Elements or compounds occurring naturally in the earth’s crust are called minerals, and those with high metal content suitable for profitable extraction are referred to as ores.

Gold, silver, platinum, and copper are found in the free state. Copper and silver are also found in the combined state as sulphide or oxide ores.

Metals at the top of the activity series are highly reactive and are never found in nature as free elements. Moderately reactive metals (Zn, Fe, Pb, etc.) are found mainly as oxides, sulphides, or carbonates in the earth’s crust.

Moderately reactive metals in the middle of the activity series include iron, zinc, lead, and copper. They are typically present as sulphides or carbonates in nature.

Metal sulphides are converted into oxides through roasting, which involves strong heating in the presence of excess air. Carbonate ores are changed into oxides through calcination, achieved by strong heating in limited air.