Element, Compound or Mixture — Elements, Compounds and Mixtures | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

Definitions and separation techniques tested consistently across both papers.

Every chemical substance falls into one of three classes:
- Element
- Compound
- Mixture
Edexcel expects you to classify a substance from its name, formula, or particle diagram

Built entirely from atoms that all share the same proton number
Each element has its own characteristic proton number (e.g. gold = 79, oxygen = 8)
Cannot be reduced to anything simpler by ordinary chemical or physical methods
118 elements are known, all set out in the Periodic Table
Examples: iron (Fe), copper (Cu), magnesium (Mg), oxygen (O₂), nitrogen (N₂), sulfur (S₈)
Diatomic and polyatomic forms are still elements
- O₂ is an element (not a compound), because every atom in it is oxygen

A pure substance formed when atoms of two or more different elements join in a fixed ratio through chemical bonds
Has its own chemical properties, different from those of its elements
Can only be broken back into its elements by another chemical reaction (not by physical means)
Millions of different compounds exist
Composition by mass is fixed
- Water is always H₂O, never H₃O or H₂O₂
- Sodium chloride is always one Na per one Cl
Examples: water (H₂O), carbon dioxide (CO₂), calcium carbonate (CaCO₃), copper(II) sulfate (CuSO₄)

Two or more substances (any combination of elements and compounds) sharing the same space, with no chemical bonds between them
Each component keeps its own chemical identity and properties
Proportions can vary (a salt solution can be weak or strong)
Components can be separated again by physical methods
Examples:
- Air (mostly N₂ and O₂, plus Ar, CO₂, water vapour)
- Seawater (water + dissolved salts)
- Iron filings + sulfur powder
- Brass (copper + zinc)

What you see	Class	Examples
One element symbol only (optionally with subscript denoting a molecule of that element)	Element	Cu, O₂, S₈, P₄
Two or more different element symbols bonded as one unit	Compound	H₂O, NaCl, CaCO₃, NH₃
Two or more formulae written separately ("+", "and", or in a list)	Mixture	Fe + S; sand + water; air