Free NCERT Solutions, Notes & MCQs for Class 9–12

by Academia Aeternum

1-2 liner Questions

Q1: What is tetravalency of carbon?

Carbon can form four covalent bonds because it has four electrons in its outer shell.

Q2: Define catenation.

Catenation is the ability of carbon atoms to bond with each other forming long chains and rings.

Q3: What is an alkane?

Alkanes are saturated hydrocarbons containing only single covalent bonds.

Q4: Name the first member of the alkene series.

Ethene is the first stable member of the alkene family.

Q5: What is the functional group in alcohols?

The functional group in alcohols is the hydroxyl group (–OH).

Q6: Write the IUPAC name of CH3OH.

The IUPAC name of \(\ce{CH3OH}\) is methanol.

Q7: What is esterification?

The reaction of an alcohol with a carboxylic acid to form an ester is called esterification.

Q8: What is the common name of ethanoic acid?

Ethanoic acid is commonly known as acetic acid.

Q9: Define micelle.

Micelles are spherical clusters formed by soap molecules in water.

Q10: What type of flame is produced during incomplete combustion?

Incomplete combustion produces a yellow sooty flame.

Q11: What is the formula of ethyne?

The molecular formula of ethyne is \(\ce{C2H2}\).

Q12: Name an oxidizing agent used in organic chemistry.

Acidified potassium permanganate \(\ce{(KMnO4)}\) is a common oxidizing agent.

Q13: What is the general formula of alkenes?

The general formula of alkenes is \(C_nH_{2n} \).

Q14: Why do detergents work in hard water?

Detergents do not form scum with calcium and magnesium ions.

Q15: What is the functional group of aldehydes?

Aldehydes contain the –CHO functional group.

Short answer Questions

Q1: What are hydrocarbons?

Hydrocarbons are organic compounds containing only carbon and hydrogen atoms.

Q2: What is meant by homologous series?

A homologous series is a group of organic compounds with the same functional group and similar properties, differing by a –CH2– unit.

Q3: Why do covalent compounds generally have low melting points?

They have weak intermolecular forces, so less energy is required to separate molecules.

Q4: Explain why graphite can conduct electricity.

Graphite has free electrons between layers that move and carry current.

Q5: What happens when ethanol reacts with sodium?

Ethanol reacts with sodium to produce sodium ethoxide and hydrogen gas.

Q6: What is saponification?

Saponification is the conversion of esters into soap and alcohol using a strong base.

Q7: Why does ethanoic acid turn blue litmus red?

Because ethanoic acid is acidic and releases hydrogen ions in solution.

Q8: What is addition reaction?

A reaction where atoms add across a double or triple bond in unsaturated compounds.

Q9: Why is carbon considered versatile?

Due to its tetravalency, catenation, and ability to form strong covalent bonds.

Q10: What is denatured alcohol?

Ethanol mixed with poisonous substances to prevent it from being used as a drink.

Long answer Questions

Q1: Explain the structure and properties of diamond.

Diamond has each carbon bonded to four others in a rigid 3D structure, making it extremely hard and non-conductive.

Q2: Describe the mechanism of soap cleansing action.

Soap molecules form micelles where hydrophobic tails trap dirt and hydrophilic heads allow water to wash it away.

Q3: Explain the difference between saturated and unsaturated hydrocarbons.

Saturated hydrocarbons have only single bonds; unsaturated ones have double or triple bonds with different reactivity.

Q4: Describe ethanol’s physical and chemical properties.

Ethanol is a clear liquid, mixes with water, burns cleanly, reacts with sodium, oxidizes to acid, and forms esters.

Q5: Discuss the formation, properties, and uses of esters.

Esters form when alcohols react with acids, have fruity smells, used in perfumes, flavourings, solvents, and soap-making.

Descriptive Questions

Q1: Explain catenation and how it creates diversity in organic compounds.

Carbon’s ability to link endlessly into chains and rings results in millions of organic structures.

Q2: Describe reaction types shown by carbon compounds with examples.

