Fossil fuels -- coal, natural gas, petroleum
0654 names three fossil fuels: coal, natural gas and petroleum. All three formed over millions of years from the remains of dead organisms, so all three are non-renewable. Methane () is the main constituent of natural gas -- it is also the simplest hydrocarbon (one carbon atom, four hydrogen atoms, four single C-H bonds), which is why natural gas burns readily and is used directly for heating and cooking.
Hydrocarbon vs petroleum -- the definitions
Hydrocarbon: a compound that contains hydrogen and carbon only. The word "only" does the work -- an alcohol contains carbon, hydrogen AND oxygen, so it is not a hydrocarbon. Petroleum: a mixture of (largely saturated) hydrocarbons. Being a mixture, its hydrocarbons are not chemically combined and are in no fixed proportion -- which is exactly why a physical process, fractional distillation, separates them with no chemical reaction.
Fractional distillation of petroleum
Petroleum is heated so that most of it vaporises. The vapours rise up a fractionating column that is hot at the base and cooler towards the top. Each hydrocarbon vapour rises until the column temperature has fallen to its own boiling point, where it condenses back to a liquid and is drawn off as one fraction. Hydrocarbons with higher boiling points condense low down; those with lower boiling points travel further up before condensing.
Drawn from real examiner reports.
Separated by boiling point, not density
A recurring wrong answer says the fractions separate "by density" -- tempting, because they do end up loosely ordered by density. The correct description is differences in boiling point: each hydrocarbon condenses at the height where the column temperature matches its own boiling point. Density is a side-effect of molecular size, not the property driving the separation.
Flagged Jun 2022 P43 Q2aii - Jun 2023 P43 Q8b
Coal is a fossil fuel, not a hydrocarbon
The three fossil fuels are named together, so candidates assume all three are hydrocarbons. Only natural gas and petroleum are made of hydrocarbons. Coal is mostly carbon with impurities, so it fails the "hydrogen and carbon only" test. Keep the categories apart: fossil fuel says where a fuel came from, hydrocarbon says what its molecules contain.
Feed the column at the base, not the top
Diagram marks are lost in predictable ways. Petroleum must be shown heated and entering near the base, never fed in at the top. The column must show a temperature gradient (hot bottom, cool top), not one uniform temperature. Vapour arrows must point upwards. Fractions must sit in boiling-point order: refinery gas at the top, bitumen at the bottom.
Hydrocarbon: "only" is required
The mark-scheme definition is "a compound that contains hydrogen and carbon only". Two errors recur. Dropping only leaves a definition that a compound of carbon, hydrogen and oxygen would also satisfy. Naming the wrong category -- calling a hydrocarbon an element, an atom or a mixture -- also scores zero; a hydrocarbon is a compound.
Flagged Nov 2022 P32 Q8c - Nov 2023 P41 Q2a
Petroleum is a mixture, not a compound
Petroleum is a mixture of hydrocarbons, not one pure compound. Calling it a compound implies its hydrocarbons are chemically bonded to each other in a fixed ratio; they are not, they are simply mixed together. That distinction carries the follow-up mark, because only a mixture can be pulled apart by a purely physical method such as distillation.
Flagged Nov 2023 P11 Q26
Fractional distillation is not cracking
Fractional distillation is a physical separation: it only sorts hydrocarbons that are already present in petroleum, using their differing boiling points, and makes no new substances. Cracking (a separate C11 leaf) is a chemical reaction that breaks large molecules into smaller alkanes and alkenes using heat and a catalyst, so it does make new substances.
(Extended) Going UP, chains get SHORTER
From the bottom to the top of the column the fractions show decreasing chain length and lower boiling points -- bitumen (longest chains, highest boiling point) at the base, refinery gas (shortest chains, lowest boiling point) at the top. Candidates routinely reverse this and write that chain length or boiling point increases going up.
Name the exact use, not a vague one
Give the specific use: refinery gas -> heating and cooking; gasoline/petrol -> fuel for cars; naphtha -> chemical feedstock; diesel oil/gas oil -> fuel for diesel engines; bitumen -> making roads. A catch-all such as "used in vehicles" identifies no fraction and earns nothing.
Either accepted fraction name scores
Two fractions carry two credited names -- gasoline / petrol and diesel oil / gas oil. Either is accepted, so write whichever you remember. The use has no such flexibility: it must match the specific wording. Naphtha is the odd one out: a feedstock, not a fuel.
Answer the command word, match the tariff
State wants the term alone; describe wants the steps of the process; explain wants the reason behind them. Give as many separate points as there are marks -- a 3-mark describe needs three. A scientifically true sentence that does not answer the question asked scores nothing.
Count every atom before you add
In relative formula mass each subscript multiplies: is , not 13. is a ratio and carries no unit, but a percentage-by-mass answer must be labelled as a percentage -- unlabelled numerical answers lose the final mark.
Cambridge 0654 spec reference: Section C11 "Organic chemistry", sub-topic C11.3 "Fuels". Core: name the fossil fuels coal, natural gas and petroleum; name methane as the main constituent of natural gas; state that hydrocarbons contain hydrogen and carbon only; state that petroleum is a mixture of hydrocarbons; describe the separation of petroleum into useful fractions by fractional distillation; name the uses of the fractions. Supplement (Extended): describe how fraction properties change from the bottom to the top of the fractionating column -- decreasing chain length, lower boiling points.
| Term | Mark-scheme-precise meaning |
|---|---|
| Fossil fuel | A fuel formed over millions of years from the remains of dead organisms; the three named in 0654 are coal, natural gas and petroleum |
| Methane () | The main constituent of natural gas |
| Hydrocarbon | A compound that contains hydrogen and carbon only |
| Petroleum | A mixture of hydrocarbons (largely saturated) |
| Fractional distillation | Separation of petroleum into fractions using differences in boiling point: the mixture is heated, vapours rise up a column that is hot at the base and cool at the top, and each fraction condenses where the column temperature matches its own boiling point |
Uses of the named fractions (Core):
| Fraction | Named use |
|---|---|
| Refinery gas | Gas used for heating and cooking |
| Gasoline / petrol | Fuel used in cars |
| Naphtha | Chemical feedstock |
| Diesel oil / gas oil | Fuel used in diesel engines |
| Bitumen | Making roads |
Extended -- property trend up the column: moving from the bottom to the top of the fractionating column, the fractions collected show: (a) decreasing chain length (shorter hydrocarbon molecules); (b) lower boiling points. The fraction collected right at the top (refinery gas) has the shortest chains and lowest boiling point of all; the fraction collected at the very bottom (bitumen) has the longest chains and highest boiling point.
Define "petroleum" as used in this topic.
Methane, , is the main constituent of natural gas.
Calculate the relative formula mass, , of methane. (2 marks)
