An element contains only one type of atom, a compound contains atoms of different elements chemically bonded in a fixed ratio, and a mixture contains two or more substances physically combined in variable proportions. Compounds require chemical reactions to separate them, whereas mixtures can be separated by physical methods.
This distinction is part of Structure 1.1: Introduction to the particulate nature of matter and applies at both SL and HL.
| Classification | Particle-level meaning | Key properties and separation |
|---|---|---|
| Element | Contains atoms with the same number of protons. Its particles may be individual atoms, such as He, or molecules, such as . | Cannot be chemically broken down into simpler substances. |
| Compound | Contains atoms of at least two different elements chemically bonded in a fixed ratio. For example, water always has the formula . | Has properties different from those of its constituent elements and can be decomposed only by chemical change. |
| Mixture | Contains two or more elements and/or compounds that are not chemically bonded to one another. Its composition can vary. | Components retain their chemical identities and can be separated physically, for example by filtration, distillation, or chromatography. |
For example, iron and sulfur form a mixture when simply stirred together. A magnet can remove the iron. After heating, they react to form iron(II) sulfide, , which is a compound with a fixed 1:1 particle ratio and cannot be separated using a magnet.
Mixtures may be homogeneous, with uniform composition, or heterogeneous, with visibly different regions. Pure elements and compounds usually have sharp melting and boiling points, while mixtures generally melt or boil over a range.
A common misconception is that every molecule is a compound. This is incorrect: is a molecule but remains an element because it contains only oxygen atoms.
Exam technique: If asked to distinguish these categories, refer to particle composition, chemical bonding, fixed or variable proportions, and the method of separation. Do not define a compound merely as “different atoms mixed together”; the atoms must be chemically bonded.
