The reactivity series
Metals can be ranked by how readily they react (lose electrons to form positive ions), from most reactive (potassium, sodium, calcium) down to least reactive (gold, unreactive metals near the bottom). A more reactive metal will displace a less reactive one from its compound in solution.
Example 1: Predict whether a reaction occurs when zinc metal is placed in copper(II) sulfate solution, and write the equation if so.
Zinc is more reactive than copper, so it displaces copper from solution: . A layer of copper forms on the zinc, and the blue colour of the solution fades as copper ions are used up.
Extraction of metals
A metal's extraction method depends on its position in the reactivity series. Very reactive metals (like aluminium) are extracted by electrolysis of their molten ore, since chemical reduction isn't powerful enough to free them. Moderately reactive metals (like iron) are extracted by reduction with carbon (in a blast furnace) or carbon monoxide, which is cheaper than electrolysis. Unreactive metals (like gold) can occur naturally as the free element and need no chemical extraction at all.
Corrosion and its prevention
Iron rusts when it reacts with both oxygen and water together, forming hydrated iron(III) oxide. Rusting is prevented by excluding oxygen and/or water from the metal's surface -- painting, oiling, galvanizing (coating with a layer of zinc, which also protects sacrificially even if scratched), or alloying with other metals (like chromium, to make stainless steel).