IGCSE CHEMISTRY DEFINITIONS
Here is a list of important terms that you need to know and understand the meaning of in Chemistry. Chemistry. Terms or words you are commonly asked to write out as a definition or an explanation are marked with an asterix (*). Note that words in bold and underlined are usually required on IGCSE papers in order to gain marks for the question if you are asked to write out the definition.
ATOMS, MOLECULES AND IONS Diffusion *
The spreading out and mixing of particles
Particle
This can mean several things, depending on the context:Electrons, protons and neutrons in an atom • Atoms, molecules or ions, e.g., in discussing diffusion experiments • or collision theory of reaction rates Pieces of solids used in reactions, e.g., marble chips or powder •
Atom *
The smallest particle of an element which shows the properties of that element. (Atoms have equal numbers of protons and neutrons and are electrically neutral.)
Molecule
Two or more atoms joined together by covalent bonds (sharing electrons). These have no electrical charge. charge.
Ion
An atom or molecule that is electrically charged because it has gained or lost electrons.
Cation
A positive ion (electrons missing)
Anion
A negative ion (extra electrons)
Atomic Number *
The number of protons in the nucleus of an atom
Mass Number *
The total number of protons plus neutrons in the nucleus of an atom Atoms of the same element (same atomic number) but with different mass numbers (different numbers of neutrons)
Isotopes *
Abundance (of isotopes)
Relative Atomic Mass, Ar
Isotopes have identical chemical properties because they have exactly the same electron configuration (arrangement) Often quoted as % values, these are the proportions of each stable isotope in the naturally occurring elements The average mass of an atom of an element (a weighted average that takes into account the mass numbers and abundances of all the stable isotopes of the element). You need to know how how to calculate this from isotope data. Atomic masses are measured relative to the carbon-12 isotope (which is taken to be exactly 12 units)
Relative Formula Mass, Mr
The average mass of a molecule (or “formula unit” for ionic compounds). It is the sum of all the individual individual relative atomic masses.
The mole (mol)
Avogadro’s number of particles (atoms, molecules, ions or electrons). (Avogadro’s Number is the number of 12C atoms in exactly 12 grams of the carbon-12 isotope.) In mass terms, the mole is also the relative mass expressed in grams.
Molar gas volume
The volume occupied by one mole of any gas. It varies with pressure and temperature. RTP is Room Temperature and Pressure (25OC and normal atmospheric pressure) and STP is Standard Temperature and Pressure (0OC and normal atmospheric pressure).
SYMBOLS, FORMULAE AND EQUATIONS Symbol (element)
A one or two letter symbol which represents an element, as given in the Periodic Table, e.g., C = Carbon and Na = sodium
State symbols
A small letter written in brackets next to a name or formula to show what physical state the substance is in: (s) = solid, (l) = liquid, (g) = gas and (aq) = in solution in water. (Try not to confuse pure liquids with solutions in water!)
Formula (used in chemical equations)
For most elements the formula is the same as the symbol. There are a few exceptions (H2 ; N2 ; O2 ; F2 ; Cl2 ; Br 2 ; I2) For compounds, the formula shows the number of a toms of each element in a molecule or the ratio of ions. Remember to use small subscript numbers, e.g., CO 2, Ca(OH)2 or CuSO4 For ions, you need to include the charge as well, e.g., Mg 2+ or CO32-
Empirical Formula *
The simplest (whole number) ratio of atoms of each element in a compound.
Molecular Formula *
The actual number of atoms of each element in a molecule of the compound. (The molecular formula is always a whole number times the empirical formula, e.g., C 3H 6 is 3 times CH 2)
Word Equation
An equation representing a reaction where the names of all the substances are used. (If you write an equation using symbols when the question asks for a word equation, you will get no marks, even if your symbol equation is correct!)
Balanced chemical or symbol Equation
An equation representing a reaction where the chemical formulae of all the substances are used. It must be balanced (the same number of every type of atom on both sides of the equation). Remember that the balancing numbers are the mole ratios of reactants and product when it comes to doing calculations.
Electrode (ionic/half) Equations
These equations use ion formulae with charges and electrons to represent the gain or loss of electrons at the electrodes in electrolysis.
