A Glossary of Chemical Terms

A

Acid: A chemical that contains hydrogen ions dissolved in water. Acids can be weak (e.g. ethanoic acid - the prime constituent of vinegar) or strong (e.g. sulphuric or nitric acid). Strong acids are corrosive and can be dangerous!

Alcohol: This usually refers to the chemical found in alcoholic drinks, which used to be called ethyl alcohol, but is now referred to as ethanol. However, alcohols form a family of chemicals of which ethanol is the second member (after methanol, the primary constituent of white spirit). All alcohols are compounds of carbon, hydrogen and oxygen and have a -OH group attached to a carbon atom somewhere.

Aldehyde: A family of compounds consisting of carbon, hydrogen and oxygen atoms only. There is a carbon atom joined to an oxygen atom with a double bond (C=O) at the end of the molecule.

Alkali: A chemical that dissolves in water and neutralises (i.e. cancels out the effects of) an acid. Alkalis include caustic soda (Sodium hydroxide) and sodium hydrogen carbonate (baking powder).

Alkane: A chemical compound consisting of nothing but carbon and hydrogen atoms. The carbon atoms are joined to each other by single covalent bonds only (C-C).

Alkene: A chemical compound consisting of nothing but carbon and hydrogen atoms. There is at least one double bond between carbon atoms (C=C)

Alpha (particle or radiation): A sub-atomic particle emitted from certain radioactive atoms when they decay. An alpha particle consists of two protons bonded to two neutrons.

Alum: A salt containing two types of metal ion, one of which has a single positive charge (such as Na⁺ or K⁺) and the other of which has three positive charges (such as Al³⁺ or Fe³⁺). An example of an alum is sodium aluminium sulphate, with the chemical formula NaAl(SO₄)₂. The sodium ion and aluminium ion together contribute 4 positive charges, and these are balanced by two sulphate ions (SO₄^2-), each of which contributes two negative charges.

Anode: The positive electrode in electrolysis. This is the point where electrons are taken out from an electrolyte.

Atomic number: Also called the Proton Number, this indicates the number of protons present in each type of atom. The atomic number of an element is the defining feature that distinguishes that element from all others.

Atom: The smallest particle of matter that can be obtained just by means of chemical reactions, consisting of a tiny electrons.

Avagadro constant (Avagadro number): The number of particles (atoms or molecules) present in one

B

Base: Any chemical that neutralises an acid. Soluble bases are called alkalis.

Beta (particle or radiation): A sub-atomic particle emitted from certain radioactive atoms when they decay. An beta particle consists of an electron formed when a neutron decays into a proton, and ejected from the nucleus of the atom at almost the speed of light!

Blast furnace: A furnace used in the industrial smelting of iron from iron ore.

C

Cathode: The negative electrode in electrolysis. This is the point where electrons are provided to the electrolyte.

Chemical equation: A system of writing down a reactants on the left side of the arrow and the Chromatography: The process of separating differently coloured compounds by some physical means, such as the different speeds at which they soak into white blotting paper. The coloured compounds "spread out" into bands or circles, and can then be identified. See also Gas Chromatography.

Coke: A form of carbon which is more pure than the form in which is usually found, i.e. coal. It is formed by heating lumps of coal in the absence of air in order to drive off the impurities.

Compound: Atoms of two or more types chemically bonded together. The atoms in a compound can only be separated using chemical reactions raher than by any purely physical process.

Covalent: This refers to a bond holding two atoms together formed when electrons orbit both atoms. Effectively, the atoms "share" electrons in order to make sure their outer shells have the correct number of electrons to be full. Usually covalent compounds include non-metallic elements such as carbon or nitrogen.

D

Diatomic: An element is said to be diatomic if its atoms are found in molecules consisting of two atoms each. Gases such as oxygen (O₂), nitrogren (N₂) and hydrogen (H₂) are diatomic when they are elements. Also, all halogens are diatomic: Fluorine, F₂, Chlorine, Cl₂, Bromine, B₂ and Iodine, I₂.

Dissociation: The splitting up of a molecule into charged ions. For instance, when a pure acid is added to water, its molecules dissociate into hydrogen ions and negative ions.

E

Electrode: Either of the two metal connections between an electrolyte and an electric circuit in electrolysis. The positive electrode is called the anode and the negative one the cathode.

Electrolysis: The process of breaking down a liquid chemically by passing an electric current through it.

Electrolyte: A liquid carrying charged particles that are free to move, and that decomposes chemically when an electric current is passed through it.

Electron: A sub-atomic particle that orbits the nucleus of all atoms. Electrons have virtually no mass at all, but a negative charge of -1 charge unit.

Endothermic: This applies to any reaction that absorbs heat.

Electron Shell: Electrons do not orbit the nucleus of an atom in a random manner. Instead they are arranged in specific orbits at particular distances from the nucleus. These are the electron shells.

Element: A substance formed of atoms of only one type. There are just over 100 known elements in the universe, listed in the Periodic Table.

Enthalpy change: The enthalpy is a measure of the chemical energy present in molecules. The enthalpy change is the amount of energy given off as heat, or absorbed as heat, during a reaction. See also exothermic and endothermic.

Equilibrium: A balance in a chemical system, usually the balance reached between products in a reversible reaction.

Exothermic: This applies to any reaction that gives off heat.

