absolute zero : the lowest possible temperature (-273.15\u00b0C).\r\nabsorption :the process by which a substance is soaked up.\r\nacid : a substance that can give a proton to another substance. Acids are compounds, containing hydrogen, that can attack and dissolve many substances. Acids are described as weak or strong, dilute or concentrated, mineral or organic. Example: hydrochloric acid (HCl). An acid in water can react with a base to form a salt and water.\r\nacidic solution : a solution with a pH lower than 7.\r\nacidity : a general term for the strength of an acid in a solution.\r\nacid radical : the negative ion left behind when an acid loses a hydrogen ion. Example: Cl- in hydrochloric acid (HCl).\r\nacid salt : An acid salt contains at least one hydrogen ion and can behave as an acid in chemical reactions. Acid salts are produced under conditions that do not allow complete neutralisation of the acid. For example, sulphuric acid may react with a sodium compound to produce a normal sodium salt, sodium sulphate (Na2SO4), or it may retain some of the hydrogen, in which case it becomes the salt sodium hydrogen sulphate (NaHSO4).\r\nactinide series or actinide metals : a series of 15 similar radioactive elements between actinium and lawrencium. They are transition metals.\r\nactivated charcoal : a form of carbon, made up of tiny crystals of graphite, which is made by heating organic matter in the absence of air. It is then processed further to increase its pore space and therefore its surface area. Its surface area is about 2000 m2\/g. Activated charcoal readily adsorbs many gases and it is therefore widely used as a filter, for example, in gas masks.\r\nactivation energy : the energy required to make a reaction occur. The greater the activation energy of a reaction, the more its reaction rate depends on temperature. The activation energy of a reaction is useful because, if the rate of reaction is known at one temperature (for example, 100\u00b0C) then the activation energy can be used to calculate the rate of reaction at another temperature (for example, 400\u00b0C) without actually doing the experiment.\r\nadsorption : the process by which a surface adsorbs a substance. The substances involved are not chemically combined and can be separated. Example: the adsorption properties of activated charcoal.\r\nalchemy : the traditional 'art' of working with chemicals that prevailed through the Middle Ages. One of the main challenges for alchemists was to make gold from lead. Alchemy faded away as scientific chemistry was developed in the 17th century.\r\nalcohol : an organic compound which contains a hydroxyl (OH) group. Example: ethanol (CH3CH2OH), also known as ethyl alcohol or grain alcohol.\r\nalkali\/alkaline : a base in (aqueous) solution. Alkalis react with, or neutralise, hydrogen ions in acids and have a pH greater than 7.0 because they contain relatively few hydrogen ions. Example: aqueous sodium hydroxide (NaOH).\r\nalkali metals : a member of Group 1 of the Periodic Table. Example: sodium.\r\nalkaline cell (or battery) : a dry cell in which the electrolyte contains sodium or potassium hydroxide.\r\nalkaline earth metal : a member of Group 2 of the Periodic Table. Example: calcium.\r\nalkane : a hydrocarbon with no carbon-to-carbon multiple bonds. Example: ethane, C2H6.\r\nalkene : a hydrocarbon with at least one carbon-to-carbon double bond. Example: ethene, C2H4.\r\nalkyne : a hydrocarbon with at least one carbon-to-carbon triple bond. Example: ethyne, C2H2.\r\nallotropes : alternative forms of an element that differ in the way the atoms are linked. Example: white and red phosphorus.\r\nalloy : a mixture of a metal and various other elements. Example: brass is an alloy of copper and zinc.\r\namalgam : a liquid alloy of mercury with another metal.\r\nAmorphous : a solid in which the atoms are not arranged regularly (i.e. glassy). Compare crystalline.\r\namphoteric\u00a0 : a metal that will react with both acids and alkalis. Example: aluminium metal.\r\n\r\nanhydrous : lacking water; water has been removed, for example, by heating. Many hydrated salts are crystalline. (Opposite of anhydrous is hydrous or hydrated.) Example: copper(ii) sulphate can be anhydrous (CuSO4) or hydrated (CuSO4\u00b75H2O).\r\nanion : a negatively charged atom or group of atoms. Examples: chloride ion (Cl-), hydroxide ion (OH-).\r\nanode : the electrode at which oxidation occurs; the negative terminal of a battery or the positive electrode of an electrolysis cell.\r\nanodising : a process that uses the effect of electrolysis to make a surface corrosion resistant. Example: anodised aluminium.\r\nantacid : a common name for any compound that reacts with stomach acid to neutralise it. Example: sodium hydrogen carbonate, also known as sodium bicarbonate.\r\nanti-bumping granules : small glass or ceramic beads, designed to promote boiling without the development of large gas bubbles.\r\nantioxidant : a substance that reacts rapidly with radicals thereby preventing oxidation of some other substance.\r\naqueous : a solution in which the solvent is water. Usually used as 'aqueous solution'. Example: aqueous solution of sodium hydroxide (NaOH(aq)).\r\naromatic hydrocarbons :\r\ncompounds of carbon that have the benzene ring as part of their structure. Examples: benzene (C6H6), naphthalene (C10H8).\r\nThey are known as aromatic because of the strong pungent smell given off by benzene.\r\natmospheric pressure : the pressure exerted by the gases in the air. Units of measurement are kilopascals (kPa), atmospheres (atm), millimetres of mercury (mm Hg) and Torr. Standard atmospheric pressure is 100 kPa, 1atm, 760 mm Hg or 760 Torr.\r\natom : the smallest particle of an element; a nucleus and its surrounding electrons.\r\natomic mass : the mass of an atom measured in atomic mass units (amu). An atomic mass unit is equal to one-twelfth of the atom of carbon-12. Atomic mass is now more generally used instead of atomic weight.\r\nExample: the atomic mass of chlorine is about 35 amu.\r\natomic number : also known as proton number. The number of electrons or the number of protons in an atom. Example: the atomic number of gold is 79 and for carbon it is 4.\r\natomic structure : the nucleus and the arrangement of electrons around the nucleus of\r\nan atom.\r\natomic weight : a common term used to mean the average molar mass of an element. This is the mass per mole of atoms. Example: the atomic weight of chlorine is about 35 g\/mol.\r\nbase : a substance that can accept a proton from another substance. Example: aqueous ammonia (NH3(aq)). A base can react with an acid in water to form a salt and water.\r\nbasic salt : a salt that contains at least one hydroxide ion. The hydroxide ion can then behave as a base in chemical reactions. Example: the reaction of hydrochloric acid (HCl) with the base, aluminium hydroxide (Al(OH)3) can form two basic salts, Al(OH)2Cl and Al(OH)Cl2.\r\nBattery : a number of electrochemical cells placed in series.\r\nBauxite : a hydrated impure oxide of aluminium (Al2O3\u00b7xH2O, with the amount of water x being variable). It is the main ore used to obtain aluminium metal. The reddish-brown colour of bauxite is mainly caused by the iron oxide impurities it contains.\r\n\r\nbeehive shelf : an inverted earthenware bowl with a hole in the upper surface and a slot in the rim. Traditionally, the earthenware was brown and looked similar to a beehive, hence its name. A delivery tube passes through the slot and a gas jar is placed over the hole. This provides a convenient way to collect gas over water in a pneumatic trough.