An acid is a substance that forms a solution with a pH value of less than 7. Acidic solutions contain an excess of hydrogen ions, H+(aq).

acid rain:

Acid rain is so called because it has a pH value below 5 due to acidic pollutant gases in the atmosphere.

activation energy:

The activation energy is the minimum amount of energy required for reactants to form products in a chemical reaction.


The alcohols are a family of organic compounds containing the −OH (hydroxide) grouping.


An alkali is a substance that dissolves in water to form a solution with a pH value greater than 7. Alkaline solutions contain an excess of hydroxide ions, OH-(aq).

alkali metals:

The alkali metals are the elements in Group 1 of the Periodic Table: lithium, sodium, potassium, rubidium, caesium, and francium.

alkaline earth metals:

The alkaline earth metals are the elements in Group 2 of the Periodic Table: beryllium, magnesium, calcium, strontium, barium, and radium.


An alkane is a saturated hydrocarbon; that is, it contains only single covalent bonds. Their general formula is CnH2N+2. For example, ethane is C2H6 and pentane is C5H5.


An alkene is an unsaturated hydrocarbon, that is a hydrocarbon whose molecules contain one or more double covalent bonds between its carbon atoms.


Allotropes are different forms of the same element. The difference is that the atoms of each allotrope are bonded together in different arrangements.


Alloys are mixtures of metals.

amphoteric oxide:

Amphoteric oxides react with both acidic and alkaline solutions.


An analgesic is a substance that acts as a painkiller, e.g. aspirin.


Anions are negatively charged ions (so called because they are attracted to the anode – the positive electrode – during electrolysis).


An anode is a positive electrode.


Anodizing is the process of coating an aluminium surface with a thicker than usual coating of aluminium oxide. This is done by making aluminium the anode (positive electrode) in an electrolytic cell.


Antacids are basic substances (such as magnesium hydroxide) used to neutralize excess acid in the stomach.


The asthenosphere is the region of the mantle that lies below the lithosphere and contains between 1 and 10 per cent molten material.


An atom is the smallest particle of an element that can still be defined as that element.

atomic number:

The atomic number is the number of protons in an atom (which equals the number of electrons).


balanced equation:

A balanced equation is a chemical reaction represented by the formulae of reactants and products, showing the same number of each type of atom before and after the reaction.
e.g. 2Mg(s)  +  O2(g)    2MgO(s)


A base is a compound that reacts with an acid to neutralize it, producing a salt and water. Ammonia and compounds containing a hydroxide group are examples of bases.


Bauxite is an ore of aluminium containing aluminium oxide.


The term 'biodegradable' is used to refer to plastics and other materials that can be decomposed naturally by bacteria, once they are thrown away.


Brine is a solution of sodium chloride (common salt)



Carbohydrates are compounds containing carbon, hydrogen, and oxygen. Many of them have the general formula Cx(H2O)y. They provide energy for the body. We find them in foods such as bread, pasta, rice (which contains starch) and sugary foods.

carboxylic acids:

The carboxylic acids are a family of organic compounds containing the –COOH grouping. They are examples of weak acids.


A catalyst is a substance that alters (usually speeds up) the rate of a chemical reaction, but remains chemically unchanged itself at the end of the reaction.

catalytic converter:

A catalytic converter is a honeycomb structure coated with precious metal catalysts, used in a car's exhaust to reduce air pollution.


A cathode is a negative electrode.


Cations are positively charged ions (so called because they are attracted to the cathode – the negative electrode – during electrolysis).

chemical change:

A chemical change is a reaction in which new substances are formed.

closed system:

A closed system is a system from which the products of reactions taking place within the system cannot escape.


A colloid is a finely dispersed mixture of two or more substances that do not dissolve in each other.


Combustion is the burning of a substance in air or oxygen to produce oxides.


A compound is a substance made up of more than one type of atom.


Corrosion is a reaction with oxygen, water, or acid that results in the disintegration of the material under attack.

covalent bond:

A covalent bond is a bond between atoms of non-metal elements formed by sharing one (or more) pairs of electrons.

crude oil:

Crude oil is a liquid formed from tiny sea creatures and plants that died millions of years ago. It contains a mixture of hydrocarbons. It is the raw material for many fuels and plastics.



