Alkane — single covalent bonds, so saturated
A hydrocarbon is a compound containing only carbon and hydrogen. An alkane is a hydrocarbon in which every carbon-carbon bond is a single covalent bond — one shared pair of electrons per bond. So every alkane is a saturated hydrocarbon. Full credit needs both elements: single covalent bonding, therefore saturated. Methane, ethane, propane and butane are the first four alkanes.
Generally unreactive — except combustion
0654 requires alkanes to be described as generally unreactive, except in terms of combustion — a single two-part statement. Complete combustion (plentiful oxygen) gives carbon dioxide and water. Incomplete combustion (limited oxygen) gives carbon monoxide and/or carbon as soot, plus water, because there is not enough oxygen to oxidise every carbon atom fully to . Both count as combustion, the one stated exception.
Bromine water — an alkane gives no change
Aqueous bromine is orange. Shaken with an alkane it shows no colour change — it stays orange, because a saturated alkane has no carbon-carbon double bond for the bromine to add across. That negative result is the standard test-tube evidence for the C11.4 statement that alkanes are unreactive. An alkene behaves oppositely (decolourised, orange to colourless), which is how the two are told apart.
Drawn from real examiner reports.
Saturated ≠ unsaturated ≠ water content
Two wrong forms recur when candidates justify "alkanes are saturated". (1) Calling an alkane unsaturated, often because it contains hydrogen — that is not the definition. (2) Explaining "saturated" as water or moisture content, importing the everyday English meaning. The only correct reason: every carbon-carbon bond in the molecule is a single bond.
Flagged Jun 2022 P41 Q2e · Nov 2023 P42 Q2c
Alkanes do not decolourise bromine water
Having seen an alkene decolourise aqueous bromine, candidates predict the same for an alkane. It does not happen: an alkane is saturated, has no C=C double bond, and the bromine water stays orange. The decolourisation is exactly what distinguishes an alkene from an alkane, so predicting it for both destroys the test.
Flagged Nov 2022 P33 Q2dii · Jun 2023 P42 Q2c
"Unreactive" alone is only half the mark
The syllabus statement has two halves: alkanes are generally unreactive, AND the exception is combustion. Answers that stop at "alkanes are unreactive" leave the second mark on the table. The opposite error is treating combustion as proof that alkanes are reactive — they still give no reaction with aqueous bromine or with dilute acid.
Complete vs incomplete combustion products
Complete combustion (plenty of oxygen) gives carbon dioxide and water only. Incomplete combustion (limited oxygen) gives carbon monoxide and/or carbon as soot, still with water. Candidates mix the two product sets, or try to balance an incomplete-combustion equation as though carbon dioxide were still forming. Read the oxygen supply given in the stem first.
"Clear" is not the same as "colourless"
When the alkane/alkene comparison is written up, the alkene result must be described as the bromine water going colourless, not "clear" — a solution can be clear and still orange, so "clear" does not describe a colour change. Pair it with the alkane result stated as no colour change, rather than leaving the alkane observation blank.
Atom-count slips when balancing
Three recurring slips: miscounting hydrogens in a formula (using 4 instead of 6 for , so comes out 28 instead of 30); leaving a fractional coefficient such as oxygen in a final equation instead of doubling everything; and writing soot as something other than plain C. Recount every element on both sides before moving on.
"Hydrocarbon" needs the word "only"
A hydrocarbon is a compound containing only carbon and hydrogen. Dropping "only" and writing "a compound of carbon and hydrogen" would also describe compounds that contain a third element such as oxygen, so it earns no mark. It must also be called a compound — not an element, an atom or a mixture.
Flagged Nov 2022 P32 Q8c · Nov 2023 P41 Q2a
Balance carbon, then hydrogen, then oxygen
Write the skeleton equation first, then set the (or CO/C) coefficient from the carbon count, the coefficient from half the hydrogen count, and only then choose the coefficient from the oxygen now needed on the right.
Clear a fractional oxygen coefficient
If balancing oxygen leaves a half, e.g. for butane, multiply every coefficient by 2 — never round it, and never add a stray molecule instead. Butane becomes .
Name the combustion type explicitly
Say "complete combustion" or "incomplete combustion" in words rather than leaving the examiner to infer it from your equation. Check the command word as well: state wants the term only, describe wants what happens, explain wants why it happens.
Mr from the formula — show the sum
Relative formula mass is plain summation from the formula, not a mole calculation — no moles are needed here. Set it out as a sum, e.g. propane , so a method mark survives an arithmetic slip. carries no unit.
Cambridge 0654 spec reference: Section C11 "Organic chemistry", sub-topic C11.4 "Alkanes". Core only, no Supplement: state that the bonding in alkanes is single covalent and that alkanes are saturated hydrocarbons; describe the properties of alkanes as being generally unreactive, except in terms of combustion.
| Term | Mark-scheme-precise meaning |
|---|---|
| Hydrocarbon | A compound containing only carbon and hydrogen atoms |
| Alkane | A hydrocarbon in which every carbon-carbon bond is a single covalent bond |
| Saturated hydrocarbon | A hydrocarbon whose molecules have all carbon-carbon bonds as single bonds -- this describes every alkane |
| Generally unreactive | Alkanes do not react with most reagents under normal conditions -- the one stated exception in 0654 is combustion |
| Complete combustion | Burning in a plentiful supply of oxygen: hydrocarbon + oxygen carbon dioxide + water |
| Incomplete combustion | Burning in a limited supply of oxygen: produces carbon monoxide () and/or carbon (soot), as well as water, because there is not enough oxygen to fully oxidise every carbon atom |
The first four alkanes (by number of carbon atoms): methane , ethane , propane , butane . Every carbon-carbon bond in each of these molecules is single, so every one of them is a saturated hydrocarbon.
State the type of bonding between carbon atoms in an alkane, and what this makes an alkane in terms of saturation.
Ethane, an alkane with molecular formula , has only single carbon-carbon and carbon-hydrogen bonds.
Calculate the relative formula mass, , of ethane. (2 marks)
