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Carbon and its Compounds

NCERT Class 10 · Science Based on NCERT Class 10 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores the element carbon, its unique bonding properties, and the vast array of compounds it forms. It explains why carbon is so versatile, covering covalent bonding, catenation, and tetravalency. The chapter then delves into the classification of carbon compounds (saturated and unsaturated), functional groups, homologous series, nomenclature, and chemical properties like combustion, oxidation, addition, and substitution. Finally, it discusses two important compounds—ethanol and ethanoic acid—and the cleansing action of soaps and detergents. The one big idea a student must walk away with is that carbon's ability to form strong covalent bonds with itself and other elements leads to millions of stable compounds, which are the basis of life, fuels, and everyday materials.
Key Concepts (15)
Covalent bond
A bond formed by the sharing of an electron pair between two atoms.
e.g. In a hydrogen molecule (H₂), two hydrogen atoms share one electron pair each to achieve a duplet.
Catenation
The unique ability of carbon to form bonds with other atoms of carbon, giving rise to large molecules.
e.g. Carbon chains in methane (CH₄), ethane (C₂H₆), and propane (C₃H₈) are examples of catenation.
Tetravalency
Carbon has a valency of four, meaning it can form four covalent bonds with other atoms.
e.g. In methane (CH₄), carbon shares its four valence electrons with four hydrogen atoms.
Saturated compounds
Compounds of carbon which are linked by only single bonds between the carbon atoms.
e.g. Ethane (C₂H₆) is a saturated hydrocarbon where all carbon-carbon bonds are single bonds.
Unsaturated compounds
Compounds of carbon having double or triple bonds between their carbon atoms.
e.g. Ethene (C₂H₄) has a double bond between the two carbon atoms, making it unsaturated.
Hydrocarbons
Carbon compounds which contain only carbon and hydrogen.
e.g. Methane (CH₄), ethane (C₂H₆), and ethene (C₂H₄) are all hydrocarbons.
Functional group
A heteroatom or a group of atoms that confers specific properties to the compound, regardless of the length and nature of the carbon chain.
e.g. The hydroxyl group (-OH) is the functional group in alcohols like ethanol (C₂H₅OH).
Homologous series
A series of compounds in which the same functional group substitutes for hydrogen in a carbon chain, and successive members differ by a -CH₂- unit.
e.g. The alcohols methanol (CH₃OH), ethanol (C₂H₅OH), propanol (C₃H₇OH) form a homologous series.
Structural isomers
Compounds with identical molecular formula but different structures.
e.g. Butane (C₄H₁₀) has two structural isomers: n-butane and isobutane.
Allotropes
Different forms of the same element with different physical properties but similar chemical properties.
e.g. Diamond and graphite are allotropes of carbon.
Oxidising agent
A substance that is capable of adding oxygen to another substance.
e.g. Alkaline potassium permanganate (KMnO₄) is an oxidising agent that converts alcohols to carboxylic acids.
Esterification
The reaction of an acid with an alcohol to form an ester and water.
e.g. Ethanoic acid reacts with ethanol in the presence of concentrated sulphuric acid to form ethyl ethanoate (ester).
Saponification
The reaction of an ester with an alkali (like sodium hydroxide) to give back alcohol and the sodium salt of the carboxylic acid.
e.g. Ethyl ethanoate reacts with NaOH to give ethanol and sodium ethanoate.
Micelle
A cluster of soap molecules in which the hydrophobic tails are in the interior and the ionic ends are on the surface.
e.g. When soap is added to water, it forms micelles that trap oily dirt in the centre for cleaning.
Denatured alcohol
Ethanol made unfit for drinking by adding poisonous substances like methanol and dyes.
e.g. Industrial ethanol is denatured by adding methanol and a blue dye to prevent misuse.
Formulas & Equations (11)
C + O₂ → CO₂ + heat and light
Reads as: Carbon plus oxygen yields carbon dioxide plus heat and light
Variables: C = carbon, O₂ = oxygen, CO₂ = carbon dioxide
Use when: Writing the combustion reaction of carbon in allotropic forms
⚠ Students forget to write 'heat and light' or write an unbalanced equation.
CH₄ + 2O₂ → CO₂ + 2H₂O + heat and light
Reads as: Methane plus two oxygen molecules yields carbon dioxide plus two water molecules plus heat and light
