The law of conservation of mass
states that matter is neither created nor destroyed in the course of a chemical reaction.
An element
is a substance all of whose atoms have the same atomic number.
The atomic number
of an element is the number of protons in the nucleus of an atom of that element.
The mass number
is the sum of the number of protons and the number of neutrons in the nucleus.
Isotopes
are atoms with the same number of protons but different numbers of neutrons.
The relative atomic mass (Ar)
of an element is the average mass of an atom of the element relative to 1/12 the mass of an atom of carbon 12.
Radioactivity
is defined as the spontaneous breaking up of certain unstable nuclei, accompanied by the emission of radiation.
Alpha particles (α)
are helium nuclei, with a positive charge and little penetrating ability.
Beta particles (β)
are electrons, with a negative charge and greater penetrating ability than alpha particles.
Gamma rays (γ)
are high-energy electromagnetic radiation, with a greater penetrating ability than beta particles.
The half life
of a radioisotope is the time taken for half of the atoms in a sample of the isotope to decay.
An energy level
is a region of definite energy within the atom that electrons can occupy.
A line spectrum
is a series of coloured lines against a dark background.
The absorption spectrum
of an element is the spectrum that is observed after light has been passed through the element.
The Heisenberg uncertainty principle
states that it is not possible to determine at the same time the exact position and velocity of an electron.
An atomic orbital
is a region in space within an atom where the probability of finding an electron is relatively high.
An energy sublevel
is a group of atomic orbital's within an atom, all of which have the same energy.
The atomic radius
of an element is half the distance between the nuclei of two atoms of the same element that are joined together by a single covalent bond.
The first ionisation energy
of an element is the minimum energy that is needed to remove the most loosely bound electron from an isolated atom of the element in its ground state.
Oxidation
is the loss of electrons and an increase in oxidation number.
Reduction
is the gain of electrons and a decrease in oxidation number.
An oxidising agent
is a substance that allows oxidation to happen by gaining electrons itself.
A reducing agent
is a substance that allows reduction to happen by losing electrons itself.
An electrolyte
is a substance that conducts electricity when dissolved in water or when molten.
Electrolysis
is the breaking up of a substance due to the passage of an electric current through it in aqueous solution or when molten.
The valency
of an element is the number of bonds an atom of the element forms when it reacts.
An ionic bond
is the electrostatic force of attraction between oppositely charged ions.
A covalent bond
is formed when two atoms share a pair of electrons.
A polar covalent bond
is a bond where electrons are shared unequally between two atoms.
Electronegativity
is the relative attraction of an atom for shared pairs of electrons.
Diffusiion
is the spontaneous spreading out of a substance, due to the natural movement of its particles.
Boyle's law
states that at a constant temperature, the volume of a given mass of any gas is inversely proportional to the pressure of the gas.
Charle's law
states that at a constant pressure, the volume of a given mass of any gas is directly proportional to the kelvin temperture.
Gay-Lussac's law of combining volumes
states that when gases react, the volumes consumed in the reaction bear a simple whole number ratio to each other, and to the volumes of any gaseous product of the reaction, all volumes being measured under the same conditions of temperature and pressure.
Avogadro's law
states that equal volumes of gases, under the same conditions of temperature and pressure, contain equal numbers of molecules.
A mole of a substance
is the amount of that substance that contains as many particles (atoms or molecules or ions) as there are atoms of C¹² in 12g of C¹².
The molar mass
of a substance is the mass in grams of a mole of the substance.
An ideal gas
is a gas that obeys all of the assumptions of the kinetic theory under all conditions of temperature and pressure.
The empirical formula of a compound
indicates what elements are present in the compound and the simplest whole number ratio in which the atoms of these elements are present.
The molecular formula of a compound
indicates the actual number of atoms of each element present in a molecule of the compound.
The structural formula of a compound
indicates the arrangement of the atoms within a molecule of the compound.
An Arrhenius acid
is a substance that dissociates in aqueous solution, forming hydrogen ions.
An Arrhenius base
is a substance that dissociates in aqueous solution, forming hydroxide ions.
A Bronsted-Lowry acid
is a proton donor.
A Bronsted-Lowry base
is a proton acceptor.