F

Fused: Molten

G

Gamma (radiation): A high-energy electromagnetic ray emitted from the nucleus of some types of radioactive atom when they decay.

Gas chromatography: The separation of a mixture of compounds into its separate components by injecting them into a moving gas stream. This stream is passed round a long tube and the compounds injected into it move at different speeds. The presence of the compounds is detected at the far end of the tube by a meter.

H

Half life: The time taken for a radioactive element to decay to half its initial mass.

Halogen: Any one of the five elements in Group VII of the Periodic Table, i.e. Fluorine, Chlorine, Bromine, Iodine or Astatine.

I

Ion: An atom or molecule that has become electrically charged, either by gaining electrons (in the case of a negative ion) or by losing them (in the case of a positive ion).

Indicator: A substance that is used to determine how acidic or alkaline a solution is by changing colour. The most common indicator is litmus paper, but others include methyl orange and universal indicator.

J

K

Ketone: A family of chemicals that are compounds of carbon, hydrogen and oxygen, and containing a carbon atom joined to an oxygen atom with a double bond (C=O). However, this double bond must not be at the end of the molecule, as that chemical would be known as an aldehyde.

L

Latent Heat: The heat that is absorbed by a chemical as it melts, evaporates or boils, or the heat that a chemical gives off when it condenses or freezes.

Litmus: An indicator made from a plant extract that turns blue when placed in an alkali solution and red in an acidic solution. Although litmus is actually a liquid, it is normally soaked into a type of paper similar to blotting paper, and called litmus paper.

M

Mixture: Two or more atoms mixed together but not chemically bonded to each other, or two or more compounds mixed together but not chemically bonded to each other. Unlike compounds, the constituent parts of a mixture can be separated by purely physical means (boiling, melting, sieving, filtering etc.)

Mole: Not a small furry animal! A mole refers to a large number, 6 x 10²³, which is the number of carbon atoms present in 12g of carbon.

Molecule: Two or more atoms, of the same type or of a different type, chemically joined together.

N

Neutron: A sub-atomic particle found in the nucleus of all atoms except hydrogen. It has a mass of 1 atomic mass unit, and a no electrical charge.

Nucleus: The central part of an atom, where most of the mass of the atom is concentrated.

O

Organic chemistry: The branch of chemistry that deals with complex compounds that the element carbon forms with other types of atom, such as hydrogen and/or oxygen.

P

Periodic Table: A list of all the chemical elements known to science, arranged into "groups" to show similarities between them.

pH (Hydrogen potential): A measure of how acidic or alkaline a solution is. It refers to the concentration of hydrogen ions in the solution, by taking the logarithm to base 10 of the concentration and removing the minus sign. In pure water (neutral, i.e. neither acidic nor alkaline), the concentration of hydrogen ions is 10^-7 moles of ions per litre. Taking the logarithm to base 10 gives -7, and removing the minus sign gives 7 as the pH of pure water.

Precipitate: A solid that is formed when a chemical that is insoluble comes out of soluble chemicals, both in solution. One of the Products: Chemicals which are formed as a result of a chemical reaction.

Proton: A sub-atomic particle found in the nucleus of all atoms. It has a mass of 1 atomic mass unit, and a charge of +1 charge unit.

Q

R

Relative Atomic Mass (RAM): The number of neutrons and protons together present in any atom. The (average) number of times heavier than a hydrogen atom that particular atom is.

Reactants: These are chemicals which react together in a chemical reaction to form products.

Reaction: A chemical process by which some chemicals (reactants) are turned into other chemicals (products). Chemical reactions are the basis of the whole of chemistry. Without them, this tutorial would be full of blank pages!

Reversible reaction: A reaction that can (and does!) proceed in both directions at the same time, i.e. while the reactants react together to form the products, the products are busy reacting to form the reactants. In practice, a balance (called an equilibrium) between products and reactants is reached.

S

soluble: Describing a chemical that can form a solution in a particular liquid. For example, sugar is soluble in water. The opposite of soluble is "insoluble".

solution: The mixture that is formed when a chemical dissolves in a liquid. For example, brine is a common term for a solution of common salt, i.e. sodium chloride dissolved in water.

Sublimation: A substance sublimes if, when heated, it turns directly from a solid to a gas without passing through the liquid phase first. Most substances don't sublime, but a few of them do, for instance, carbon dioxide (CO₂) and iodine (I₂). Although it is possible to get liquid carbon dioxide by heating the solid under immense pressure, when carbon dioxide reaches its melting point under normal air pressure, it turns straight into a gas. Similarly, when carbon dioxide or iodine gas are cooled, they pass straight from gaseous form into a solid without going through a liquid phase.

T

Titration: A process to determine the concentration (in moles/litre) of a solution by noting what volume of the solution reacts with another solution of a known volume and concentration. Typically, this involves calculating the concentration of an alkali (or acid) by finding what volume of it reacts with a known volume of acid (alkali) of known concentration.

U

Universal Indicator: An indicator which indicates a wide variety of pH values by displaying several different colours.

V

W

X, Y, Z

X-Rays: A type of electromagnetic radiation with a very short wavelength and high frequency. They are more penetrative than light or ultra-violet radiation (which is why radiologists use them to take X-rays of bones and the like), but can cause cell damage at high concentrations.