GCSE Combined Science - AQA

acid

A substance that contains replaceable hydrogen atoms which form H+ ions when the acid is dissolved in water. It has a pH less than 7.

activation energy

The minimum energy that must be provided before a reaction can take place.

addition polymer

A polymer that is made when molecules of a single monomer join together in large numbers.

alkali metal

A Group I element.

alkane

A hydrocarbon in which the carbon atoms are bonded together by single bonds only.

alkene

A hydrocarbon that contains a carbon- carbon double bond.

alloy

A mixture of a metal and one or more other elements.

anhydrous

Literally means "without water" - a compound, usually a salt, with no water of crystallisation.

anion

A negatively charged ion.

anode

A positively charged electrode in electrolysis.

atom

The smallest particle of an element. Atoms are made of protons, electrons and neutrons.

boiling

The change of state from liquid to gas.

boiling point

The temperature of a boiling liquid — the highest temperature that the liquid can reach and the lowest temperature that the gas can reach.

burning

The reaction of a substance with oxygen in a flame.

calorimetry

A method for determining energy changes in reactions or when substances are mixed together.

carbonate

A salt formed by the reaction of carbon dioxide with alkalis in solution.

catalyst

A chemical that is added to speed up a reaction, but remains unchanged at the end.

catalytic cracking

The process by which long-chain alkanes are broken down to form more useful short-chain alkanes and alkenes, using high temperatures and a catalyst.

cathode

A negatively charged electrode in electrolysis.

cation

A positive ion.

chemical change

A change that is not easily reversed because new substances are made.

chemical formula

The combination of element symbols that represents a compound or molecule.

chemical reaction

A chemical change that produces new substances and which is not usually easily reversed.

chemical symbol

A unique symbol that represents a particular chemical element.

chromatogram

A visible record (usually a coloured chart or graph) showing the separation of a mixture using chromatography.

chromatography

The process for separating dissolved solids using a solvent and filter paper (in the school laboratory).

collision theory

A theory used to explain differences in the rates of reactions as a result of the frequency and energy associated with the collisions between the reacting particles.

combustion

The reaction that occurs when a substance (usually a fuel) burns in oxygen.

compound

A pure substance formed when elements react together.

condensation

The change of state from gas to liquid.

control variable

Something that is fixed and is unchanged in an investigation.

covalent bond

A bond that forms when electrons are shared between the atoms of two non-metals.

dependent variable

A variable that changes as a result of changes made to value of the independent variable.

desalination

The separation of salt from sea water by evaporation of the water.

diatomic

Two atoms combined together (for example, in a molecule).

displacement reaction

A reaction in which one element takes the place of another in a compound, removing (displacing) it from the compound.

dissociation

The splitting of a molecule to form smaller molecules or, in the presence of water, ions.

distillation

The process for separating a liquid from a solid (usually when the solid is dissolved in the liquid) or a liquid from a mixture of liquids.

effective collision

A collision between particles with enough energy to cause a chemical reaction.

electrode

The carbon or metal material that delivers electric charge in electrolysis reactions.

electrolysis

The breaking down of a compound by passing an electric current through it.

electrolyte

A substance that allows electric current to pass through it when it is molten or dissolved in water.

electron

Negatively charged particle with a negligible mass that forms the outer part of all atoms.

electronic configuration

The arrangement of electrons in an atom, molecule or ion.

element

A substance that cannot be broken down into other substances by any chemical change.

endothermic

A type of reaction in which energy is taken in from the surroundings.

enthalpy change (ΔH)

The heat energy change when the reactants shown in a chemical equation react together.

enzyme

A chemical that speeds up certain reactions in biological systems, such as digestive enzymes that speed up the chemical digestion of food.

evaporation

When liquid changes to gas at a temperature lower than its boiling point.

exothermic

A type of reaction in which energy is transferred out to the surroundings.

filtrate

The clear solution produced by filtering a mixture.

fossil fuel

Fuel made from the remains of decayed animal and plant matter compressed over millions of years.

fraction

A collection of hydrocarbons that have similar molecular masses and boil at similar temperatures.

fractional distillation

A process for separating liquids with different boiling points.

freezing

Changing a liquid to a solid.

freezing point

The temperature at which a liquid changes to a solid.

functional group

A part of an organic molecule which is responsible for the characteristic reactions of the molecule.

gas

The state of matter in which the substance has no volume or shape.

global warming

The rise in the average temperature of the Earth's atmosphere and oceans.

greenhouse effect

The trapping of long-wave radiation emitted from the Earth's surface by gases in the atmosphere.

greenhouse gas

A gas that can trap long-wave radiation emitted from the Earth's surface.

group

A vertical column of elements in the Periodic Table.

halogens

The Group VII elements (F, Cl, Br, I, At).

homologous series

A group of organic compounds with the same general formula, similar chemical properties and physical properties that change gradually from one member of the series to the next.

hydrocarbon

A compound containing only hydrogen atoms and carbon atoms.

independent variable

A variable that is deliberately changed in an investigation and, as a result, causes changes to the dependent variable.

indicator

A substance that changes colour in either an acid or alkali and so can be used to identify acids or alkalis.

intermolecular force

The force of attraction or repulsion between molecules.

intramolecular bond

A bond within a molecule.

ion

A charged atom or molecule.

ionic bond

A bond that involves the transfer of electrons to produce electrically charged ions.

ionic compound

A compound formed by the reaction between a metal and one or more non- metals.

ionic equation

A chemical equation showing how the ions involved react together.

liquid

The state of matter in which a substance has a fixed volume but no definite shape.

litmus

An indicator that has different colours in acids (red) and alkalis (blue).

mass number

The number of protons and neutrons in an atom (also known as the nucleon number).

melting

Changing a solid into a liquid at its melting point.

melting point

The temperature at which a solid changes to a liquid.

metal

An element with particular properties (usually hard, shiny and a good conductor of heat and electricity).

mineral

A solid inorganic substance that occurs naturally.

mixture

Two or more substances combined without a chemical reaction — they can be separated easily.

molecule

A group of two or more atoms covalently bonded together.

monatomic

An element composed of separate atoms.

monomer

Small molecules that can be joined in a chain to make a polymer.

neutralisation

A reaction in which an acid reacts with a base or alkali to form a salt and water.

neutron

Particle in the nucleus of atoms that have mass but no charge.

noble gas

Group 0 elements (He, Ne, Ar, Kr, Xe, Rn). They have full outer electron shells.

non-metal

An element with particular properties (usually a gas or soft solid and a poor conductor of heat and electricity).

non-renewable

A fuel that cannot be made again in a short time span.

nucleus, atomic

The tiny centre of an atom, typically made up of protons and neutrons.

nucleon number (mass number)

The total number of protons and neutrons in an atom.

ore

A mineral from which a metal may be extracted.

oxidation state

The degree of oxidation of an element.

oxidation

The addition of oxygen in a chemical reaction.

particle theory

The theory describing the movement of particles in solids, liquid and gases.

period

A row in the Periodic Table, from an alkali metal to a noble gas.

Periodic Table

The modern arrangement of the chemical elements in groups and periods.

periodicity

The gradual change in properties of the elements across each row (period) of the Periodic Table.

pH scale

A scale measuring the acidity (lower than 7) or alkalinity of a solution (higher than 7). It is a measure of the concentration of hydrogen ions in a solution.

photosynthesis

A reaction that plants carry out to make food.