Explosives : As defined earlier, explosives release a large amount of gas and energy in a very short time. These are of four basic types:

Mixed gases - Usually the mixture is of a gas with air or hydrogen. Any' flammable liquid as a vapour or aerosol will similarly create an explosive mixture although some of these mixtures may, in the strictest sense, be considered to burn very quickly, e.g. a petrol lair mixture. It would take only the smallest spark, such as the static spark of a nylon lab coat, to ignite this kind of mixture and anyone who witnesses that kind of accident probably wouldn't be interested in the scientific differences between an explosion and a rapid burn - the devastation in the laboratory would be the same!

Flammable dust - When mixed with air, this can cause two explosions. The first generates more dust which then causes a second, more violent explosion. Ordinary dusts may also cause severe explosion when mixed with air, and this is one of the commonest causes of explosion in industrial environments, e.g. flour, wood dust, dycopodium powder. They can cause lung problems too!

Oxidiser/Reducer mixture - Mixture of strong oxidising and reducing agents invariably produce highly explosive results, e.g. gunpowder.

Unstable compounds - Such as picrates, sodamide, potassium metal, azides, acetylides, etc. The list of potentially explosive substances of an explosive mixture may be present as an impurity and is therefore "an unknown", e.g.

Leclanche cells utilising manganese dioxide which can contain carbon dust as an impurity.