Which represents calcination?

Concept: Calcination is the thermal decomposition of a carbonate or hydroxide ore, heated in limited or absent air below its melting point, into the corresponding metal oxide with loss of a volatile gas (CO2 or H2O). It is distinct from roasting, which heats a sulfide ore in excess oxygen and gives off SO2.

Application: Applying this to the given reaction: heating decomposes the carbonate ore magnesium carbonate into magnesium oxide, releasing carbon dioxide gas (MgCO3 → MgO + CO2) — exactly the calcination pattern of a carbonate breaking down into its oxide.

Cross-check: Contrasting the other reactions confirms none of them fit the calcination pattern:

• 2Ag + 2HCl + [O] → 2AgCl + H2O is a redox/chlorination reaction of a metal with an acid and an oxidant — no carbonate or hydroxide ore is being decomposed, and no metal oxide is formed.

• 2Zn + O2 → 2ZnO is the direct oxidation of the free metal zinc to its oxide — calcination always starts from an ore compound such as a carbonate, never from the free metal.

• 2ZnS + 3O2 → 2ZnO + 2SO2 heats a sulfide ore in excess oxygen and releases sulfur dioxide — this is roasting, the process most often confused with calcination, not calcination itself.

Hence, MgCO3 → MgO + CO2 is the reaction that represents calcination.