Oxidation, reduction and everyday effects

Heat a copper powder in a china dish and its surface turns black: copper has gained oxygen and become copper oxide. Gaining oxygen is oxidation.

2Cu + O₂ → 2CuO

Now pass hydrogen over the hot black copper oxide. The black coating turns brown again: the copper oxide has lost oxygen. Losing oxygen is reduction.

CuO + H₂ → Cu + H₂O

In that second reaction, two things happen at once. Copper oxide is reduced (it loses oxygen), and hydrogen is oxidised (it gains oxygen). One substance can't lose oxygen unless another takes it. So oxidation and reduction always come together: a redox reaction. Another example: ZnO + C → Zn + CO, where zinc oxide is reduced and carbon is oxidised.

Corrosion is oxidation you see every day. Iron gets a reddish-brown coat of rust, silver turns black, copper turns green. Think of an old iron gate: rust flakes off, fresh iron underneath is exposed, and it rusts in turn. That is why corrosion is costly, and why paint must cover every bit of the metal to keep air and moisture out.

Rancidity is oxidation of food. Leave something oily out too long and the fats and oils are oxidised: it smells and tastes bad. To slow it down:

• antioxidants are added to foods with fats and oils;
• food is kept in airtight containers;
• chips packets are filled with nitrogen, an unreactive gas, so there is no oxygen to oxidise the chips. That is why a bag of chips feels half full of air.