\r\nbell jar : a tall glass jar with an open bottom and a wide, stoppered neck that is used in conjunction with a beehive shelf and a pneumatic trough in some experiments involving gases. The name derives from historic versions of the apparatus, which resembled a bell in shape.\r\nblast furnace : a tall furnace charged with a mixture of iron ore, coke and limestone and used for the refining of iron metal. The name comes from the strong blast of air introduced during smelting.\r\nbleach : a substance that removes colour in stains on materials, either by oxidising or reducing the staining compound. Example: sulphur dioxide (SO2).\r\nblock : one of the main divisions of the Periodic Table. Blocks are named for the outermost, occupied electron shell of an element. Example: The Transition Metals all belong to the d-block.\r\nboiling point : the temperature at which a liquid boils, changing from a liquid to a gas. Boiling points change with atmospheric pressure. Example: The boiling point of pure water at standard atmospheric pressure is 100 \u00b0C.\r\nboiling tube : A thin glass tube closed at one end and used for chemical tests, etc. The composition and thickness of the glass is such that it cannot sustain very high temperatures and is intended for heating liquids to boiling point.\r\nbond : chemical bonding is either a transfer or sharing of electrons by two or more atoms. There are a number of types of chemical bond, some very strong (such as covalent and ionic bonds), others weak (such as hydrogen bonds). Chemical bonds form because the linked molecule is more stable than the unlinked atoms from which it formed. Example: the hydrogen molecule (H2) is more stable than single atoms of hydrogen, which is why hydrogen gas is always found as molecules of two hydrogen atoms.\r\nBoyle's Law : at constant temperature, and for a given mass of gas, the volume of the gas (V) is inversely proportional to pressure that builds up (P): P 1\/V.\r\nbrine : a solution of salt (sodium chloride, NaCl) in water.\r\nB\u00fcchner flask : a thick-walled side-arm flask designed to withstand the changes in pressure that occur when the flask is connected to a suction pump.\r\nB\u00fcchner funnel : a special design of plastic or ceramic funnel which has a flat stage on which a filter paper can be placed. It is intended for use under suction with a B\u00fcchner flask.\r\nbuffer (solution) : a mixture of substances in solution that resists a change in the acidity or alkalinity of the solution when small amounts of an acid or alkali are added.\r\nburette : a long, graduated glass tube with a tap at one end. A burette is used vertically, with the tap lowermost. Its main use is as a reservoir for a chemical during titration.\r\nburn : a combustion reaction in which a flame is produced. A flame occurs where gases combust and release heat and light. At least two gases are therefore required if there is to be a flame. Example: methane gas (CH4) burns in oxygen gas (O2) to produce carbon dioxide (CO2) and water (H2O) and give out heat and light.\r\ncalorimeter : an insulated container designed to prevent heat gain or loss with the environment and thus allow changes of temperature within reacting chemicals to be measured accurately. It is named after the old unit of heat, the calorie.\r\ncapillary : a very small diameter (glass) tube. Capillary tubing has a small enough diameter to allow surface tension effects to retain water within the tube.\r\ncapillary action : the tendency for a liquid to be sucked into small spaces, such as between objects and through narrow-pore tubes. The force to do this comes from surface tension.\r\ncarbohydrate : a compound containing only carbon, hydrogen and oxygen. Carbohydrates have the formula Cn(H2O)n, where n is variable. Example: glucose (C6H12O6).\r\n\r\ncarbonate : a salt of carbonic acid. Carbonate ions have the chemical formula CO32-. Examples: calcium carbonate CaCO3 and sodium carbonate Na2CO3.\r\ncatalyst : a substance that speeds up a chemical reaction, but itself remains unaltered at the end of the reaction. Example: copper in the reaction of hydrochloric acid with zinc.\r\ncatalytic converter : a device incorporated into some exhaust systems. The catalytic converter contains a framework and\/or granules with a very large surface area and coated with catalysts that convert the pollutant gases passing over them into harmless products.\r\ncathode : the electrode at which reduction occurs; the positive terminal of a battery or the negative electrode of an electrolysis cell.\r\ncathodic protection : the technique of protecting a metal object by connecting it to a more readily oxidisable metal. The metal object being protected is made into the cathode of a cell. Example: iron can be protected by coupling it with magnesium. Iron forms the cathode and magnesium the anode.\r\ncation : a positively charged ion. Examples: calcium ion (Ca2+), ammonium ion (NH4+).\r\ncaustic : a substance that can cause burns if it touches the skin. Example:\u00a0 Sodium hydroxide, caustic soda (NaOH).\r\ncell : a vessel containing two electrodes and an electrolyte that can act as an electrical conductor.\r\nCelsius scale (\u00b0C) : a temperature scale on which the freezing point of water is at 0 degrees and the normal boiling point at standard atmospheric pressure is 100 degrees.\r\ncentrifuge : an instrument for spinning small samples very rapidly. The fast spin causes the components of a mixture that have a different density to separate. This has the same effect as filtration.\r\nceramic : a material based on clay minerals which has been heated so that it has chemically hardened.\r\nchalcogens : the members of Group 6 of the Periodic Table: oxygen, sulphur, selenium and tellurium. The word comes from the Greek meaning 'brass giver', because all these elements are found in copper ores, and copper is the most important metal in making brass.\r\nchange of state : a change between two of the three states of matter, solid, liquid and gas. Example: when water evaporates it changes from a liquid to a gaseous state.\r\nCharles's Law : the volume (V) of a given mass of gas at constant pressure is directly proportional to its absolute temperature (T): V T.\r\nchromatography : a separation technique using the ability of surfaces to adsorb substances with different strengths. The substances with the least adherence to the surface move faster and leave behind those that adhere more strongly.\r\n\u00a0coagulation : a term describing the tendency of small particles to stick together in clumps.\r\ncoherent : meaning that a substance holds together or sticks together well, and without holes or other defects. Example: Aluminium appears unreactive because, as soon as new metal is exposed to air, it forms a very complete oxide coating, which then stops further reaction occurring.\r\ncoinage metals : the elements copper, silver and gold, used to make coins.\r\ncoke : a solid substance left after the gases have been extracted from coal.\r\nColloid : a mixture of ultramicroscopic particles dispersed uniformly through a second substance to form a suspension which may be almost like a solution or may set to a jelly (gel). The word comes from the Greek for glue.\r\ncolorimeter : an instrument for measuring the light-absorbing power of a substance. The absorption gives an accurate indication of the concentration of some coloured solutions.\r\ncombustion : a reaction in which an element or compound is oxidised to release energy. Some combustion reactions are slow, such as the combustion of the sugar we eat to provide our energy. If the combustion results in a flame, it is called burning. A flame occurs where gases combust and release heat and light. At least two gases are therefore required if there is to be a flame. Example: the combustion or burning of methane gas (CH4) in oxygen gas (O2) produces carbon dioxide (CO2) and water (H2O) and gives out heat and light. Some combustion reactions produce light and heat but do not produce flames. Example: the combustion of carbon in oxygen produces an intense red-white light but no flame.