Variables: CH₄ = methane, O₂ = oxygen, CO₂ = carbon dioxide, H₂O = water
Use when: Writing the balanced combustion reaction of methane
⚠ Students often forget to balance the equation, especially the number of water molecules.
CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O + heat and light
Reads as: Ethanol plus three oxygen molecules yields two carbon dioxide plus three water plus heat and light
Variables: CH₃CH₂OH = ethanol, O₂ = oxygen, CO₂ = carbon dioxide, H₂O = water
Use when: Writing the balanced combustion reaction of ethanol
⚠ Students incorrectly balance the carbon and hydrogen atoms, leading to wrong coefficients.
2Na + 2CH₃CH₂OH → 2CH₃CH₂O⁻Na⁺ + H₂
Reads as: Two sodium plus two ethanol yields two sodium ethoxide plus hydrogen gas
Variables: Na = sodium, CH₃CH₂OH = ethanol, CH₃CH₂O⁻Na⁺ = sodium ethoxide, H₂ = hydrogen gas
Use when: Writing the reaction of sodium with ethanol to test for the presence of alcohol
⚠ Students forget to write the ionic form of sodium ethoxide or omit the hydrogen gas.
CH₃CH₂OH → CH₂=CH₂ + H₂O (Hot Conc. H₂SO₄, 443 K)
Reads as: Ethanol yields ethene plus water in the presence of hot concentrated sulphuric acid at 443 Kelvin
Variables: CH₃CH₂OH = ethanol, CH₂=CH₂ = ethene, H₂O = water, H₂SO₄ = sulphuric acid (catalyst/dehydrating agent)
Use when: Writing the dehydration reaction of ethanol to form an unsaturated hydrocarbon
⚠ Students forget to mention the conditions (hot conc. H₂SO₄, 443 K) or write the wrong product.
CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O (Acid catalyst)
Reads as: Ethanoic acid plus ethanol in equilibrium with ethyl ethanoate plus water in the presence of an acid catalyst
Variables: CH₃COOH = ethanoic acid, CH₃CH₂OH = ethanol, CH₃COOCH₂CH₃ = ethyl ethanoate (ester), H₂O = water
Use when: Writing the esterification reaction
⚠ Students forget the acid catalyst (conc. H₂SO₄) or write the ester formula incorrectly.
CH₃COOCH₂CH₃ + NaOH → CH₃CH₂OH + CH₃COONa
Reads as: Ethyl ethanoate plus sodium hydroxide yields ethanol plus sodium ethanoate
Variables: CH₃COOCH₂CH₃ = ester, NaOH = sodium hydroxide, CH₃CH₂OH = ethanol, CH₃COONa = sodium ethanoate
Use when: Writing the saponification reaction
⚠ Students write the wrong salt or forget that the reaction is reversible in some contexts.
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
Reads as: Two ethanoic acid plus sodium carbonate yields two sodium ethanoate plus water plus carbon dioxide
Variables: CH₃COOH = ethanoic acid, Na₂CO₃ = sodium carbonate, CH₃COONa = sodium ethanoate, H₂O = water, CO₂ = carbon dioxide
Use when: Writing the reaction of carboxylic acid with a carbonate
⚠ Students forget to balance the equation or write the wrong salt formula.
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
Reads as: Ethanoic acid plus sodium hydrogencarbonate yields sodium ethanoate plus water plus carbon dioxide
Variables: CH₃COOH = ethanoic acid, NaHCO₃ = sodium hydrogencarbonate, CH₃COONa = sodium ethanoate, H₂O = water, CO₂ = carbon dioxide
Use when: Writing the reaction of carboxylic acid with a hydrogencarbonate
⚠ Students confuse this with the carbonate reaction and write wrong coefficients.
CH₄ + Cl₂ → CH₃Cl + HCl (in the presence of sunlight)
Reads as: Methane plus chlorine yields chloromethane plus hydrogen chloride in the presence of sunlight
Variables: CH₄ = methane, Cl₂ = chlorine, CH₃Cl = chloromethane, HCl = hydrogen chloride
Use when: Writing a substitution reaction of alkanes
⚠ Students forget to mention the condition 'sunlight' or write the wrong product.
C₂H₄ + H₂ → C₂H₆ (Ni/Pd catalyst)
Reads as: Ethene plus hydrogen yields ethane in the presence of nickel or palladium catalyst
Variables: C₂H₄ = ethene, H₂ = hydrogen, C₂H₆ = ethane, Ni/Pd = nickel or palladium catalyst
Use when: Writing an addition reaction (hydrogenation) of unsaturated hydrocarbons
⚠ Students forget the catalyst or write the wrong product (e.g., C₂H₄ instead of C₂H₆).
How It Works
Formation of a Covalent Bond
  1. Two atoms come close together.
  2. Each atom contributes one electron from its valence shell to form a shared pair of electrons.
  3. Both atoms achieve a noble gas configuration (duplet or octet) by sharing.
  4. The shared pair of electrons constitutes a single covalent bond.
💡 Covalent bonds involve sharing, not transfer, of electrons. No ions are formed.
Formation of a Double Bond (e.g., in Oxygen)
  1. Each oxygen atom has six valence electrons and needs two more to complete its octet.