Dehydration is the removal of H2O in a chemical reaction.


Electrons are said to be delocalized if they are not associated with one particular atom.


An enzyme is said to be denatured when its shape is disrupted (e.g. by the effect of heat or pH changes), resulting in its lack of activity.


A diatomic molecule is made of two atoms bonded together, e.g. Cl2.


Diffusion is the process whereby one substance moves through another. The substances become mixed without us needing to stir them together.

displacement reaction:

A displacement reaction is one in which a more reactive element takes the place of a less reactive element in one of its compounds.


Dissociation is the breaking of chemical bonds. For example, acidic molecules dissociate in water to form aqueous ions (one of which will be H+(aq))


Distillation is the process of separating a liquid from a mixture by evaporation followed by condensation.

double bond:

A double bond is a type of covalent bond formed between atoms by sharing two pairs of electrons, e.g. O2.

Down's cell:

A Down's cell is an electrolytic cell in which sodium is extracted from molten sodium chloride (mixed with calcium chloride to lower its melting point).


A ductile material can be drawn out into wire.

dynamic equilibrium:

Dynamic equilibrium is the position reached by a reversible reaction in a closed system when the rate of the forward reaction is the same as the rate of the reverse reaction.


E number:

E numbers are a way of classifying chemicals added to food (for example, colourings, preservatives, emulsifiers).


Electrolysis is the breakdown of a substance by electricity.


An electrolyte is a liquid that conducts electricity and is decomposed in the process, i.e. a liquid that undergoes electrolysis.


Electrons are tiny, negatively charged particles that orbit the nucleus of an atom in energy levels (or shells).

electronic configuration:

The electronic configuration is a description of the arrangement of the electrons in an atom, starting from the lowest energy level (shell). For example, the electronic structure of sodium is 2, 8, 1. (Also known as an electronic structure.)

electronic structure:

The electronic structure is a description of the arrangement of the electrons in an atom, starting from the lowest energy level (shell). For example, the electronic structure of sodium is 2, 8, 1. (Also known as an electronic configuration.)


Electroplating is the coating of a metal object by a thin layer of another metal(s) during electrolysis.


An element is a substance made from only one type of atom. An element cannot be broken down into any simpler substances.

empirical formula:

The empirical formula gives the simplest ratio of the number of moles of each element present in a compound.


A endothermic reaction absorbs energy from its surroundings, resulting in a fall in temperature.


An enzyme is a biological catalyst which speeds up reactions in living things. An enzyme is a protein molecule with a specific shape to accommodate reactant molecules.


Erosion is the wearing away of rock as a result of the movement of pieces of weathered rock.


Esterification is the reaction between an alcohol and a carboxylic acid (with a strong acid catalyst) which produces an ester plus water.


Eutrophication is the pollution of rivers and lakes caused by leaching of fertilizers, detergents, or sewage into the water.


An exothermic reaction gives out energy to its surroundings, resulting in a rise in temperature.



Fermentation is the breakdown of glucose (and other sugars) into alcohol (ethanol) and carbon dioxide by enzymes in yeast (in the absence of oxygen).


The fraction is the mixture of hydrocarbons collected between a certain range of temperatures in the fractional distillation of crude oil.

fuel cell:

A fuel cell is a cell in which the chemical energy stored in a fuel is converted directly into electrical energy.



Galvanizing is a way of protecting iron from rusting by coating it with zinc metal.

greenhouse gas:

A greenhouse gas is a gas, such as carbon dioxide, in the Earth's atmosphere that absorbs infrared radiation and contributes to global warming.


Groups are the vertical columns in the Periodic Table, consisting of elements with similar properties (chemical 'families').



Haematite is a mineral composed of iron (III) oxide. It is the major ore of iron.


A half-equation describes the reaction at an electrode during electrolysis (or during any reduction/oxidation reaction) that includes the electrons involved, e.g. Na+ + e Na.


Halides are ionic salts of the Group 7 elements, e.g. sodium fluoride, potassium iodine.