\r\ncombustion spoon : also known as a deflagrating spoon, it consists of a long metal handle with a small cup at the end. Its purpose is to allow the safe introduction of a (usually heated) substance into a gas jar filled with gas, when the reaction is likely to be vigorous. Example: the introduction of a heated sodium pellet into a gas jar containing chlorine.\r\ncompound : a chemical consisting of two or more elements chemically bonded together. Example: Calcium can combine with carbon and oxygen to make calcium carbonate (CaCO3), a compound of all three elements.\r\ncondensation : the formation of a liquid from a gas. This is a change of state, also called a phase change.\r\n\r\ncondensation nuclei : microscopic particles of dust, salt and other materials suspended in the air, that attract water molecules. The usual result is the formation of water droplets.\r\ncondensation polymer : a polymer formed by a chain of reactions in which a water molecule is eliminated as every link of the polymer is formed. Examples:polyesters, proteins, nylon.\r\nconduction : (i) the exchange of heat (heat conduction) by contact with another object, or (ii) allowing the flow of electrons (electrical conduction).\r\nconductivity : the ability of a substance to conduct. The conductivity of a solution depends on there being suitable free ions in the solution. A conducting solution is called an electrolyte. Example: dilute sulphuric acid.\r\nconvection : the exchange of heat energy with the surroundings produced by the flow of a fluid due to being heated or cooled.\r\ncorrosion : the oxidation of a metal. Corrosion is often regarded as unwanted and is more generally used to refer to the slow decay of a metal resulting from contact with gases and liquids in the environment. Example: Rust is the corrosion of iron.\r\ncorrosive : causing corrosion.\r\ncovalent bond : this is the most common form of strong chemical bonding and occurs when two atoms share electrons. Example: oxygen (O2)\r\ncracking : breaking down complex molecules into simpler compounds, as in oil refining.\r\ncrucible : a small bowl with a lip, made of heat-resistant white glazed ceramic. It is used for heating substances using a Bunsen flame.\r\ncrude oil : a chemical mixture of petroleum liquids. Crude oil forms the raw material for an oil refinery.\r\ncrystal : a substance that has grown freely so that it can develop external faces. Compare crystalline, where the atoms are not free to form individual crystals and amorphous, where the atoms are arranged irregularly.\r\ncrystalline : a solid in which the atoms, ions or molecules are organised into an orderly pattern without distinct crystal faces. Examples: copper(ii) sulphate, sodium chloride. Compare amorphous.\r\ncrystallisation : the process in which a solute comes out of solution slowly and forms crystals.\r\ncrystal systems : seven patterns or systems into which all crystals can be grouped: cubic, hexagonal, rhombohedral, tetragonal, orthorhombic, monoclinic and triclinic.\r\ncubic crystal system : groupings of crystals that look like cubes.\r\ncurrent : an electric current is produced by a flow of electrons through a conducting solid or ions through a conducting liquid. The rate of supply of this charge is measured in amperes (A).\r\ndecay (radioactive decay) : the way that a radioactive element changes into another element due to loss of mass through radiation. Example: uranium 238 decays with the loss of an alpha particle to form thorium 234.\r\ndecomposition :the break down of a substance (for example, by heat or with the aid of a catalyst) into simpler components. In such a chemical reaction only one substance is involved. Example: hydrogen peroxide (H2O2(aq)) into oxygen (O2(g)) and water (H2O(l)).\r\ndecrepitation : when, as part of the decomposition of a substance, cracking sounds are also produced. Example: heating of lead nitrate (Pb(NO3)2).\r\ndehydration : the removal of water from a substance by heating it, placing it in a dry atmosphere or using a drying (dehydrating) reagent such as concentrated sulphuric acid.\r\ndensity : the mass per unit volume (e.g. g\/cm3).\r\ndesalinisation : the removal of all the salts from sea water, by reverse osmosis or heating the water and collecting the distillate. It is a very energy-intensive process.\r\ndesiccant : a substance that absorbs water vapour from the air. Example: silica gel.\r\ndesiccator : a glass bowl and lid containing a shelf. The apparatus is designed to store materials in dry air. A desiccant is placed below the shelf and the substance to be dried is placed on the shelf. The lid makes a gas-tight joint with the bowl.\r\ndestructive distillation : the heating of a material so that it decomposes entirely to release all of its volatile components. Destructive distillation is also known as pyrolysis.\r\ndetergent : a chemical based on petroleum that removes dirt.\r\nDevarda's alloy : zinc with a trace of copper, which acts as a catalyst for reactions with the zinc.\r\ndiaphragm : a semipermeable membrane - a kind of ultrafine mesh filter - that allows only small ions to pass through. It is used in the electrolysis of brine.\r\ndiffusion : the slow mixing of one substance with another until the two substances are evenly mixed. Mixing occurs because of differences in concentration within the mixture. Diffusion works rapidly with gases, very slowly with liquids.\r\n\r\ndiffusion combustion : the form of combustion that occurs when two gases only begin to mix during ignition. As a result the flame is hollow and yellow in colour. Example: a candle flame.\r\ndilute acid : an acid whose concentration has been reduced in a large proportion of water.\r\ndisinfectant : a chemical that kills bacteria and other microorganisms.\r\ndisplacement reaction : a reaction that occurs because metals differ in their reactivity. If a more reactive metal is placed in a solution of a less reactive metal compound, a reaction occurs in which the more reactive metal displaces the metal ions in the solution. Example: when zinc metal is introduced into a solution of copper(ii) sulphate (which thus contains copper ions), zinc goes into solution as zinc ions, while copper is displaced from the solution and forced to precipitate as metallic copper.\r\ndissociate : to break bonds apart. In the case of acids, it means to break up, forming hydrogen ions. This is an example of ionisation. Strong acids dissociate completely. Weak acids are not completely ionised, and a solution of a weak acid has a relatively low concentration of hydrogen ions.\r\ndissolve :\u00a0 to break down a substance in a solution without causing a reaction.\r\ndistillation : the process of separating mixtures by condensing the vapours through cooling.\r\ndistilled water : distilled water is nearly pure water and is produced by distillation of tap water. Distilled water is used in the laboratory in preference to tap water because the distillation process removes many of the impurities in tap water that may influence the chemical reactions for which the water is used.\r\nDreschel bottle : a tall bottle with a special stopper, designed to allow a gas to pass through a liquid. The stopper contains both inlet and outlet tubes. One tube extends below the surface of the liquid so that the gas has to pass through the liquid before it can escape to the outlet tube.