AQA · International A Level · HL/SL
Chemistry — Keywords & Key Terms — Definitions Glossary (2026)
IB Diploma Programme Chemistry (HL/SL)
Topic-by-topic keywords, key terms and definitions for precise exam language—separate from our revision checklists (topic coverage) and formula sheets (equations).
Keywords & Key Terms — definitions
Examiner-style keywords and definitions organised by syllabus topic. Terms are tagged Essential (start here), Core (typical exam standard), and Advanced for harder distinctions — tick each row when you can recall it. Your progress is saved in this browser for this list.
IB Diploma Programme Chemistry (HL/SL)
IB Diploma Programme Chemistry (HL/SL)
IB Diploma Programme Chemistry (HL/SL)
Topics follow the IB DP Chemistry syllabus (first assessment 2025). HL content is clearly flagged. The IB data booklet provides constants and equations — keywords focus on conceptual understanding and precise language.
Mark schemes: IB Chemistry mark schemes demand precise chemical language. 'Ions are free to move' earns marks; 'particles move' does not. Always name specific particles. HL questions often require mechanistic explanations.
Active recall: 0 / 54 terms ticked
| Recalled | Topic | Level | Keyword | Definition |
|---|---|---|---|---|
| Atomic Structure and Periodicity | Essential | Atom | Smallest particle of an element that retains the chemical properties of that element. | |
| Atomic Structure and Periodicity | Essential | Proton | Positively charged particle in the nucleus; relative charge +1, relative mass 1. | |
| Atomic Structure and Periodicity | Essential | Neutron | Uncharged particle in the nucleus; relative charge 0, relative mass 1. | |
| Atomic Structure and Periodicity | Essential | Electron | Negatively charged particle surrounding the nucleus; relative charge –1, negligible mass. | |
| Atomic Structure and Periodicity | Essential | Atomic number (Z) | Number of protons in the nucleus of an atom; defines the element. | |
| Atomic Structure and Periodicity | Essential | Mass number (A) | Total number of protons and neutrons in the nucleus. | |
| Atomic Structure and Periodicity | Core | Isotope | Atoms of the same element with the same number of protons but different numbers of neutrons. | |
| Atomic Structure and Periodicity | Core | Relative atomic mass (Ar) | Weighted average mass of one atom relative to 1/12 the mass of carbon-12. | |
| Atomic Structure and Periodicity | Core | Electron configuration | Arrangement of electrons in orbitals around the nucleus (e.g. 1s²2s²2p⁶). | |
| Atomic Structure and Periodicity | Core | Ionisation energy | Energy required to remove one mole of electrons from one mole of gaseous atoms. | |
| Atomic Structure and Periodicity | Core | Electronegativity | Measure of the attraction of a bonded atom for the shared pair of electrons. | |
| Atomic Structure and Periodicity | Core | Periodic trend | Systematic variation in a property across a period or down a group of the periodic table. | |
| Atomic Structure and Periodicity | Advanced | Orbital | Region of space around the nucleus where there is a high probability of finding an electron (s, p, d, f types). | |
| Atomic Structure and Periodicity | Advanced | Degenerate orbitals | Orbitals of the same subshell with equal energy. | |
| Chemical Bonding | Essential | Ionic bond | Electrostatic attraction between oppositely charged ions formed by electron transfer. | |
| Chemical Bonding | Essential | Covalent bond | Shared pair of electrons between two atoms. | |
| Chemical Bonding | Core | Metallic bond | Attraction between positive metal ions (cations) and a sea of delocalised electrons. | |
| Chemical Bonding | Core | Coordinate (dative) bond | Covalent bond where both electrons are donated by the same atom. | |
| Chemical Bonding | Core | Lewis structure | Diagram showing all valence electrons as dots and bonding pairs as lines. | |
| Chemical Bonding | Core | VSEPR theory | Valence Shell Electron Pair Repulsion — electron domains arrange to minimise repulsion. | |
| Chemical Bonding | Core | Molecular polarity | Uneven distribution of electron density in a molecule due to polar bonds and asymmetric shape. | |
| Chemical Bonding | Core | Hydrogen bond | Strong intermolecular force between a δ+ hydrogen bonded to N, O or F and a lone pair on N, O or F. | |
| Chemical Bonding | Core | Van der Waals forces | Weak intermolecular forces including London dispersion and dipole–dipole interactions. | |
| Chemical Bonding | Core | London dispersion force | Temporary dipole–induced dipole force; increases with molar mass and contact area. | |
| Chemical Bonding | Advanced | Sigma bond (σ) | Bond formed by direct head-on overlap of atomic orbitals along the internuclear axis. | |
| Chemical Bonding | Advanced | Pi bond (π) | Bond formed by sideways overlap of p orbitals above and below the internuclear axis. | |
| Chemical Bonding | Advanced | Hybridisation | Mixing of atomic orbitals to form equivalent hybrid orbitals (sp, sp², sp³). | |
| Energetics and Thermodynamics | Essential | Exothermic reaction | Reaction that releases energy to the surroundings; ΔH < 0. | |
| Energetics and Thermodynamics | Essential | Endothermic reaction | Reaction that absorbs energy from the surroundings; ΔH > 0. | |
