Extraction of metals

Hematite is iron's ore (iron(III) oxide); bauxite is aluminium's (aluminium oxide). The pair is routinely swapped, or one name is forgotten and only the compound is offered. Writing "aluminium oxide" where the ore name was asked does not earn the mark -- aluminium oxide is what is extracted FROM bauxite, not the ore's own name.

Aluminium sits above carbon in the reactivity series, so carbon cannot displace it from its oxide. Aluminium is extracted by electrolysis of the molten compound, which supplies the energy carbon cannot. Importing iron's "reduction with carbon" method onto aluminium is a straight error, and giving only the ore or only the method loses the other mark.

Carbon can extract a metal only if carbon is more reactive than it -- that is, carbon sits above it in the series. So carbon reduces the oxides of zinc, iron, lead and copper, but not those of aluminium, magnesium, calcium, sodium or potassium. "Carbon reduces iron because iron is more reactive than carbon" has the comparison the wrong way round and scores nothing.

Given a reactivity order worked out from displacement data, candidates assume it automatically runs least-reactive to most-reactive down the page. It does not. Read the table the way the data actually orders it before deciding which metal displaces which, or which metals carbon is able to extract.

Reduction of $\text{CO}_2$ to $\text{CO}$, and reduction of the iron(III) oxide by $\text{CO}$, are two separate furnace stages and must not be merged. The first makes the reducing agent; the second is where that carbon monoxide strips oxygen from the ore. Writing carbon reducing the iron oxide directly is not the syllabus pathway and does not earn the stage (c) mark.

In the iron-reducing step the reducing agent is carbon monoxide, not iron(III) oxide. The reducing agent is the substance that is itself oxidised while removing oxygen from something else: $\text{CO}$ gains oxygen to become $\text{CO}_2$, while the ore loses oxygen to become iron. Candidates routinely name the ore instead.

Silica reacts with calcium oxide, not with calcium carbonate. The limestone must thermally decompose first; only then does the resulting basic calcium oxide react with the acidic silica to form slag. Writing calcium carbonate reacting directly with silica collapses two reactions into one and loses the decomposition mark.