\r\ndropper funnel : a special funnel with a tap to allow the controlled release of a liquid. Also known as a dropping funnel or tap funnel.\r\ndye : a coloured substance that will stick to another substance so that both appear coloured.\r\neffervesce : to give off bubbles of gas.\r\neffloresce : to lose water and turn to a fine powder on exposure to the air. Example: Sodium carbonate on the rim of a reagent bottle stopper.\r\nelectrical potential : the energy produced by an electrochemical cell and measured by the voltage or electromotive force (emf).\r\nelectrochemical cell : a cell consisting of two electrodes and an electrolyte. It can be set up to generate an electric current (usually known as a galvanic cell, an example of which is a battery), or an electric current can be passed through it to produce a chemical reaction (in which case it is called an electrolytic cell and can be used to refine metals or for electroplating).\r\nelectrochemical series : the arrangement of substances that are either oxidising or reducing agents in order of strength as a reagent, for example, with the strong oxidising agents at the top of the list and the strong reducing agents at the bottom.\r\nelectrode : a conductor that forms one terminal of a cell.\r\nelectrolysis : an electrical-chemical process that uses an electric current to cause the break-up of a compound and the movement of metal ions in a solution. The process happens in many natural situations (as for example in rusting) and is also commonly used in industry for purifying (refining) metals or for plating metal objects with a fine, even metal coating.\r\nelectrolyte : an ionic solution that conducts electricity.\r\nelectromotive force (emf) : the force set up in an electric circuit by a potential difference.\r\nelectron : a tiny, negatively charged particle that is part of an atom. The flow of electrons through a solid material such as a wire produces an electric current.\r\nelectron configuration : the pattern in which electrons are arranged in shells around the nucleus of an atom. Example: chlorine has the configuration 2, 8, 7.\r\nelectroplating : depositing a thin layer of a metal on to the surface of another substance using electrolysis.\r\nelement : a substance that cannot be decomposed into simpler substance by chemical means. Examples: calcium, iron, gold.\r\nemulsion : tiny droplets of one substance dispersed in another. One common oil in water emulsion is called milk. Because the tiny droplets tend to come together, another stabilising substance is often needed. Soaps and detergents are such agents, wrapping the particles of grease and oil in a stable coat. Photographic film is an example of a solid emulsion.\r\nendothermic reaction : a reaction that takes in heat. Example: when ammonium chloride is dissolved in water.\r\nend point : the stage in a titration when the reaction between the titrant (added from a burette) and the titrate (in the flask) is complete. The end point is normally recognised by use of an indicator which has been added to the titrate. In an acid-base reaction this is also called the neutralisation point.\r\n\r\nenzyme : biological catalysts in the form of proteins in the body that speed up chemical reactions. Every living cell contains hundreds of enzymes that help the processes of life continue.\r\nester : organic compounds formed by the reaction of an alcohol with an acid and which often have a fruity taste. Example: ethyl acetate (CH3COOC2H5).\r\nevaporation : the change of state of a liquid to a gas. Evaporation happens below the boiling point and is used as a method of separating the materials in a solution.\r\nexcess, to : if a reactant has been added to another reactant in excess, it has exceeded the amount required to complete the reaction.\r\nexothermic reaction : a reaction that gives out substantial amounts of heat.\u00a0 Example: sucrose and concentrated sulphuric acid.\r\nexplosive : a substance which, when a shock is applied to it, decomposes very rapidly, releasing a very large amount of heat and creating a large volume of gases as a shock wave.\r\nfats : semisolid, energy-rich compounds derived from plants or animals, made of carbon, hydrogen and oxygen. These are examples of esters.\r\nferment : to break down a substance by microorganisms in the absence of oxygen. Example: fermentation of sugar to ethanol during the production of alcoholic drinks.\r\nfiltrate : the liquid that has passed through a filter.\r\nfiltration : the separation of a liquid from a solid using a membrane with small holes (i.e. a filter paper).\r\nflame : a mixture of gases undergoing burning. A solid or liquid must produce a gas before it can react with oxygen and burn with a flame.\r\nflammable (also inflammable) : able to burn (in air). Opposite: non-flammable.\r\nflocculation : the grouping together of small particles in a suspension to form particles large enough to settle out as a precipitate. Flocculation is usually caused by the presence of a flocculating agent. Example: calcium ions are the flocculating agent for suspended clay particles.\r\nfluid : able to flow; either a liquid or a gas.\r\nfluorescent : a substance that gives out visible light when struck by invisible waves, such as ultraviolet rays.\r\nflux : a material used to make it easier for a liquid to flow. A flux dissolves metal oxides and so prevents a metal from oxidising while being heated.\r\nfoam : a substance that is sufficiently gelatinous to be able to contain bubbles of gas. The gas bulks up the substance, making it behave as though it were semirigid.\r\nfossil fuels : hydrocarbon compounds that have been formed from buried plant and animal remains. High pressures and temperatures lasting over millions of years are required. Examples: The fossil fuels are coal, oil and natural gas.\r\nfraction : a group of similar components of a mixture. Example: In the petroleum industry the light fractions of crude oil are those with the smallest molecules, while the medium and heavy fractions have larger molecules.\r\nfractional distillation : the separation of the components of a liquid mixture by heating them to their boiling points.\r\nfractionating column : a glass column designed to allow different fractions to be separated when they boil. In industry, it may be called a fractionating tower.\r\nfree radical : a very reactive atom or group with a 'spare' electron.\u00a0 Example: methyl, CH3\u00b7.\r\nfreezing point : the temperature at which a substance undergoes a phase change from a liquid to a solid. It is the same temperature as the melting point.\r\nfuel : a concentrated form of chemical energy. The main sources of fuels (called fossil fuels because they were formed by geological processes) are coal, crude oil and natural gas.\r\nfuel rods : the rods of uranium or other radioactive material used as a fuel in nuclear power stations.\r\nfume chamber or fume cupboard : a special laboratory chamber fitted with a protective glass shield and containing a powerful extraction fan to remove toxic fumes.\r\nfuming : an unstable liquid that gives off a gas. Very concentrated acid solutions are often fuming solutions. Example: fuming nitric acid.\r\ngalvanising : applying a thin zinc coating to protect another metal.\r\ngamma rays : waves of radiation produced as the nucleus of a radioactive element rearranges itself into a tighter cluster of protons and neutrons. Gamma rays carry enough energy to damage living cells.\r\ngangue : the unwanted material in an ore.\r\ngas\/gaseous phase : a form of matter in which the molecules form no definite shape and are free to move about to uniformly fill any vessel they are put in. A gas can easily be compressed into a much smaller volume.