CAIE IGCSE Chemistry – Key Vocabulary

A comprehensive set of vocabulary flashcards covering fundamental terms, processes, and principles required for CAIE IGCSE Chemistry (2023–2025 syllabus).

Solid

State of matter with fixed shape and volume; particles vibrate in a regular lattice

Liquid

State of matter with fixed volume but no fixed shape; particles move past one another

Gas

State of matter with neither fixed shape nor volume; particles move randomly and rapidly

Boiling

Endothermic change of a liquid to gas at its boiling point throughout the liquid

Evaporation

Slow endothermic change of a liquid to gas only at the surface below boiling point

Condensation

Exothermic change of gas to liquid occurring at the boiling point

Melting

Endothermic change of solid to liquid at its melting point

Freezing

Exothermic change of liquid to solid at the freezing/melting point

Sublimation

Direct change of solid to gas or gas to solid without passing through liquid phase

Cooling Curve

Temperature–time graph showing phase changes while a substance cools

Heating Curve

Temperature–time graph showing phase changes while a substance is heated

Diffusion

Net movement of particles from high to low concentration due to random motion

Relative Molecular Mass Effect

Lighter gas molecules diffuse faster than heavier ones at the same temperature

Pressure (gas)

Force per unit area caused by gas molecules colliding with container walls

Element

Pure substance made of atoms with the same proton number; cannot be decomposed chemically

Compound

Pure substance of two or more elements chemically bonded in fixed ratio

Mixture

Combination of substances not chemically bonded and separable by physical means

Atom

Smallest particle of an element retaining its properties

Proton Number (Z)

Number of protons in an atom’s nucleus; equals electron number in a neutral atom

Nucleon (Mass) Number (A)

Total number of protons and neutrons in the nucleus

Isotope

Atoms of same element with same proton number but different neutron numbers

Relative Atomic Mass (Ar)

Weighted average mass of an element’s isotopes relative to 1⁄12 of carbon-12

Electron Shell

Energy level around nucleus where electrons are found (2,8,8 rule for IGCSE)

Metallic Bonding

Electrostatic attraction between positive metal ions and delocalised electrons

Giant Covalent Structure

Macromolecular lattice where atoms are covalently bonded throughout, e.g., diamond

Diamond

Carbon allotrope where each atom is bonded to four others; hard, non-conductive

Graphite

Carbon allotrope of layered hexagonal sheets with delocalised electrons; conducts electricity

Silicon(IV) Oxide

Giant covalent lattice similar to diamond; very high melting point, non-conductor

Covalent Bond

Shared pair(s) of electrons between non-metal atoms to achieve noble configuration

Ionic Bond

Strong electrostatic attraction between oppositely charged ions

Cation

Positively charged ion (usually metal) attracted to cathode

Anion

Negatively charged ion (usually non-metal) attracted to anode

Empirical Formula

Simplest whole-number ratio of atoms/ions in a compound

Molecular Formula

Actual number of each type of atom in one molecule

Relative Molecular Mass (Mr)

Sum of Ar values of all atoms in a molecule

Mole

Amount containing 6.02×10²³ entities; mass in g equals Ar or Mr

Avogadro Constant

6.02×10²³ – number of particles in one mole of substance

Molar Mass

Mass of one mole of a substance in g mol⁻¹

Concentration (mol dm⁻³)

Amount of solute per dm³ of solution

Electrolysis

Decomposition of ionic substance (molten/aqueous) by electricity

Electrolyte

Molten or aqueous ionic compound that conducts and is decomposed during electrolysis

Anode

Positive electrode where oxidation occurs

Cathode

Negative electrode where reduction occurs

OIL RIG

Mnemonic: Oxidation Is Loss, Reduction Is Gain (of electrons)

PANIC

Mnemonic: Positive Anode, Negative Is Cathode

Electroplating

Using electrolysis to coat object (cathode) with a metal from anode

Hydrogen-Oxygen Fuel Cell

Electrochemical cell producing electricity and water from H₂ and O₂

Exothermic Reaction

Releases heat to surroundings; ΔH negative

Endothermic Reaction

Absorbs heat from surroundings; ΔH positive

Activation Energy (Ea)

