Acid: A substance that produces hydrogen ions (H+) as the only positive ions when dissolved in water.


Activation energy: The minimum energy that molecules must possess during their collisions in order for a chemical reaction to occur.


Addition reaction: A reaction in which a molecule (element or compound) adds to an unsaturated compound to form a single new compound.


Alcohol: An organic compound containing the hydroxyl group, -OH.


Alkali: A base that is soluble in water.


Alkali metals: The elements in Group 1 of the Periodic Table.


Alkane: Hydrocarbon having the general formula CnH2n+2


Alkene: Hydrocarbon that contains one or more carbon-carbon double bonds. Alkenes with only one carbon-carbon double bond have the general formula CnH2n.


Alloy: A mixture of a metal with non-metals or other metals.


Anhydrous: Anhydrous salts are salts without water of crystallization.


Anion: A negatively charged ion which moves towards the anode during electrolysis.


Anode: A positively charged electrode in an electrolytic cell.


Aqueous: Describing the solution of a substance in water, i.e. the aqueous solution. In chemical equations, aqueous solutions are represented by the symbol (aq).


Atom: The smallest particle of an element.


Avogadro’s constant: The number of particles in one mole of a substance. Its value is 6 x 1023.


Avogadro’s law: At constant temperature, the volume of a gas is directly proportional to the number of moles of the gas present.


Base: A substance that reacts with an acid to form a salt and water only.


Boiling point: The temperature at which a liquid turns rapidly to its vapour.


Carboxylic acid: An organic acid containing the carboxyl group, -COOH.


Cathode: A negatively charged electrode in an electrolytic cell.


Cation: A positively charged ion which moves towards the cathode during electrolysis.


Chromatography: A method of separating the components in a mixture.


Collision theory: A chemical reaction can occur only if the reacting particles collide with one another.


Combustion: The chemical name for burning. Burning occurs when a substance reacts very rapidly with oxygen.


Compound: A substance formed in a chemical change when two or more elements are joined together.


Condensation: The process by which a vapour or a gas turns to liquid on cooling.


Corrosion: The wearing away of the surface of a metal by chemical reaction.


Covalent bond: The type of bond formed when electrons are shared between two non-metal atoms.


Cracking: The breaking down of long chain hydrocarbon molecules with heat and/or catalyst to produce smaller hydrocarbon molecules and/or hydrogen.


Decomposition: A chemical reaction that results in the breaking down of a compound into two or more components.


Diatomic molecule: A molecule that consists of two atoms.


Displacement reaction: A reaction in which an atom or molecule takes the place of another atom or molecule in a compound.


Distillation: A process of obtaining the pure solvent from a solution. When the solution is boiled, the solvent is vaporized and the vapour condenses to reform the pure liquid. 


Electrode: A rod or a plate which carries electricity in or out of an electrolyte during electrolysis.


Electrolysis: A process in which electrical energy is used to cause a chemical reaction to occur, typically to separate the electrolyte into its elements.


Electron: A negatively charged sub-atomic particle that surrounds the nucleus of an atom.


Electronic configuration: The arrangement of electrons in the various shells of an atom or a molecule.


Element: A substance made from only one type of atom. It cannot be separated into simpler substances by chemical processes or by electricity.


Endothermic reaction: A reaction which absorbs heat from the surroundings.


Evaporation: The process by which a liquid changed to its vapour on the surface of the liquid.


Exothermic reaction: A process that gives off heat to the surroundings.


Fermentation: The conversion of glucose by microorganisms such as yeast into ethanol and carbon dioxide.


Filtrate: The clear liquid which passes through the filter during filtration.


Filtration: The process of separating a solid from a liquid or a solution.


Fossil fuels: Fuels produced many millions of years ago from the decaying remains of animals or plants, includes oil, natural gas and coal.


Fractional distillation: A process that separates the components in a mixture on the bases of their different boiling points. The component with the lowest boiling point boils off first and is distilled over.


Freezing point: The temperature at which a liquid changes to a solid.


Fuel: A substance that burns easily to produce energy.


Functional group: An atom or group of atoms that gives characteristic properties to an organic compound.


Giant structure: A three-dimensional network of atoms or ions packed together in a regular pattern.


Group: A vertical column of elements in the Periodic Table.


Halogen: The non-metallic elements in Group VII (7) of the Periodic Table.


Homologous series: A family of organic compounds with members of the family having the same functional group and similar chemical properties.


Hydrated salts: Salts that contain water of crystallization.


Hydrocarbons: Organic compounds made up from the elements hydrogen and carbon only.


Hydrogenation: The addition of a hydrogen molecule across a double bond.


