Elements, Compounds and Mixtures
Summary: Elements are single-atom-type substances, compounds are chemically bonded combinations of elements in fixed ratios, and mixtures are physically combined substances with variable composition that can be separated by physical means. Tags: igcse chemistry Created: 2026-07-18
All matter can be classified as an Element, a Compound or a Mixture, and distinguishing between the three is fundamental to understanding chemistry at every level. An element is a substance made of only one type of atom and cannot be broken down into anything simpler by chemical means, while a compound consists of two or more different elements chemically bonded together in a fixed proportion, giving it entirely new properties that differ from those of its constituent elements. A mixture, by contrast, contains two or more substances (elements, compounds, or both) that are physically combined but not chemically bonded, so each component keeps its own properties and can be recovered using Separation Techniques such as filtration, distillation or chromatography. The distinction matters because it determines whether a chemical reaction (such as Electrolysis) or merely a physical process is needed to separate a substance into its parts. Whether the particles involved are single atoms, molecules or ions arranged in giant lattices also follows from this classification, since bonding — ionic, covalent or metallic — occurs within elements and compounds but never between the components of a mixture.
Differences Between Elements, Compounds and Mixtures
| Feature | Element | Compound | Mixture |
|---|---|---|---|
| Definition | A substance made of only one type of atom; cannot be broken down into simpler substances by chemical means | A substance made of two or more elements chemically bonded together | Two or more substances (elements and/or compounds) physically combined, not chemically bonded |
| Composition | One type of atom only | Two or more types of atom, always in a fixed ratio (e.g. H₂O is always 2 H : 1 O) | Two or more substances in any proportion |
| Bonding | Atoms may be bonded to identical atoms (covalent in molecules like O₂; metallic in metals) or exist as single atoms (noble gases) | Chemical bonds between different elements — ionic or covalent | No chemical bonds between the components; only weak physical interactions |
| How to separate | Cannot be broken down chemically into simpler substances | Only by chemical reactions, e.g. Electrolysis or thermal decomposition | By physical Separation Techniques: filtration, evaporation, distillation, chromatography, using a magnet, etc. |
| Fixed composition? | Yes — a pure element is 100% one type of atom | Yes — fixed ratio of elements given by its chemical formula | No — composition can vary (e.g. salt water can be more or less concentrated) |
| Properties compared to constituents | Not applicable — it has no simpler chemical constituents | Completely different from the elements it contains (e.g. NaCl is edible; sodium is a reactive metal, chlorine a toxic gas) | Each component keeps its own properties (e.g. iron in an iron–sulfur mixture is still magnetic) |
| Examples | Oxygen (O₂), iron (Fe), helium (He), carbon (C) | Water (H₂O), carbon dioxide (CO₂), sodium chloride (NaCl), Ammonia (NH₃) | Air, salt water, crude oil, alloys such as brass, iron and sulfur powders mixed together |
Elements
An Element is the simplest form of a substance: it contains only one type of atom, defined by its proton (atomic) number — see Atomic Structure. Elements are listed in the Periodic Table and their behaviour follows Periodic Table Trends. They may exist as single atoms (the Group 0 Noble Gases), as molecules of identical atoms (O₂, N₂, Cl₂), or as giant structures such as metallic lattices held together by delocalised electrons via Metallic Bonding. No chemical reaction can split an element into simpler substances.
Compounds
A Compound forms when atoms of two or more different elements bond chemically — through ionic bonding (electron transfer, forming a giant ionic lattice) or Covalent Bonding (electron sharing). A compound always has a fixed composition given by its formula, and its formation usually involves an energy change, indicating a chemical reaction has occurred. Crucially, the properties of a compound are entirely different from those of its constituent elements: water is nothing like hydrogen or oxygen gas. Compounds can only be broken back into their elements by chemical means, for example Electrolysis of molten lead(II) bromide.
Mixtures
A Mixture contains two or more substances that are not chemically bonded, so no new substance is formed when they are combined. The proportions can vary freely — sea water, air (see Composition of Air) and crude oil all have variable compositions. Because each component retains its own properties (melting point, solubility, magnetism, boiling point), mixtures can be separated by physical Separation Techniques chosen to exploit those differences: filtration for insoluble solids, simple or fractional distillation for liquids, crystallisation for dissolved salts, and Paper Chromatography for coloured mixtures. Mixtures also melt and boil over a range of temperatures rather than at a sharp fixed point, which is a useful test of purity.
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.1 Core 1, Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Air is a compound.” | Air is a mixture of gases (mainly nitrogen, oxygen, argon and carbon dioxide). The gases are not chemically bonded to each other, their proportions can vary, and they can be separated physically by fractional distillation of liquid air. |
| ”Mixtures have a fixed composition.” | Only compounds have a fixed composition, set by their chemical formula. Mixtures can be made in any proportions — salt water, for example, can contain any amount of dissolved salt up to saturation. |
| ”A compound has the same properties as the elements in it (e.g. sodium chloride is dangerous because sodium and chlorine are).” | A compound’s properties are completely different from its constituent elements. Sodium chloride is a safe, edible white solid, even though sodium is a violently reactive metal and chlorine is a toxic green gas. It is the components of a mixture that keep their original properties. |