CLASS 10 SCIENCE — CARBON AND ITS COMPOUNDS

Quality Revision Notes

1. Introduction to Carbon

Carbon is one of the most important elements because a huge variety of substances around us contain carbon. Food, medicines, fuels, clothing materials and living organisms are largely associated with carbon compounds.

Although the quantity of carbon naturally available in the Earth’s crust and atmosphere is relatively small, its importance is extremely high. The special chemical properties of carbon allow it to form an enormous variety of stable compounds.

2. Bonding in Carbon — Covalent Bond

Why does carbon form covalent bonds?

Carbon has atomic number 6.
Its electronic configuration is 2, 4.
Therefore, carbon has four valence electrons.
It needs four additional electrons to complete its outer shell.

Carbon does not normally form stable compounds by simply gaining or losing four electrons because:

Gaining four electrons would produce a highly charged negative ion that would be difficult for the carbon nucleus to hold.
Losing four electrons would require a very large amount of energy.

Therefore, carbon generally achieves stability by sharing electrons with other atoms.

Covalent bond

A covalent bond is formed when two atoms share one or more pairs of electrons.

Examples:

H₂ → one shared pair → single bond
O₂ → two shared pairs → double bond
N₂ → three shared pairs → triple bond
CH₄ → carbon shares electrons with four hydrogen atoms.

General properties of covalent compounds

Covalently bonded compounds generally:

have strong bonds within individual molecules,
have relatively weak forces between molecules,
have comparatively low melting and boiling points,
are generally poor conductors of electricity because they do not produce free ions.

3. Allotropes of Carbon

Allotropes are different structural forms of the same element.

Important carbon allotropes discussed in the chapter include:

Diamond

In diamond:

each carbon atom is bonded to four other carbon atoms;
the atoms form a rigid three-dimensional network;
it is extremely hard;
it does not conduct electricity like graphite.

Graphite

In graphite:

each carbon atom is bonded to three other carbon atoms;
carbon atoms form hexagonal layers;
the layers are arranged one above another;
graphite is soft and slippery;
graphite conducts electricity.

Thus, different arrangements of carbon atoms produce very different physical properties.

Fullerene

Fullerenes are another group of carbon allotropes.

A famous example is C₆₀, in which carbon atoms form a structure resembling a football. It is known as Buckminsterfullerene.

4. Versatile Nature of Carbon

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

A. Catenation

Catenation is the ability of carbon atoms to bond with one another and form long structures.

Carbon can form:

straight chains,
branched chains,
rings,
structures containing single bonds,
structures containing double bonds,
structures containing triple bonds.

The carbon-carbon bond is strong and stable, which helps carbon form large and diverse molecules.

B. Tetravalency

Carbon has a valency of four.

Therefore, one carbon atom can form bonds with four other atoms.

Carbon can combine with elements such as:

hydrogen,
oxygen,
nitrogen,
sulphur,
chlorine and other elements.

The small size of the carbon atom allows its nucleus to hold shared electrons strongly, contributing to the stability of many carbon compounds.

Key exam point

The huge variety of carbon compounds is mainly due to:

Tetravalency
Catenation

5. Organic Compounds

Carbon compounds are generally studied under organic chemistry.

Historically, scientists thought organic compounds could only be produced by living organisms and that a special “vital force” was necessary.

This idea was disproved when Friedrich Wöhler prepared urea from ammonium cyanate in 1828.

6. Saturated and Unsaturated Carbon Compounds

Saturated compounds

Carbon compounds containing only single bonds between carbon atoms are called saturated compounds.

Example:

Ethane: C₂H₆

Saturated compounds are generally less reactive.

Unsaturated compounds

Carbon compounds containing one or more double or triple bonds between carbon atoms are called unsaturated compounds.

Examples:

Ethene → C₂H₄ → contains a double bond
Ethyne → C₂H₂ → contains a triple bond

Unsaturated compounds are generally more reactive than saturated compounds.

Important comparison

Saturated	Unsaturated
Only single C–C bonds	At least one C=C or C≡C bond
Generally less reactive	Generally more reactive
Example: ethane	Examples: ethene, ethyne

7. Carbon Chains — Straight, Branched and Cyclic

Carbon atoms can be arranged in different ways.

Straight-chain compounds

Carbon atoms form a continuous chain.

Examples:

Methane — CH₄
Ethane — C₂H₆
Propane — C₃H₈
Butane — C₄H₁₀
Pentane — C₅H₁₂
Hexane — C₆H₁₄

Branched-chain compounds

A carbon skeleton can contain branches.

Cyclic compounds

Carbon atoms may also form rings.

