Oxidation, reduction, and everyday life

Oxidation is gain of oxygen and reduction is loss of oxygen, and in a reaction one cannot happen without the other. Rusting of iron and rancidity of fats are oxidation reactions in everyday life.

Gaining and losing oxygen

Heat copper powder in a china dish in air. Its surface turns black. Copper has combined with oxygen from the air to form copper(II) oxide, which is black. A substance that gains oxygen in a reaction is said to be oxidised.

2Cu + O₂ → 2CuO

Now pass hydrogen gas over the hot black copper oxide. The black solid turns reddish-brown, the colour of copper metal. The copper oxide has lost oxygen. A substance that loses oxygen in a reaction is said to be reduced.

CuO + H₂ → Cu + H₂O

Oxidation and reduction happen together

In the second reaction, copper oxide lost oxygen, so it was reduced. The oxygen atom did not disappear. It combined with hydrogen to form water. So the hydrogen gained oxygen: it was oxidised.

This is true of every such reaction. Oxygen gained by one substance must come from another substance that loses it. So whenever one substance is oxidised, another is reduced in the same reaction.

A reaction in which oxidation and reduction both occur is called a redox reaction, from reduction and oxidation.

Corrosion

When a metal reacts with substances around it, such as oxygen, water or acids, and its surface is gradually worn away, the process is called corrosion.

The corrosion of iron is called rusting. Iron reacts with oxygen and water to form rust, a reddish-brown hydrated iron(III) oxide. Iron in dry air, or in water with no dissolved oxygen, does not rust. Both oxygen and water are needed.

Other metals corrode too. Copper slowly gains a green coating, mainly basic copper carbonate, from reaction with moist air and carbon dioxide. Silver turns black because it reacts with sulphur compounds in the air to form silver sulphide.

Rust is weaker than iron and flakes off, exposing fresh iron below, so rusting continues. Corrosion damages car bodies, bridges, ships and pipes. A 2016 study by NACE International estimated the worldwide cost of corrosion at about 2.5 trillion US dollars a year, around 3% of world GDP. Painting iron keeps oxygen and water away from its surface, so rusting cannot begin there.

Rancidity

Fats and oils in food also react with oxygen from the air. The products have an unpleasant smell and taste. Food in this condition is called rancid. Fried snacks left in an open packet for a few days taste stale for this reason.

Rancidity is slowed by reducing contact between the fat and oxygen:

• Adding antioxidants. An antioxidant is a substance that reacts with oxygen, or with the reactive products of fat oxidation, more readily than the fat does. It is used up in place of the fat.
• Storing food in airtight containers, so less fresh air reaches it.
• Filling packets with nitrogen instead of air. Nitrogen does not react with the fat.

The unit in summary

This unit started with how to recognise a reaction. It then covered writing equations and balancing them using conservation of mass. It described the main types: combination, decomposition, displacement and double displacement, and the heat change in each. It ended with oxidation and reduction, which explain rusting and rancidity.

Milk turning sour, respiration in your cells, iron rusting and chips going stale are all chemical reactions of these kinds.