\r\ngas syringe : a glass syringe with a graduated cylinder designed to collect and measure small amounts of gases produced during an experiment.\r\ngelatinous precipitate : a precipitate that has a jelly-like appearance. Example: iron(iii) hydroxide. Because a gelatinous precipitate is mostly water, it is of a similar density to water and will float or lie suspended in the liquid.\r\n\r\nglass : a transparent silicate without any crystal growth. It has a glassy lustre and breaks with a curved fracture. Note that some minerals have all these features and are therefore natural glasses. Household glass is a synthetic silicate.\r\nglucose : the most common of the natural sugars (C6H12O6). It occurs as the polymer known as cellulose, the fibre in plants. Starch is also a form of glucose.\r\ngranular precipitate : a precipitate that has a grain-like appearance. Example: lead(ii) hydroxide.\r\ngravimetric analysis : a quantitative form of analysis in which the mass (weight) of the reactants and products is measured.\r\ngroup : a vertical column in the Periodic Table. There are eight groups in the table. Their numbers correspond to the number of electrons in the outer shell of the atoms in the group. Example: Group 2 contains beryllium, magnesium, calcium, strontium, barium and radium.\r\nGreenhouse Effect : an increase in the global air temperature as a result of heat released from burning fossil fuels being absorbed by carbon dioxide in the atmosphere.\r\nGreenhouse gas : any of the various gases that contribute to the Greenhouse Effect. Example: carbon dioxide.\r\nhalf-life : the time it takes for the radiation coming from a sample of a radioactive element to decrease by half.\r\nhalide : a salt of one of the halogens.\r\nhalogen : one of a group of elements including chlorine, bromine, iodine and fluorine in Group 7 of the Periodic Table.\r\nheat : the energy that is transferred when a substance is at a different temperature to that of its surroundings.\r\nheat capacity : the ratio of the heat supplied to a substance, compared with the rise in temperature that is produced.\r\nheat of combustion : the amount of heat given off by a mole of a substance during combustion. This heat is a property of the substance and is the same no matter what kind of combustion is involved. Example: heat of combustion of carbon is 94.05 kcal (x 4.18 = 393.1 kJ).\r\nhydrate : a solid compound in crystalline form that contains water molecules. Hydrates commonly form when a solution of a soluble salt is evaporated. The water that forms part of a hydrate crystal is known as the 'water of crystallisation'. It can usually be removed by heating, leaving an anhydrous salt.\r\nhydration : the process of absorption of water by a substance. In some cases hydration makes the substance change colour; in many other cases there is no colour change, simply a change in volume. Example: dark blue hydrated copper(ii) sulphate (CuSO4\u00b75H2O) can be heated to produce white anhydrous copper(ii) sulphate (CuSO4).\r\n\r\nhydride : a compound containing just hydrogen and another element, most often a metal. Examples: water (H2O), methane (CH4) and phosphine (PH3).\r\nhydrous : hydrated with water.\r\nhydrocarbon : a compound in which only hydrogen and carbon atoms are present. Most fuels are hydrocarbons, as is the simple plastic, polyethene. Example:methane CH4.\r\nhydrogen bond : a type of attractive force that holds one molecule to another. It is one of the weaker forms of intermolecular attractive force. Example:hydrogen bonds occur in water.\r\nignition temperature : the temperature at which a substance begins to burn.\r\nimmiscible : will not mix with another substance. e.g. oil and water.\r\nincandescent : glowing or shining with heat. Example: tungsten filament in an incandescent light bulb.\r\nincomplete combustion : combustion in which only some of the reactant or reactants combust, or the products are not those that would be obtained if all the reactions went to completion. It is uncommon for combustion to be complete and incomplete combustion is more frequent. Example: incomplete combustion of carbon in oxygen produces carbon monoxide and not carbon dioxide.\r\nindicator (acid-base indicator) : a substance or mixture of substances used to test the acidity or alkalinity of a substance. An indicator changes colour depending on the acidity of the solution being tested. Many indicators are complicated organic substances. Some indicators used in the laboratory include Universal Indicator, litmus, phenolphthalein, methyl orange and bromothymol.\r\ninduction period : the time taken for a reaction to reach ignition temperature. During this period, no apparent reaction occurs, then the materials appear to undergo spontaneous combustion.\r\ninert : unreactive.\r\ninhibitor : a substance that prevents a reaction from occurring.\r\ninorganic substance : a substance that does not contain carbon and hydrogen. Examples: NaCl, CaCO3.\r\ninsoluble : a substance that will not dissolve.\r\nion : an atom, or group of atoms, that has gained or lost one or more electrons and so developed an electrical charge. Ions behave differently from electrically neutral atoms and molecules. They can move in an electric field, and they can also bind strongly to solvent molecules such as water. Positively charged ions are called cations; negatively charged ions are called anions. Ions can carry an electrical current through solutions.\r\nionic bond : the form of bonding that occurs between two ions when the ions have opposite charges. Example: sodium cations bond with chloride anions to form common salt (NaCl) when a salty solution is evaporated. Ionic bonds are strong bonds except in the presence of a solvent.\r\nionic compound : a compound that consists of ions. Example: NaCl.\r\n\r\nionisation : a process that creates ions.\r\nionise : to break up neutral molecules into oppositely charged ions or to convert atoms into ions by the loss of electrons.\r\nisotope : one of two or more atoms of the same element that have the same number of protons in their nucleus (atomic number), but which have a different number of neutrons (atomic mass). Example: carbon-12 and carbon-14.\r\nKipp's apparatus : a special piece of glassware consisting of three chambers, designed to provide a continuous and regulated production of gas by bringing the reactants into contact in a controlled way.\r\nlanthanide series or lanthanide metals : a series of 15 similar metallic elements between lanthanum and lutetium. They are transition metals and are also called rare earths.\r\nlatent heat : the amount of heat that is absorbed or released during the process of changing state between gas, liquid or solid. For example, heat is absorbed when a substance melts and it is released again when the substance solidifies.\r\nlattice : a regular arrangement of atoms, ions or molecules in a crystalline solid.\r\nleaching : the extraction of a substance by percolating a solvent through a material. Example: when water flows through an ore, some of the heavy metals in it may be leached out causing environmental pollution.\r\nLiebig condenser : a piece of glassware consisting of a sloping water-cooled tube. The design allows a volatile material to be condensed and collected.\r\nliquefaction : to make something liquid.\r\nliquid\/liquid phase : a form of matter that has a fixed volume but no fixed shape.\r\nlime (quicklime) : calcium oxide (CaO). A white, caustic solid, manufactured by heating limestone and used for making mortar, fertiliser or bleach.\r\nlimewater : an aqueous solution of calcium hydroxide, used especially to detect the presence of carbon dioxide.