K2KARAKORAM
मराठीEnglishहिंदी
Open the app

CBSE · Class 10 · Science

Chapter 4: Carbon and its Compounds

5 concepts · 40 practice questions with answers · Free to read

This lesson is also in: मराठी · हिंदी

Covalent bonding & properties

In daily life

LPG, petrol, sugar and plastics are all carbon compounds; they burn or melt easily and do not conduct electricity.

Simple definition

A covalent bond is formed when two atoms share electrons so that each gets a full outer shell. Carbon (2,4) cannot easily lose or gain 4 electrons, so it shares them. It forms single, double or triple bonds depending on the number of pairs shared.

Equation / formula : H₂: H–H (1 pair). O₂: O=O (2 pairs). N₂: N≡N (3 pairs). CH₄: carbon shares 4 pairs with 4 hydrogens.

Single, double, triple bondsSingle, double, triple bondsHHH₂singleOOO₂doubleNNN₂triple

In the figure : More shared pairs means a stronger, shorter bond.

Rule 1 : Showing sharing of electrons — Count the electrons each atom needs to reach 8 (or 2 for hydrogen) and share that many.

  • Easy : HCl: H needs 1, Cl needs 1, so one shared pair.
  • Medium : H₂O: O needs 2, so it shares one pair with each of two H.
  • For exams : CH₄: C needs 4, so four single bonds with four H. C₂H₄: C=C double bond and two C–H on each carbon.

Rule 2 : Properties of covalent compounds — Molecules attract each other only weakly, so melting and boiling points are low. No free ions, so they do not conduct.

  • Easy : Naphthalene melts easily; sugar solution does not conduct.
  • Medium : Many dissolve in organic solvents (petrol) and not in water.
  • For exams : Exceptions: graphite conducts, diamond has a very high melting point (giant network, not small molecules).
Remember

Covalent compounds are mostly poor conductors; ionic compounds conduct when molten or dissolved.

Common mistake

Drawing a double bond for N₂. Nitrogen needs 3 electrons, so N≡N is triple.

Shortcut

Needs 1 → single. Needs 2 → double. Needs 3 → triple. Memory clue : "Share to be Stable": the atoms hold hands (shared pairs) instead of giving away electrons.

Spot the difference

PointCovalentIonic
Formed bysharing electronstransfer of electrons
Melting pointlowhigh

Practice questions

Easy
  1. 1Define covalent bond.
  2. 2Why does carbon form covalent bonds?
  3. 3How many shared pairs are in a nitrogen molecule?
Medium
  1. 4Draw the electron-sharing of H₂O in words.
  2. 5Why are covalent compounds poor conductors?
  3. 6Why do covalent compounds have low boiling points?
Challenging
  1. 7Give one covalent compound that conducts or has a high melting point and say why.
  2. 8How many covalent bonds are in C₂H₄ and what types?
Show answers (try them yourself first)
  1. 1Bond formed by sharing electrons between atoms.
  2. 2It cannot easily lose or gain 4 electrons.
  3. 33
  4. 4O shares one pair with each H.
  5. 5No free ions or electrons.
  6. 6Weak forces between molecules.
  7. 7Graphite conducts; diamond melts very high (giant structure).
  8. 86 bonds: 1 double (C=C) + 4 C–H single.

Versatile nature of carbon (catenation, allotropes, isomers)

In daily life

Diamond in a ring and graphite in a pencil are both pure carbon, yet look and feel completely different.

Simple definition

Carbon is versatile because of catenation (carbon atoms joining in chains, branches and rings) and tetravalency (4 bonds). Allotropes are different forms of the same element. Isomers are compounds with the same molecular formula but different structures.

Equation / formula : C₄H₁₀ has two isomers: butane (CH₃–CH₂–CH₂–CH₃) and 2-methylpropane (CH₃–CH(CH₃)–CH₃).

Allotropes of carbonAllotropes of carbon💎Diamond:rigid 3D, hardest✏️Graphite:layers, conducts⚽Fullerene C₆₀:football shape🔗Isomers:same formula,different structure

In the figure : In diamond each carbon has four bonds; in graphite each has three bonds with hexagonal layers.

