Boiling point

The temperature at which a liquid boils, when the pressure of the gas created above the liquid equals atmospheric pressure.

Intermolecular forces

The weak attractive forces which act between molecules. These forces are much weaker than covalent bonds. When simple molecular substances melt or boil, it is these weak intermolecular forces that are overcome to allow the molecules to break loose and move.

Melting point

The temperature at which a solid turns into a liquid and which has the same value as the freezing point. A pure substance has a sharp melting point.

Relative molecular mass (Mr)

The sum of all the relative atomic masses of the atoms present in a molecule. The relative atomic mass (Ar) is the average mass of naturally occurring atoms of an element on a scale where the carbon-12 atom has a mass of exactly 12 units.

Capillary action

Capillary action is the ascension or movement of liquids through a slim tube, cylinder or permeable substance such as chromatography paper, due to adhesive and cohesive forces interacting between the liquid and the surface.

Centrifugation

The separation of an insoluble solid from a liquid by rapid spinning during which the solid collects at the bottom of the sample tubes – the liquid can then be decanted off carefully.

Condenser

This is the part of the distillation apparatus that removes heat from the system by continual pumping of cold water. The hot vapours condenser as they come into contact with the cold glass of the condenser.

Immiscible

Two or more liquids which do not mix together are immiscible. If two liquids form two layers when they are mixed then they are immiscible. This usually occurs for polar and non-polar substances e.g. oil and water.

Locating agent

A compound that reacts with invisible, colourless spots separated by chromatography to produce a coloured product which can be seen. Amino acids and other biological substances often require the use of locating agents.

Miscible

If two liquids form a completely uniform mixture when added together, they are said to be miscible.

Allotropes

Different atomic or molecular arrangements of the same element in the same physical state.

Atomic number (or proton number) (Z)

The number of protons in the nucleus of an atom; it is also the number of electrons present in an atom and determines the position of the element in the Periodic Table.

Metallic Lattice

Giant three-dimensional lattice structure of positive ions surrounded by a sea of delocalised electrons.

Nucleon number (mass number) (A)

The total number of protons and neutrons in the nucleus of an atom.

Polarity

Polarity refers to the distribution of electrons in a bond or molecule. In covalent substances where electrons are shared equally, there is an even distribution of electrons so the negative charge produced by the electrons is also spread out evenly. Covalent substances are thus nonpolar. This doesn´t occur in ionic substances as the electrons are transferred from one atom to another, not shared. Ionic substances thus tend to be polar.

Relative atomic mass (Ar)

The average mass of naturally occurring atoms of an element on a scale where the carbon 12 atom has a mass of exactly 12 units.

Relative formula / molecular mass (Mr)

The sum of all the relative atomic masses of the atoms present in a formula unit of a substance.

Ionic equation

This is an equation which shows only those ions and molecules that take part in the reaction.

Spectator ion

This is an ion which is present in a reaction mixture but which does not take part in the reaction.

Valency

Valency refers to the combining power of an atom and depends on the atom’s position on the Periodic Table. It can also be applied to groups of atoms. In ionic compounds the valency of each ion is the same as its charge. In covalent molecules the valency is the number of bonds that atom makes.

Binary compound

A compound consisting of only 2 elements e.g.: CO, HCl.

Cryolite

This is the compound sodium aluminium fluoride (Na3AlF6) which is a naturally occurring mineral found mainly in Greenland.

Bond energy

This is the energy required to break one mole of covalent bonds between two particular atoms and is measured in kJ per mole.

Calorimetry

The measurement of heat transfers in chemical reactions.

Enthalpy

This is the total amount of heat energy that a chemical system contains and considers both reactants and products.

Heat of combustion

The heat change which takes place when one mole of a substance is completely burnt in oxygen.

Activation energy

This is the minimum energy required for particles to react when they collide.

Anhydrous salts

Anhydrous means when a substance does not contain any water. Anhydrous salts are those that have lost their water of crystallisation, usually by heating, in which the salt becomes dehydrated.

Catalyst

A substance that speeds up a chemical reaction without being used up or chemically altered in the reaction.

Enzyme

Enzymes are biological catalysts, usually proteins.