\r\nlitmus : an indicator obtained from lichens. Used as a solution or impregnated into paper (litmus paper), which is dampened before use. Litmus turns red under acid conditions and purple in alkaline conditions. Litmus is a crude indicator when compared with Universal Indicator.\r\nload (electronics) : an impedance or circuit that receives or develops the output of a cell or other power supply.\r\nlustre : the shininess of a substance.\r\nmalleable : able to be pressed or hammered into shape.\r\nmanometer : a device for measuring gas pressure. A simple manometer is made by partly filling a U-shaped rubber tube with water and connecting one end to the source of gas whose pressure is to be measured. The pressure is always relative to atmospheric pressure.\r\nmass : the amount of matter in an object. In everyday use the word weight is often used (somewhat incorrectly) to mean mass.\r\nmatter : anything that has mass and takes up space.\r\nmelting point : the temperature at which a substance changes state from a solid phase to a liquid phase. It is the same as freezing point.\r\nmembrane : a thin, flexible sheet. A semipermeable membrane has microscopic holes of a size that will selectively allow some ions and molecules to pass through but hold others back. It thus acts as a kind of filter. Example: a membrane used for osmosis.\r\nmeniscus : the curved surface of a liquid that forms in a small bore or capillary tube. The meniscus is convex (bulges upwards) for mercury and is concave (sags downwards) for water.\r\nmetal : a class of elements that is a good conductor of electricity and heat, has a metallic lustre, is malleable and ductile, forms cations and has oxides that are bases. Metals are formed as cations held together by a sea of electrons. A metal may also be an alloy of these elements. Example: sodium, calcium, gold.\r\nmetallic bonding : cations reside in a 'sea' of mobile electrons. It allows metals to be good conductors and means that they are not brittle.\r\nmetalloid : a class of elements intermediate in properties between metals and non-metals. Metalloids are also called semi-metals or semiconductors. Example:silicon, germanium, antimony.\r\nmicronutrient : an element that the body requires in small amounts. Another term is trace element.\r\nmineral : a solid substance made of just one element or compound. Example: calcite is a mineral because it consists only of calcium carbonate; halite is a mineral because it contains only sodium chloride.\r\nmineral acid : an acid that does not contain carbon and which attacks minerals. Hydrochloric, sulphuric and nitric acids are the main mineral acids.\r\nmiscible : capable of being mixed.\r\nmixing combustion : the form of combustion that occurs when two gases thoroughly mix before they ignite and so produce almost complete combustion.Example: when a Bunsen flame is blue.\r\nmixture : a material that can be separated into two or more substances using physical means. Example: a mixture of copper(ii) sulphate and cadmium sulphide can be separated by filtration.\r\nmolar mass : the mass per mole of atoms of an element. It has the same value and uses the same units as atomic weight. Example: molar mass of chlorine is 35.45 g\/mol.\r\nmole : 1 mole is the amount of a substance which contains Avagadro's number (6 x 10 23) of particles. Example: 1 mole of carbon-12 weighs exactly 12 g.\r\nmolecule : a group of two or more atoms held together by chemical bonds. Example: O2.\r\nmonoclinic system : a grouping of crystals that look like double-ended chisel blades.\r\nmonomer : a small molecule and building block for larger chain molecules or polymers ('mono' means one, 'mer' means part). Examples: tetrafluoroethene for teflon, ethene for polyethene.\r\nnative element : an element that occurs in an uncombined state. Examples: sulphur, gold.\r\nnative metal : a pure form of a metal, not combined as a compound. Native metal is more common in poorly reactive elements than in those that are very reactive. Examples: copper, gold.\r\nnet ionic reaction : the overall, or net, change that occurs in a reaction, seen in terms of ions.\r\nneutralisation : the reaction of acids and bases to produce a salt and water. The reaction causes hydrogen from the acid and hydroxide from the base to be changed to water. Example: hydrochloric acid reacts with, and neutralises, sodium hydroxide to form the salt sodium chloride (common salt) and water. The term is more generally used for any reaction in which the pH changes toward 7.0, which is the pH of a neutral solution.\r\nneutron : a particle inside the nucleus of an atom that is neutral and has no charge.\r\n\r\nnewton (N) : the unit of force required to give one kilogram an acceleration of one metre per second every second (1 ms-2).\r\nnitrate : a compound that includes nitrogen and oxygen and contains more oxygen than a nitrite. Nitrate ions have the chemical formula NO3-. Examples:sodium nitrate NaNO3 and lead nitrate Pb(NO3)2.\r\nnitrite : a compound that includes nitrogen and oxygen and contains less oxygen than a nitrate. Nitrite ions have the chemical formula NO2-. Example: sodium nitrite NaNO2.\r\nnoble gases : the members of Group 8 of the Periodic Table: helium, neon, argon, krypton, xenon, radon. These gases are almost entirely unreactive.\r\nnoble metals : silver, gold, platinum and mercury. These are the least reactive metals.\r\nnon-combustible : a substance that will not combust or burn. Example: carbon dioxide.\r\nnon-metal : a brittle substance that does not conduct electricity. Examples: sulphur, phosphorus, all gases.\r\nnormal salt : salts that do not contain a hydroxide (OH-) ion, which would make them basic salts, or a hydrogen ion, which would make them acid salts.Example: sodium chloride (NaCl).\r\nnucleus : the small, positively charged particle at the centre of an atom. The nucleus is responsible for most of the mass of an atom.\r\nopaque : a substance that will not transmit light so that it is impossible to see through it. Most solids are opaque.\r\nore : a rock containing enough of a useful substance to make mining it worthwhile. Example: bauxite, aluminium ore.\r\norganic acid : an acid containing carbon and hydrogen. Example: methanoic (formic) acid (HCOOH).\r\norganic chemistry : the study of organic compounds.\r\norganic compound (organic substance; organic material) : a compound (or substance) that contains carbon and usually hydrogen. (The carbonates are usually excluded.) Examples: methane (CH4), chloromethane (CH3Cl), ethene (C2H4), ethanol (C2H5OH), ethanoic acid (C2H3OOH), etc.\r\norganic solvent : an organic substance that will dissolve other substances. Example: carbon tetrachloride (CCl4).\r\nosmosis : a process whereby molecules of a liquid solvent move through a semipermeable membrane from a region of low concentration of a solute to a region with a high concentration of a solute.\r\noxidation : combination with oxygen or a reaction in which an atom, ion or molecule loses electrons to an oxidising agent. (Note that an oxidising agent does not have to contain oxygen.) The opposite of oxidation is reduction.\r\noxidation number (oxidation state) : the effective charge on an atom in a compound. An increase in oxidation number corresponds to oxidation, and a decrease to reduction. Shown in Roman numerals. Example: manganate(iv).\r\noxidation-reduction reaction (redox reaction) : reaction in which oxidation and reduction occurs; a reaction in which electrons are transferred. Example:copper and oxygen react to produce copper(ii) oxide. The copper is oxidised, and oxygen is reduced.\r\noxide : a compound that includes oxygen and one other element. Example: copper oxide (CuO).