CDS Chemistry · Teaching notes
Carbon and Its Compounds — CDS Chemistry
Carbon and Its Compounds has 21 past-year questions in CDS General Knowledge, from 2016 (II) to 2026 (I), and six of them are HARD, the most of any chemistry chapter. The hard ones sit in organic chemistry beyond Class 10: functional isomers, Fischer projections, Lassaigne's test, octane numbers. The easy marks are the allotropes, where one contrast answers almost every question: diamond makes four bonds per carbon and insulates, graphite makes three and conducts.
Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.
Subtopic notes
Carbon and Its Allotropes
7 PYQsDiamond, graphite, fullerene and graphene: how each is built, why only graphite conducts, and what is not an allotrope.
Hydrocarbons and Fuels
6 PYQsThe general formulas of alkanes, alkenes and alkynes, benzene's structure, why some fuels burn with a sooty flame, and what octane and cetane numbers measure.
Functional Groups and Isomerism
5 PYQsThe functional groups that name an organic compound, which classes can be functional isomers of each other, the three ways of drawing a molecule in 3-D, and how Lassaigne's test finds nitrogen, sulphur and halogens.
Soaps, Micelles and Hydrogenation
3 PYQsHow a soap molecule cleans by forming micelles, why micelles stay spread out, and how nickel turns vegetable oil into solid fat.
Formula & revision sheet
1 formulas · 9 reference tables · 13 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
1 formulas · 9 reference tables · 13 gotchas across all subtopics — the exam-eve cheat-sheet
Reference tables (2)
The allotropes of carbon5 rows
| Form | Bonds per carbon | Conducts electricity? |
|---|---|---|
| Diamond | 4 (3-D network) | No |
| Graphite | 3 (flat layers) | Yes |
| Fullerene (C₆₀) | 3 (closed cage) | No |
| Graphene | 3 (one layer) | Yes, very wellQ CDS 2025 (II): 'a wide band-gap semiconductor' is the property graphene does NOT have. |
| Coal | Not an allotrope: a mixture | Not a single form of carbonQ CDS 2016 (II): coal is the one that is not an allotrope of carbon. |
Diamond and graphite compared5 rows
| Property | Diamond | Graphite |
|---|---|---|
| Structure | 3-D tetrahedral network | Flat hexagonal layers |
| Bonds per carbon | 4 | 3, all in one planeQ CDS 2019 (II): in graphite each carbon is bonded to three others in the same plane, giving a hexagonal array. |
| Hardness | Hardest natural substance | Soft, slippery |
| Electrical conduction | Insulator | Good conductor |
| Percentage of carbon | 100% (pure) | 100% (pure) |
Watch out for (4)
- The coal answer: a carbon-rich rock is not an allotrope→ The allotropes of carbon
- Graphene has no band gap→ The allotropes of carbon
- Same element, different properties→ Diamond and graphite compared
- Diamond's bonds are all the same length→ Diamond and graphite compared
Reference tables (2)
Clean and sooty flames3 rows
| Fuel | Flame | Why |
|---|---|---|
| Saturated (methane, LPG) | Blue, clean | Burns completely |
| Unsaturated hydrocarbons | Yellow, sooty | Incomplete combustionQ |
| Naphthalene (aromatic) | Yellow, sooty | High carbon to hydrogen ratio, so incomplete combustionQ CDS 2023 (I): the reason is incomplete combustion. 'The carbon to hydrogen ratio is low' states the reverse. |
Watch out for (4)
- CₙH₂ₙ₊₁ is a group, not a compound→ Alkanes, alkenes, alkynes and benzene
- Benzene has no isomers of its own→ Alkanes, alkenes, alkynes and benzene
- Sooty means carbon-rich, not carbon-poor→ Clean and sooty flames
- Longer chain, lower octane→ Octane and cetane numbers
Reference tables (3)
Functional groups5 rows
| Group | Formula | Example |
|---|---|---|
| Alcohol | –OH | Ethanol, C₂H₅OH |
| Aldehyde | –CHO | Methanal, HCHO |
| Ketone | >C=O | Propanone, CH₃COCH₃ |
| Carboxylic acid | –COOH | Propanoic acid, C₂H₅COOHQ |
| Haloalkane | –X (Cl, Br, I) | Chloromethane, CH₃Cl |
Catenation, isomerism and 3-D projections5 rows
| Class | Functional isomer |
|---|---|
| Alcohol (C₂H₆O: ethanol) | Ether (dimethyl ether) |
| Aldehyde (C₃H₆O: propanal) | Ketone (propanone) |
| Carboxylic acid | Ester |
| Cyanide (–CN) | Isocyanide (–NC) |
| Alkyl halide | None possibleQ CDS 2019 (I), HARD: alkyl halides cannot show functional isomerism. |
Lassaigne's test for N, S and halogens4 rows
| Halogen | Silver halide | Seen as |
|---|---|---|
| Chlorine | AgCl | White precipitate |
| Bromine | AgBr | Pale yellow precipitate |
| Iodine | AgI | Yellow precipitate |
| Fluorine | AgF | No precipitate (soluble)Q |
Watch out for (2)
- Methanol's damage comes from methanal→ Functional groups
- Fischer is the least stable view→ Catenation, isomerism and 3-D projections
Reference tables (2)
Soap molecules and micelles5 rows
| Part or idea | What it is |
|---|---|
| Soap | Na or K salt of a long-chain carboxylic acid |
| Hydrophilic part | Ionic head (–COO⁻ Na⁺) |
| Hydrophobic part | Long hydrocarbon tail |
| Grease in a micelle | Trapped in the centre |
| Why micelles stay dispersed | Like charges on the heads repelQ CDS 2026 (I): 'micelles precipitate out because of ion-ion repulsion' is the false statement. |
Hydrogenation of oils2 rows
| Process | Reagent or catalyst | Result |
|---|---|---|
| Hydrogenation of oil | H₂ with nickel | Solid saturated fat (vanaspati)Q |
| Esterification | Concentrated H₂SO₄ (removes water) | Ester + water |
Watch out for (3)
- The hydrophilic part is the ionic head, not the oil→ Soap molecules and micelles
- Repulsion keeps micelles apart→ Soap molecules and micelles
- The catalyst is nickel→ Hydrogenation of oils
PYQ weightage by concept
10 concepts · 21 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
10 concepts · 21 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| The allotropes of carbon | 4 | 19% |
| Diamond and graphite compared | 3 | 14% |
| Concept | PYQs | Share |
|---|---|---|
| Alkanes, alkenes, alkynes and benzene | 2 | 10% |
| Clean and sooty flames | 2 | 10% |
| Octane and cetane numbers | 2 | 10% |
| Concept | PYQs | Share |
|---|---|---|
| Functional groups | 2 | 10% |
| Catenation, isomerism and 3-D projections | 2 | 10% |
| Lassaigne's test for N, S and halogens | 1 | 5% |
| Concept | PYQs | Share |
|---|---|---|
| Soap molecules and micelles | 2 | 10% |
| Hydrogenation of oils | 1 | 5% |
Test yourself on Carbon and Its Compounds
10 past CDS questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.