CDS Chemistry · Formula sheet
Carbon and Its Compounds formulas
1 formula, 9 reference tables and 13 common traps for CDS Chemistry Carbon and Its Compounds, grouped by subtopic.
Carbon and Its Allotropes
Learn this subtopic in the notesThe allotropes of carbon
| 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 compared
| 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) |
Common traps
The coal answer: a carbon-rich rock is not an allotrope
An allotrope is a pure form of one element. Coal contains hydrogen, sulphur, nitrogen and minerals, so it is not an allotrope. Diamond, graphite, graphene and fullerene are.
Graphene has no band gap
Graphene is a zero band-gap semimetal, which is why it conducts so well. Calling it a wide band-gap semiconductor is the false statement.
Same element, different properties
'Diamond and graphite have similar physical and chemical properties' is false for physical properties: one is hard and insulating, the other soft and conducting. Their chemistry is the same because both are pure carbon.
Diamond's bonds are all the same length
Every C–C bond in diamond is the same length in every direction. A statement that diamond has 'different carbon to carbon distance in all directions' is untrue.
Hydrocarbons and Fuels
Learn this subtopic in the notesAlkanes, alkenes, alkynes and benzene
General formulas
Clean and sooty flames
| 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. |
Common traps
CₙH₂ₙ₊₁ is a group, not a compound
Formulas with an odd number of hydrogens, such as CₙH₂ₙ₊₁, describe an alkyl group (methyl, ethyl), not a stable hydrocarbon. Saturated hydrocarbons are CₙH₂ₙ₊₂.
Benzene has no isomers of its own
All six carbons and all six hydrogens in benzene are equivalent, so 'benzene has six isomers' is false. Its C–C bonds are all the same length.
Sooty means carbon-rich, not carbon-poor
A sooty flame comes from a high carbon-to-hydrogen ratio and incomplete combustion. Excess air would give a cleaner flame, not a sootier one.
Longer chain, lower octane
For straight-chain alkanes the octane number drops as carbons are added: butane > pentane > hexane. A ring (cyclohexane) raises it above the matching chain.
Functional Groups and Isomerism
Learn this subtopic in the notesFunctional groups
| 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 projections
| 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. |
CDS 2019 (I), HARD: a Fischer projection is the eclipsed, least stable conformation, not the most stable.
Lassaigne's test for N, S and halogens
| Halogen | Silver halide | Seen as |
|---|---|---|
| Chlorine | AgCl | White precipitate |
| Bromine | AgBr | Pale yellow precipitate |
| Iodine | AgI | Yellow precipitate |
| Fluorine | AgF | No precipitate (soluble)Q |
Common traps
Methanol's damage comes from methanal
Methanol is toxic because the liver turns it into methanal, which coagulates protoplasm. It is not converted to acetic acid or carbon monoxide.
Fischer is the least stable view
A Fischer projection draws the molecule eclipsed, which has the most strain. 'The Fischer projection is the most stable conformation' is the false statement.
Soaps, Micelles and Hydrogenation
Learn this subtopic in the notesSoap molecules and micelles
| 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 oils
| Process | Reagent or catalyst | Result |
|---|---|---|
| Hydrogenation of oil | H₂ with nickel | Solid saturated fat (vanaspati)Q |
| Esterification | Concentrated H₂SO₄ (removes water) | Ester + water |
Common traps
The hydrophilic part is the ionic head, not the oil
'The hydrophilic part of soap is oil' is false. The ionic head is hydrophilic; the hydrocarbon tail is hydrophobic and dissolves the oil.
Repulsion keeps micelles apart
Like charges on the micelle surfaces make them repel, which keeps them dispersed. Repulsion does not make them settle out.
The catalyst is nickel
Hydrogenation of oils uses nickel (or platinum, palladium). Options of copper, iron or zinc with hydrogen are distractors.