\r\noxidise : to combine with or gain oxygen or to react such that an atom, ion or molecule loses electrons to an oxidising agent.\r\noxidising agent : a substance that removes electrons from another substance being oxidised (and therefore is itself reduced) in a redox reaction. Example:chlorine (Cl2).\r\nozone : a form of oxygen whose molecules contain three atoms of oxygen. Ozone is regarded as a beneficial gas when high in the atmosphere because it blocks ultraviolet rays. It is a harmful gas when breathed in, so low level ozone which is produced as part of city smog is regarded as a form of pollution. The ozone layer is the uppermost part of the stratosphere.\r\npartial pressure : the pressure a gas in a mixture would exert if it alone occupied a flask. Example: oxygen makes up about a fifth of the atmosphere. Its partial pressure is therefore about a fifth of normal atmospheric pressure.\r\npascal : the unit of pressure, equal to one newton per square metre of surface.\r\npatina : a surface coating that develops on metals and protects them from further corrosion. Example: the green coating of copper carbonate that forms on copper statues.\r\npercolate : to move slowly through the pores of a rock.\r\nperiod : a row in the Periodic Table.\r\nPeriodic Table : a chart organising elements by atomic number and chemical properties into groups and periods.\r\npestle and mortar : a pestle is a ceramic rod with a rounded end, a mortar is a ceramic dish. Pestle and mortar are used together to pound or grind solids into fine powders.\r\nPetri dish : a shallow glass or plastic dish with a lid.\r\npetroleum : a natural mixture of a range of gases, liquids and solids derived from the decomposed remains of plants and animals.\r\npH : a measure of the hydrogen ion concentration in a liquid. Neutral is pH 7.0; numbers greater than this are alkaline; smaller numbers are acidic.\r\npH meter : a device that accurately measures the pH of a solution. A pH meter is a voltmeter that measures the electric potential difference between two electrodes (which are attached to the meter through a probe) when they are submerged in a solution. The readings are shown on a dial or digital display.\r\nphase : a particular state of matter. A substance may exist as a solid, liquid or gas and may change between these phases with addition or removal of energy.Examples: ice, liquid and vapour are the three phases of water. Ice undergoes a phase change to water when heat energy is added.\r\nphosphor : any material that glows when energised by ultraviolet or electron beams, such as in fluorescent tubes and cathode ray tubes. Phosphors, such as phosphorus, emit light after the source of excitation is cut off.\r\nThis is why they glow in the dark. By contrast, fluorescers, such as fluorite, only emit light while they are being excited by ultraviolet light or an electron beam.\r\nphotochemical smog : photochemical reactions are caused by the energy of sunlight. Photochemical smog is a mixture of tiny particles and a brown haze caused by the reaction of colourless nitric oxide from vehicle exhausts and oxygen of the air to form brown nitrogen dioxide.\r\nphoton : a parcel of light energy.\r\nphotosynthesis : the process by which plants use the energy of the Sun to make the compounds they need for life. In six molecules of carbon dioxide from the air combine with six molecules of water, forming one molecule of glucose (sugar) and releasing six molecules of oxygen back into the atmosphere.\r\npipe-clay triangle : a device made from three small pieces of ceramic tube which are wired together in the shape of a triangle. Pipe-clay triangles are used to support round-bottomed dishes when they are heated in a Bunsen flame.\r\npipette : a log, slender, glass tube used, in conjunction with a pipette filler, to draw up and then transfer accurately measured amounts of liquid.\r\nplastic (material) : a carbon-based substance consisting of long chains (polymers) of simple molecules. The word plastic is commonly restricted to synthetic polymers. Examples: polyvinyl chloride, nylon.\r\nplastic (property) : a material is plastic if it can be made to change shape easily. Plastic materials will remain in the new shape. (Compare with elastic, a property whereby a material goes back to its original shape.)\r\npneumatic trough : a shallow water-filled glass dish used to house a beehive shelf and a gas jar as part of the apparatus for collecting a gas over water.\r\npolar solvent : a solvent in which the atoms have partial electric charges.\u00a0 Example: water.\r\n\r\npolymer : a compound that is made of long chains by combining molecules (called monomers) as repeating units. ('Poly' means many, 'mer' means part.)Examples: polytetrafluoroethene or Teflon from tetrafluoroethene, Terylene from terephthalic acid and ethane-1,2-diol (ethylene glycol).\r\npolymerisation : a chemical reaction in which large numbers of similar molecules arrange themselves into large molecules, usually long chains. This process usually happens when there is a suitable catalyst present. Example: ethene gas reacts to form polyethene in the presence of certain catalysts.\r\npolymorphism : (meaning many shapes) the tendency of some materials to have more than one solid form. Example: carbon as diamond, graphite and buckminsterfullerene.\r\nporous : a material containing many small holes or cracks. Quite often the pores are connected, and liquids, such as water or oil, can move through them.\r\npotential difference : a measure of the work that must be done to move an electric charge from one point to the other in a circuit. Potential difference is measured in volts (V).\r\nprecious metal : silver, gold, platinum, iridium and palladium. Each is prized for its rarity.\r\nprecipitate : a solid substance formed as a result of a chemical reaction between two liquids or gases. Example: iron(iii) hydroxide is precipitated when sodium hydroxide solution is added to iron(iii) chloride.\r\npreservative : a substance that prevents the natural organic decay processes from occurring. Many substances can be used safely for this purpose, including sulphites and nitrogen gas.\r\nPressure : the force per unit area measured in pascals.\r\nproduct : a substance produced by a chemical reaction. Example: when the reactants copper and oxygen react, they produce the product, copper oxide.\r\nproton : a positively charged particle in the nucleus of an atom that balances out the charge of the surrounding electrons.\r\nproton number : this is the modern expression for atomic number.\r\npurify : to remove all impurities from a mixture, perhaps by precipitation, or filtration.\r\npyrolysis : chemical decomposition brought about by heat. Example: decomposition of lead nitrate.\r\npyrometallurgy : refining a metal from its ore using heat. A blast furnace or smelter is the main equipment used.\r\nquantitative : measurement of the amounts of constituents of a substance, for example by mass or volume.\r\nradiation : the exchange of energy with the surroundings through the transmission of waves or particles of energy. Radiation is a form of energy transfer that can happen through space; no intervening medium is required (as would be the case for conduction and convection).\r\nradical : an atom, molecule, or ion with at least one unpaired electron. Example: nitrogen monoxide (NO).\r\nradioactive : emitting radiation or particles from the nucleus of its atoms.\r\nradioactive decay : a change in a radioactive element due to loss of mass through radiation. For example, uranium decays (changes) to lead.