Example:

Cyclohexane — C₆H₁₂

Some cyclic compounds may be unsaturated as well; benzene (C₆H₆) is an important example.

8. Hydrocarbons

Compounds containing only carbon and hydrogen are called hydrocarbons.

They are classified as:

Alkanes

Saturated hydrocarbons containing only single bonds.

General formula:

CₙH₂ₙ₊₂

Examples:

CH₄
C₂H₆
C₃H₈

Alkenes

Unsaturated hydrocarbons containing one or more double bonds.

General formula for the simple alkene series:

CₙH₂ₙ

Examples:

C₂H₄
C₃H₆
C₄H₈

Alkynes

Unsaturated hydrocarbons containing one or more triple bonds.

General formula for the simple alkyne series:

CₙH₂ₙ₋₂

Example:

C₂H₂

The textbook specifically identifies saturated hydrocarbons as alkanes, double-bond hydrocarbons as alkenes and triple-bond hydrocarbons as alkynes.

9. Structural Isomerism

Sometimes two compounds have:

the same molecular formula,
but different arrangements of atoms.

Such compounds are called structural isomers.

Example: Butane

Molecular formula:

C₄H₁₀

It can have two different carbon skeletons:

straight-chain structure,
branched-chain structure.

Therefore, both have the same molecular formula but different structures.

10. Functional Groups

In carbon compounds, hydrogen atoms can sometimes be replaced by other atoms or groups.

Atoms such as chlorine, bromine, oxygen, nitrogen and sulphur may act as heteroatoms.

A functional group is an atom or group of atoms attached to a carbon chain that gives the compound characteristic properties.

Important functional groups from the chapter include:

Class	Functional group / naming
Haloalkane	–Cl, –Br
Alcohol	–OH
Aldehyde	–CHO
Ketone	>C=O
Carboxylic acid	–COOH

The functional group strongly influences the chemical properties of the compound.

11. Homologous Series

A homologous series is a family of organic compounds having:

the same functional group,
similar chemical properties,
a common general formula,
successive members differing by a –CH₂– unit.

Example: Alcohol series

CH₃OH → methanol
C₂H₅OH → ethanol
C₃H₇OH → propanol
C₄H₉OH → butanol

The members have similar chemical properties because they contain the same functional group.

Properties of a homologous series

As molecular mass increases:

melting point generally increases,
boiling point generally increases,
other physical properties may show gradual changes.

However, chemical properties remain broadly similar because they are mainly determined by the functional group.

12. Nomenclature of Carbon Compounds

The name of a carbon compound depends on:

Number of carbon atoms.
Type of bond present.
Functional group, if present.

Basic carbon prefixes

Carbon atoms	Prefix
1	meth-
2	eth-
3	prop-
4	but-
5	pent-
6	hex-

Important suffixes

Compound	Suffix
Alcohol	–ol
Aldehyde	–al
Ketone	–one
Carboxylic acid	–oic acid
Alkene	–ene
Alkyne	–yne

Examples

Propane → 3-carbon saturated hydrocarbon
Propene → 3-carbon hydrocarbon with a double bond
Propyne → 3-carbon hydrocarbon with a triple bond
Propanol → alcohol
Propanal → aldehyde
Propanone → ketone
Propanoic acid → carboxylic acid

For unsaturated compounds, –ane is replaced by –ene or –yne according to the type of multiple bond.

13. Chemical Properties of Carbon Compounds

The chapter discusses four important types of reactions:

Combustion
Oxidation
Addition
Substitution

A. Combustion

Combustion is the burning of a substance in oxygen.

Carbon and many carbon compounds burn to produce carbon dioxide, heat and light. Complete combustion of many hydrocarbons also produces water.

Examples:

C + O₂ → CO₂ + heat + light

CH₄ + O₂ → CO₂ + H₂O + heat + light

CH₃CH₂OH + O₂ → CO₂ + H₂O + heat + light

Carbon compounds are therefore important sources of fuels.

Flame and combustion

Adequate oxygen supply generally promotes cleaner combustion.
Unsaturated compounds may produce a yellow, smoky flame.
Limited oxygen can cause incomplete combustion and soot formation.
A blackened cooking vessel can indicate incomplete combustion and wastage of fuel.

B. Oxidation

Oxidation may involve the addition of oxygen to a substance.

Certain substances called oxidising agents promote oxidation.

The chapter identifies:

alkaline potassium permanganate,
acidified potassium dichromate

as oxidising agents that can oxidise alcohols to acids.

C. Addition Reaction

Unsaturated hydrocarbons can add hydrogen in the presence of catalysts such as:

nickel,
palladium.