  2. Each oxygen atom shares two electrons with the other oxygen atom.
  3. This gives rise to two shared pairs of electrons, forming a double bond.
💡 A double bond is stronger than a single bond but is more reactive in some contexts.
Formation of a Triple Bond (e.g., in Nitrogen)
  1. Each nitrogen atom has five valence electrons and needs three more to complete its octet.
  2. Each nitrogen atom shares three electrons with the other nitrogen atom.
  3. This gives rise to three shared pairs of electrons, forming a triple bond.
💡 A triple bond is the strongest covalent bond but is also a site for addition reactions.
Hydrogenation (Addition Reaction)
  1. An unsaturated hydrocarbon (e.g., vegetable oil with double bonds) is taken.
  2. Hydrogen gas is passed over the unsaturated compound in the presence of a catalyst (Ni or Pd).
  3. Hydrogen adds across the double or triple bond, converting it into a single bond.
  4. The unsaturated compound becomes saturated (e.g., oil becomes fat).
💡 This reaction is used industrially to convert vegetable oils into margarine or vanaspati ghee.
Substitution Reaction (e.g., Chlorination of Methane)
  1. Methane (CH₄) is mixed with chlorine (Cl₂).
  2. In the presence of sunlight, chlorine atoms replace hydrogen atoms one by one.
  3. The products include chloromethane (CH₃Cl), dichloromethane (CH₂Cl₂), chloroform (CHCl₃), and carbon tetrachloride (CCl₄).
  4. Hydrogen chloride (HCl) is also produced as a by-product.
💡 This is a characteristic reaction of saturated hydrocarbons (alkanes).
Oxidation of Alcohol to Carboxylic Acid
  1. An alcohol (e.g., ethanol) is taken in a test tube and warmed gently in a water bath.
  2. An oxidising agent like alkaline potassium permanganate (KMnO₄) is added drop by drop.
  3. The purple colour of KMnO₄ disappears initially as it oxidises the alcohol.
  4. The alcohol is converted to the corresponding carboxylic acid (e.g., ethanoic acid).
💡 The colour of KMnO₄ disappears because it is reduced; excess KMnO₄ will persist once all alcohol is oxidised.
Esterification
  1. A carboxylic acid (e.g., ethanoic acid) and an alcohol (e.g., ethanol) are mixed.
  2. A few drops of concentrated sulphuric acid are added as a catalyst.
  3. The mixture is warmed in a water bath for about five minutes.
  4. An ester (e.g., ethyl ethanoate) and water are formed. The ester has a sweet, fruity smell.
💡 The reaction is reversible. The ester can be hydrolysed back to acid and alcohol.
Saponification
  1. An ester is treated with an alkali like sodium hydroxide (NaOH).
  2. The ester is converted back to the alcohol and the sodium salt of the carboxylic acid.
  3. This reaction is used in the preparation of soap (sodium or potassium salts of long-chain carboxylic acids).
💡 Saponification literally means 'soap-making'. The products are soap and glycerol.
Cleaning Action of Soap (Micelle Formation)
  1. Soap molecules have a hydrophilic (ionic) head and a hydrophobic (long hydrocarbon) tail.
  2. When soap is added to water, the hydrophobic tails cluster together away from water, while the ionic heads face outward.
  3. This forms a spherical structure called a micelle.
  4. Oily dirt is trapped in the hydrophobic centre of the micelle.
  5. The micelles remain suspended in water as a colloid due to ion-ion repulsion.
  6. The dirt is rinsed away with water.
💡 Soap does not dissolve oil; it emulsifies it. Micelles form only in water, not in organic solvents like ethanol.
Exam Traps — Don't Make These Mistakes
⚠ Confusing Catenation with Tetravalency
✗ Carbon forms a large number of compounds because it is tetravalent.
→ Tetravalency (valency of 4) allows carbon to bond with four other atoms, but it is catenation (self-linking) that specifically allows long chains, branches, and rings.
✓ Carbon forms a large number of compounds due to two properties: catenation (the ability to form bonds with other carbon atoms) and tetravalency (the ability to form four bonds).
⚠ Writing the Wrong Product for Dehydration of Ethanol
✗ CH₃CH₂OH → CH₃CHO + H₂O (with conc. H₂SO₄)
→ Dehydration of ethanol removes water to form an alkene (ethene), not an aldehyde. Aldehyde formation is an oxidation reaction.
✓ CH₃CH₂OH → CH₂=CH₂ + H₂O (Hot Conc. H₂SO₄, 443 K)
⚠ Forgetting Conditions in Substitution Reaction
✗ CH₄ + Cl₂ → CH₃Cl + HCl
→ The reaction requires sunlight (or UV light) to proceed. Without mentioning the condition, the answer is incomplete.
