In the study of chemical reactions, many processes involve the transfer of atoms or electrons between substances. A redox reaction (short for reduction-oxidation) is a fundamental type of reaction where oxidation and reduction occur simultaneously. One substance cannot be oxidized unless another is reduced. The IGCSE 0654 Core syllabus focus is on defining these processes in terms of oxygen transfer and identifying the oxidized and reduced species in simple reactions.

Historically, oxidation and reduction were defined by the movement of oxygen atoms. While modern chemistry uses electron transfer (Extended tier), the oxygen-based definitions remain a vital starting point for Core chemistry.

Oxidation: The Gain of Oxygen

Oxidation is defined as the gain of oxygen by a substance.

• Example: When magnesium is burned in air, it reacts with oxygen to form magnesium oxide.

In this reaction, magnesium has been oxidized.

Reduction: The Loss of Oxygen

Reduction is defined as the loss of oxygen from a substance.

• Example: When hydrogen gas is passed over heated copper(II) oxide, the copper oxide loses its oxygen to form copper metal and water.

In this reaction, copper(II) oxide has been reduced.

In most reactions involving oxygen transfer, both processes occur at the same time. Consider the reaction between lead(II) oxide and carbon:

• Lead(II) Oxide ($PbO$) loses oxygen to become lead ($Pb$). It has been reduced.
• Carbon ($C$) gains oxygen to become carbon dioxide ($CO_2$). It has been oxidized.

Because reduction and oxidation happen together, this is a redox reaction.

Identifying which substance has been oxidized or reduced is a critical analytical skill.

1. Identify the reactants: Look at the starting materials.
2. Compare to products: Determine which reactant gained oxygen (oxidized) and which lost oxygen (reduced).
3. Naming Convention: We always say the reactant is oxidized or reduced, not the product.

In Core chemistry, the concept of oxidation number is primarily used to name ions of elements that can have different charges (variable valency). This is common among transition metals.

• Iron(II) Oxide ($FeO$): The iron ion has an oxidation state of +2.
• Iron(III) Oxide ($Fe_2O_3$): The iron ion has an oxidation state of +3.

The Roman numerals indicate the oxidation state of the metal ion in that specific compound.

Redox reactions are the driving force behind many essential processes, from the combustion of fuels to the extraction of metals from their ores. By mastering the definitions of oxidation and reduction in terms of oxygen transfer, we can track the movement of chemical energy and matter through the environment, providing a foundation for more advanced electrochemical studies.