9. Definition of terms
Below is an explanation of some terms you may find in this resource and in other
material used to help you plan for chemical emergencies:
Acid - an agent able to produce positively charged hydrogen ions (H+). [Since the
hydrogen ion is a bare proton, it usually exists in a solvated form
(such as H3O+).]
Auto-ignition or Ignition Temperature - the temperature to which, under ideal conditions, a substance has to be heated
to initiate self-sustained combustion, independent of any ignition
Carcinogenic - a carcinogen is a chemical known or believed to cause cancer in mammals. Some
chemicals are proven carcinogens, but many more are suspected to be
Chain reaction - reactions which proceed by means of a set of repeating cyclical steps, e.g.
the free radical addition of hydrogen bromide to an alkene.
Corrosive – a material that is capable of eroding or eating away by chemical action
Cryogenic liquid - A refrigerated, liquefied gas that has a boiling point colder than -90°C (-130°F) at atmospheric pressure
Ecotoxic - Toxic to the environment
Explosive – a material that expands with force and noise because of rapid chemical change
or decomposition. A material that is capable of undergoing an exothermic
reaction in which the production of a gas results in a rapid increase
in pressure in a confined space.
Flammable liquid - A liquid that has a flash point of between 21 and 55°C.
Highly Flammable liquid: A liquid that has a flash point of less than 21°C.
Flash point - Lowest temperature at which a liquid or solid gives off vapour in such a concentration
that, when the vapour combines with air near the surface of the liquid
or solid, a flammable mixture is formed. Hence, the lower the flash
point, the more flammable the material.
Fume - Airborne particulate formed by the vaporisation of solid materials (usually
refers to metals).
Gas - A state of matter in which the material has a very low density and viscosity,
diffuses easily into other gases, and is readily distributed throughout
Harmful – capable of causing injury.
Irritant – a material that causes a body part to become unusually sensitive, inflamed or
LD50 - (Lethal Dose 50) – This is the dose of a chemical that kills 50% of a population. This figure is
used to indicate the theoretical toxicity of a chemical.
LC50 - (Lethal Concentration 50) – This is the concentration of a chemical within a medium (such as air) that kills
50% of a sample population.
MEL – the maximum limit to which personnel may be exposed. Employers must reduce exposure
as far below this limit as is possible.
Mist – Aerosolised liquid droplets generated either by condensation of gases to liquids
or by fine dispersion of a liquid through splashing or atomising.
Mutagenic - a mutagen is a chemical that changes the hereditary genetic material that
is a part of each living cell. The mutation may be the first step in
the sequence of events that ultimately leads to the development of
OES – This is the level of exposure that is deemed to demonstrate adequate control.
Exposure may exceed this standard level so long as steps are taken
to conform with it as soon as is reasonably practicable.
Oxidation - a chemical process in which the proportion of electronegative substituents
in a compound is increased, or the charge is made more positive, or
the oxidation number is increased.
Oxidiser - A chemical which supplies its own oxygen and which helps other combustible
material burn more readily.
pH - pH is a value that represents the acidity or alkalinity of a aqueous solution.
Pure water has a pH of 7. A pH value below 7 indicates an acid solution
(a pH of 1 is extremely acidic). A pH above 7 indicates an alkaline
solution (a pH of 14 is extremely alkaline). Acids and alkalis (bases)
are commonly referred to as corrosive materials.
Photochemical reaction - a chemical reaction brought about by the action of light.
Powder – Any solid matter in a state of minute subdivision.
Reduction - chemical processes in which the proportion of more electronegative constituents
is decreased, or the charge is made more negative, or the oxidation
number is lowered.
Sensitiser - a material that brings a part of the body to a state where it is excessively
influenced by external agencies. A material for which repeated exposures
result in escalating levels of reaction
Toxic - a poisonous substance.
Vapour – The gaseous form of materials that are normally liquids or solids at room temperature
and pressure (e.g. steam)
Vapour pressure - Pressure at which a liquid and its vapour are in equilibrium at a given temperature.
(Liquids with high vapour pressures evaporate rapidly).