A conjugate acid
is formed when a proton (H⁺) is added to a Bronsted-Lowry base.
A conjugate base
is formed when a proton (H⁺) is removed from a Bronsted-Lowry acid
A conjugate acid-base pair
is an acid and base that differ by the presence or absence of a proton.
Concentration
is defined as the amount of solute in a specified amount of solution.
A standard solution
is a solution whose concentration is accurately known.
A primary standard
is a water-soluble substance that is stable and available in pure form.
Oxidation number
is the charge that an atom appears to have when the electrons are distributed according to certain rules.
Hydrocarbons
are compounds consisting of carbon and hydrogen only, bonded together covalently.
A homologous series
is a family of organic compounds with the same general formula, similar chemical properties, and successive members differing by CH₂.
Saturated molecules
are molecules that contain only single bonds.
Unsaturated molecules
are molecules that contain a double or triple bond.
Structural isomers
are compounds that have the same molecular formula but different structural formulas.
Exothermic reaction
is a reaction in which heat is given out.
Endothermic reaction
is a reaction in which heat is taken in.
The heat of reaction, ΔH,
of a chemical reaction is the heat in kilojoules released or absorbed when the number of moles of reactants indicated, in the balanced equation describing the reaction react completely.
The heat of combustion
of a substance is the heat change in kilojoules, when one mole of the substance is completely burned in excess oxygen.
The heat of formation
is the heat change in kilojoules, when one mole of a substance is formed from its elements in their standard states.
Bond energy
is the amount of energy in kilojoules needed to break one mole of bonds of the same type, all species being in the gaseous state.
The kilogram calorific value
of a fuel is the heat energy produced per kilogram of fuel.
The law of conservation of energy
states that energy cannot be created of destroyed, but only changed from one form to another.
Hess's law
states that the heat change of a reaction depends only on the initial and final states of the reaction and is independent of the route by which the reaction may occur.
Auto-ignition
is ignition in an internal combustion engine before a spark is produced.
The octane number
is a measure of the tendency of a fuel to auto-ignite or cause knocking.
The rate of reaction
is the change in concentration in unit time of any one reactant or product.
The instantaneous rate of reaction
is the rate at a particular point in time during the reaction.
A catalyst
is a substance that alters the rate of a chemical reaction but is not consumed in the reaction.
Auto-catalysis
occurs when a product of a reaction increases the rate of the reaction.
Activation energy
is the minimum energy with which particles need to collide to cause a reaction.
Aliphatic hydrocarbons
are hydrocarbons that do not contain a benzene ring.
Aromatic hydrocarbons
are hydrocarbons that contain a benzene ring.
A reversible reaction
is a reaction that can take place in either direction.
Chemical equilibrium
is said to be dynamic because, at equilibrium, there are reactions continually occurring. The rate of the forward reaction equals the rate of the reverse reaction.
Le Chatelier's principle
states that when a system at equilibrium is subjected to a stress, the equilibrium shifts in such a way as to minimize the effect of the stress.
The pH of a solution
is defined at the negative logarithm to the base 10 of the hydrogen ion concentration.
An acid-base indicator
is a weak acid or base that has a different colour when it is dissociated into its ions than when it is undissociated.
Hard water
is water that dose not easily form a lather with soap.
Temporary hardness
is hardness in water that can be removed by boiling.
Permanent hardness
is hardness in water that can be removed by methods other than boiling.
Eutrophication
is caused by the over enrichment of water by nutrients such as phosphates and nitrates.
The biochemical oxygen demand (BOD) of a water sample
is defined as the amount of dissolved oxygen in p.p.m. used up by the sample over a period of five days at 20°C in the dark.
A batch process
is an industrial process in which an amount of the product is made in a reaction vessel during a particular time interval.
A continuous process
is an industrial process in which the reactants are continuously fed into the reaction vessel, and the products are continuously removed.
A semi continuous process
is an industrial process in which part of the process is a continuous process, and part is a batch process.
The feedstock
is the reactants in an industrial process.
Fixed costs
in an industrial process are those costs that have to be paid regardless of the rate of production.
Variable costs
in an industrial process are those costs that depend directly on the level of plant output.
The residence time
is the length of time a pollutant remains in the atmosphere, this is an important factor in the damage a pollutant can cause.