Redox = oxidation and reduction together
Redox means REDuction and OXidation happening simultaneously — one substance is oxidised while a different substance is reduced, in the same reaction. At Core level the change is tracked through oxygen: oxidation = gain of oxygen, reduction = loss of oxygen. When iron(III) oxide reacts with carbon monoxide in a blast furnace, the oxide loses oxygen (reduced) while the carbon monoxide gains oxygen (oxidised).
(Extended) Electron transfer — OIL RIG
Extended candidates define redox by electron transfer, which also covers reactions with no oxygen at all. Oxidation = loss of electrons (an increase in oxidation number); reduction = gain of electrons (a decrease in oxidation number). Determination of oxidation numbers is not required. OIL RIG — Oxidation Is Loss, Reduction Is Gain — fixes the direction, but it is a memory aid, not a mark-scheme definition.
Oxidation number only names ions
Where an element forms ions of more than one charge, a Roman numeral after its name gives that charge: iron(II) = (so ), iron(III) = (), copper(I) = (), copper(II) = (). You only READ the numeral to get the charge and build a charge-neutral formula — calculating an oxidation number is never required.
Drawn from real examiner reports.
Oxidised ≠ oxidising agent
The substance oxidised loses electrons (or gains oxygen). The oxidising agent is the OTHER reactant, which causes that oxidation by accepting the electrons — and is itself reduced. So in zinc + copper(II) sulfate, is the oxidising agent and zinc the reducing agent; calling zinc the oxidising agent because it gets oxidised is backwards.
Flagged Jun 2022 P23 Q18 · Jun 2023 P11 Q20 · Nov 2023 P43 Q11aii
Gain and loss the wrong way round
Examiners repeatedly flag the direction being reversed: "oxidation = loss of oxygen" and "oxidation = gain of electrons" both invert the definition. A second version of the same slip is labelling by reactivity instead of by transfer — calling magnesium "reduced" in Mg + CuO because it is the reactive metal that gets used up, when magnesium gains oxygen and is oxidised.
Flagged Nov 2023 P43 Q8a · Jun 2022 P23 Q18 · Jun 2023 P11 Q20 · Nov 2023 P43 Q11aii
"Adds oxygen" is too vague
Mark schemes want the transfer named and directed. "Oxygen is added", "it reacts with oxygen" or "the substance changes" earn nothing — the credited forms are "gain of oxygen by the substance" and, at Extended, "loss of electrons by the substance". Saying something is simply lost or gained without naming oxygen or electrons is equally incomplete.
Half a redox answer scores half
A redox reaction has BOTH halves. "A redox reaction is one where oxidation happens" omits the simultaneous reduction and is not the definition. Likewise an "identify the oxidation and the reduction" question needs both substances named with a reason each — candidates commonly give only that iron(III) oxide is reduced, never that carbon monoxide is oxidised.
Flagged Jun 2022 P23 Q18 · Jun 2023 P11 Q20 · Nov 2023 P43 Q11aii
(Extended) No oxygen still means redox
Candidates argue that zinc added to copper(II) sulfate cannot be redox "because there is no oxygen". The electron-transfer definition does not need oxygen: zinc loses electrons (oxidised) and gains them (reduced), so oxidation and reduction still occur simultaneously. The oxygen definition is the narrower special case, not the test.
(Extended) Electrons on the wrong side
In a half-equation, electrons on the left are gained, so that half is reduction: . Electrons on the right are lost, so that half is oxidation: . Putting them on the wrong side reverses the answer, and dropping the coefficient (writing for ) leaves the charge unbalanced.
Flagged Jun 2022 P43 Q5 · Nov 2023 P41 Q11cii
CuO vs Cu2O — read the numeral
Copper(II) oxide is : a 2+ charge and a 2- charge already balance 1:1, so no subscripts are needed. is copper(I) oxide, a different compound. Build the formula from the charge the Roman numeral states, not from habit — extra subscripts unbalance the charge and lose the mark.
(Extended) Count electrons instead
If the question gives an ionic or half-equation rather than a molecular one, switch routes: count electrons instead of oxygens. Electrons on the reactant side = gained = reduction; on the product side = lost = oxidation. A word or molecular equation signals the oxygen route.
Check atoms and charge balance
Count each element on both sides before committing — the number of atoms of every element is unchanged in a reaction. For ionic and half-equations, total charge must match across the arrow too. The check catches a mis-written formula before it costs the identification marks.
Carbon only reduces metals below it
"Carbon is a reducing agent" alone does not explain why carbon extracts copper but not aluminium. State the comparison: carbon is more reactive than copper, so it takes the oxygen from copper(II) oxide; aluminium is more reactive than carbon, so carbon cannot reduce its oxide.
Track the oxygen, name both
Follow the oxygen through the equation. Which reactant ends up with oxygen it did not have (oxidised)? Which loses the oxygen it had (reduced)? Write both, each with its reason: "carbon monoxide is oxidised — it gains oxygen; iron(III) oxide is reduced — it loses oxygen."
Cambridge 0654 spec reference: Section C6 "Chemical reactions", sub-topic C6.3 "Redox". Core: define redox as simultaneous oxidation and reduction; define oxidation as gain of oxygen, reduction as loss of oxygen; identify redox reactions and identify oxidation/reduction within them (oxidation number limited to naming ions, e.g. iron(II), iron(III), copper(II)). Extended (Supplement): define oxidation as electron loss / increase in oxidation number, reduction as electron gain / decrease in oxidation number (determination of oxidation numbers not required).
| Term | Mark-scheme-precise meaning |
|---|---|
| Redox reaction | A reaction in which oxidation and reduction occur simultaneously -- one substance is oxidised while another is reduced, in the same reaction |
| Oxidation (Core) | Gain of oxygen by a substance |
| Reduction (Core) | Loss of oxygen by a substance |
| Oxidation (Extended) | Loss of electrons by a substance (equivalently, an increase in oxidation number) |
| Reduction (Extended) | Gain of electrons by a substance (equivalently, a decrease in oxidation number) |
| Oxidising agent | A substance that causes oxidation of another substance (by accepting electrons from it) -- the oxidising agent itself is reduced |
| Reducing agent | A substance that causes reduction of another substance (by giving electrons to it) -- the reducing agent itself is oxidised |
| Oxidation number (0654 use only) | A Roman numeral written after an element's name to show the charge on its ion where more than one is possible, e.g. iron(II) = , iron(III) = , copper(II) = . Calculating/determining an oxidation number is NOT required by this spec. |
OIL RIG (a mnemonic, not itself a mark-scheme definition): Oxidation Is Loss, Reduction Is Gain -- of electrons.
Atom and charge conservation. In any chemical equation (redox or not), the number of atoms of each element is unchanged (atoms are rearranged, never created or destroyed); in an ionic (half-)equation, total charge must also balance across the arrow.
Define a redox reaction.
Copper(II) oxide contains ions and oxide ions, .
Use the oxidation number in the name to write the formula of copper(II) oxide. (1 mark)
