What is the difference between an element, a compound and a mixture, and how do we tell a chemical reaction has happened?
Classifying substances as elements, compounds or mixtures, reading chemical formulae, and distinguishing physical changes from chemical reactions.
A focused answer to the WJEC GCSE Science Double Award Unit 2 topic on substances, covering how to classify elements, compounds and mixtures, read chemical formulae, and tell a physical change from a chemical reaction using evidence.
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What this dot point is asking
WJEC Double Award Unit 2 wants you to classify substances as elements, compounds or mixtures, read chemical formulae, and distinguish physical changes from chemical reactions using evidence.
Elements, compounds and mixtures
The key differences are how the parts are joined and how they can be separated:
- Element: one type of atom (e.g. copper, oxygen). Found on the Periodic Table.
- Compound: elements chemically bonded; has different properties from the elements in it; separated only by chemical reactions or electrolysis.
- Mixture: substances physically mixed; keep their own properties; separated by physical methods such as filtering, evaporating or distillation.
Reading chemical formulae
A chemical formula shows the type and number of atoms in a substance. The small numbers (subscripts) tell you how many of each atom:
- has 2 hydrogen atoms and 1 oxygen atom.
- has 1 carbon atom and 2 oxygen atoms.
- has 1 calcium, 2 oxygen and 2 hydrogen atoms (the bracket subscript multiplies everything inside).
A formula like is still an element (one type of atom), just two atoms joined; the "2" does not make it a compound.
Separating mixtures
Because the parts of a mixture are not chemically bonded, they can be separated by physical methods that exploit a difference in their properties:
- Filtration: separates an insoluble solid from a liquid (such as sand from water), using filter paper.
- Evaporation/crystallisation: gets a dissolved solid back from a solution (such as salt from salt water) by evaporating the water.
- Distillation: separates a liquid from a solution (such as pure water from salt water) by boiling and condensing the vapour.
- Chromatography: separates a mixture of dissolved substances (such as the dyes in an ink).
Knowing which method to choose, and why, is a common exam point: it depends on whether the parts are soluble and whether you want the solid or the liquid.
Compounds have new properties
A useful idea is that a compound has different properties from the elements it is made from. Sodium is a soft, dangerously reactive metal and chlorine is a poisonous green gas, yet the compound sodium chloride (table salt) is a safe white solid we eat. This is because the atoms are chemically bonded in the compound, changing their behaviour completely. In a mixture, by contrast, each substance keeps its own properties because nothing is bonded.
Physical changes and chemical reactions
Evidence that a chemical reaction has happened includes:
- a colour change,
- a gas given off (bubbling or fizzing),
- a temperature change (it gets hotter or colder),
- a precipitate (an insoluble solid) forming,
- the change being difficult to reverse.
Try this
Q1. State how a mixture can be separated, and how a compound can be separated. [2 marks]
- Cue. A mixture: by physical methods (e.g. filtering, distillation). A compound: only by chemical means (e.g. a reaction or electrolysis).
Q2. How many atoms in total are in ? [1 mark]
- Cue. Three (1 carbon and 2 oxygen).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksClassify each of the following as an element, compound or mixture, giving a reason for each: oxygen gas (O2), water (H2O), air.Show worked answer →
A Unit 2 classification question worth 4 marks. Reward: oxygen (O2) is an element because it contains only one type of atom (1); water (H2O) is a compound because it is two elements chemically bonded in a fixed ratio (1); air is a mixture because it contains several substances (nitrogen, oxygen and others) not chemically bonded together (1); and a clear reason for any third (1). Markers credit the correct class and the reason. A common error is to call oxygen gas a compound because the formula has a "2".
WJEC style3 marksGive three pieces of evidence that a chemical reaction, rather than a physical change, has taken place.Show worked answer →
A Unit 2 evidence question. Reward any three of: a colour change, a gas given off (bubbling/fizzing), a temperature change, a precipitate (solid) forming, or a change that is difficult to reverse (1 each). Markers credit three valid signs. A common error is to give "the substance changed state" (melting), which is a physical change, not a chemical reaction.
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