✓ CH₄ + Cl₂ → CH₃Cl + HCl (in the presence of sunlight)
⚠ Confusing Saturated and Unsaturated Hydrocarbon Tests
✗ Saturated hydrocarbons give a yellow, sooty flame.
→ Saturated hydrocarbons generally give a clean blue flame. Unsaturated hydrocarbons give a yellow, sooty flame due to incomplete combustion.
✓ Saturated hydrocarbons burn with a clean blue flame, while unsaturated hydrocarbons burn with a yellow, sooty flame.
⚠ Thinking Soap Works in Hard Water Like Detergent
✗ Soap works well in hard water.
→ Soap reacts with calcium and magnesium ions in hard water to form an insoluble precipitate (scum), reducing its effectiveness. Detergents do not form such precipitates.
✓ Soap is less effective in hard water because it forms scum with Ca²⁺ and Mg²⁺ ions. Detergents are effective in hard water as they do not form insoluble precipitates.
Compare & Contrast
Nature of carbon-carbon bonds
Saturated hydrocarbons (Alkanes)
Unsaturated hydrocarbons (Alkenes/Alkynes)
Saturated hydrocarbons have only single bonds between carbon atoms, while unsaturated hydrocarbons have at least one double (alkene) or triple (alkyne) bond.
Reaction with sodium carbonate/hydrogencarbonate
Ethanol (C₂H₅OH)
Ethanoic acid (CH₃COOH)
Ethanoic acid reacts with Na₂CO₃/NaHCO₃ to produce CO₂ gas (which turns lime water milky), while ethanol does not react with carbonates or hydrogencarbonates.
Effectiveness in hard water
Soap
Detergent
Soap forms an insoluble precipitate (scum) with Ca²⁺ and Mg²⁺ ions in hard water, reducing its cleaning action. Detergents do not form such precipitates and remain effective in hard water.
Exam Focus
Marks pattern: Typically 2-3 MCQs (1 mark each), 1-2 Very Short Answer Questions (1 mark each), 1 Short Answer Question (3 marks), and 1 Long Answer Question (5 marks). Total weightage: approximately 8-10 marks.
Most asked: Covalent bonding and its properties · Catenation and tetravalency of carbon · Saturated vs unsaturated hydrocarbons · Functional groups and homologous series · Nomenclature of carbon compounds · Chemical properties: combustion, oxidation, addition, substitution · Ethanol and ethanoic acid: properties and reactions · Soaps and detergents: cleansing action and micelle formation
Don't skip: Draw electron dot structures of given compounds (e.g., CO₂, C₂H₆, C₂H₄, C₂H₂, CH₃Cl).; Explain the formation of covalent bonds with examples.; Differentiate between saturated and unsaturated hydrocarbons with examples and tests.; Write the reactions of ethanol and ethanoic acid (with Na, conc. H₂SO₄, esterification, saponification, carbonates).; Explain the mechanism of the cleansing action of soap (micelle formation).
One-Line Revision (read aloud before the exam)
  1. Carbon forms covalent bonds by sharing electrons to achieve a noble gas configuration.
  2. Catenation is the property of carbon to form bonds with other carbon atoms, leading to long chains.
  3. Tetravalency means carbon can form four covalent bonds with other atoms.
  4. Saturated hydrocarbons have only single bonds and are called alkanes.
  5. Unsaturated hydrocarbons have double (alkenes) or triple (alkynes) bonds.
  6. A functional group is an atom or group of atoms that determines the chemical properties of a compound.
  7. A homologous series is a family of compounds with the same functional group, differing by a -CH₂- unit.
  8. Structural isomers have the same molecular formula but different structural arrangements.
  9. Combustion of carbon compounds produces CO₂, H₂O, heat, and light.
  10. Unsaturated hydrocarbons undergo addition reactions, like hydrogenation.
  11. Saturated hydrocarbons undergo substitution reactions, like chlorination in sunlight.
  12. Ethanol reacts with sodium to produce hydrogen gas and sodium ethoxide.
  13. Dehydration of ethanol with conc. H₂SO₄ at 443 K gives ethene.
  14. Ethanoic acid reacts with carbonates to produce CO₂ gas, which turns lime water milky.
  15. Esterification is the reaction of a carboxylic acid with an alcohol to form an ester.
  16. Saponification is the alkaline hydrolysis of an ester to give soap and alcohol.
  17. Soap molecules form micelles in water, trapping oily dirt for cleaning.
  18. Detergents are effective in hard water, while soaps form scum with Ca²⁺ and Mg²⁺ ions.
  19. Diamond and graphite are allotropes of carbon with different physical properties.
  20. Fullerenes are another class of carbon allotropes, like C₆₀ buckminsterfullerene.