Carbon compounds show addition (ethene + \(\ce{H2}\)), substitution (methane + \(\ce{Cl2}\)), oxidation (ethanol \(\rightarrow\) acid), and esterification.

Q3: Explain formation and importance of micelles.

Soap molecules arrange into micelles trapping grease inside, helping water wash away oily dirt.

Q4: Discuss physical behaviour of covalent compounds.

They have low melting/boiling points, are poor conductors, and show varied solubility depending on polarity.

Q5: Describe the importance of carbon compounds in daily life.

Fuels, plastics, medicines, detergents, alcohols, and biomolecules rely on carbon chemistry and support modern living.

Text Book Questions

Q1: What would be the electron dot structure of carbon dioxide which has the formula \(\ce{CO2}\) ?

Carbon dioxide ( \(\ce{CO2}\) consists of one carbon atom bonded to two oxygen atoms. In its electron dot structure, the carbon atom shares two pairs of electrons with each oxygen atom, forming two double covalent bonds. Each oxygen completes its octet by sharing electrons with carbon, and carbon also achieves a stable octet through this shared electron arrangement.
The electron dot structure can be shown as:\[ \begin{array}{cccc} ..&&&&..\\ :O:&=&:C:&=&:O: \end{array} \]Here, the symbol : : beside oxygen atoms represents their lone pairs of electrons, while the pairs of dots between carbon and oxygen represent the shared electrons forming double bonds.

Q2: What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur?

Sulphur atoms normally form two covalent bonds because each sulphur has six valence electrons and needs two more to complete its octet. In the solid state, eight sulphur atoms join together to form a closed ring known as an S8 molecule.
In this structure, each sulphur atom is linked to two neighbouring sulphur atoms by single covalent bonds, forming a crown-shaped ring. Every sulphur atom has two shared bonding pairs (one on each side) and three lone pairs of electrons that complete its octet.
The electron-dot representation of S8 therefore shows eight sulphur atoms arranged in a ring, each sulphur having:

Two bonding pairs (shared electrons)—one with each adjacent atom
Three lone pairs (six dots) placed around it

A simplified Lewis dot pattern for the \(\ce{S8}\) ring is:

S8-lewis-dot-structure — \(\mathrm{S_8}\): Lewis dot structure

Q3: How many structural isomers can you draw for pentane?

Answer — Structural Isomers of Pentane

Pentane has the molecular formula \(\ce{C5H12}\). It can be arranged in three different structural isomers, each showing a unique pattern of carbon bonding.

n-Pentane — A straight-chain arrangement of all five carbon atoms:
Structure: \(\ce{CH3-CH2-CH2-CH2-CH3}\)
2-Methylbutane (Isopentane) — A four-carbon main chain with one methyl branch on carbon-2:
Structure: \(\ce{(CH3)2CH-CH2-CH3}\)
2,2-Dimethylpropane (Neopentane) — A compact structure where two methyl groups branch from the central carbon:
Structure: \(\ce{C(CH3)4}\)

Thus, pentane has three structural isomers, all sharing the formula \(\ce{C5H12}\) but differing in how their carbon atoms are connected.

Q4: What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?

Carbon forms an exceptionally large number of compounds because of two special properties:

Catenation

Carbon atoms have the remarkable ability to bond with one another and form long, stable chains and rings. This repeated linking (catenation) allows the formation of millions of organic molecules ranging from simple gases to complex biomolecules.

Tetravalency

A carbon atom has four valence electrons, enabling it to form four covalent bonds with other atoms. This allows carbon to combine with a wide variety of elements such as hydrogen, oxygen, nitrogen, chlorine, and also with other carbon atoms, giving rise to countless structural possibilities.

Together, these two properties make carbon one of the most versatile elements, responsible for the enormous diversity of compounds found in nature and in everyday life.

Q5: What will be the formula and electron dot structure of cyclopentane?

Cyclopentane is a cyclic hydrocarbon made of five carbon atoms arranged in a closed ring. Its molecular formula is: \[\ce{C5H10}\] This formula shows that cyclopentane has two fewer hydrogen atoms than pentane because the chain forms a ring, eliminating two terminal hydrogens.