Minimum energy required for reactants to collide successfully

Bond Energy

Energy required to break one mole of a specific bond in gaseous molecules

Collision Theory

Rate depends on frequency and energy of successful particle collisions

Catalyst

Substance that increases reaction rate by lowering activation energy and remains unchanged

Enzyme

Biological catalyst, protein that operates at optimum pH and temperature

Le Châtelier’s Principle

Equilibrium shifts to oppose applied change in conditions

Equilibrium

State in closed system where forward and reverse reaction rates are equal

Haber Process

Industrial synthesis of ammonia at 450 °C, 200 atm, iron catalyst

Contact Process

Industrial manufacture of sulfuric acid; SO₂ + ½O₂ ⇌ SO₃ with V₂O₅ catalyst

Redox Reaction

Simultaneous oxidation and reduction involving electron transfer

Oxidising Agent

Species that accepts electrons and is reduced

Reducing Agent

Species that donates electrons and is oxidised

Acid

Proton donor producing H⁺ in water; pH < 7

Base

Proton acceptor; produces OH⁻ in water (alkali if soluble)

Strong Acid

Completely dissociates in water releasing many H⁺ ions

Weak Acid

Partially dissociates in water releasing few H⁺ ions

Salt

Compound formed when H⁺ of an acid is replaced by a metal or NH₄⁺

Indicator

Dye that changes colour with pH, e.g., litmus, methyl orange

Amphoteric Oxide

Oxide that reacts with both acids and bases, e.g., Al₂O₃

Neutralisation

Reaction of acid with base forming salt and water

Precipitation

Formation of insoluble solid when two solutions react

Water of Crystallisation

Fixed water molecules chemically bonded in hydrated salts

Periodic Table

Arrangement of elements in order of increasing atomic number showing periodicity

Group

Vertical column; elements share same number of outer electrons

Period

Horizontal row; indicates number of electron shells

Alkali Metals

Group I metals; very reactive, soft, form +1 ions

Halogens

Group VII non-metals; diatomic, form −1 ions, decreasing reactivity down group

Transition Elements

Central block metals with variable oxidation states and coloured compounds

Noble Gases

Group VIII inert gases with full outer shells

Metal

Element that forms positive ions, good conductor, malleable and ductile

Alloy

Mixture of metals (or metal + non-metal) with improved properties

Reactivity Series

Ranking of metals by ability to displace others or lose electrons

Rusting

Corrosion of iron forming hydrated iron(III) oxide; needs water and oxygen

Galvanising

Protecting iron by coating with zinc which acts sacrificially

Blast Furnace

Industrial extraction of iron from haematite using coke, limestone and air

Bauxite

Main ore of aluminium (Al₂O₃) used in electrolytic extraction

Cryolite

Na₃AlF₆; dissolves Al₂O₃ lowering its melting point in aluminium extraction

Greenhouse Gas

Gas that absorbs infrared radiation, e.g., CO₂, CH₄

Catalytic Converter

Device with Pt/Rh catalysts converting CO, NOx and hydrocarbons to CO₂, N₂, H₂O

Fertiliser

Substance supplying N, P, K nutrients to enhance plant growth

Fractional Distillation (crude oil)

Separation of petroleum into fractions by boiling point differences

Alkane

Saturated hydrocarbon with formula CnH₂n+₂

Substitution Reaction

Reaction where an atom/group in molecule is replaced, e.g., halogenating alkanes with UV

Alkene

Unsaturated hydrocarbon with at least one C=C double bond; formula CnH₂n

Addition Reaction

Reaction where atoms add across a double bond forming one product

Cracking

Thermal decomposition of long-chain alkanes into shorter alkanes and alkenes

Bromine Water Test

Unsaturated compounds decolourise orange bromine water; saturated do not

Alcohol

Organic compound with hydroxyl (–OH) functional group

Fermentation

Anaerobic conversion of glucose to ethanol and CO₂ by yeast

Hydration of Ethene

Industrial method to make ethanol by adding steam to ethene with H₃PO₄ catalyst

Carboxylic Acid

Organic acid with –COOH functional group