Chemistry DEFINITIONS (IGCSE EDEXCEL CORE)

Diffusion

the net movement of particles from an area of higher concentration to an area of lower concentration, caused by the random movement of particles

Melting

on heating, particles gain energy and vibrate more until the forces of attraction holding them in fixed positions are overcome

Freezing

on cooling, particles lose energy and move more slowly until the forces of attraction between them are strong enough to pull them into fixed positions in a regular arrangement, changing the state from liquid to solid

Boiling/evaporation

the change of state from liquid to gas

Condensation

on cooling, particles lose energy and move more slowly, so the forces of attraction between them pull them closer together into a random, close arrangement, changing the state from gas to liquid

Sublimation

the change of state directly between solid and gas without passing through the liquid state

Solvent

the liquid in which a solute dissolves to form a solution

Solute

the substance that dissolves in a solvent

Solution

a mixture formed when a solute dissolves in a solvent

Saturated solution

a solution in which no more solute can dissolve at that temperature, with undissolved solute present

Solubility

the mass of solute that dissolves in 100 g of solvent to form a saturated solution at a stated temperature

Element

a substance made of only one type of atom, which cannot be broken down into simpler substances by chemical means

Compound

a substance formed when two or more elements are chemically combined together in fixed proportions

Mixture

two or more elements or compounds that are not chemically combined together and can be separated by physical means

Pure substance

a single element or compound not mixed with any other substance, having a fixed melting and boiling point

Filtration

separates an insoluble solid from a liquid, using filter paper to trap the solid

Crystallisation

separates a soluble solid from a solution by evaporating the solvent until crystals form

Simple distillation

separates a solvent from a solution by evaporation followed by condensation

Fractional distillation

separates a mixture of miscible liquids with different boiling points, using a fractionating column

Chromatography

separates substances in a mixture based on their differing solubility in, and attraction to, the mobile and stationary phases

Rf value

the distance travelled by a substance divided by the distance travelled by the solvent

Atom

the smallest particle of an element that can exist and still show the chemical properties of that element

Molecule

two or more atoms chemically bonded together

Atomic number

the number of protons in the nucleus of an atom

Mass number

the total number of protons and neutrons in the nucleus of an atom

Isotopes

atoms of the same element with the same number of protons but a different number of neutrons

Relative atomic mass (Ar)

the average mass of an atom of an element compared to 1/12th the mass of an atom of carbon-12, taking into account the abundance of its isotopes

Period

a horizontal row of the periodic table, with elements arranged in order of increasing atomic number

Group

a vertical column of the periodic table containing elements with the same number of electrons in their outer shell, giving them similar chemical properties

Metal

an element that loses electrons to form positive ions and typically forms basic oxides

Non-metal

an element that gains (or shares) electrons to form negative ions and typically forms acidic or neutral oxides

Noble gases

the Group 0 elements, which are unreactive because they have a full outer shell of electrons

Mole

the amount of a substance that contains 6.02 x 10^23 particles (Avogadro's number) of that substance

Relative formula mass (Mr)

the sum of the relative atomic masses of all the atoms shown in a formula

Empirical formula

the simplest whole-number ratio of atoms of each element present in a compound

Molecular formula

the actual number of atoms of each element present in a molecule

Percentage yield

the actual yield divided by the theoretical yield, multiplied by 100

Concentration

the amount of solute, in mol or g, dissolved per dm3 of solution

Molar volume

the volume occupied by one mole of any gas, equal to 24 dm3 at room temperature and pressure

Ion

an atom or group of atoms that has lost or gained electrons and so carries an electric charge

Ionic bond

the strong electrostatic force of attraction between oppositely charged ions

Giant ionic lattice

a regular three-dimensional structure of oppositely charged ions held together by strong electrostatic forces acting in all directions

Covalent bond

a shared pair of electrons between two atoms, held together by the electrostatic attraction between the nuclei and the shared pair of electrons

Simple molecular structure

a structure with strong covalent bonds within each molecule but only weak intermolecular forces between molecules, giving low melting and boiling points

Intermolecular forces

the weak forces of attraction that exist between molecules

Giant covalent structure

a structure in which many atoms are joined by strong covalent bonds in a repeating three-dimensional lattice, giving very high melting and boiling points

Metallic bond

the strong electrostatic force of attraction between positive metal ions and the delocalised (free-moving) electrons surrounding them

Alloy

a mixture of a metal with one or more other elements, usually other metals or carbon

Electrolysis

the breakdown (decomposition) of an ionic compound, when molten or in aqueous solution, by the passage of electricity

Electrolyte

a molten ionic compound or an ionic solution that conducts electricity and is broken down in the process