Rule 1 : Allotropes compared — Same atoms, different arrangement, different properties.

  • Easy : Diamond: hard, insulator, cutting tools. Graphite: slippery, conductor, pencil lead, lubricant.
  • Medium : Fullerene C₆₀ has 60 carbon atoms in a hollow football-like cage.
  • For exams : Burning each in oxygen gives only CO₂, which shows both are pure carbon.

Rule 2 : Isomers — Rearranging the same atoms gives different compounds. Pentane (C₅H₁₂) has three isomers.

  • Easy : C₄H₁₀: butane (straight chain) and 2-methylpropane (branched).
  • Medium : C₃H₈ has no isomers; C₄H₁₀ has 2; C₅H₁₂ has 3.
  • For exams : Isomers have different boiling points: branching lowers the boiling point.
Remember

Carbon forms millions of compounds because of strong C–C bonds and the ability to link endlessly.

Common mistake

Calling two drawings of the same chain "isomers". Rotating or bending the chain does not make an isomer.

Shortcut

Same formula + different joining = isomers. Memory clue : "Dia-hard, Graphite-slides": diamond is hard, graphite slides.

Spot the difference

PointDiamondGraphite
Bonds per C43
Conductornoyes

Practice questions

Easy
  1. 1What is catenation?
  2. 2Name two allotropes of carbon.
  3. 3What is the shape of C₆₀ fullerene?
Medium
  1. 4Why is graphite used as a lubricant?
  2. 5Write the two isomers of C₄H₁₀.
  3. 6Why does carbon show tetravalency?
Challenging
  1. 7Both diamond and graphite burn to give only CO₂. What does this prove?
  2. 8Explain why diamond is a bad conductor but graphite is a good conductor.
Show answers (try them yourself first)
  1. 1Linking of carbon atoms in chains, branches or rings.
  2. 2Diamond, graphite
  3. 3Football-shaped cage
  4. 4Layers slide over each other.
  5. 5Butane and 2-methylpropane
  6. 6Its outer shell has 4 electrons.
  7. 7Both are forms of pure carbon.
  8. 8Graphite has a free electron in each layer; diamond uses all four.

Saturated/unsaturated, homologous series & nomenclature

In daily life

Cooking gas (propane/butane) burns with a clean blue flame, but a candle-like or kerosene flame is yellow and sooty.

Simple definition

Saturated compounds (alkanes) have only single C–C bonds; unsaturated compounds have a double (alkenes) or triple (alkynes) bond. A homologous series is a family with the same functional group, the same general formula, and each member differs from the next by CH₂ (14 u).

Equation / formula : Alkanes CₙH₂ₙ₊₂ (CH₄, C₂H₆, C₃H₈), alkenes CₙH₂ₙ (C₂H₄, C₃H₆), alkynes CₙH₂ₙ₋₂ (C₂H₂, C₃H₄). Prefixes: meth(1) eth(2) prop(3) but(4) pent(5) hex(6).

Hydrocarbon familiesHydrocarbon families➖AlkaneCₙH₂ₙ₊₂ · -ane═AlkeneCₙH₂ₙ · -ene≡AlkyneCₙH₂ₙ₋₂ · -yne🧩Functional groups:–OH, –CHO, >C=O, –COOH

In the figure : Name = prefix (carbon count) + suffix (type): propane, ethene, ethyne, ethanol, ethanoic acid.

Rule 1 : Naming a carbon compound — Count the carbon chain, choose the prefix, then add the ending for the type of bond or group.

  • Easy : CH₃CH₂CH₃ → propane ; CH₂=CH₂ → ethene
  • Medium : CH₃CH₂OH → ethanol ; CH₃CHO → ethanal ; CH₃COOH → ethanoic acid
  • For exams : Number of carbons in "butyne" is 4; formula C₄H₆ from CₙH₂ₙ₋₂ (2×4 − 2 = 6).

Rule 2 : Homologous series — Members have similar chemical properties and a regular change in physical properties.

  • Easy : CH₄ → C₂H₆: difference is one CH₂.
  • Medium : Formula of the alkane with 8 carbons: C₈H₁₈ (2×8 + 2).
  • For exams : Boiling point rises with chain length as the molecular mass increases.
Remember

Saturated hydrocarbons burn with a clean blue flame; unsaturated ones with a sooty yellow flame.

