Lighting a fire, iron rusting and even our own breathing are all examples of a kind of chemical reaction called oxidation. Substances that gain oxygen or lose hydrogen atoms are said to be oxidized, while those that lose oxygen or gain hydrogen atoms are said to be reduced. Both always happen at the same time and are called "redox" reactions, a mixture of the two words "reduction" and "oxidation".

Electron transfer

Both oxidation and reduction always involve the movement of electrons from one atom to another. During oxidation, an atom loses electrons; during reduction an atom gains them. In fact, oxidation describes all chemical reactions in which electrons are lost, whether or not oxygen is involved.

Reactions with oxygen

Oxygen is a colourless gas, but it joins up easily with many other elements in chemical reactions to form compounds called oxides. Sometimes this is a slow process. Iron, for example, reacts with oxygen in water to form iron oxide, more commonly known as the rust we see in iron objects left out in damp air over a long period.
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Antioxidants

The chemicals in food can react with oxygen in the air—another example of oxidation. This process, which scientists call rancidification, is what makes food "go off"—to get a bad smell or taste. To prevent this from happening, chemicals called antioxidants are added. These react with the oxygen to become oxidized themselves, leaving the food to stay fresh for longer. An example of an antioxidant is ascorbic acid, a form of Vitamin C. Antioxidants are also naturally present in many foods, including fruits and vegetables.

Hydrogenation

Treating vegetable oils with hydrogen is a type of chemical reaction called hydrogenation. It is commonly done to make a liquid vegetable oil—for example, sunflower oil—into a more solid fat, a spreadable such as margarine.

The addition of hydrogen has the effect of changing the double bonds that previously existed between the carbon atoms to single bonds. This is called saturation. The higher the saturation, the more viscous (thicker) the substance becomes.