MCQs — 10 Questions with Answers

Q1. A student burns a piece of charcoal in a test tube and passes the gas produced through lime water. The lime water turns milky white. What does this observation indicate about the composition of charcoal?

  • A. Charcoal contains carbon which combines with oxygen to form carbon dioxide ✓
  • B. Charcoal is a pure form of hydrogen that reacts with oxygen
  • C. Charcoal releases water vapor that reacts with lime water
  • D. Charcoal contains calcium carbonate that dissolves in water

Answer: A — The milky precipitate in lime water is calcium carbonate formed when CO₂ reacts with Ca(OH)₂, confirming carbon combustion; students often incorrectly attribute the reaction to hydrogen or moisture in charcoal.

Q2. A chemist examines the melting point of methane (90 K) and sodium chloride (1074 K). Why is the melting point of methane significantly lower than that of sodium chloride?

  • A. Methane has covalent bonds with weak intermolecular forces, while NaCl has strong ionic bonds ✓
  • B. Methane contains more atoms than sodium chloride
  • C. Sodium chloride is denser and therefore requires more energy to melt
  • D. Methane is a gas at room temperature while NaCl is a solid

Answer: A — Covalent compounds like methane have weak van der Waals forces between molecules requiring less energy to melt, whereas ionic compounds have strong electrostatic attractions; density and physical state are consequences, not causes of melting point differences.

Q3. Assertion (A): Carbon can form four covalent bonds by sharing its valence electrons with other atoms. Reason (R): Carbon has four electrons in its outermost shell and cannot easily gain or lose electrons to form ions. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A ✓
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false
  • D. A is false but R is true

Answer: A — Carbon's four valence electrons and difficulty in forming C⁴⁺ or C⁴⁻ ions directly explain why it forms covalent bonds instead; this is the fundamental reason for carbon's bonding behavior.

Q4. In a laboratory, a student observes that ethanol (C₂H₅OH) conducts electricity poorly, while copper sulfate solution conducts electricity well. What does this tell us about the nature of bonding in ethanol?

  • A. Ethanol contains covalent bonds and does not form free ions in solution ✓
  • B. Ethanol has ionic bonds that are too weak to conduct electricity
  • C. Ethanol is a non-conductor because it has no valence electrons
  • D. Ethanol molecules repel electricity due to their structure

Answer: A — Poor electrical conductivity indicates absence of free ions, which is characteristic of covalent compounds like ethanol; students often confuse weak conductivity with weak bonds rather than recognizing the absence of mobile ions.

Q5. A scientist prepares two samples: one containing a carbon compound (methane) and another containing an ionic compound (NaCl). Both are dissolved in water at room temperature. Which statement correctly explains why methane has a lower boiling point than NaCl?

  • A. Methane molecules experience only weak intermolecular forces, whereas NaCl has strong ionic attractions between ions ✓
  • B. Methane is lighter in molecular weight than NaCl, so it boils at lower temperatures
  • C. Methane evaporates completely in water while NaCl remains dissolved
  • D. Ionic compounds always have higher boiling points because they contain more atoms

Answer: A — Boiling point directly correlates with strength of intermolecular/interionic forces; methane's weak van der Waals forces require less energy to overcome than NaCl's ionic bonds; molecular weight alone does not determine boiling point.

