Cameroon GCE O-Level Chemistry key terms

The smallest particle of an element that can take part in a chemical change, consisting of a central nucleus of protons and neutrons surrounded by electrons.

A pure substance made of only one kind of atom, which cannot be split into simpler substances by ordinary chemical means.

A pure substance formed when two or more different elements are chemically joined together in fixed proportions by mass, with properties different from those of its elements.

Two or more substances placed together without chemical bonding, in no fixed proportion, which can usually be separated by physical methods such as filtration or distillation.

The smallest particle of an element or compound that can exist on its own and still keep the chemical properties of that substance, made of atoms joined by covalent bonds.

An atom or group of atoms that has gained or lost electrons, giving it a net electrical charge, such as Na+ or Cl- or the sulphate ion SO4 2-.

One of two or more atoms of the same element that have the same number of protons but different numbers of neutrons, and so different mass numbers.

The total number of protons and neutrons in the nucleus of an atom, written as a superscript before the symbol, for example 23 in sodium-23.

The number of protons in the nucleus of an atom, which is also the number of electrons in a neutral atom and decides the position of the element in the Periodic Table.

The average mass of the naturally occurring atoms of an element compared with one twelfth of the mass of an atom of carbon-12, taking isotope abundance into account.

An arrangement of the elements in order of increasing atomic number, in rows called periods and columns called groups, so that elements with similar properties fall in the same group.

A vertical column of the Periodic Table whose members have the same number of electrons in their outer shell and therefore similar chemical properties.

A horizontal row of the Periodic Table in which the elements are arranged in order of increasing atomic number; the period number equals the number of occupied electron shells.

An electron in the outermost shell of an atom, which is involved in bonding and largely decides the chemical behaviour of the element.

The strong electrostatic attraction between oppositely charged ions, formed when a metal atom transfers electrons to a non-metal atom, as in sodium chloride.

A bond formed when two atoms share one or more pairs of electrons, usually between non-metal atoms, as in the hydrogen molecule or a water molecule.

The attraction between positively charged metal ions in a regular lattice and the sea of delocalised electrons moving freely among them, which explains conduction and malleability.

The amount of substance that contains the same number of particles as there are atoms in exactly 12 grams of carbon-12, that is the Avogadro number of particles.

The mass of one mole of a substance in grams per mole, found by adding the relative atomic masses of all the atoms shown in its chemical formula.

The number of particles in one mole of any substance, approximately 6.02 x 10^23 per mole, used to convert between numbers of particles and amounts in moles.

A shorthand showing the elements in a compound and the ratio of their atoms using symbols and subscripts, such as H2O or CaCO3.

The simplest whole-number ratio of the atoms of each element present in a compound, obtained from experimental masses or percentage composition.

The reactant that is completely used up first in a chemical reaction, and which therefore determines the maximum amount of product that can be formed.

The mass of product actually obtained expressed as a percentage of the maximum mass predicted from the balanced equation, showing how efficient the reaction was.

A substance that releases hydrogen ions (H+) when dissolved in water, turning blue litmus red and reacting with bases to form a salt and water.

A substance that can neutralise an acid to form a salt and water; a soluble base such as sodium hydroxide is called an alkali and releases hydroxide ions in water.

A scale from 0 to 14 measuring how acidic or alkaline a solution is: values below 7 are acidic, 7 is neutral, and values above 7 are alkaline.

The reaction between an acid and a base in which hydrogen ions and hydroxide ions combine to form water, leaving a salt in solution.

The compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion, for example sodium chloride from hydrochloric acid and sodium hydroxide.

A volumetric technique in which a solution of known concentration is added from a burette to a measured volume of another solution until the end-point is signalled by an indicator.

A weak acid or base that changes colour over a particular pH range, used to show the end-point of an acid-base titration, such as phenolphthalein or methyl orange.

A reaction in which a substance loses electrons, gains oxygen or loses hydrogen; the oxidation number of the element involved increases.

A reaction in which a substance gains electrons, loses oxygen or gains hydrogen; the oxidation number of the element involved decreases.

A reaction in which oxidation and reduction happen at the same time, because electrons lost by one substance are gained by another.

The decomposition of a compound in molten or dissolved state by passing a direct electric current through it, using inert or reactive electrodes.

A substance that conducts electricity when molten or dissolved in water because it contains freely moving ions, and is decomposed by the current.

A chemical reaction that releases heat energy to the surroundings, so the temperature rises and the products have less chemical energy than the reactants.

A chemical reaction that absorbs heat energy from the surroundings, so the temperature falls and the products have more chemical energy than the reactants.

The minimum energy that colliding particles must have before a reaction can take place, shown as the height of the energy barrier on a reaction profile.

A substance that speeds up a chemical reaction by providing an alternative path of lower activation energy, but is not used up in the reaction.

The change in amount or concentration of a reactant or product per unit time, increased by higher temperature, higher concentration, smaller particle size or a catalyst.

The state of a reversible reaction in a closed system when the forward and backward rates are equal, so the amounts of reactants and products stay constant.

A mixture of a metal with one or more other elements, made to improve properties such as hardness or resistance to corrosion, for example brass or steel.

A naturally occurring rock or mineral from which a metal can be extracted economically, often as an oxide or sulphide of the metal.

A family of organic compounds with the same general formula and similar chemical properties, in which each member differs from the next by a CH2 group.

The particular atom or group of atoms in an organic molecule that gives the compound its characteristic reactions, such as the OH group in alcohols.

One of two or more compounds that have the same molecular formula but different arrangements of atoms, and therefore different physical or chemical properties.

A separation method that passes a mixture through filter paper to trap an insoluble solid while the liquid or solution runs through as the filtrate.

A separation method in which a liquid is boiled to vapour and then condensed back to liquid, used to separate a solvent from a solution or liquids with different boiling points.

A qualitative test in which a clean wire dipped in a sample is held in a Bunsen flame; the colour identifies the metal ion, for example lilac for potassium.

An insoluble solid formed when two solutions are mixed and react, used in qualitative analysis to identify ions such as chloride or sulphate.