The unsaturated compound becomes saturated.

This process is called hydrogenation.

Industrial application

Hydrogenation is used in processing vegetable oils. Vegetable oils generally contain unsaturated carbon chains, while animal fats generally contain more saturated chains.

D. Substitution Reaction

Saturated hydrocarbons are comparatively unreactive.

However, under suitable conditions such as sunlight, chlorine can replace hydrogen atoms in hydrocarbons.

Example:

CH₄ + Cl₂ → CH₃Cl + HCl

This is called a substitution reaction because one atom or group takes the place of another.

14. Ethanol

Formula

C₂H₅OH

Ethanol is a liquid at room temperature and is commonly referred to as alcohol.

Uses

Ethanol is used:

as a solvent,
in some medicinal preparations,
in tincture iodine,
in cough syrups and tonics,
as a component/additive in some fuels.

The chapter also stresses that consumption of ethanol can have serious health effects and that methanol is highly poisonous.

Reactions of Ethanol

1. Reaction with sodium

Ethanol reacts with sodium to produce:

sodium ethoxide,
hydrogen gas.

2Na + 2CH₃CH₂OH → 2CH₃CH₂O⁻Na⁺ + H₂

Hydrogen evolution is an important observation in this reaction.

2. Dehydration of ethanol

When ethanol is heated with excess concentrated sulphuric acid at about 443 K, it loses water and forms ethene.

Ethanol → Ethene + Water

Concentrated sulphuric acid acts as a dehydrating agent in this reaction.

15. Ethanoic Acid

Formula

CH₃COOH

Ethanoic acid is commonly called acetic acid.

It belongs to the family of carboxylic acids.

A solution containing about 5–8% acetic acid in water is called vinegar and is widely used as a preservative. Pure ethanoic acid can solidify in cold conditions, giving rise to the name glacial acetic acid.

Carboxylic acids are weak acids compared with strong mineral acids such as HCl.

16. Reactions of Ethanoic Acid

A. Esterification

Ethanoic acid reacts with ethanol in the presence of an acid catalyst to form an ester.

General idea:

Carboxylic acid + Alcohol → Ester + Water

Esters often have pleasant smells and are used in:

perfumes,
flavouring substances.

B. Reaction with sodium hydroxide

Ethanoic acid reacts with sodium hydroxide to form:

sodium ethanoate,
water.

CH₃COOH + NaOH → CH₃COONa + H₂O

C. Reaction with carbonates

Ethanoic acid reacts with sodium carbonate to produce:

sodium ethanoate,
water,
carbon dioxide.

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

With sodium hydrogencarbonate:

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

The evolved carbon dioxide can be identified using lime-water.

17. Soaps

Soaps are generally sodium or potassium salts of long-chain carboxylic acids.

A soap molecule has two different parts:

Hydrophilic end

interacts with water,
generally the ionic end.

Hydrophobic end

avoids water,
interacts with oils and hydrocarbons.

This dual nature allows soap to remove oily dirt from surfaces.

18. Micelle Formation and Cleaning Action

When soap is added to water:

The hydrophobic tails are attracted towards oily dirt.
The hydrophilic ionic ends remain oriented towards water.
Soap molecules arrange themselves around oil droplets.
A spherical cluster called a micelle is formed.
The oily dirt becomes trapped towards the interior.
Agitation helps detach and suspend dirt.
The dirt-containing micelles can then be washed away with water.

Micelles remain dispersed because of repulsion between their charged ends.

Important terms

Hydrophilic: attracted to/interacts with water.

Hydrophobic: avoids water and interacts with hydrocarbons/oily substances.

19. Soap and Hard Water

Hard water contains calcium and magnesium salts.

When soap is used with hard water:

calcium and magnesium ions react with soap,
an insoluble substance called scum is produced,
soap is consumed in the reaction,
less soap remains available for cleaning,
therefore more soap may be required.

This makes soaps less effective in hard water.

20. Detergents

Detergents are another class of cleansing agents.

They generally contain long hydrocarbon chains and charged groups.

Advantage over soap

Detergents remain effective in hard water because their charged ends do not form insoluble precipitates with calcium and magnesium ions in the same way soap does.

Therefore:

Soap + hard water → scum formation

Detergent + hard water → remains effective

Detergents are commonly used in cleaning products, including those for clothes and shampoos.

