A-level chemistry definitions
Dr Lavan Ganeshkumar, PhD · updated 4 October 2026
Chemistry mark schemes reward exact wording. Two answers can describe the same idea and only one scores, because the other leaves out a word such as "gaseous", "one mole" or "standard conditions". These pages set out each definition in its exact wording, the words that must appear, and the term it is most often confused with.
They are written for the AQA course, and the other sciences work the same way. Each page is our own explanation, and the official specification is on the AQA website.
Energetics
Every term in this section links below. Read the definition, cover it, write it from memory, and check the words that must appear.
Common questions
Where can I find A-level chemistry definitions?
These pages set out key A-level chemistry definitions in exact wording, starting with energetics, with the reason each word matters.
Why do definitions matter so much in A-level chemistry?
Mark schemes reward specific wording, so an answer that leaves out a key word can lose the mark even when the idea is right.
Trayna does this automatically. You enter your subjects, your exam dates and how long you have each day, and it builds the route from today to those exams, then adapts it as you go. Maths follows Edexcel; Chemistry, Physics and Biology follow AQA.
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Read next
- Enthalpy change of hydration
- Lattice enthalpy
- Enthalpy of atomisation
- Electron affinity
- Born-Haber cycle
- Enthalpy change
- Standard conditions
- Standard enthalpy change of formation
- Standard enthalpy change of combustion
- Hess law
- Mean bond enthalpy
- Entropy
- Gibbs free energy
- Apps and tools for A-level chemistry
- How to revise A-level chemistry