Electron Dot Structure (Lewis Structure)

In cyclopentane, each carbon atom:

forms two single covalent bonds with neighbouring carbon atoms to complete the ring, and bonds with two hydrogen atoms to complete its valence of four.

A simplified electron dot representation is:

\[\scriptsize\ \begin{array}{ccccc} &&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}\\ &&|&&|&&|&&|&&|\\ \ce{&\color{red}-&C&-&C&-&C&-&C&-&C&\color{red}-&}\\ &&|&&|&&|&&|&&|\\ &&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H} \end{array} \]

This shows:
each carbon shares electrons with two neighbouring carbon atoms, and each carbon shares electrons with two hydrogens, ensuring all atoms satisfy the octet/duplet rule.

Q6: How would you name the following compounds?
(i) CH3 —CH2 —Br
(ii) \[ \begin{array}{cccccc} &&H\\ &&|\\ H&-&C&=&O \end{array} \] (iii) \[\tiny \begin{array}{cccccccccc} &&H&&H&&H&&H\\ &&|&&|&&|&&|\\ \ce{H&-&C&-&C&-&C&-&C&-&C&#&C&-&H}\\ &&|&&|&&|&&|\\ &&H&&H&&H&&H \end{array} \]

How would you name the following compounds?

\( \ce{CH3-CH2-Br} \)
This molecule consists of two carbon atoms in a straight chain, with a bromine atom substituted for a terminal hydrogen.

Name: Bromoethane
- The parent chain is ethane (\( \ce{C2H6} \)).
- Bromine substitutes for one hydrogen atom, hence the prefix "bromo-".
- No position number is needed, as only one structure is possible.
\[ \begin{array}{cccccc} &&H\\ &&|\\ H & - & C & = & O \end{array} \] \( \ce{H2C=O} \)
This structure represents a single carbon atom double-bonded to oxygen and bonded to two hydrogens—the simplest aldehyde.

Name: Methanal
- Contains one carbon atom, double bonded to oxygen and bonded to two hydrogens (\( \ce{H2C=O} \)).
- Named "methanal" according to IUPAC nomenclature (formaldehyde is the common name).
- The aldehyde functional group is represented by \( \ce{-CHO} \).
\[\tiny \begin{array}{cccccccccc} &&H&&H&&H&&H\\ &&|&&|&&|&&|\\ \ce{H&-&C&-&C&-&C&-&C&-&C&#&C&-&H}\\ &&|&&|&&|&&|\\ &&H&&H&&H&&H \end{array} \]
\( \ce{C6H10} \)

The given structure \(\ce{CH3CH2CH2CH2C#CH}\) is a straight chain of six carbon atoms, with a triple bond between the first and second carbon from the end.

Name: Hex-1-yne

This is named hex-1-yne according to IUPAC nomenclature ("-yne" suffix for alkynes, and the triple bond starts at carbon 1 when numbered from the end closest to the triple bond).

Q7: Why is the conversion of ethanol to ethanoic acid an oxidation reaction?

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

The conversion of ethanol \( \ce{CH3CH2OH} \) to ethanoic acid \( \ce{CH3COOH} \) is considered an oxidation reaction because, during this process, oxygen is added to the ethanol molecule or, equivalently, hydrogen is removed.

\[\ce{CH3CH2OH ->[Oxidation] CH3COOH} \]

Ethanol is an alcohol containing a hydroxyl \( \ce{-OH} \) group.
When ethanol is treated with an oxidizing agent (e.g., potassium permanganate or potassium dichromate), its alcohol group changes into a carboxylic acid group.
The reaction involves the gain of an oxygen atom (forming the double-bonded oxygen in \( \ce{CH3COOH} \)) or loss of hydrogen atoms.
Addition of oxygen or removal of hydrogen both define oxidation in chemistry.

Summary: This reaction is termed oxidation because ethanol becomes ethanoic acid by gaining oxygen and/or losing hydrogen, which are the defining characteristics of oxidation according to modern chemical concepts.

Q8: A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?