\r\nreactant : a starting material that takes part in, and undergoes, change during a chemical reaction. Example: hydrochloric acid and calcium carbonate are reactants; the reaction produces the products calcium chloride, carbon dioxide and water.\r\nreaction : the recombination of two substances using parts of each substance to produce new substances. Example: the reactants sodium chloride and sulphuric acid react and recombine to form the products sodium sulphate, chlorine and water.\r\nreactivity : the tendency of a substance to react with other substances. The term is most widely used in comparing the reactivity of metals. Metals are arranged in a reactivity series.\r\nreactivity series : the series of metals organised in order of their reactivity, with the most reactive metals, such as sodium, at the top and the least react metals, such as gold, at the bottom. Hydrogen is usually included in the series for comparative purposes.\r\nreagent : a commonly available substance (reactant) used to create a reaction. Reagents are the chemicals normally kept on chemistry laboratory benches. Many substances called reagents are most commonly used for test purposes.\r\nredox reaction (oxidation-reduction reaction) : a reaction that involves oxidation and reduction; a reactions in which electrons are transferred.\r\nreducing agent : a substance that gives electrons to another substance being reduced (and therefore itself being oxidised) in a redox reaction.\u00a0 Example:hydrogen sulphide (H2S).\r\nreduction : the removal of oxygen from, or the addition of hydrogen to, a compound. Also a reaction in which an atom, ion or molecule gains electrons from an reducing agent. (The opposite of reduction is oxidation.)\r\nreduction tube : a boiling tube with a small hole near the closed end. The tube is mounted horizontally, a sample is placed in the tube and a reducing gas, such as carbon monoxide, is passed through the tube. The oxidised gas escapes through the small hole.\r\nrefining : separating a mixture into the simpler substances of which it is made.\r\nreflux distillation system : a form of distillation using a Lie big condenser placed vertically, so that all the vapours created during boiling are condensed back into the liquid, rather than escaping. In this way, the concentration of all the reactants remains constant.\r\nrelative atomic mass : In the past a measure of the mass of an atom on a scale relative to the mass of an atom of hydrogen, where hydrogen is 1. Nowadays a measure of the mass of an atom relative to the mass of one twelfth of an atom of carbon-12. If the relative atomic mass is given as a rounded figure, it is called an approximate relative atomic mass.\u00a0 Examples: chlorine 35, calcium 40, gold 197.\r\nreversible reaction : a reaction in which the products can be transformed back into their original chemical form. Example: heated iron reacts with steam to produce iron oxide and hydrogen. If the hydrogen is passed over this heated oxide, it forms iron and steam. 3Fe + 4H2O Fe3O4 + 4H2.\r\nroast : heating a substance for a long time at a high temperature, as in a furnace.\r\nrust : the product of the corrosion of iron and steel in the presence of air and water.\r\nsalt : a compound, often involving a metal, that is the reaction product of an acid and a base, or of two elements. (Note 'salt' is also the common word for sodium chloride, common salt or table salt.)\u00a0 Example: sodium chloride (NaCl) and potassium sulphate (K2SO4)\r\nsalt bridge : a permeable material soaked in a salt solution that allows ions to be transferred from one container to another. The salt solution remains unchanged during this transfer. Example: sodium sulphate used as a salt bridge in a galvanic cell.\r\nsaponification : a reaction between a fat and a base that produces a soap.\r\nsaturated : a state in which a liquid can hold no more of a substance. If any more of the substance is added, it will not dissolve.\r\nsaturated hydrocarbon : a hydrocarbon in which the carbon atoms are held with single bonds. Example: ethane (C2H6).\r\nsaturated solution : a solution that holds the maximum possible amount of dissolved material. When saturated, the rate of dissolving solid and that of recrystallisation solid are the same, and a condition of equilibrium is reached. The amount of material in solution varies with the temperature; cold solutions can hold less dissolved solid material than hot solutions. Gases are more soluble in cold liquids than in hot liquids.\r\n\r\nSediment: material that settles out at the bottom of a liquid when it is still. A precipitate is one form of sediment.\r\nsemiconductor : a material of intermediate conductivity. Semiconductor devices often use silicon when they are made as part of diodes, transistors or integrated circuits. Elements intermediate between metals and non-metals are also sometimes called semiconductors.\r\nExample: germanium oxide, germanium.\r\nsemipermeable membrane : a thin material that acts as a fine sieve or filter, allowing small molecules to pass, but holding large molecules back.\r\nseparating column : used in chromatography. A tall glass tube containing a porous disc near the base and filled with a substance (for example, aluminium oxide, which is known as a stationary phase) that can adsorb materials on its surface. When a mixture is passed through the column, fractions are retarded by differing amounts, so that each fraction is washed through the column in sequence.\r\nseparating funnel : a pear-shaped, glassware funnel designed to permit the separation of immiscible liquids by simply pouring off the more dense liquid while leaving the less dense liquid in the funnel.\r\nseries circuit : an electrical circuit in which all of the components are joined end to end in a line.\r\nshell : the term used to describe the imaginary ball-shaped surface outside the nucleus of an atom that would be formed by a set of electrons of similar energy. The outermost shell is known as the valence shell. Example: neon has shells containing 2 and 8 electrons.\r\nside-arm boiling tube : a boiling tube with an integral glass pipe near its open end. The side arm is normally used for the entry or exit of a gas.\r\nsimple distillation : the distillation of a substance when only one volatile fraction is to be collected. Simple distillation uses a Liebig condenser arranged almost horizontally. When the liquid mixture is heated and vapours are produced, they enter the condenser and then flow away from the flask and can be collected. Example: simple distillation of ethanoic acid.\r\nslag : a mixture of substances that are waste products of a furnace. Most slags are composed mainly of silicates.\r\nSmelting: roasting a substance in order to extract the metal contained in it.\r\nsmog : a mixture of smoke and fog. The term is used to describe city fogs in which there is a large proportion of particulate matter (tiny pieces of carbon from exhausts) and also a high concentration of sulphur and nitrogen gases and probably ozone.\r\nsmokeless fuel : a fuel which has been subjected to partial pyrolysis, such that there is no more loose particulate matter remaining. Example: Coke is a smokeless fuel.\r\nsolid\/solid phase : a rigid form of matter which maintains its shape, whatever its container.\r\nsolubility : a measure of the maximum amount of a substance that can be contained in a solvent.\r\nsoluble : readily dissolvable in a solvent.\r\nsolute : a substance that has dissolved. Example: sodium chloride in water.\r\nsolution : a mixture of a liquid (the solvent) and at least one other substance of lesser abundance (the solute). Mixtures can be separated by physical means, for example, by evaporation and cooling.\r\nsolvent : the main substance in a solution.