How metals react

With oxygen: most metals burn or slowly react to form metal oxides: 2Cu + O₂ → 2CuO (black). Metal oxides are basic. A few, like aluminium oxide and zinc oxide, react with both acids and bases: they are amphoteric.

Metals react with oxygen at very different speeds. Sodium and potassium react so fast that they catch fire in air, so they are kept under kerosene. Magnesium, aluminium, zinc and lead form a thin oxide layer that protects the metal underneath. The outer layer sacrifices itself to save the inside. Anodising makes this layer thicker on aluminium.

With water, the reactivity shows clearly, going down from the most reactive:

• cold water: sodium and potassium react violently (and catch fire); calcium reacts gently;
• hot water only: magnesium;
• steam only: aluminium, iron, zinc;
• no reaction at all: lead, copper, silver, gold.

Calcium and magnesium float: bubbles of hydrogen stick to their surface and carry them up.

With dilute acids: metal + acid → salt + hydrogen. In dilute hydrochloric acid, magnesium bubbles fastest, then aluminium, zinc and iron. Copper does not react at all.

A rule with an exception inside an exception: nitric acid gives no hydrogen with metals, because it is a strong oxidising agent and turns the hydrogen into water. Except magnesium and manganese, which do give hydrogen with very dilute nitric acid.

Aqua regia ("royal water") is a fresh mixture of concentrated hydrochloric and nitric acids in the ratio 3 : 1. It can dissolve gold and platinum, which neither acid can dissolve alone.

Metal + salt of another metal: if metal A pushes metal B out of B's salt solution, A is more reactive. Iron displaces copper from copper sulphate; copper does not displace iron. Putting all these results in order gives the reactivity series:

K, Na, Ca, Mg, Al, Zn, Fe, Pb, (H), Cu, Hg, Ag, Au

Metals above hydrogen give hydrogen with dilute acids; metals below it don't.