The halogens are the elements of Group 7 in the Periodic Table: fluorine, chlorine, bromine, iodine, and astatine.

homologous series:

A homologous series is a family of organic molecules all containing the same groupings of atoms (for example, the alcohols, which all contain the –OH group).


Hydration is the addition of H2O to a molecule in a chemical reaction.


A hydrocarbon is a compound containing only hydrogen and carbon.


Hydrolysis is the breakdown of a compound by water.


igneous rock:

Igneous rock is rock formed when molten rock cools down and solidifies.


Liquids are said to be immiscible when they do not mix, for example oil and water.

incomplete combustion:

Incomplete combustion is the burning of a substance in a limited supply of air or oxygen.


'Inert' means unreactive.


An inhibitor is a substance that slows down a chemical reaction (a type of 'negative catalyst').

intermolecular forces:

Intermolecular forces (or bonds) are the forces between molecules.


An ion is a charged particle made when an atom (or group of atoms) loses or gains one or more electrons, e.g. Na+ and Cl.

ionic compound:

A ionic compound is a compound made up of oppositely charged ions bonded to each other in a giant lattice.

ionic equation:

An ionic equation is one which shows the atoms and ions that actually change in a reaction, e.g.
Zn(s)  +  Cu2+(aq)    Zn2+(aq)  +  Cu(s).


Isomers are molecules that have the same chemical formula but different arrangements of atoms. For example:


Isotopes are atoms of the same element with different numbers of neutrons. They have the same atomic number but different mass numbers.



Kroll process:

The Kroll process is a method of extracting titanium metal, using magnesium as the reducing agent.



Lava is molten rock that has erupted from the surface of the Earth.


The lithosphere is the Earth's crust and uppermost part of the mantle.



Magma is the name given to molten rocks beneath the Earth's surface.


A malleable material can be hammered into different shapes.


The mantle is the region inside the Earth that lies between the outer crust and the core.

mass number:

The mass number is the number of protons plus neutrons in the nucleus of an atom.

membrane cell:

A membrane cell is an electrolytic cell in which brine is electrolysed producing chlorine, hydrogen and forming sodium hydroxide solution (an alkali).


Metalloids are elements with properties of both metals and non-metals.

metamorphic rock:

Metamorphic rock is rock formed as a result of heat and/or pressure, changing the structure and/or mineral composition (without the rock melting).


A mineral is a naturally occurring solid that consists entirely of a single element or compound.


One mole is the number of carbon atoms in exactly 12 g of carbon (6.02 × 1023).

molecular formula:

The molecular formula gives the actual number of atoms of each element present in a molecule.


A molecule is a group of two or more atoms bonded together.


A monatomic gas is one that exists as single atoms.


Monomers are small, reactive molecules that react together to form very large molecules called polymers.



'Native' is the term used to describe metal that is found in nature as the metal itself.

noble gases:

The noble gases are the elements in Group 0 (sometimes called Group 8) in the Periodic Table: helium, neon, argon, krypton, xenon, and radon.


The nucleus is the centre of an atom, containing protons and neutrons.



An ore is a rock containing a metal or metal compound from which it is economically viable to extract the metal.


Oxidation is the loss of electrons from an atom or ion (or addition of oxygen in a reaction).


percentage composition:

The percentage composition is the proportions of each element present in a compound by mass (expressed as percentages).


The periods are the horizontal rows in the Periodic Table.

Periodic Table:

The Periodic Table is a table listing the elements in order of atomic number, arranged so that similar elements appear in columns.

pH value:

The pH value of a solution is a number on a scale indicating how strongly acidic or alkaline the solution is. The lower the pH value, the more acidic it is. pH values less than 7 are acidic, pH values over 7 are alkaline, and a pH value of 7 indicates a neutral solution.


Photosynthesis is the generation of glucose (and oxygen) from carbon dioxide and water in green plants. The reaction needs sunlight and chlorophyll to take place.

physical change:

A physical change is one in which no new substances are made, e.g. changes of state.

pig iron:

Pig iron is impure iron from a blast furnace.

polar molecule:

A polar molecule is a molecule with an imbalance of electrical charge.


Polymers are very large molecules made from repeating units formed when many small, reactive molecules (called monomers) join together.


