• Carbon basics: Symbol C, Atomic no. 6, Mass 12, Electron config \(2,4\), Valency 4 (forms covalent bonds by sharing electrons).
• Occurrence: Free (diamond, graphite); Combined (CO₂, carbonates like CaCO₃, fossil fuels, biomolecules—carbohydrates, proteins, fats; natural fibres).
• Allotropes:
  • Diamond: each C bonded to 4 C (3D tetrahedral network) → hardest, dense (~3.5 g/cm³), high mp (~\(3500^\circ\)C), electrical insulator.
  • Graphite: layered hexagonal sheets (graphene); soft, slippery, density ~1.9–2.3 g/cm³; good electrical conductor within layers.
  • Fullerenes: C₆₀ (buckyball) & nanotubes; soluble in some organics; uses in materials, catalysis.
• Amorphous forms: Coal (peat → lignite → bituminous → anthracite), Charcoal, Coke (reducing agent; makes water/producer gas).
• Hydrocarbons: Saturated (single C–C; e.g., ethane, propane) vs Unsaturated (≥1 multiple bond; e.g., ethene, ethyne). Methane: \( \mathrm{CH_4} \) (tetrahedral), major in natural/biogas.
• Key reactions (balanced):
  Combustion: \( \mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}} \)
  Methane chlorination (UV): \( \mathrm{CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl} \)
  CO₂ prep: \( \mathrm{CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\uparrow} \)
  CO₂ + NaOH: \( \mathrm{2NaOH + CO_2 \rightarrow Na_2CO_3 + H_2O} \)
  CO₂ + \( \mathrm{Na_2CO_3} \): \( \mathrm{Na_2CO_3 + H_2O + CO_2 \rightarrow 2NaHCO_3} \)
• CO₂ properties/uses: colorless, faint odor, denser than air; turns limewater milky; used in aerated drinks, dry ice, fire extinguishers, decaffeination, dry cleaning, photosynthesis.
• Biogas: 55–60% methane (rest mostly CO₂); anaerobic digestion → clean fuel + good manure.

1) Valency of carbon?

4 (forms four covalent bonds by sharing electrons).

2) Two free-state forms of carbon?

Diamond and Graphite (also allotrope Fullerene).

3) What is allotropy?

Existence of one element in different structural forms with same chemical but different physical properties.

4) Hardest natural substance?

Diamond.

5) Electrical conductor among allotropes?

Graphite (good conductor within layers).

6) One use of diamond in industry?

Glass cutting and rock drilling; microsurgery knives.

7) Structural unit of graphite layer?

Graphene (hexagonal carbon sheet).

8) Simplest hydrocarbon?

Methane, \( \mathrm{CH_4} \).

9) Saturated hydrocarbon example?

Ethane, Propane, Butane, etc.

10) Unsaturated hydrocarbon example?

Ethene (C=C), Ethyne (C≡C), Propene, etc.

11) Formula for methane chlorination (main step)?

\( \mathrm{CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl} \).

12) CO₂ turns which test milky?

Limewater (due to CaCO₃ formation).

13) Density: CO₂ vs air?

CO₂ is denser than air.

14) Two uses of CO₂ (solid/liquid)?

Dry ice for cooling/special effects; liquid CO₂ as solvent/dry cleaning.

15) Four stages of coal (in order)?

Peat → Lignite → Bituminous → Anthracite.

16) What is coke?

Porous carbon-rich solid obtained from coal (after removing coal gas/tar).

17) Main component of natural gas?

Methane (~87%).

18) Biogas contains chiefly?

Methane (≈55–60%) and CO₂ (balance).

19) Type of bonds in saturated hydrocarbons?

Only single (C–C) covalent bonds.

20) Essential element in all organic compounds?

Carbon.

1) Why is diamond an electrical insulator?

All valence electrons localized in σ-bonds; no free electrons.

2) Why is graphite soft and slippery?

Weak forces between layers allow sheets to slide.

3) Density comparison: diamond vs graphite?

Diamond ~3.5 g/cm³; Graphite ~1.9–2.3 g/cm³.

4) Fullerene C₆₀ nickname?

Buckyball (Buckminsterfullerene).

5) Define hydrocarbon.

Compound containing only C and H.

6) Write methane combustion.

\( \mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + heat} \).

7) What is washing soda (ion)?

Sodium carbonate, \( \mathrm{Na_2CO_3} \).

8) What is baking soda (ion)?

