saponification reaction
Answer: B. Addition reaction — Unsaturated hydrocarbons (alkenes and alkynes) readily undergo addition reactions across their double or triple bonds, where atoms are added without the loss of any other atoms.
MCQ 18. The functional group -COOH represents a:
- A. Alcohol
- B. Aldehyde
- C. Ketone
- D. Carboxylic acid
Answer: D. Carboxylic acid — The -COOH group, consisting of a carbonyl group attached to a hydroxyl group, is the characteristic functional group of carboxylic acids.
MCQ 19. What part of a soap molecule is responsible for dissolving grease and oil?
- A. Hydrophilic head
- B. Hydrophobic tail
- C. Ionic bond
- D. Covalent bond
Answer: B. Hydrophobic tail — The hydrophobic (water-repelling) hydrocarbon tail of a soap molecule is attracted to and dissolves in grease/oil, while the hydrophilic (water-attracting) ionic head remains in water. This leads to micelle formation.
MCQ 20. Which of the following is a characteristic property of ethanoic acid (acetic acid)?
- A. It is a strong mineral acid.
- B. It does not react with sodium carbonate.
- C. It turns blue litmus red.
- D. It has a sweet smell.
Answer: C. It turns blue litmus red. — Ethanoic acid is a weak organic acid, so it will turn blue litmus paper red. It reacts with carbonates to produce CO2 and has a pungent, vinegar-like smell, not sweet.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.