A precipitate is an insoluble solid formed when two solutions react together.

preferential order of discharge:

The preferential order of discharge is a list of negatively charged ions arranged in the order that they are likely to be oxidized at the anode during electrolysis.


A product is a substance formed in a chemical reaction.



raw materials:

Raw materials are the starting materials for an industrial process, obtained from nature.


A reactant is a substance we start with before a chemical reaction.

Reactivity Series:

The Reactivity Series is a list of metals in order of their reactivity.

reducing agent:

A reducing agent is an element or compound that removes oxygen from a compound (or provides electrons in a reaction).


Reduction is the removal of oxygen from a compound (or the addition of electrons to an atom or ion).

relative atomic mass:

The relative atomic mass, Ar, is the mass of one mole of atoms of an element (taking into account the proportions of each isotope in a naturally occurring sample of the element).

relative formula mass:

The relative formula mass is the sum of the relative atomic masses of the elements indicated by a chemical formula. For example, the relative formula mass of Na2O = 2 × Ar(Na) + 1 × Ar(O). Symbol = Mr.

reversible reactions:

Reversible reactions are reactions in which reactants form products and the products can also react with each other to re-form the reactants.


Rusting is a reaction between iron (or steel) with oxygen and water resulting in corrosion.


sacrificial protection:

Sacrificial protection is the use of a metal more reactive than iron to protect an iron object from rusting.


Salts are compounds formed in neutralization reactions. They are ionic compounds, containing positive metal ions (or ammonium ions) bonded to negatively charged ions originating from an acid. Examples include sodium chloride, zinc nitrate, and copper sulfate.

sedimentary rock:

Sedimentary rock is rock formed from layers of sediment by compaction (pressure) and cementation (by insoluble minerals that bind the particles of sediment together).


Slag is the solid waste product from the extraction of iron and manufacture of steel.


A solvent is a liquid that dissolves another substance.


A sonorous material makes a 'ringing' sound when struck with a hard object.

spectator ions:

Spectator ions are ions that do not change in a reaction. They do not appear in the ionic equation for the reaction.

stainless steel:

Stainless steel does not rust. It has nickel and chromium added in its manufacture.

state symbol:

State symbols are used in chemical equations to show the state of reactants and products.

(s) = solid, (l) = liquid, (g) = gas
(aq) = dissolved in water

For example,
2Na(s)  +  2H2O(l)    2NaOH(aq)  +  H2(g)


Steel is a metal composed mainly of iron, with a little carbon and sometimes other metals, mixed in.


A substrate is a reactant molecule in an enzyme-catalysed reaction.


A suspension is a mixture between a liquid (usually water) and fine particles of insoluble solid spread throughout the liquid.



Tarnish is a dulling of a metal's shiny surface as a result of reactions at its surface between the metal and oxygen (and other gases in the air).

tectonic plates:

Tectonic plates are huge slabs of rock that make up the lithosphere (the Earth's crust and uppermost part of the mantle). They move at a rate of a few centimetres per year.

thermal decomposition:

Thermal decomposition is the breakdown of the compound by heat.


A thermoplastic substance softens when subjected to heat and can be remoulded into different shapes.


A thermoset plastic is heat resistant because of extensive cross-linking between its molecules by strong covalent bonds.


A thermosetting plastic is heat resistant because of extensive cross-linking between its molecules by strong covalent bonds.


A thermosoftening plastic softens when subjected to heat and can be remoulded into different shapes.


Titanium is a corrosion-resistant transition metal used in high performance alloys.


unsaturated hydrocarbon:

Unsaturated hydrocarbons are compounds made up of molecules containing carbon and hydrogen atoms only, with one or more double covalent bonds between their carbon atoms.




Weathering is the breakdown of rock in nature by physical, chemical, and biological means.

word equation:

A word equation is a way of describing what we start with and what is formed in a chemical equation, e.g. magnesium  +  oxygen    magnesium oxide.




Yeast is a fungus whose enzymes aid the breakdown of sugar (glucose) into alcohol (ethanol) and carbon dioxide in the absence of oxygen. It is used in the brewing and baking industries.