STRUCTURE AND BONDING Structure
There are two types of structure: Giant lattice (a very large number or atoms or ions forming one huge crystal) or Simple Molecular
Metallic bonding
A giant lattice of close-packed metal ions (+ve) surrounded by a “sea” of delocalised valence (outer) electrons. Electrostatic attraction of the cations for the negatively charged “sea” of electrons is strong.
Ionic bonding
Ionic compounds are formed between metals and non-metals and they form giant lattices. The strong electrostatic force of attraction between +ve and –ve ions is called ionic bonding.
Covalent bond *
A pair of electrons shared between two atoms. Simple molecules have one or more covalent bonds in them. There are a few giant covalent lattices, most importantly diamond and graphite.
Allotropes *
Different structural forms of the same element, in particular you need to know about diamond and graphite as allotropes of carbon.
ORGANIC (CARBON) CHEMISTRY AND CRUDE OIL
Homologous Series *
A family of carbon compounds which:have the same general formula. • differ from each other by a –CH 2 each time. • have the same functional group • have similar chemical properties (because same functional group). • show a trend in physical properties (e.g., boiling points). • (Examples are the Alkanes, the Alkenes and Alcohols)
General Formula
For Homologous Series it shows the relationship between the number of C atoms and H atoms in the compounds, e.g., for Alkanes it is CnH2n+2
Displayed or Structural Formula
This means drawing out the molecule using sticks to represent the bonds between atoms so it is clear how the atoms are joined up.
Isomers *
Compounds (or molecules) which have the same molecular formula, but have different structural formulae (the atoms are joined up to give different structures).
Hydrocarbon *
Compounds containing ONLY the elements carbon and hydrogen.
Saturated
Carbon compounds where all the C-C bonds are single
Unsaturated
Carbon compounds containing one or more C=C double bonds
Combustion
Burning. It means reaction with oxygen. Complete combustion of carbon compounds forms only CO 2 + H2O • Incomplete combustion forms CO and soot (carbon) •
Fractional distillation
A process which PHYSICALLY separates out the different components of crude oil. It works because the different components have different boiling points.
Cracking
A process which CHEMICALLY breaks up larger molecules into smaller ones.
Addition reaction
A reaction where C=C double bonds open up to allow another molecule to join across it. The product has C-C single bonds.
Addition polymer
A very long chain molecule formed when many individual molecules containing C=C double bonds (monomers) join together.
ACIDS, BASES & SALTS Acid
A substance which has pH < 7 and produces hydrogen ions (H +) in solution in water.
Base
The opposite of an acid. It is a substance which will neutralise an acid to form a salt and water. (Bases are metal oxides, metal hydroxides, metal carbonates and ammonia. Most are insoluble in water.)
Alkali
A base which is soluble in water. (The oxides, hydroxides and carbonates of Group I metals and ammonia gas, NH 3.)
Salt
The type of substance formed when an acid is neutralised by a base.
Neutralisation
The process of forming water from an acid and a base.
Precipitate
The solid formed when solutions are mixed together. (It makes the solution go cloudy and opaque.)
GENERAL Oxidation
The gain of oxygen or the loss of electrons.
Reduction
The loss of oxygen or the gain of electrons
Exothermic
A reaction which gives out heat energy - the temperature will rise.
Endothermic
A reaction which takes in energy - either the temperature will drop or the reaction needs a constant input of energy (heat or electricity).
Heat change,
A measure of the energy change in a reaction. Usually given in units of kJ per mole. A negative sign means an exothermic reaction. A positive sign means an endothermic reaction.
H
Bond Energy
The amount of energy needed to break 1 mole of covalent bonds in a molecule. (Remember that breaking bonds requires energy input and making bonds gives energy out.)
Catalyst
A substance which speeds up a chemical reaction, but is not chemically changed itself or used up in the reaction.
Dynamic equilibrium
A state of dynamic equilibrium is reached for reversible reactions when the rates of the forward and reverse reactions are equal. Both reactions continue, but the concentrations of all products and reactants in the mixture remain unchanged.
Le Chatelier’s Principle
This states that “a system at equilibrium will respond in such a way as to oppose any change made to it.”
