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Absorption spectrum
A spectrum of electromagnetic radiation transmitted through a substance, showing dark lines or bands due to absorption at specific wavelengths.
Acid (Arrhenius theory)
A substance that dissociates in water to provide H+ ions.
Acid (Bronsted-Lowry theory)
A proton donor.
Acid-base indicator
Substances which change colour with pH.
Activation energy (E_ACT)
The minimum amount of energy which colliding molecules must have before they can react together.
Addition polymer
A polymer made by the combining together of small unsaturated molecules of the same compound.
Addition reaction
A reaction where the double or triple bond of an unsaturated compound opens up and other atoms add on.
Alcohols
Organic compounds in which the –OH (hydroxyl group) is bound to a saturated carbon atom.
Aldehydes
Organic compounds containing a functional group with the structure –CHO (carbonyl group).
Aliphatic hydrocarbon
Compound whose molecules contain only chains of carbon atoms.
Alkali Metals
The elements of Group 1 of the Periodic Table.
Alkanes
Hydrocarbons with only single bonds between carbon atoms.
Alkenes
Hydrocarbons with a double bond between two carbon atoms.
Alkynes
Hydrocarbons with a triple bond between two carbon atoms.
Allotropes
Different physical forms of the same element due to different arrangements of the atoms.
Alloy
A mixture of metals.
Alpha particle
A high-speed, positively charged helium nucleus.
Amphoteric substance
A substance that shows both acidic and basic properties.
Anion
The ion attracted to the anode.
Anode
The positive electrode in electrolysis.
Anodising
A process to thicken the oxide layer of aluminium for corrosion protection.
Aromatic compound
Compounds with a 6-membered ring of carbon atoms.
Atom
The smallest particle of a chemical element that can exist.
Atomic number
The number of protons that is characteristic of an element.
Atomic orbital
A region in space where there is a high probability of finding an electron.
Atomic radius
Half the distance between the nuclei of bonded atoms of that element.
Aufbau principle
Electrons occupy the lowest available energy level.
Auto-ignition
The explosion of the petrol-air mixture in the engine before the spark is produced.
Autocatalysis
Catalysis of a reaction by one of the products of that reaction.
Avogadro constant
The number of particles in a mole of a substance, approximately 6×10^23.
Avogadro’s law
Equal volumes of gases under the same conditions contain equal numbers of molecules.
Balmer Series
Visible line spectrum of Hydrogen.
Base (Arrhenius theory)
A substance that dissociates in water to produce OH– ions.
Base (Bronsted-Lowry theory)
A proton acceptor.
Base hydrolysis of esters (saponification)
Preparation of soap by alkaline hydrolysis of fats.
Beta particle
A high-speed electron emitted from the nucleus of an unstable atom.
Biochemical oxygen demand (BOD)
A measure of the amount of organic pollution in water.
Bond energy
The average energy required to break one mole of bonds in gaseous state.
Boyle’s law
For a fixed gas mass at constant temperature, the volume is inversely proportional to pressure.
Carbonium ion
Unstable positive carbon intermediate.
Carboxylic acid
An organic compound containing a carboxyl group (-COOH).
Catalyst
A substance that alters the rate of a chemical reaction without being consumed.
Catalyst poison
A substance that destroys the activity of a catalyst.
Catalytic converter
Device that transforms harmful exhaust gases into less harmful molecules.
Catalytic cracking
Breaking down long-chain hydrocarbons into shorter, more desirable molecules using heat and catalysts.
Cathode
The negative electrode in electrolysis.
Cation
The ion attracted to the cathode.
Charles’s law
For a fixed gas mass at constant pressure, volume is directly proportional to absolute temperature.
Chemical equilibrium (dynamic)
A state where the rate of forward reaction equals the rate of reverse reaction.
Chlorofluorocarbons (CFCs)
Compounds damaging ozone in the atmosphere, releasing chlorine radicals.
Chromatography
A separation technique using a mobile phase and a stationary phase.
Compound
A substance made of two or more elements chemically combined.
Conjugate acid
A base that has accepted a proton.
Conjugate base
An acid that has donated a proton.
Conjugate acid-base pair
Two substances differing by a single H+ (proton).
Co-product
Substances formed alongside the main product in a reaction.
Covalent bond
Involves sharing pairs of electrons to achieve a full outer shell.
Dehydration
An elimination reaction where an alcohol forms an alkene.
Dehydrocyclisation
Conversion of straight-chained alkanes into cyclic compounds using catalysts.
Deionised water
Water without ions but can have soluble covalent compounds.
Dibasic acid
A molecule dissociating in solution to produce two H+ ions.
Diffusion
The spontaneous spreading out of a substance due to particle movement.
Dipole-dipole force
Attraction between permanently polarised molecules.
Dissociation constant (K_a)
Equilibrium constant measuring the tendency of a larger object to separate.
Distilled water
Completely pure water with all impurities removed.
Electrochemical series
List of elements in order of ease with which they lose electrons.
Electrolysis
Decomposition of an electrolyte by passing a current through it.
Electrolyte
A compound that conducts electricity in liquid state.
Electroplating
Covering one metal with a thin layer of a more expensive metal.
Electronegativity
Measure of an atom's attraction for shared electrons in a bond.
Element
A substance that cannot be chemically broken down further.
Elimination reaction
When a small molecule is removed from another, forming a double bond.
Emission spectrum
Spectrum obtained from light passed through a prism showing emitted wavelengths.
Empirical formula
Formula showing the ratio of different types of atoms in a molecule.
Endothermic reaction
A reaction where heat is absorbed.
Energy level
Measure of the energy possessed by an electron in an atom.
Energy sub-level
Sublevels arranged from lowest to highest energy within a main energy level.
Enzyme
A biological catalyst such as amylase.
Equation of state for an ideal gas
pV = nRT, relating pressure, volume, moles, and temperature.
Equilibrium constant (K_c)
Constant indicating the extent of a reversible reaction at equilibrium.
Esterification
A reaction forming an ester and water from an acid and alcohol.
Eutrophication
Rapid algae growth due to nutrient enrichment, leading to oxygen depletion in water.
Excited state
State of an atom with electrons in higher energy levels.
Exothermic reaction
A reaction where heat is released.
Fermentation
Chemical breakdown of a substance by microorganisms involving heat release.
First ionisation energy
Energy required to remove one mole of loosely held electrons from gaseous atoms.
Flocculation
Coagulation of small particles in water to form a precipitate.
Fluoridation
Addition of fluorine to drinking water.
Fractional distillation
Separation of liquid mixtures into components by boiling point differences.
Free chlorine
Hypochlorous acid (HOCl) and hypochlorite (OCl-) used for water disinfection.
Free radical
Atom or molecule with a single unpaired electron.
Functional group
Group of atoms defining the properties of a compound.
Gamma radiation
Electromagnetic radiation with high penetrating power and no charge.
Gay Lussac’s law of combining volumes
In gas reactions, volumes are in the ratio of small whole numbers.
Ground state
State of an atom with electrons in their lowest energy levels.
Half-life
Time taken for half of undecayed atoms in a sample to decay.
Hard water
Water that does not form lather with soap due to calcium or magnesium salts.
Heat of combustion
Heat change when one mole of a substance is burned in excess oxygen.
Heat of formation
Heat change when one mole of a compound is formed from its elements.
Heat of reaction
Heat change during a reaction as per the chemical equation.
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