Equilibrium position

The equilibrium position describes the relative concentrations of reactants and products at equilibrium.

Hydrated salt

Hydrated means when there is water present in a substance. Hydrated salts contain water of crystallisation which affects their molecular shape and colour.

Ionic equation

This is a simplified equation for a reaction involving ionic substances in which only those ions that actually take part in the reaction are shown.

Photochemical reaction

This is a reaction which is catalysed by light or needs light in order to occur.

Photosynthesis

The process where plants use sunlight to produce glucose and oxygen from carbon dioxide and water in the presence of chlorophyll.

Reversible reaction

A reaction in which the reactants combine to form products and the products can also react together to form the original reactants.

Spectator ions

Ions that are present in a chemical equation but do not take part in the overall reaction and do not change their physical or chemical status.

Water of crystallisation

This is the water that is stoichiometrically included in the structure of some salts during the crystallisation process. A common example is copper(II) sulfate which crystallises forming the salt copper(II) sulfate pentahydrate, CuSO4.5H2O. Water of crystallisation is indicated with a dot written in between the salt molecule and the surrounding water molecules.

Acid

A substance that dissolves in water, producing H+(aq) ions – a solution of an acid turns litmus red and has a pH below 7; in their reactions acids act as proton donors.

Acidic oxide

An oxide that reacts with a base to form a salt and water. When dissolved in water it forms a solution with a low pH.

Amphoteric oxide

An oxide that reacts with both acids and bases to form a salt and water.

Base (Alkali)

A substance that neutralises an acid, producing a salt and water as the only products; in their reactions, bases act as proton acceptors.

Basic oxide

An oxide that reacts with an acid to form a salt and water. When dissolved in water it forms a solution with a high pH.

Decomposition

A type of chemical reaction where a compound breaks down into simpler substances.

Dissociate

When molecules, usually ionic, split up into smaller particles or ions in aqueous solution. This is usually a reversible process and the particles can recombine under different conditions.

Flame test

A test for a particular metal cation by heating the sample of a compound in a blue Bunsen flame. Characteristic colours are produced indicating the identity of the metal in the compound.

Indicator

A substance which changes colour when added to acidic or alkaline solutions; for example, litmus or phenolphthalein.

Neutral oxide

An oxide which does not react with an acid or a base.

Precipitation reaction

A reaction in which a solid product is formed when two solutions are mixed together.

Water of crystallisation

This is the water that is stoichiometrically included in the structure of some salts during the crystallisation process. A common example is copper(II) sulfate which crystallises forming the salt copper(II) sulfate pentahydrate, CuSO4.5H2O. Water of crystallisation is indicated with a dot written in between the salt molecule and the surrounding water molecules.

Displacement reaction

A reaction in which a more reactive element displaces a less reactive element from a solution of its salt.

Electron (arrangement) configuration

A shorthand method of describing the arrangement of electrons within the energy levels of an atom; also referred to as electronic structure.

Electrostatic forces

Strong forces of attraction between particles with opposite charges – such forces are involved in ionic bonding.

Group

Vertical columns of the Periodic Table containing elements with similar properties; atoms of elements in the same Group have the same number of electrons in their outer energy levels.

Halide

Halide refers to the compound formed when a halogen combines with another atom, such as a metal. A halide ion is the negatively charged ion formed when a halogen atom accepts an electron.

Inert

An inert substance does not react chemically with any other substance.

Metalloid (semi-metal)

Element which shows some of the properties of metals and some of non-metals; for example, boron and silicon.

Oxidation state

A number given to show whether an element has been oxidised or reduced; the oxidation state of an ion is simply the charge on the ion.

Period

A horizontal row of the Periodic Table.

Periodic table

A table of elements arranged in order of increasing proton number (atomic number) to show the similarities of the chemical elements with related electron configurations.

Sublimation

The direct change of state from solid to gas or gas to solid: the liquid phase is bypassed.

Substitution reaction

A reaction in which an atom (or atoms) of a molecule is (are) replaced by different atom(s), without changing the molecule’s general structure.

Transition metal

Elements from the central region of the Periodic Table – they are hard, strong, dense metals that form compounds that are often coloured. They are also able to have more than one oxidation state.