Immiscible: Two liquids that do not mix.


Indicators: Compounds that have distinctly different colours in acidic and alkaline solutions.


Ion: A positively or negatively charged particle. It is formed when an atom or group of atoms loses or gains electrons.


Ionic bond: The electrostatic force that holds positive and negative ions together in an ionic compound.


Isotopes: Atoms of the same element that have the same atomic number but different mass/nucleon number.


Melting point: The temperature at which a solid changes to a liquid.


Metal: An element that is shiny and conducts electricity in the solid state. Metals burn in oxygen to form basic oxides or amphoteric oxides.


Mixture: A substance made by mixing other substances together. The components in a mixture can be easily separated by physical methods because they are not chemically joined together like in compounds.


Mole: The amount of a substance which contains 6 x 1023 particles.


Molecule: A group of atoms held together by covalent bonds. Molecules may be elements or compounds.


Nucleon number: Also known as the mass number. It is the sum of the number of protons and neutrons in the nucleus of an atom.


Neutralization: The reaction between an acid and a base to produce a salt and water only.


Neutron: A sub-atomic particle in the nucleus of an atom. It has a mass but no electrical charge.


Organic chemistry: The branch of chemistry that deals with carbon compounds.


Oxidation: A reaction where a substance gains oxygen or loses hydrogen. Oxidation is also defined as the loss of electron(s) or the increase in the oxidation state of the element.


Oxides: Compounds of an element with oxygen.


Oxidizing agent: A substance that brings about oxidation. It is itself reduced. An oxidizing agent is an acceptor of  electrons.


Period: A horizontal row of elements in the Periodic Table.


Periodic table: A table that contains horizontal rows and vertical columns of elements. The elements are arranged in order of their atomic numbers and in accordance with their chemical properties.


pH scale: A scale that measures the acidity or alkalinity of a solution.


Pollution: The presence in the environment of toxic substances which are harmful to living things.


Polymer: A very large molecule built up of a number of repeating units called monomers.


Polymerization: A chemical reaction in which simple molecules, called monomers, react with each other to form larger molecules called polymers.


Polyunsaturated: Vegetable oils that contain many carbon-carbon double bonds in their molecules.


Precipitate: An insoluble solid that is produced in a solution as a result of a chemical reaction.


Protein: A polymer of amino acids.


Protons: Positively charged sub-atomic particles found in the nucleus of an atom.


Proton number: The number of protons in the nucleus of an atom.


Pure substance: A single substance which is not mixed with other substances. It has definite melting and boiling points. (e.g. pure water boils at exactly 100°C and freezes at 0°C)


Reactivity series: A list of elements in order of their reactivity. The more reactive the element, the higher its position in the series. An element higher up the series will displace a less reactive one from a solution of its salt.


Redox reaction: A reaction where both oxidation and reduction take place simultaneously.


Reducing agent: A substance that brings about reduction. It is itself oxidized. A reducing agent is a donor of electrons.


Reduction: The removal of oxygen, the addition of hydrogen, the gain of electrons, or the decrease in the oxidation state of the substance.


Relative atomic mass: The number of times the mass of one atom of an element is heavier than 1/12 of the mass of a carbon-12 atom.


Relative molecular mass: The sum of the relative atomic masses of each of the atoms in one molecule of a substance.


Residue: The solid which remains on the filter paper after filtration.


Respiration: The slow combustion of food in the cells of living organisms to release energy.


Rusting: The slow oxidation of iron in the presence of air and water to form hydrated iron (III) oxide (rust).


Salt: The ionic compound formed by the replacement of one or more hydrogen ions of an acid by a metallic ion or an ammonium ion.


Saturated hydrocarbons: Hydrocarbons that contain only single bonds between carbon atoms.


Solute: The substance that dissolves in a solvent to form a solution.


Solvent: The liquid in which a solute dissolves.


Steel: An alloy of iron and carbon.


Structural formula: A formula which shows how the atoms are arranged in a molecule.


Sublimation: The process of changing from the solid state directly to the gaseous state without passing through the liquid state.


Suspension: A mixture of a liquid and an insoluble solid where the insoluble solid remains suspended throughout the solution.


Titration: The gradual addition of a solution from a burette to another solution in a conical flask until the chemical reaction between the two solutions is complete; the 2 solutions tend to be an acid and an alkali.


Unsaturated molecule: Any hydrocarbon that contains one or more carbon-carbon double bonds.


Valence electrons: Electrons in the outer shell that are used by the atom for forming chemical bonds.


Water of crystallization: Water molecules that are chemically bonded in the crystals of some salts.