| Energetics and Thermodynamics | Core | Enthalpy change (ΔH) | Heat energy exchanged at constant pressure during a chemical reaction. | |
| Energetics and Thermodynamics | Core | Standard enthalpy of combustion | Enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions. | |
| Energetics and Thermodynamics | Core | Standard enthalpy of formation | Enthalpy change when one mole of a substance is formed from its elements in their standard states. | |
| Energetics and Thermodynamics | Core | Hess's Law | Total enthalpy change of a reaction is independent of the pathway taken; ΔH is a state function. | |
| Energetics and Thermodynamics | Core | Bond enthalpy | Energy required to break one mole of a specific bond in gaseous molecules. | |
| Energetics and Thermodynamics | Advanced | Entropy (S) | Measure of the dispersal of energy or disorder in a system. | |
| Energetics and Thermodynamics | Advanced | Gibbs free energy (ΔG) | ΔG = ΔH – TΔS; a reaction is spontaneous when ΔG < 0. | |
| Kinetics and Equilibrium | Core | Rate of reaction | Change in concentration of reactant or product per unit time. | |
| Kinetics and Equilibrium | Core | Activation energy (Ea) | Minimum energy required for a collision to result in a reaction. | |
| Kinetics and Equilibrium | Core | Catalyst | Substance that increases reaction rate by providing an alternative pathway with lower activation energy; not consumed in the reaction. | |
| Kinetics and Equilibrium | Core | Collision theory | Reactions occur when particles collide with sufficient energy (≥Ea) and correct orientation. | |
| Kinetics and Equilibrium | Core | Chemical equilibrium | State where forward and reverse reaction rates are equal; concentrations remain constant. | |
| Kinetics and Equilibrium | Core | Le Chatelier's principle | When a system at equilibrium is disturbed, it shifts to oppose the change. | |
| Kinetics and Equilibrium | Core | Equilibrium constant (Kc) | Ratio of product concentrations to reactant concentrations at equilibrium, each raised to stoichiometric coefficients. | |
| Kinetics and Equilibrium | Advanced | Rate constant (k) | Proportionality constant in the rate equation; depends on temperature not concentration. | |
| Kinetics and Equilibrium | Advanced | Rate-determining step | Slowest step in a reaction mechanism; determines the overall rate. | |
| Acids and Bases | Essential | Acid (Brønsted-Lowry) | Proton (H⁺) donor. | |
| Acids and Bases | Essential | Base (Brønsted-Lowry) | Proton (H⁺) acceptor. | |
| Acids and Bases | Core | Strong acid | Acid that completely dissociates in water (e.g. HCl, H₂SO₄, HNO₃). | |
| Acids and Bases | Core | Weak acid | Acid that partially dissociates in water; equilibrium lies to the left. | |
| Acids and Bases | Core | pH | Measure of hydrogen ion concentration: pH = –log[H⁺]. | |
| Acids and Bases | Core | Neutralisation | Reaction between an acid and a base producing salt and water. | |
| Acids and Bases | Core | Buffer solution | Solution that resists changes in pH when small amounts of acid or base are added. | |
| Acids and Bases | Advanced | Ka (acid dissociation constant) | Equilibrium constant for the dissociation of a weak acid in water; pKa = –log Ka. | |
| Acids and Bases | Advanced | Amphoteric | Substance that can act as both an acid and a base (e.g. water, amino acids). |
Pair this with our revision checklists, formula sheets hub and past paper finder.
Chemistry (HL/SL) — Keywords & Key Terms FAQ
- What is on this AQA International A Level Chemistry keywords and key terms list?
- It is a topic-organised glossary of important chemistry terms with short, exam-style definitions aligned to IB Diploma Programme Chemistry (HL/SL) (HL/SL). It is designed for “define”, “state”, “outline” and “explain” questions where precise vocabulary earns marks.
- How should I use this Chemistry glossary alongside past papers?
- Tick terms when you can recall them without reading the answer, then check your wording against mark schemes. Pair vocabulary practice with past papers for International A Level Chemistry (HL/SL) so you apply terms in context.
- Is this the same as a revision checklist or a formula sheet?
- No. Revision checklists help you track which syllabus topics you have covered and your confidence—separate pages on Tutopiya. Formula sheets summarise equations and quantitative relationships. This page is only a definitions and key-terms glossary for Chemistry. Quantitative relationships belong on formula sheets; this list emphasises language and concepts.
- Can I download this Chemistry keywords and key terms list for free?
- Yes. After a quick free sign-up you can download a UTF-8 CSV (opens in Excel or Google Sheets) or open a print-friendly page and save as PDF. Browsing the list on the page is free.
- Is this Chemistry list aligned to the HL/SL specification?
- Topic groupings and wording follow IB Diploma Programme Chemistry (HL/SL) for AQA International A Level. Always confirm final learning objectives and any regional options in your official specification and recent examiner reports for your exam session.
- Why focus on definitions instead of full notes?
- Mark schemes reward correct technical terms and clear links between ideas. A compact glossary lets you drill the exact language examiners expect for Chemistry at International A Level, separate from longer notes or topic trackers.