67 of the Chemistry glossary’s terms, each with the part of the syllabus where you first meet it.
The mass of waste produced per unit mass of product in a process, counting by-products, solvents, excess reagents and losses.
The bubbling seen when a gas is produced in a liquid, for example when a reactive metal reacts with a dilute acid.
A collision in which no kinetic energy is lost; the total kinetic energy of the particles is the same before and after.
The ability of a substance to carry an electric current, which needs mobile charged particles: delocalized electrons in metals and graphite, mobile ions in molten or dissolved ionic compounds.
An arrangement in which oxidation and reduction take place at two separate electrodes joined by an external circuit; voltaic and electrolytic cells are the two types.
A list of redox couples in order of their standard electrode potentials; oxidizing agents become stronger towards the positive end and reducing agents towards the negative end.
A conductor, usually a metal or graphite, through which electrons enter or leave the electrolyte in an electrochemical cell; oxidation or reduction takes place at its surface.
The decomposition of a molten or dissolved ionic compound (an electrolyte) by a direct electric current, with ions gaining or losing electrons at the electrodes.
A substance that conducts electricity when molten or in aqueous solution, because it contains mobile ions, and is decomposed by the current.
An electrochemical cell in which an external dc supply drives a non-spontaneous redox reaction, converting electrical energy into chemical energy; its anode is positive and its cathode negative.
Purifying a metal by electrolysis with the impure metal as the anode and a thin sheet of pure metal as the cathode, in a solution of the metal's ions.
The full range of electromagnetic radiation, from gamma rays (shortest wavelength, highest frequency and photon energy) through X-rays, ultraviolet, visible light, infrared and microwaves to radio waves (longest wavelength).
A subatomic particle with a relative charge of −1 and a negligible mass (about 1/1836 of a proton), occupying the space around the nucleus.
The arrangement of the electrons of an atom or ion in sublevels, written with superscripts for the number of electrons in each, for example for sodium.
A region of electron density around a central atom: a lone pair, a single bond or a multiple bond each count as one domain.
The arrangement of all the electron domains, bonding and non-bonding, around a central atom: linear, trigonal planar or tetrahedral for two, three or four domains.
A property of an electron with two possible values, drawn as up and down arrows; two electrons in the same orbital must have opposite spins.
A measure of the ability of an atom to attract a shared pair of electrons in a covalent bond; it has no units (Pauling scale).
The difference between the electronegativities of two bonded elements, ; the larger it is, the greater the ionic character of the bond.
A reactant that forms a bond to its reaction partner (the nucleophile) by accepting both bonding electrons from it; it is electron-deficient and may be positive, such as and
An addition reaction in which an electrophile attacks the electron-rich C=C double bond of an alkene and the two parts of the reagent add to the two carbon atoms, giving a saturated product.
A reaction in which an electrophile replaces an atom (usually hydrogen) on an aromatic ring, through a carbocation intermediate, the ring keeping its delocalized π system; for example the nitration of benzene.
Using electrolysis to coat an object with a thin layer of metal; the object is the cathode, the plating metal the anode, and the electrolyte contains ions of the plating metal.
A substance made of only one kind of atom (all with the same atomic number), which cannot be broken down into simpler substances by chemical reactions.
The particle being counted in a mole: an atom, molecule, ion, electron, formula unit or other specified particle or group of particles.
A single molecular event in a reaction mechanism: one collision, or one particle breaking apart, in which bonds are broken and formed.
The light emitted by excited atoms, seen as bright lines of specific wavelengths on a dark background; each line comes from electrons falling between two energy levels.
A formula that gives the simplest whole-number ratio of the atoms of each element in a compound.
One of a pair of stereoisomers that are non-superimposable mirror images of each other. Enantiomers rotate plane-polarized light by equal angles in opposite directions.
The point in a titration at which the indicator changes colour (or a self-indicating titrant gives a permanent colour); ideally it coincides with the equivalence point.