Anode

the positive electrode, at which oxidation (loss of electrons) occurs

Cathode

the negative electrode, at which reduction (gain of electrons) occurs

Anion

a negatively charged ion, which moves towards the anode during electrolysis

Cation

a positively charged ion, which moves towards the cathode during electrolysis

Oxidation

the loss of electrons, or the gain of oxygen, by a substance

Reduction

the gain of electrons, or the loss of oxygen, by a substance

Redox reaction

a reaction in which oxidation and reduction both take place at the same time

Oxidising agent

a substance that oxidises another substance by accepting electrons from it, and is itself reduced

Reducing agent

a substance that donates electrons

Reactivity series

a list of metals arranged in order of their reactivity, with the most reactive metal first

Displacement reaction

a reaction in which a more reactive element takes the place of a less reactive element in a compound

Rusting

the corrosion of iron, which requires the presence of both oxygen and water

Galvanising

coating iron or steel with a layer of zinc, which acts as a barrier and provides sacrificial protection against rusting

Sacrificial protection

attaching a more reactive metal to iron or steel so that the more reactive metal corrodes in preference to the iron, protecting it

Ore

a rock that contains enough of a metal or metal compound to make it economically worthwhile to extract the metal

Acid

a substance that releases (donates) hydrogen ions, H+, in aqueous solution

Base

a substance that can neutralise an acid

Alkali

a base that is soluble in water and releases hydroxide ions, OH-, in aqueous solution

Neutralisation

a reaction between an acid and a base in which they react together to form a salt and water

pH scale

a scale, from 0 to 14, used to measure how acidic or alkaline a solution is

Salt

a compound formed when the hydrogen ion(s) of an acid are replaced by metal ions or ammonium ions

Titration

a technique used to accurately measure the volume of one solution that reacts exactly with a known volume of another solution

Flame test

a test used to identify metal cations, based on the characteristic colour a metal compound produces when heated in a flame

Greenhouse gas

a gas, such as carbon dioxide, that absorbs and re-emits infrared radiation, trapping heat in the Earth's atmosphere

Acid rain

rain that is more acidic than normal (below pH 5.6), caused by sulfur dioxide and oxides of nitrogen dissolving in atmospheric water

Exothermic reaction

a reaction that transfers heat energy to the surroundings, so the temperature of the surroundings increases

Endothermic reaction

a reaction that takes in heat energy from the surroundings, so the temperature of the surroundings decreases

Activation energy

the minimum amount of energy that colliding particles must have for a reaction to occur

Catalyst

a substance that increases the rate of a reaction, without being chemically changed at the end of the reaction, by providing an alternative reaction pathway with a lower activation energy

Rate of reaction

the speed at which reactants are converted into products, measured as the change in amount of reactant or product per unit time

Collision theory

the theory that particles must collide with sufficient energy (at least equal to the activation energy) and the correct orientation for a reaction to take place

Reversible reaction

a reaction in which the products of the reaction can react together to reform the original reactants

Dynamic equilibrium

the state reached in a reversible reaction in a closed system when the forward and reverse reactions occur at exactly the same rate, so the concentrations of reactants and products remain constant

Hydrocarbon

a compound containing hydrogen and carbon atoms only

Homologous series

a family of compounds with the same general formula and similar chemical properties, showing a gradual change in physical properties as the number of carbon atoms increases

Functional group

an atom or group of atoms in a molecule that is responsible for its characteristic chemical reactions

Isomers

compounds that have the same molecular formula but a different structural formula

Saturated hydrocarbon

a hydrocarbon that contains only single covalent bonds between carbon atoms

Unsaturated hydrocarbon

a hydrocarbon that contains at least one carbon-to-carbon double bond

Alkane

a saturated hydrocarbon with the general formula CnH2n+2

Alkene

an unsaturated hydrocarbon, containing a C=C double bond, with the general formula CnH2n

Addition reaction

a reaction in which two molecules join together to form a single new product, with no other substance produced

Substitution reaction

a reaction in which an atom, or group of atoms, in a molecule is replaced by a different atom or group of atoms

Complete combustion

the burning of a fuel in a plentiful supply of oxygen, producing carbon dioxide and water and releasing the maximum amount of energy

Incomplete combustion

the burning of a fuel in a limited supply of oxygen, producing carbon monoxide and/or carbon (soot) as well as water

Cracking

the process in which long-chain, saturated hydrocarbons are broken down into shorter, more useful alkanes and alkenes, using heat and a catalyst

Crude oil

a mixture of hydrocarbons, mostly alkanes, formed from the remains of ancient organisms

Fraction

a mixture of hydrocarbons with similar boiling points, separated from crude oil by fractional distillation

Alcohol

an organic compound containing the functional group -OH

Fermentation

the process in which yeast converts glucose into ethanol and carbon dioxide, in the absence of oxygen