Common mistake

Writing ethane as C₂H₄. Ethane is C₂H₆; C₂H₄ is ethene.

Shortcut

Alkane 2n+2, alkene 2n, alkyne 2n−2. Memory clue : "ane = All single; ene = Extra one double; yne = triple (y three)".

Spot the difference

PointSaturatedUnsaturated
Bondssingle onlydouble or triple
Flameclean blueyellow, sooty

Practice questions

Easy
  1. 1What is a homologous series?
  2. 2Give the general formula of alkenes.
  3. 3Name CH₃CH₂CH₂CH₃.
Medium
  1. 4Write the formula of the alkane with 6 carbons.
  2. 5Why do unsaturated compounds burn with a sooty flame?
  3. 6Name the functional group in ethanol and ethanoic acid.
Challenging
  1. 7A hydrocarbon has formula C₅H₁₀. Which family and name?
  2. 8Explain with CH₂ difference why CH₄, C₂H₆, C₃H₈ form a homologous series.
Show answers (try them yourself first)
  1. 1Family with same functional group and CH₂ difference.
  2. 2CₙH₂ₙ
  3. 3Butane
  4. 4C₆H₁₄
  5. 5High carbon content burns incompletely.
  6. 6–OH (alcohol) and –COOH (carboxylic acid)
  7. 7Alkene; pentene
  8. 8Each differs from the next by one CH₂ and all are alkanes.

Chemical properties of carbon compounds

In daily life

A gas stove burns LPG to cook food; ghee made from oil is firmer than the oil itself.

Simple definition

Carbon compounds undergo combustion (burn in oxygen), oxidation (gain oxygen with an oxidising agent), addition (unsaturated compounds add hydrogen etc.) and substitution (one atom replaced by another in saturated compounds).

Equation / formula : CH₄ + 2O₂ → CO₂ + 2H₂O + heat. C₂H₄ + H₂ →(Ni) C₂H₆. CH₄ + Cl₂ →(sunlight) CH₃Cl + HCl. C₂H₅OH →(alkaline KMnO₄ / acidified K₂Cr₂O₇) CH₃COOH.

Four reaction typesFour reaction types🔥Combustion:+ O₂ → CO₂ + H₂O⬆️Oxidation:alcohol → acid➕Addition:C=C + H₂ → C–C🔄Substitution:H replaced by Cl

In the figure : Addition needs a double/triple bond; substitution happens in saturated compounds.

Live figure

Chemical properties of carbon compounds

In the app you can move this figure with your finger.

Open in the app

Rule 1 : Combustion and oxidation — Complete burning gives CO₂ + water + heat/light; alcohols are oxidised to acids by oxidising agents.

  • Easy : C + O₂ → CO₂ ; CH₃CH₂OH + O₂ → CO₂ + H₂O (balance it)
  • Medium : C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
  • For exams : Alcohol → ethanoic acid: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O (orange K₂Cr₂O₇ turns green).

Rule 2 : Addition and substitution — Unsaturated compounds add atoms across the double bond; saturated ones swap an H for another atom.

  • Easy : C₂H₄ + H₂ →(Ni) C₂H₆
  • Medium : CH₄ + Cl₂ →(sunlight) CH₃Cl + HCl
  • For exams : Hydrogenation of vegetable oil with Ni catalyst makes solid fat (ghee-like).
Remember

Combustion of carbon compounds releases heat; incomplete combustion gives soot and poisonous CO.

Common mistake

Writing substitution for ethene + hydrogen. That is addition.

Shortcut

Double bond? Addition. All single? Substitution. Oxygen? Combustion. Memory clue : "COAS": Combustion, Oxidation, Addition, Substitution.

