Corrosion and extraction basics

Highly reactive metals such as sodium, calcium and aluminium are more reactive than carbon. Therefore, their compounds cannot be reduced by carbon. They are generally extracted by electrolysis of their molten compounds. Reduction with carbon is suitable for metals of moderate reactivity. Exam tip: Remember that metals placed above carbon in the reactivity series are commonly extracted by electrolysis.

The unwanted earthy impurities such as sand and clay present in an ore are called gangue. Removing gangue to increase the proportion of the metal-bearing material is called concentration of ore. Roasting and calcination involve heating ores, whereas reduction produces the metal from its compound. Exam tip: if the question mentions removal of gangue, choose concentration of ore.

Rusting requires both oxygen and water (or moisture). Iron reacts slowly in their presence to form hydrated iron(III) oxide, called rust. Carbon dioxide alone is not sufficient to cause rusting. Exam tip: remember that rusting needs oxygen + water together.

The process of applying a protective zinc coating to iron or steel is called galvanization. Zinc prevents iron from coming into contact with air and moisture; even if the coating is scratched, zinc oxidises preferentially and protects the iron. Electroplating is a general method for depositing metals, whereas the specific zinc coating on iron is called galvanization. Exam tip: zinc-coated iron is known as galvanised iron.

Rusting is a slow oxidation process in which iron needs both oxygen and water (moisture). Moisture helps the electrochemical reaction occur. Oxygen alone, without moisture, does not normally cause rusting. Exam tip: Remember the required conditions as oxygen + water.