For welding purposes, a mixture of ethyne (\( \ce{C2H2} \)) and oxygen is preferred because it produces a very hot, clean, and efficient flame. When ethyne is burnt with pure oxygen, the temperature of the oxy-acetylene flame can reach approximately 3000°C, which is sufficient to melt and join metals.

If ethyne is burned with air instead of oxygen, the flame temperature is much lower (about 1300°C), since air contains only about 21% oxygen, and the rest is mostly nitrogen.
The incomplete combustion of ethyne in air produces a sooty, luminous, yellow flame. This flame contains unburnt carbon (soot) and is not hot enough for efficient welding.
Soot deposited from a luminous flame can contaminate the weld and weaken the joint, making it unsuitable for high-quality metal work.
The oxy-acetylene flame (from ethyne and oxygen) is well-controlled and produces little or no soot, giving a clean weld.

Conclusion: A mixture of ethyne and air is not used for welding because the resulting flame is less hot, produces soot, and cannot efficiently melt and join metals. The combination of ethyne and oxygen is essential for creating a high-temperature, clean flame required for effective welding.

Q9: How would you distinguish experimentally between an alcohol and a carboxylic acid?

Alcohols and carboxylic acids are two different types of organic compounds, and they can be distinguished using simple laboratory tests.

Litmus Paper Test:
Carboxylic acids are acidic in nature. When a drop of a carboxylic acid is placed on blue litmus paper, the paper turns red, indicating the presence of acidity.
Alcohols do not affect the color of blue or red litmus paper, as they are neutral compounds.
Reaction with Sodium Bicarbonate:
Carboxylic acids react with sodium bicarbonate (\( \ce{NaHCO3} \)), producing brisk effervescence of carbon dioxide gas:
\[\scriptsize\ce{CH3COOH + NaHCO3 \rightarrow CH3COONa + H2O + CO2\uparrow} \]
Alcohols do not produce carbon dioxide when reacted with sodium bicarbonate; there is no effervescence observed.
Odor:
Many carboxylic acids (like ethanoic acid) have a sharp, vinegar-like smell, while alcohols (like ethanol) have a characteristic spirit-like odor.

Conclusion: The most reliable experimental distinction is to add sodium bicarbonate to both samples. The evolution of carbon dioxide gas (bubbles) confirms the presence of a carboxylic acid, whereas the alcohol will show no reaction. This test is quick, visible, and highly specific.

Q10: What are oxidising agents?

Oxidising agents are chemical substances that bring about oxidation in other substances during a chemical reaction. An oxidising agent achieves this by either adding oxygen to another substance or by removing hydrogen from it.

They work by accepting electrons from other molecules or ions. In this process, the oxidising agent itself gets reduced.
Common examples include substances like potassium permanganate (\( \ce{KMnO4} \)), potassium dichromate (\( \ce{K2Cr2O7} \)), and chlorine (\( \ce{Cl2} \)).
In organic reactions, oxidising agents are often used to convert alcohols to acids or aldehydes, and to remove hydrogen atoms from compounds.
During redox reactions, oxidising agents gain electrons while the substance being oxidised loses electrons.

Summary: An oxidising agent is any substance that can oxidise another substance by either adding oxygen or removing hydrogen, itself being reduced in the process.

Q11: Draw the structures for the following compounds.
(i) Ethanoic acid
(ii) Bromopentane
(iii) Butanone
(iv) Hexanal
Are structural isomers possible for bromopentane? "

Draw the structures for the following compounds:

Ethanoic Acid (Acetic Acid)
Ethanoic acid has molecular formula \( \mathrm{CH_3COOH}\).

\[\scriptsize \begin{array}{ccccc} &\mathrm{H}&&\mathrm{O}\\ &|&&||\\ \ce{H-&C&-&C&-&OH}\\ &|\\ &\mathrm{H} \end{array} \]

Condensed formula: \( \mathrm{CH_3COOH} \)
- Carbon-1: attached to three hydrogen atoms.
- Carbon-2: double bonded to oxygen, single bonded to \( \mathrm{-OH} \).
Bromopentane
Bromopentane (straight chain) has formula \( \mathrm{C_5H_{11}Br} \).

\[\tiny \begin{array}{ccccc} &&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}\\ &&|&&|&&|&&|&&|\\ \ce{Br&-&C&-&C&-&C&-&C&-&C&-&H}\\ &&|&&|&&|&&|&&|\\ &&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H} \end{array} \]
Bromine is attached to the terminal (first) carbon.
Other isomers: bromine can attach to carbon 2 or 3 (e.g., 2-bromopentane, 3-bromopentane).

