Learning focus
Develop precise chemical language, connect particle and bonding models to observations, interpret diagrams and data, and apply ideas to unfamiliar examples.
Elements
An element contains only one type of atom and cannot be chemically broken into simpler substances. An element may exist as single atoms, molecules such as O2, or a giant structure such as a metal or diamond.
Compounds
A compound contains atoms of two or more different elements chemically combined in a fixed ratio. The compound has properties different from its constituent elements. Sodium chloride is a stable solid even though sodium is a reactive metal and chlorine is a toxic gas.
Mixtures
A mixture contains two or more substances not chemically combined. Its composition can vary and its components retain many individual properties. Physical methods such as filtration, distillation, crystallisation and chromatography can separate mixtures.
Reading particle diagrams
An element contains one atom type, even when its atoms are bonded into molecules. A compound contains identical particles made from different atom types in a fixed ratio. A mixture contains more than one kind of particle.
Formulae
A compound has a chemical formula that shows fixed composition. A mixture has no single formula because the proportions can change. Air and seawater are mixtures, whereas carbon dioxide and pure water are compounds.
Practical or data skill
Classify named samples and particle diagrams as elements, compounds or mixtures. Give evidence based on atom types, bonding and whether composition is fixed.
Examination tip
O2 is an element because only oxygen atoms are present, even though each particle is a molecule.
Review questions and suggested answers
Question 1
Define a compound.
Suggested answer
A substance containing two or more elements chemically combined in a fixed ratio.
Question 2
Why can a mixture be separated physically?
Suggested answer
Its components are not chemically bonded.
Question 3
Is oxygen gas an element or compound?
Suggested answer
An element.