Sodium bicarbonate, \( \mathrm{NaHCO_3} \).

9) Why CO₂ used in extinguishers?

Heavier than air, displaces oxygen, non-conductive, non-corrosive.

10) CO₂ effect on universal indicator in water?

Turns slightly acidic (color shifts accordingly).

11) Valence electrons in carbon?

4 (configuration \(2,4\)).

12) Why are covalent compounds poor conductors?

Lack free ions/electrons in molecular state.

13) Anthracite property?

Hardest, ~95% carbon, highest calorific value among coals.

14) Why methane called ‘marsh gas’?

Produced naturally in marshy areas by anaerobic decomposition.

15) One industrial use of graphite?

Electrodes in arc lamps/batteries.

16) One domestic use of methane?

Cooking fuel (PNG/biogas).

17) Reaction: CO₂ + NaOH (aq)?

\( \mathrm{2NaOH + CO_2 \rightarrow Na_2CO_3 + H_2O} \).

18) Reaction: CO₂ bubbled into \( \mathrm{Na_2CO_3} \) (aq)?

\( \mathrm{Na_2CO_3 + H_2O + CO_2 \rightarrow 2NaHCO_3} \).

19) What remains after destructive distillation of coal?

Coke (plus coal tar, coal gas as by-products).

20) Give one test to detect CO₂ gas.

Turns limewater milky (CaCO₃).

1) Explain why carbon forms covalent bonds.

Carbon (2,4) achieves octet by sharing four electrons; gaining/losing four is energetically unfavorable → forms covalent bonds.

2) How does structure make diamond hardest?

3D tetrahedral network of strong C–C σ-bonds throughout; no planes of weakness → extreme hardness.

3) Why is graphite a conductor?

Each C is sp²; one electron delocalized over the layer → mobile electrons conduct electricity.

4) List two uses each: diamond & graphite.

Diamond: cutting/drilling, surgical blades. Graphite: lubricants, electrodes (also pencil lead).

5) Distinguish crystalline vs amorphous carbon.

Crystalline: regular lattice (diamond/graphite/fullerene), sharp mp; Amorphous: irregular (coal/charcoal/coke), broad softening.

6) Name & order coal types with carbon % trend.

Peat < Lignite < Bituminous < Anthracite (carbon content increases along the series).

7) Write balanced equation for CO₂ preparation in lab.

\( \mathrm{CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\uparrow} \).

8) Why does limewater turn milky with CO₂?

Formation of insoluble calcium carbonate: \( \mathrm{Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O} \).

9) Give two properties & uses of CO₂ as dry ice.

Sublimes, very cold → cold storage/transport; stage fog/special effects.

10) Explain methane formation in biogas plant.

Anaerobic microbes convert organic waste → organic acids → methanogens convert to methane + CO₂.

11) Why are saturated hydrocarbons less reactive than unsaturated?

C–C and C–H σ-bonds are strong; unsaturated have π-bonds which are more reactive (addition reactions).

12) Solubility pattern of covalent compounds?

Generally insoluble in water; soluble in non-polar organic solvents (like dissolves like).

13) State two uses of coke.

As reducing agent in metallurgy; to make water gas/producer gas.

14) Why is methane a cleaner fuel than many others?

Highest H/C ratio among common fuels → less CO₂ per energy; burns with blue, smokeless flame.

15) Write sequential reactions from NaOH to NaHCO₃ using CO₂.

\( \mathrm{2NaOH + CO_2 \rightarrow Na_2CO_3 + H_2O} \), then \( \mathrm{Na_2CO_3 + H_2O + CO_2 \rightarrow 2NaHCO_3} \).

16) Why CO₂ useful in electrical fires?

Non-conductive, leaves no residue, blankets the fire by displacing oxygen.

17) Write electron-dot structure of methane (description).

Central C with four shared pairs to four H atoms; tetrahedral arrangement (109.5°).

18) State one environmental benefit of biogas.

Manages organic waste and reduces indoor air pollution versus solid fuels; also yields bio-manure.

19) Why is fullerene named so?

C₆₀ resembles geodesic domes designed by R. Buckminster Fuller.

20) Give one test to show graphite conducts electricity.

Insert pencil lead in a simple circuit; bulb glows indicating conduction.