Q6. Assertion (A): Nitrogen gas (N₂) contains a triple covalent bond between its two atoms. Reason (R): Each nitrogen atom has seven valence electrons and needs to share three electrons to complete its octet. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A ✓
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false
  • D. A is false but R is true

Answer: A — Nitrogen's seven valence electrons require three additional electrons for an octet of eight, so each atom contributes three electrons forming three shared pairs (triple bond); the reason directly explains the assertion.

Q7. A teacher shows students a data table of boiling points: Acetic acid (391 K), Chloroform (334 K), Ethanol (351 K), Methane (111 K). A student claims all these compounds must have the same type of bonding because they are all listed together. What is the flaw in this reasoning?

  • A. All are carbon compounds with covalent bonding, but varying boiling points reflect different strengths of intermolecular forces, not different bonding types ✓
  • B. These compounds have different bonding types: ionic, covalent, and metallic
  • C. The boiling point has no relationship to the type of bonding present
  • D. Carbon compounds always conduct electricity, so they must all have ionic bonding

Answer: A — All listed compounds are covalently bonded carbon compounds, but their different boiling points reflect different intermolecular force strengths (e.g., methane has only van der Waals forces while acetic acid has hydrogen bonding); students often confuse bonding type with intermolecular force strength.

Q8. Assertion (A): A molecule of hydrogen (H₂) is formed when two hydrogen atoms share one pair of electrons. Reason (R): This shared pair of electrons allows each hydrogen atom to attain the electronic configuration of helium. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A ✓
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false
  • D. A is false but R is true

Answer: A — Hydrogen atoms achieve helium's 1s² configuration by sharing electrons in a single covalent bond; this is the fundamental driving force for H₂ formation and directly explains the assertion.

Q9. During an experiment, a student observes that when 10 grams of sugar (a carbon compound) is heated strongly in air, only a small amount of white ash remains while gases escape. A classmate suggests the compound must be ionic. Why is this conclusion incorrect?

  • A. The escape of gases indicates decomposition of covalent bonds in an organic compound, not an ionic compound which would melt rather than decompose with gas evolution ✓
  • B. Sugar is always crystalline, proving it is covalent, not ionic
  • C. Ionic compounds never form gases when heated
  • D. The white ash proves the compound is covalent because covalent compounds leave mineral residue

Answer: A — Gas evolution during strong heating indicates decomposition of covalent organic compounds; ionic compounds typically melt and don't produce gases; students often confuse crystal appearance with bonding type.

Q10. A student analyzes why oxygen gas (O₂) contains a double bond rather than two single bonds. What is the correct explanation based on valence electron theory?

  • A. Oxygen has six valence electrons and requires two more to achieve an octet; each oxygen atom shares two electrons with the other, creating a double bond ✓
  • B. Oxygen atoms are too large to form single bonds, so they must form double bonds
  • C. Double bonds are always stronger than single bonds, so oxygen naturally forms double bonds
  • D. Oxygen needs to share four electrons total, and a double bond is the only way to do this

Answer: A — Each oxygen atom's six valence electrons plus two shared pairs equals eight electrons (octet); this electron-counting directly explains the double bond necessity; students often assume double bonds are always stronger or preferred without considering electron configuration requirements.

Flashcards

What is a covalent bond?

A bond formed by sharing a pair of electrons between two atoms so both attain noble gas configuration.

Why does carbon not form C⁴⁺ or C⁴⁻ ions?

C⁴⁻ is unstable because 6 protons cannot hold 10 electrons, and C⁴⁺ requires excessive energy to remove 4 electrons.

What is the valency of carbon and why?

Carbon has valency 4 because it has 4 valence electrons in its outermost shell that it can share.

How many electrons does oxygen need to complete its octet?

Oxygen needs 2 more electrons; it has 6 in its L shell and requires 8 for a complete octet.

What type of bond exists between two oxygen atoms in O₂?

A double bond (two shared pairs of electrons) because each oxygen atom contributes two electrons for sharing.

Why do carbon compounds have low melting and boiling points?

Covalent bonds within molecules are strong, but intermolecular forces between molecules are weak, resulting in low m.p. and b.p.

What is the structure of methane (CH₄) and what bonds does it contain?

Methane has one carbon atom bonded to four hydrogen atoms with four single covalent bonds.

Define a triple bond with an example.

A triple bond consists of three shared pairs of electrons between two atoms; nitrogen (N₂) forms a triple bond.

Why are most carbon compounds poor conductors of electricity?

Covalent bonding does not produce ions; without mobile charged particles, these compounds cannot conduct electricity.

What percentage of carbon is present in Earth's crust and atmosphere?