Butanone (Methyl Ethyl Ketone)

Butanone has molecular formula \( \mathrm{CH_3COCH_2CH_3} \).

\[\tiny \begin{array}{ccccc} &&\mathrm{H}&&\mathrm{O}&&\mathrm{H}&&\mathrm{H}\\ &&|&&||&&|&&|\\ \ce{H&-&C&-&C&-&C&-&C&-&H}\\ &&|&&&&|&&|\\ &&\mathrm{H}&&&&\mathrm{H}&&\mathrm{H} \end{array} \]

Carbonyl group \( \mathrm{C=O} \) is bonded to second carbon in the chain.

Hexanal

Hexanal is an aldehyde with formula \( \mathrm{CH_3CH_2CH_2CH_2CH_2CHO} \).

\[\tiny \begin{array}{ccccc} &&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{O}\\ &&|&&|&&|&&|&&|&&||\\ \ce{H&-&C&-&C&-&C&-&C&-&C&-&C&-&H}\\ &&|&&|&&|&&|&&|\\ &&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H} \end{array} \]

Terminal carbon (sixth) is part of the aldehyde (\( \mathrm{-CHO} \)) group.

Are structural isomers possible for bromopentane?

Yes, structural isomers are possible for bromopentane.
Bromopentane (\( \mathrm{C_5H_{11}Br} \)) can have bromine attached to different carbon atoms in the chain:

1-bromopentane: Br on first carbon
2-bromopentane: Br on second carbon
3-bromopentane: Br on third carbon
Branched isomers: bromine on branched chains like 2-methylbutane or 3-methylbutane

Structural isomerism arises due to the ability of bromine to bond to different positions and branching of the pentane carbon chain.

Q12: People use a variety of methods to wash clothes. Usually after adding the soap, they ‘beat’ the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?

Agitation is necessary during washing because it helps to physically remove dirt, grease, and stains from the fabric surface. Simply soaking clothes in soap or detergent solution allows for some loosening of dirt, but real cleaning happens when the garments are moved, scrubbed, beaten, or brushed.

Role of agitation: The movement (by beating, scrubbing, or swirling in a washing machine) forces water and soap or detergent molecules deep into the fabric. This helps to break up and detach the dirt particles which are then carried away by the cleansing solution.
Soap and detergent action: Soap and detergent molecules surround oily dirt particles (forming micelles), but it’s agitation that actually lifts and dislodges these micelles from the fibers so they can be rinsed away.
Better results: Without agitation, many stains and particles remain stuck to the fabric, resulting in incomplete cleaning.

Conclusion: Agitation (beating, scrubbing, or machine movement) is essential to physically separate dirt and stains from clothes, ensuring thorough cleaning by helping soap and detergent work effectively.

Q13: Would you be able to check if water is hard by using a detergent?

No, you would generally not be able to check if water is hard by using a detergent. The reason is that most detergents are specially formulated to work efficiently in both soft and hard water. Unlike soaps, detergents do not form scum or lose their cleansing action in the presence of calcium and magnesium ions present in hard water.

Soaps: When used with hard water, soaps react with the minerals (mainly calcium and magnesium ions) to form an insoluble substance called scum. This makes the soap less effective, and the formation of scum is a clear test for hard water.
Detergents: Detergents contain synthetic surfactants which do not react with the minerals in hard water. They continue to produce foam and clean effectively, whether the water is soft or hard.

Conclusion: Because detergents work well in both hard and soft water, you cannot use them as a test for water hardness. The inability to produce scum or reduced performance is not observed with detergents, so they do not reveal the hardness of water.

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