21. Quick Comparison: Soap vs Detergent

Soap	Detergent
Salts of long-chain carboxylic acids	Cleansing compounds with long hydrocarbon chains and charged groups
Forms scum with Ca²⁺/Mg²⁺ in hard water	Generally remains effective in hard water
Less effective in hard water	More effective in hard water
Cleansing action involves micelles	Cleansing action also involves interaction with oily dirt

22. Important Chemical Equations

Combustion

C + O₂ → CO₂ + heat + light

Methane combustion

CH₄ + O₂ → CO₂ + H₂O + heat + light

Ethanol combustion

CH₃CH₂OH + O₂ → CO₂ + H₂O + heat + light

Ethanol + sodium

2Na + 2CH₃CH₂OH → 2CH₃CH₂O⁻Na⁺ + H₂

Ethanol dehydration

C₂H₅OH → C₂H₄ + H₂O

Ethanoic acid + sodium hydroxide

CH₃COOH + NaOH → CH₃COONa + H₂O

Ethanoic acid + sodium carbonate

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

Ethanoic acid + sodium hydrogencarbonate

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

Substitution

CH₄ + Cl₂ → CH₃Cl + HCl
(in sunlight)

23. Must-Remember Definitions

Covalent bond: A bond formed by sharing of electron pairs between atoms.

Catenation: Ability of carbon atoms to bond with one another and form chains, branches and rings.

Tetravalency: The ability of carbon to form four covalent bonds.

Saturated compound: Carbon compound having only single bonds between carbon atoms.

Unsaturated compound: Carbon compound containing one or more double or triple bonds.

Hydrocarbon: Compound containing only carbon and hydrogen.

Functional group: Atom or group of atoms that gives characteristic chemical properties to a carbon compound.

Homologous series: Series of organic compounds with the same functional group and similar chemical properties, where successive members differ by –CH₂–.

Structural isomers: Compounds having the same molecular formula but different structures.

Hydrogenation: Addition of hydrogen to an unsaturated compound in the presence of a suitable catalyst.

Esterification: Reaction of an acid with an alcohol to form an ester and water.

Saponification: Conversion of an ester into an alcohol and the salt of a carboxylic acid using an alkali; the process is associated with soap preparation.

Micelle: Cluster of soap molecules in which hydrophobic tails are directed inward and hydrophilic ionic ends face the surrounding water.

24. High-Value Exam Points

Why does carbon form such a large number of compounds?

Because carbon has:

tetravalency, and
strong catenation ability.

Why are carbon compounds generally poor conductors?

Their covalent bonding does not generally produce free ions or charged particles capable of carrying electric current.

Why are unsaturated compounds more reactive?

Their carbon-carbon double or triple bonds allow reactions such as addition reactions and generally make them more reactive than saturated compounds.

Why is soap less effective in hard water?

Calcium and magnesium ions in hard water react with soap and form insoluble scum.

Why are detergents preferred for hard water?

Their charged ends generally do not form insoluble precipitates with calcium and magnesium ions, so they remain effective.

Why are carbon compounds useful as fuels?

Many carbon compounds undergo combustion and release considerable heat and light.

25. One-Page Final Revision

Carbon → atomic number 6 → valency 4

Two reasons for versatility:
→ Tetravalency
→ Catenation

Bond:
→ Covalent bond = sharing of electrons

Carbon structures:
→ Straight chain
→ Branched chain
→ Ring

Hydrocarbons:
→ Alkane = single bond
→ Alkene = double bond
→ Alkyne = triple bond

Homologous series:
→ Same functional group
→ Similar chemical properties
→ Consecutive members differ by –CH₂–

Major reactions:
→ Combustion
→ Oxidation
→ Addition
→ Substitution

Ethanol:
→ C₂H₅OH
→ reacts with sodium
→ dehydration gives ethene

Ethanoic acid:
→ CH₃COOH
→ vinegar contains dilute acetic acid
→ reacts with bases and carbonates
→ reacts with ethanol to form ester

Soap:
→ hydrophilic head + hydrophobic tail
→ forms micelles
→ removes oily dirt

Hard water:
→ Ca²⁺/Mg²⁺
→ soap forms scum
→ detergents work better

26. Exam Checklist

Before the exam, make sure you can explain:

☐ Why carbon forms covalent bonds
☐ Meaning of tetravalency and catenation
☐ Diamond vs graphite
☐ Saturated vs unsaturated compounds
☐ Alkanes, alkenes and alkynes
☐ Structural isomerism
☐ Functional groups
☐ Homologous series and –CH₂– difference
☐ Basic nomenclature
☐ Combustion reaction
☐ Oxidation reaction
☐ Addition and hydrogenation
☐ Substitution reaction
☐ Properties and reactions of ethanol
☐ Properties and reactions of ethanoic acid
☐ Esterification and saponification
☐ Soap micelles and cleaning action
☐ Soap vs detergent in hard water