Spot the difference

PointAdditionSubstitution
Compoundunsaturatedsaturated
Productone producttwo products

Practice questions

Easy
  1. 1Write the complete combustion of methane.
  2. 2What catalyst is used in hydrogenation?
  3. 3What is formed when ethanol is oxidised?
Medium
  1. 4Write the substitution reaction of methane with chlorine.
  2. 5How do vegetable ghee and oil differ chemically?
  3. 6Why is incomplete combustion dangerous?
Challenging
  1. 7Predict the product when propene reacts with H₂ using Ni.
  2. 8A student sees the orange colour of K₂Cr₂O₇ turn green with ethanol on warming. What does it show?
Show answers (try them yourself first)
  1. 1CH₄ + 2O₂ → CO₂ + 2H₂O
  2. 2Nickel (Ni)
  3. 3Ethanoic acid
  4. 4CH₄ + Cl₂ → CH₃Cl + HCl
  5. 5Oil is unsaturated; ghee is saturated (hydrogenated).
  6. 6It gives poisonous CO and soot.
  7. 7Propane
  8. 8Ethanol was oxidised to ethanoic acid.

Ethanol, ethanoic acid, soaps & detergents

In daily life

Vinegar in pickles is dilute ethanoic acid. Soap cleans oily dirt, but leaves a scum in hard water.

Simple definition

Ethanol (C₂H₅OH) is a colourless liquid used in medicines and as fuel. Ethanoic acid (CH₃COOH) is a weak acid; 5–8% solution is vinegar. Soaps are sodium/potassium salts of long-chain fatty acids; detergents are sulphonate or ammonium salts.

Equation / formula : 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂. C₂H₅OH →(conc. H₂SO₄, 443 K) C₂H₄ + H₂O. CH₃COOH + C₂H₅OH →(conc. H₂SO₄) CH₃COOC₂H₅ + H₂O. Ester + NaOH → alcohol + sodium ethanoate (saponification).

How soap cleansHow soap cleansoilwater-loving head(–COO⁻Na⁺)oil-loving tail(C₁₇H₃₅–)micelle: tails in the oil, heads out in water

In the figure : The oily dirt is trapped inside the micelle and rinsed away.

Rule 1 : Properties and uses of ethanol and ethanoic acid — Ethanol reacts with sodium (H₂ gas). Ethanoic acid reacts with carbonates (CO₂) and forms sweet-smelling esters with alcohols.

  • Easy : 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
  • Medium : CH₃COOH + NaOH → CH₃COONa + H₂O
  • For exams : Denatured alcohol has poison/dye added to prevent drinking. Glacial ethanoic acid freezes at 290 K (16.6 °C) into ice-like crystals in cold weather.

Rule 2 : Soaps, detergents and hard water — Soap reacts with calcium and magnesium salts in hard water to form insoluble scum; detergents do not.

  • Easy : Soap: sodium stearate C₁₇H₃₅COONa.
  • Medium : Saponification: oil/fat + NaOH → soap + glycerol.
  • For exams : A micelle has hydrophobic tails inside and hydrophilic heads outside, so it dissolves oil in water.
Remember

Alcohol abuse is harmful: it damages the liver and nervous system. Never drink industrial alcohol (methanol).

Common mistake

Mixing up "ethanol" and "ethanoic acid". Ethanol is the alcohol (–OH); ethanoic acid is the acid (–COOH).

Shortcut

Alcohol + acid → ester + water (sweet smell). Memory clue : "Soap has a Head that loves water and a Tail that loves oil."

Spot the difference

PointSoapDetergent
Hard waterforms scumno scum
Made fromoils/fatspetrochemicals

Practice questions

Easy
  1. 1Write the formula of ethanol.
  2. 2What gas is released when ethanol reacts with sodium?
  3. 3What is vinegar?
Medium
  1. 4Write the esterification reaction of ethanol with ethanoic acid.
  2. 5What is saponification?
  3. 6Why does soap form scum with hard water but detergent does not?
Challenging
  1. 7Explain how a micelle removes oily dirt.
  2. 8A liquid with a fruity smell is formed from an acid and an alcohol. Name the class and give an example equation.
Show answers (try them yourself first)
  1. 1C₂H₅OH
  2. 2Hydrogen
  3. 35–8% solution of ethanoic acid in water.
  4. 4CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O
  5. 5Making soap from oil/fat with NaOH.
  6. 6Soap forms insoluble Ca/Mg salts; detergents salts are soluble.
  7. 7Tails trap oil inside; heads face water; micelle is washed away.
  8. 8Ester; CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O