chemistry definitions

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.