Earth's crust contains 0.02% carbon (as minerals, coal, petroleum) and atmosphere contains 0.03% as carbon dioxide.

Important Board Questions

Define a covalent bond. Why does carbon form covalent bonds instead of ionic bonds? [2 marks]

Explain that a covalent bond is a shared electron pair. State that carbon cannot easily form C⁴⁻ (unstable) or C⁴⁺ (requires too much energy), so it shares electrons with other atoms to achieve noble gas configuration.

Draw the electron dot structure of a water molecule (H₂O) and identify the types of bonds present. Explain how each atom achieves a noble gas configuration. [3 marks]

Show oxygen bonded to two hydrogen atoms with two single covalent bonds. Explain that oxygen needs 2 electrons (6 valence + 2 from H atoms = 8 = octet), and each hydrogen achieves 2 electrons (duet) like helium. Focus on electron sharing for noble gas config.

Compare the properties of ionic compounds with carbon (covalent) compounds in terms of melting points, boiling points, and electrical conductivity. Explain the reasons for these differences using bonding concepts. [5 marks]

State that ionic compounds have high m.p./b.p. due to strong electrostatic forces between ions and conduct electricity in molten/aqueous states because of mobile ions. Covalent compounds have low m.p./b.p. because intermolecular forces are weak (despite strong intramolecular bonds) and don't conduct because no free ions exist. Link these properties to the nature of bonding: ionic = electron transfer (ions), covalent = electron sharing (molecules).

Assertion-Reason Questions for Carbon and its Compounds

Q1. Assertion (A):

Carbon forms covalent bonds instead of ionic bonds.

Reason (R):

Carbon has four valence electrons and it is energetically unfavorable for it to gain or lose four electrons.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. Carbon indeed forms covalent bonds, and the reason correctly explains why—it cannot easily gain or lose four electrons, making electron sharing the most favorable option. The reason is the correct explanation for the assertion.

Q2. Assertion (A):

Carbon compounds have low melting and boiling points.

Reason (R):

Covalent bonds within carbon molecules are very weak.

Answer: C — A is true but R is false — The assertion is true—carbon compounds do have low melting and boiling points. However, the reason is false. Covalent bonds within molecules are actually strong; it is the intermolecular forces between molecules that are weak. This weak intermolecular bonding is the actual reason for low melting and boiling points, not weak covalent bonds.

Q3. Assertion (A):

Nitrogen forms a triple bond in N₂ molecule.

Reason (R):

Each nitrogen atom has five valence electrons and shares three electrons with another nitrogen atom to achieve noble gas configuration.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both assertion and reason are true. Nitrogen with five valence electrons needs three more to complete its octet (eight electrons). In N₂, each atom contributes three electrons, forming three shared pairs (triple bond). The reason correctly explains why the triple bond forms.

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Case-Based Questions (CBQ) for Carbon and its Compounds

Case Study:

A student conducted an experiment to test the nature of bonding in various compounds. She measured the melting points of four substances: sodium chloride (ionic compound), methane (CH₄), ethanol (CH₃CH₂OH), and chloroform (CHCl₃). The data collected are: Sodium chloride: 1074 K, Methane: 90 K, Ethanol: 156 K, Chloroform: 209 K. The student also tested whether each substance conducts electricity in its pure liquid state. Ionic sodium chloride conducted electricity, while the three organic compounds did not conduct electricity.

Q1. Why do methane, ethanol, and chloroform have significantly lower melting points than sodium chloride?

  • A. These compounds have stronger covalent bonds
  • B. These compounds have weak intermolecular forces between molecules ✓
  • C. These compounds have higher atomic masses
  • D. These compounds are non-metallic elements

Answer: B

Q2. What explains why methane, ethanol, and chloroform do not conduct electricity in their pure liquid state?

  • A. These compounds do not contain electrons
  • B. These compounds form covalent bonds with no free ions or mobile charge carriers ✓
  • C. These compounds have too many atoms in their molecules
  • D. These compounds are insoluble in water

Answer: B

Q3. Among methane, ethanol, and chloroform, ethanol has a higher melting point than methane. This suggests:

  • A. Ethanol has stronger covalent bonds than methane
  • B. Ethanol has fewer molecules than methane
  • C. Ethanol is more ionic than methane
  • D. Ethanol has stronger intermolecular forces than methane ✓

Answer: D

Q4. If the student had tested molten sodium chloride and aqueous solutions of the organic compounds, which would show electrical conductivity?