1) Select the proper option and complete the statements.

a) A carbon atom forms a covalent bond; the two atoms share electrons.
b) All the carbon bonds in a saturated hydrocarbon are single bonds.
c) At least one carbon bond in an unsaturated hydrocarbon is multiple (double/triple).
d) Carbon is the essential element in all organic compounds.
e) Hydrogen is present in most organic compounds.

2) Answer the following.

a) Why are carbon and its compounds used as fuels?
High calorific value; hydrocarbons burn efficiently to CO₂ + H₂O releasing large heat; methane/PNG/biogas are clean-burning.

b) In which forms does carbon occur?
Free: diamond, graphite, fullerenes; Combined: CO₂, carbonates (CaCO₃), fossil fuels (coal, petroleum, natural gas), biomolecules, fibres; amorphous: coal, charcoal, coke.

c) Uses of diamond:
Cutting/drilling tools, surgical blades, jewelry, polishing (diamond dust), radiation windows in space instruments.

3) Explain the difference.

a) Diamond vs Graphite: Diamond—3D tetrahedral, hardest, insulator, transparent, dense; Graphite—layered hexagonal, soft, conductor, opaque, lower density.
b) Crystalline vs Non-crystalline carbon: Crystalline—regular lattice (diamond/graphite/fullerene), definite mp; Non-crystalline—irregular (coal/charcoal/coke), broad softening.

4) Write scientific reasons.

a) Graphite conducts electricity due to delocalized electrons in sp² layers.
b) Graphite not used in ornaments—black, soft, marks surfaces, lacks luster/hardness of gems.
c) Limewater turns milky with CO₂ as \( \mathrm{CaCO_3} \) precipitate forms.
d) Biogas is eco-friendly—renewable, clean combustion, manages waste, provides bio-manure.

5) Explain the following.

a) Diamond, graphite, fullerenes are crystalline (ordered atomic arrangements).
b) Methane is marsh gas—produced in marshes by anaerobic decomposition.
c) Petrol, diesel, coal are fossil fuels—formed from ancient biomass over geological time.
d) Uses of various allotropes—Diamond (cutting/jewelry); Graphite (electrodes/lubricant/pencils); Fullerenes (materials, catalysis).
e) Use of CO₂ in extinguisher—displaces O₂, non-conductive, leaves no residue.
f) Practical uses of CO₂—aerated drinks, dry ice, decaffeination, dry cleaning, photosynthesis enhancer in greenhouses.

6) Write two physical properties each.

a) Diamond: very hard; high refractive index/brightness; high mp; dense (~3.5 g/cm³).
b) Charcoal: porous; black; good adsorbent; low density.
c) Fullerene: molecular crystals; soluble in CS₂/chlorobenzene; may show superconductivity at specific conditions.

7) Complete the chemical reactions.

1) \( \boxed{\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Heat}}} \)
2) \( \boxed{\mathrm{CH_4 + Cl_2 \xrightarrow{Light} CH_3Cl + HCl}} \)
3) \( \boxed{\mathrm{2NaOH + CO_2 \rightarrow Na_2CO_3 + H_2O}} \)

8) Write answers in detail.

a) Types of coal & uses: Peat (low C) → Lignite (60–70% C) → Bituminous (70–90% C) → Anthracite (~95% C). Uses: fuel, power generation, coke/tar/gas production.

b) Prove graphite conducts electricity: Build a simple circuit with battery, bulb, wires; use pencil lead as conductor—bulb glows, confirming conduction.

c) Properties of carbon: Tetravalent; catenation (forms chains/rings); allotropy; forms covalent compounds; organic compound backbone.

d) Classify carbon: Crystalline (diamond, graphite, fullerenes) and Non-crystalline (coal, charcoal, coke).

9) How will you verify properties of CO₂?

Prepare CO₂ by \( \mathrm{CaCO_3 + 2HCl} \); test: colorless/odor-light; extinguishes flame; turns limewater milky; turns blue litmus red (acidic in water); denser than air (collect by upward displacement).

• Combustion of methane: \( \displaystyle \mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}} \)
• Chlorination (first step): \( \displaystyle \mathrm{CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl} \)
• CO₂ with NaOH: \( \displaystyle \mathrm{2NaOH + CO_2 \rightarrow Na_2CO_3 + H_2O} \)
• CO₂ with \( \mathrm{Na_2CO_3} \): \( \displaystyle \mathrm{Na_2CO_3 + H_2O + CO_2 \rightarrow 2NaHCO_3} \)
• CO₂ prep: \( \displaystyle \mathrm{CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\uparrow} \)