Displayed formula — every atom, every bond
A displayed formula draws the molecule out in full: every atom shown individually — never a shorthand group like — and every bond drawn as its own line, each line being one shared pair of electrons and a double bond two parallel lines. It also shows the relative positions of the atoms. A molecular formula such as gives only atom counts, with no bonds and no arrangement, so only the displayed formula lets you read off saturation.
Saturated vs unsaturated — C-C bonds only
Saturated: every carbon-carbon bond in the molecule is a single bond, so each carbon is bonded to as many other atoms as possible. Unsaturated: at least one carbon-carbon bond is not single — most commonly a C=C double bond. Carbon-hydrogen bonds are always single and are irrelevant to the decision, so classify by tracing the carbon backbone alone. One non-single C-C bond anywhere makes the whole compound unsaturated.
(Extended) Homologous series — the two characteristics
A family of similar compounds with similar chemical properties — both halves are needed. Its two characteristics: (a) every member fits the same general formula (0654 does not require recall of the specific formulas, only that one exists); (b) members show a trend in physical properties, e.g. boiling point rises as the chain lengthens, because longer molecules have more electrons and stronger intermolecular forces. Chemical properties stay similar.
Drawn from real examiner reports.
Saturated ≠ unsaturated
Candidates swap the two terms, calling a molecule that contains a C=C double bond "saturated". Fix the direction once and check it every time: saturated = ALL carbon-carbon bonds single; unsaturated = AT LEAST ONE carbon-carbon bond not single. If you can find even one double line between two carbon atoms in the displayed formula, the compound is unsaturated.
Flagged Jun 2022 P41 Q2e · Nov 2023 P42 Q2c
"Saturated" has nothing to do with water
In everyday English a saturated cloth is one soaked with water, and candidates carry that meaning into the exam — describing a hydrocarbon in terms of its moisture or water content. In organic chemistry "saturated" refers only to the type of carbon-carbon bonding present. No water, no soaking, no solution: just single C-C bonds.
Flagged Jun 2022 P41 Q2e · Nov 2023 P42 Q2c
One C=C is enough to be unsaturated
Shown a three-carbon chain where the first C-C bond is a double line and the second is a single line, candidates count the majority and answer "saturated". The classification is not a majority vote: one non-single carbon-carbon bond anywhere makes the entire compound unsaturated. Saturated requires every carbon-carbon bond to be single, without exception.
A double bond needs two lines
Asked to draw ethene, candidates place all six atoms correctly but join the two carbon atoms with a single line, losing the C=C. A displayed formula must show every bond explicitly, so a double bond is drawn as two parallel lines. Correct atoms with the wrong bond type does not earn full credit — the atoms and the bonds carry separate marks.
Flagged Jun 2023 P32 Q8b
(Extended) Half the homologous-series definition
The definition carries two marking points: (1) a family of similar compounds, AND (2) with similar chemical properties. Answers that stop at "a group of compounds that are alike" score one mark of two. A 2-mark "define" question wants two distinct elements, so check your sentence names the chemical properties explicitly before moving on.
(Extended) The trend is physical, not chemical
Candidates reverse the two properties, writing that members of a homologous series show a trend in chemical properties. It is the other way round: the chemical properties stay similar throughout the series — that is the defining feature — while the physical properties, such as melting point and boiling point, change gradually as the chain lengthens.
Scan the carbon backbone
Trace every bond that joins one carbon directly to the next, and ignore all C-H bonds — they are always single and never decide the answer. If every backbone bond is a single line, answer saturated. If any of them is a double line, answer unsaturated. You only need to find one.
Name the bond that proves it
A bare "unsaturated" loses the second mark on a state-and-justify question. Point at the evidence: "unsaturated, because the displayed formula shows a carbon-carbon double bond". Justifying by counting hydrogen atoms scores nothing — the C-C bond type is the defining feature.
Give every carbon four bonds
Before you move on from a displayed formula, count the lines meeting each atom: four at every carbon, one at every hydrogen. This catches the two commonest slips — a missing C-H bond on an end carbon, and writing a shorthand group instead of drawing each atom out individually.
Answer the command word, count the marks
"State" wants the term only; "describe" wants the features; "explain" wants a reason. Across 0654 sittings the recurring losses are answers that are scientifically true but off-question, and answers offering fewer points than the mark tariff allows. One mark, one distinct point.
Cambridge 0654 spec reference: Section C11 "Organic chemistry", sub-topic C11.1 "Formulas and terminology". Core: draw and interpret the displayed formula of a molecule to show all atoms and all bonds; state that a saturated compound has all carbon-carbon bonds single, and an unsaturated compound has one or more carbon-carbon bonds that are not single. Supplement (Extended): state that a homologous series is a family of similar compounds with similar chemical properties; describe its general characteristics -- same general formula (specific formulas not required), and a trend in physical properties.
| Term | Mark-scheme-precise meaning |
|---|---|
| Displayed formula | Shows all the atoms in a molecule and all the bonds between them, drawn out individually (a double bond = two parallel lines) |
| Saturated compound | A compound whose molecules have all carbon-carbon bonds as single bonds |
| Unsaturated compound | A compound whose molecules have one or more carbon-carbon bonds that are NOT single bonds (commonly a C=C double bond) |
| Homologous series (Ext) | A family of similar compounds with similar chemical properties |
| General formula (Ext, recall not required) | An algebraic formula (e.g. relating carbon and hydrogen atom counts) that fits every member of a homologous series |
| Trend in physical properties (Ext) | Physical properties (e.g. melting point, boiling point) change gradually and predictably as you move along a homologous series |
Displayed vs molecular formula: a molecular formula (e.g. for ethene) gives only the total number of each type of atom -- it shows no bonds and no arrangement. A displayed formula draws out every individual atom AND every individual bond, so you can see directly whether the compound is saturated or unsaturated.
Define "displayed formula".
The displayed formula of methane shows one carbon atom bonded singly to four hydrogen atoms, giving the molecular formula .
Calculate the relative formula mass, , of methane. (2 marks)