  • A. Only molten sodium chloride
  • B. Only aqueous solutions of the organic compounds
  • C. Both molten sodium chloride and aqueous solutions of organic compounds if they dissociate ✓
  • D. None of these would show conductivity

Answer: C

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Previous Year Questions (PYQ) for Carbon and its Compounds

Real CBSE board exam questions from past papers — free to attempt.

CBSE 2026 · Set 31/4/1 · 1 mark

Assertion (A): Pure ethanoic acid is also known as glacial acetic acid. Reason (R): Pure ethanoic acid often freezes during winter due to its low melting point of 290 K.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

CBSE 2026 · Set 31/4/1 · 1 mark

(b) Write chemical formula of Plaster of Paris. At what temperature gypsum is heated to obtain Plaster of Paris?

CBSE 2026 · Set 31/4/1 · 5 marks

(A) (i) Carbon cannot form C4+ cation and C4– anion. Explain. (ii) Define homologous series of carbon compounds. Why are the melting and boiling points of C4H10 higher than C3H8 or C2H6? (iii) What happens when ethanol is heated with acidified potassium dichromate?

CBSE 2026 · Set 31/4/3 · 5 marks

(i) Carbon cannot form C4+ cation and C4– anion. Explain. (ii) Define homologous series of carbon compounds. Why are the melting and boiling points of C4H10 higher than C3H8 or C2H6? (iii) What happens when ethanol is heated with acidified potassium dichromate?

CBSE 2026 · Set 31/5/1 · 1 mark

Which of the following set of compounds does not belong to same homologous series?

  • CH4 and C4H10
  • C2H6 and C3H8
  • C3H8 and C5H12
  • C4H8 and C5H12

CBSE 2026 · Set 31/5/1 · 1 mark

Which of the following will not undergo addition reaction?

  • C4H8
  • C2H2
  • C3H8
  • C2H4

CBSE 2026 · Set 31/5/1 · 4 marks

Alcohol forms a homologous series with general formula CnH2n + 1 –OH and –OH group as functional group. Ethanol is commonly called alcohol and is used in alcoholic drinks. It is good solvent, used in medicines, cough syrups, tonics etc. (a) Write structural formula and name of 4th member of alcohol homologous series. (b) What happens when ethanol is heated with alkaline KMnO4? Write chemical equation involved. (c) Write the chemical equation of reaction of ethanol with ethanoic acid in the presence of concentrated H2SO4. Write the name of this reaction.

CBSE 2026 · Set 31/5/2 · 1 mark

Which of the following set of compounds does not belong to same homologous series ?

  • CH4 and C4H10
  • C2H6 and C3H8
  • C3H8 and C5H12
  • C4H8 and C5H12

CBSE 2026 · Set 31/5/2 · 1 mark

Which of the following will not undergo addition reaction ?

  • C4H8
  • C2H2
  • C3H8
  • C2H4

CBSE 2026 · Set 31/5/2 · 4 marks

Alcohol forms a homologous series with general formula CnH2n + 1 –OH and –OH group as functional group. Ethanol is commonly called alcohol and is used in alcoholic drinks. It is good solvent, used in medicines, cough syrups, tonics etc. (a) Write structural formula and name of 4th member of alcohol homologous series. (b) What happens when ethanol is heated with alkaline KMnO4 ? Write chemical equation involved. (c) Write the chemical equation of reaction of ethanol with ethanoic acid in the presence of concentrated H2SO4. Write the name of this reaction.

CBSE 2026 · Set 31/5/3 · 4 marks

Alcohol forms a homologous series with general formula CnH2n + 1 –OH and –OH group as functional group. Ethanol is commonly called alcohol and is used in alcoholic drinks. It is good solvent, used in medicines, cough syrups, tonics etc. (a) Write structural formula and name of 4th member of alcohol homologous series. (b) What happens when ethanol is heated with alkaline KMnO4? Write chemical equation involved.

CBSE 2026 · Set 31/2/1 · 1 mark

Assertion (A): Carbon compounds can form chains, branched and ring structures. Reason (R): Carbon exhibits the property of catenation.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

See all 74 board PYQs for this chapter — open PYQ tab →

Class 10 Science Formula Sheet

Every formula and chemical equation from the 10 chapters on one page, checked against the NCERT textbook and the CBSE 2026-27 syllabus. Free PDF download.

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Class 10 Science Previous Year Questions, Chapter-wise

All 875 CBSE board questions from 2022–2026, sorted by chapter and question type. Free PDF download.

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