IGCSE Chemistry Definitions – LEARN THESE!
Melting - Solid changing into a liquid
Freezing - Liquid changing into a solid
Condensation - Gas/vapour changing into a liquid
Evaporation - Liquid changing into a gas/vapour
Melting Point - Temperature at which a solid turns into a liquid
Freezing Point - Temperature at which a liquid changes into a solid
Boiling Point - Temperature at which a liquid changes into a vapour at
atmospheric pressure
Diffusion - The random movement of particles from an area of a high
concentration to an area of low concentration
Filtration - Separating an insoluble solid from a liquid
Filtrate - is fluid that has passed through a filter.
Solute - the material that dissolves in a solvent to form a solution.
Solution - the mixture formed when a solute dissolves in a solvent.
Solvent - the liquid in which the solute dissolves to form a solution.
Fractional Distillation - Separating liquids of different boiling points by
evaporation and condensing
Galvanization – The coating of iron by zinc to protect from rusting. A type
of sacrificial protection.
Atomic number/Proton number- The number of protons in an atom
Atom - The smallest unit of an element, having all the characteristics of
that element
Mass number/Nucleon number - The sum of the protons and neutrons
Nucleus - The centre of the atom which contains protons and neutrons
Isotope - Atoms of the same element with the same atomic number and
different number of neutrons
Element - Made of only one type of atom
Compound - Made of two or more different atoms chemically joined
together
Mixture - Made of two or more different components which are not
chemically joined together
Group - The columns in the Periodic Table. The Group Number equals the
number of electrons in the outer shell
Period - The rows of the Periodic Table. The Period Number equals the
number of shells used by the electrons of the atom.
Ionic Bonding - Transfer of electrons so as to achieve a full outer shell of
electrons. This results in the formation of ions.
Covalent Bonding - Sharing of an electron pair between atoms. This
results in the formation of a simple covalent molecule or a giant covalent
structure.
Ion - A charged particle resulting from ionic bonding. The number of
electrons are not equal to the number of protons.
Molecule - A particle formed from covalent bonding which is neutral.
Rate Of Reaction - Change in product or reactant concentration
per unit time. The more frequent the collision of particles the greater the
rate of reaction.
Catalyst - Substance which increases the rate of reaction without
chemically being changed. It lowers the activation energy of the reaction.
Enzyme - A biological catalyst e.g. Amylase
Relative Formula Mass - The sum of the atomic masses of elements in a
compound. The formula mass expressed in grams.
Mole - One mole of a substance contains the same number of
molecules/atoms as in 12 grams of carbon-12. This number (of atoms or
molecules) is known as the Avogadro constant (NA) which is equal to 6.02 x
1023
Exothermic Reaction - A reaction which gives out heat (ΔH is negative)
Endothermic Reaction - A reaction which takes in heat (ΔH is positive).
pH - A scale of 0 – 14 which identifies solution as acidic, neutral or alkaline.
Base - A substance with a pH higher than 7, they react with acids to form a
salt and water (called neutralization). Metal hydroxides, oxides and
carbonates are all bases.
Acid - They have a pH less than 7 and neutralize bases or
alkalis to form salt and water. Acidity is caused by a high concentration of
hydrogen ions.
Alkali - They have a pH greater than 7 and neutralize acids to
form salt and water. They have a high concentration of hydroxyl ions (OH -).
They are soluble bases.
Neutralization - A reaction where acids react to form salts and water.
H+ + OH- → H2O
Combustion - A reaction where a substance burns in oxygen combining to
form the oxide and giving out energy.
Displacement - A reaction where a more reactive element displaces a
less reactive element from a solution of its compound.
Thermal Decomposition - A reaction where a compound is broken down
into simpler substances using heat e.g.
Metal carbonates → Metal Oxide + Carbon Dioxide
Oxidation - A reaction where oxygen is added to a substance e.g.
Copper + Oxygen → Copper Oxide
Oxidation also means a loss of electrons.
Reduction - A reaction where oxygen is removed from a substance.
Reduction also means a gain in electrons.
Electrolysis - is the break-down of a substance or solution by electricity
Electrolyte - a molten or aqueous solution through which an electrical
current can flow.
Anode - The positive terminal (electrode).
Cathode - The negative terminal (electrode)
Anion – negative ion (attracted to the anode)
Cation – Positive ion (metal) attracted to the cathode.
Reversible Reaction - A reaction where reactants change to products
which can then change back to reactants.
Dynamic equilibrium – The state of a reversible reaction when the rate of
the forward reaction is equal to the rate of the backward reaction.
Fertilizers - Chemical compounds (e.g. Ammonium compounds)
which are added to soil to increase the quantity and quality of crops.
Eutrophication - Process where lakes and rivers which are rich in
nutrients due to leaching of fertilizers, encourages the growth of plant life
which is decomposed by bacteria using oxygen in the water.
Crude Oil - A mixture of hydrocarbons formed from the remains
of dead sea life which was covered with silt on the sea bed and
compressed over millions of years
Fossil Fuel - A substance formed from the dead remains of plants
or animals which will burn in air e.g. Coal, natural gas
Hydrocarbon - A substance which contains carbon and hydrogen
atoms only.
Cracking - Process where long chain hydrocarbons of little value
are broken down by heat and a catalyst into smaller, more useful
hydrocarbons
Saturated - A hydrocarbon which only has single covalent bonds
between all atoms.
Unsaturated - A hydrocarbon which has at least one double bond
between two neighbouring carbon atoms.
Alkane - A saturated hydrocarbon e.g. CH4 – methane
Alkene - An unsaturated hydrocarbon e.g. C2H4 – ethene
Addition Polymerization - A reaction where many small alkene molecules
(monomers) join up to form a long chain molecule of repeating units
(polymer).
Condensation Polymerization - A reaction where different monomers join
up to form a long chain molecule of repeating units (polymer) whilst also
producing smaller molecules.
Homologous Series - a group of molecules with similar physical and
chemical properties, the same general formula and the same functional
group (albeit with trends e.g. increasing boiling point with increasing carbon
chain length)