Valence shell

This is the outer shell of electrons of an atom.

Alloy

A mixture of a metal with another metal or nonmetal.

Displacement reaction

A reaction in which a more reactive element displaces a less reactive element from a solution of its salt.

Galvanising

The protection of iron and steel objects by coating with a layer of zinc.

Reactivity series of metals

An order of reactivity, giving the most reactive metal first, based on results from a range of experiments involving metals reacting with oxygen, water, dilute hydrochloric acid and metal salt solutions.

Thermal decomposition

The breakdown of a compound due to heating, where two or more products are formed.

Thermal stability

The stability of a molecule at high temperatures. A molecule with high thermal stability is resistant at high temperatures and does not decompose.

Anhydrous

A substance is anhydrous when it contains absolutely no water.

Global warming

The warming of the atmosphere due to greenhouse gases trapping infrared radiation emitted from the Earth’s surface.

Greenhouse gases

These are gases which absorb infrared radiation emitted from the Earth’s surface, trapping the heat and leading to global warming.

Internal combustion engine

Petrol, diesel, oil or other fuel engines where the fuel is combusted inside the engine, the resulting explosive force being used to drive the engine pistons and generate motive power.

Contact process

The industrial process of making sulphuric acid using a vanadium(V) oxide catalyst and high temperature.

Dibasic acid (diprotic acid)

An acid that contains two replaceable hydrogen atoms per molecule of the acid; for example, sulfuric acid, H2SO4.

Addition reaction

A reaction in which a simple molecule adds across the carbon–carbon double bond of an alkene.

Amide link (or peptide link)

The link between monomers in a protein or nylon, formed by a condensation reaction between a carboxylic acid group on one monomer and an amine group on the next monomer.

Amino acids

Naturally occurring organic compounds which possess both an amino (—NH2) group and an acid (—COOH) group in the molecule; there are 20 naturally occurring amino acids and they are polymerised in cells to make proteins.

Branching

Branching occurs in an organic molecule when the longest chain is nonlinear as branches with groups of atoms or smaller chains sprout from the main chain.

Carbohydrates

General name given for simple and complex sugars having the general formula Cx(H2O)y.

Catalytic cracking

Simply the splitting of larger molecules to simpler ones.

Denature

Disrupt the characteristic properties of a protein or other biological molecule by excess heat, acidity, or pressure.

Enzymes

Protein molecules that act as biological catalysts.

Ester link

The link produced when an ester is formed from a carboxylic acid and an alcohol; also found in polyesters and in the esters present in fats and vegetable oils.

Fraction

A group of molecules with a defined boiling point range which distils off at the same place during fractional distillation.

Functional group

The atom or group of atoms responsible for the characteristic reactions of an organic compound.

Gradation

A series of small differences or changes in characteristics or a specific property.

Locating agent

A compound that reacts with invisible, colourless spots separated by chromatography to produce a coloured product which can be seen. Amino acids and other biological substances often require the use of locating agents.

Long-chain hydrocarbon

Large number of hydrocarbon molecules. More viscous and less flammable so less useful.

Monomer

A small molecule, such as ethene, which can be polymerised to make a polymer.

Polyamide

A polymer where the monomer units are joined together by amide (peptide) links; for example, nylon and proteins.

Polyester

A polymer where the monomer units are joined together by ester links; for example, Terylene.

Polymer

A substance consisting of very large molecules made by polymerising a large number of repeating units or monomers.

Polymerisation

The chemical reaction in which molecules (monomers) join together to form a long-chain polymer.

Proteins

Condensation polymers formed from amino acids which are joined by peptide (amide) groups.

Reflux

Refers to the technique involving the condensation of vapors and the return of this condensate to the reaction system, usually in the same vessel.

Saturated compound

A compound containing only carbon-carbon single bonds with no carbon-carbon double or triple bonds.

Short-chain hydrocarbons

Small numbers of hydrocarbon molecules which are more useful.

Unsaturated compound

Compounds that contain one or more carbon-carbon double or triple bonds.

Viscosity

The viscosity of a liquid is a description of its resistance to flow. A viscous liquid like honey for example flows much more slowly than a non-viscous liquid like ethanol.

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.