Describes a change that absorbs energy from the surroundings, such as melting, vaporization or sublimation; the surroundings get colder and is positive.
A diagram showing two or more routes between the same initial and final states, used with Hess's law to calculate an unknown enthalpy change.
The energy released per unit volume of a fuel when it burns, for example in ; it equals the specific energy multiplied by the density.
One of the fixed (quantized) energies that an electron in an atom can have; electrons can move between levels but cannot have energies in between.
A graph of potential energy against reaction coordinate showing the reactants, the transition state and the products, with the activation energy and .
The additional warming caused by increased concentrations of greenhouse gases in the atmosphere as a result of human activities, mainly the burning of fossil fuels.
The heat content of a system at constant pressure, symbol . Only changes in enthalpy can be measured.
The heat transferred between a system and its surroundings at constant pressure, ; negative for exothermic and positive for endothermic changes.
The enthalpy change when hydrogen adds to the double bonds of one mole of an unsaturated compound to give the saturated compound, for example about for cyclohexene.
The enthalpy change when an acid and a base react to form one mole of water; about for a strong acid with a strong base.
The enthalpy change when one mole of a substance dissolves in a solvent, usually water, to form a dilute solution; it can be exothermic or endothermic, for example for ammonium nitrate.
The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state, for example
A measure of the dispersal or distribution of matter and/or energy in a system, symbol , in (or
The change in entropy of the surroundings caused by heat transferred from or to a reaction at temperature : .
A biological catalyst: a protein that catalyses a specific reaction in living cells, working best at body temperature and close to neutral pH.
One of the three positions around the middle of a trigonal bipyramid, at 120° to each other; lone pairs occupy these positions because they have only two neighbouring domains at 90°.
The constant given by the equilibrium law for a reaction at a particular temperature; its size shows the extent of reaction, and it changes only with temperature.
The fraction for written from a balanced equation: products over reactants, each concentration in square brackets raised to the power of its coefficient.
At equilibrium at a given temperature, the product concentrations raised to the powers of their coefficients, divided by the reactant concentrations raised to the powers of theirs, is a constant, .
The mixture of reactants and products present in a system at equilibrium, whose composition stays constant while conditions are unchanged.
The point in a titration at which the reactants have been added in exactly the mole ratio of the balanced equation.
Hydrogen atoms in the same chemical environment, related by the symmetry of the molecule or by free rotation; they give a single NMR signal and do not split each other.
A line drawn through a plotted point on a graph to show the range of its uncertainty.
The marking rule that a wrong answer used correctly in a later step does not lose the later marks, so correct method on a wrong earlier value still scores.
A compound containing the ester group, R–COO–R′, made from a carboxylic acid and an alcohol and named alkyl alkanoate, such as ethyl ethanoate.
The functional group –COO–: a C=O whose carbon is also bonded to an oxygen atom that is bonded to another carbon atom.
The group that joins the residues of an acid and an alcohol in an ester or a polyester.
A compound with an oxygen atom bonded to two carbon atoms, R–O–R′, general formula
Vaporization from the surface of a liquid at any temperature below its boiling point, as the most energetic particles escape; it cools the remaining liquid. Also used to remove a solvent from a solution.
A reactant present in more than the amount needed to react with the limiting reactant, so some of it remains when the reaction is complete.
An arrangement of electrons with more energy than the ground state, in which one or more electrons have been promoted to a higher energy level.
Describes a change that releases energy to the surroundings, such as freezing, condensation or deposition; the surroundings get warmer and is negative.
A valence shell with more than eight electrons around a central atom, as in (10) and
The amount or mass of product actually obtained from a reaction.
How far a reaction has gone towards products when equilibrium is reached; measured by the size of .
The wire and any meter or device that connect the two electrodes of a cell; electrons flow through it from the anode to the cathode.
Extending a line or trend on a graph beyond the measured data, for example extending a cooling line back to the time of mixing to correct for heat loss.