Compound – IGCSE Chemistry Definition

A compound is a substance formed when two or more elements are chemically combined. Recognising whether a substance is an element, compound, or mixture is a fundamental skill tested throughout the 0620 syllabus, from Paper 1 definitions to Paper 4 and 6 practical contexts.

The 0620 definition

A compound is a substance made of two or more elements chemically combined (bonded) in a fixed ratio. It can only be separated into its elements by chemical reactions, not by physical methods.

Key properties of compounds

Property	Detail
Composition	Fixed ratio of atoms (shown by its formula)
Properties	Different from those of its constituent elements
Separation	Chemical methods only (e.g. electrolysis, thermal decomposition)
Bonding	Atoms joined by ionic or covalent bonds
Formula	Represented by a chemical formula showing the ratio of atoms

Compound vs element vs mixture

Feature	Element	Compound	Mixture
Made of	One type of atom only	Two or more types of atom, chemically bonded	Two or more substances, not chemically bonded
Ratio	N/A	Fixed ratio	Variable proportions
Separation	Cannot be broken down further by chemical means	Chemical methods	Physical methods (filtration, distillation, etc.)
Properties	Characteristic of that element	Different from constituent elements	Each substance retains its own properties

Examples

• Water, H₂O: compound of hydrogen and oxygen in a 2:1 atom ratio. It is a liquid at room temperature, whereas hydrogen and oxygen are both gases.
• Carbon dioxide, CO₂: compound of carbon and oxygen. A gas at room temperature, unlike solid carbon (graphite or diamond).
• Sodium chloride, NaCl: compound of sodium (a reactive metal) and chlorine (a toxic gas). The compound is a harmless white solid.

Worked exam question

Magnesium burns in oxygen to form magnesium oxide. (a) Is magnesium oxide an element, compound, or mixture? Explain your answer. (2) (b) How could magnesium oxide be broken down into its elements? (1)

Mark scheme

(a) Compound [1]; because it contains two (different) elements (magnesium and oxygen) chemically combined / bonded together [1]

(b) By electrolysis (of the molten oxide) [1]

Common exam mistakes

• Calling a compound a “mixture of elements”. The elements in a compound are chemically bonded, not just mixed.
• Suggesting a compound can be separated by physical methods. A compound requires a chemical reaction (such as electrolysis or thermal decomposition) to break it into elements.
• Forgetting that the properties of a compound differ from those of its elements. Always state that new properties emerge when a compound forms.