Fire, Arson and Explosives Questions and Answers

Answer: A

The O-nitro compounds are (a), (b) and (c) only: nitrocellulose, nitroglycerine and PETN.

Explosives are classed by the atom the nitro group is attached to. In O-nitro compounds, the nitrate esters, the \(\mathrm{NO_{2}}\) group is bonded to oxygen: nitrocellulose (cellulose nitrate), nitroglycerine (glyceryl trinitrate) and PETN (pentaerythritol tetranitrate) are all nitrate esters made by nitrating alcohols.

TNT (trinitrotoluene) is a C-nitro compound, with its nitro groups bonded directly to carbon atoms of the benzene ring, so options including (d) are wrong. RDX and HMX would be N-nitro compounds (nitramines).

Answer: A

The dye is a dialkyl amino anthraquinone dye.

Subsidised kerosene supplied through the public distribution system is coloured blue so that its diversion, for example to adulterate diesel or petrol, can be detected. The blue colouring used for petroleum products is a dialkylamino anthraquinone dye, an oil-soluble anthraquinone dye that gives a stable blue colour.

Phenyl azo 2-naphthol (Sudan I) is an orange-red azo dye, methylene blue is a water-soluble dye that does not dissolve in kerosene, and 'green dye' names no particular compound.

Answer: A

The primary high explosive here is lead azide.

A percussion cap or primer must explode when struck, so it uses a primary (initiating) explosive that detonates from a mild shock or friction. Lead azide is such an explosive, and it is used with or alongside lead styphnate in primers and detonators.

TNT, PETN and RDX are secondary high explosives. They are relatively insensitive and need a detonator to set them off, which is why they are used as main charges and boosters rather than in percussion caps.

Answer: A

The common booster explosive is pentolite.

A booster is a charge of sensitive secondary high explosive placed between the detonator and the main charge, to amplify the detonator's shock enough to set off a less sensitive main charge. Pentolite, a mixture of PETN and TNT, is widely used in cast boosters, especially for ANFO and emulsion explosives in mining.

ANFO is itself an insensitive main-charge explosive that needs a booster. Dynamite is a blasting explosive used as a main charge, and petrol is a fuel, not an explosive used in boosters.

Answer: D

Diphenylamine gives a blue colour with TATP.

Diphenylamine in concentrated sulphuric acid is oxidised to a blue product (a quinoidal diphenylbenzidine dye) by oxidising substances. Triacetone triperoxide (TATP) is a peroxide explosive and an oxidiser, so it gives the blue colour, as do nitrates, chlorates and other oxidisers.

The test is a general oxidiser test rather than a specific one, so a blue result is only presumptive. Green, red and orange are not the colours this reagent gives.

Answer: D

The element added in the fire tetrahedron is the uninhibited chain reaction.

The fire triangle says a fire needs fuel, heat and oxygen. The fire tetrahedron adds a fourth element: the self-sustaining chemical chain reaction of free radicals in the flame. This explains why some extinguishing agents, such as dry chemical powders and halons, work by breaking the chain rather than by cooling or smothering.

Fuel, heat and oxygen are the three sides of the original triangle, so they are present in both models.

Answer: A

Pyrolysis is the decomposition of solid organic matter by heat.

When a solid such as wood or plastic is heated, it breaks down chemically into volatile, combustible gases and a carbon-rich char, with little or no oxygen involved. These gases then burn in the flame. Pyrolysis is therefore the step that lets a solid fuel feed a fire, and pyrolysis-GC is used to identify polymers and paints.

Evaporation of a liquid is a physical change, the red glow of charcoal is glowing (smouldering) combustion, and the burning of a cigarette is a smouldering fire. None of these is the thermal decomposition itself.

Answer: C

The mode that needs no medium is radiation.

Radiant heat travels as electromagnetic waves, mainly infrared, which can cross a vacuum. That is how the Sun's heat reaches the Earth, and how a fire spreads to objects across a room without touching them.

Conduction passes heat through a material from molecule to molecule, and convection carries it by the movement of a fluid such as hot air or smoke, so both need a medium. Bloating is not a mode of heat transfer.

Answer: B

Deflagration is a very rapid oxidation reaction accompanied by the generation of a low-intensity pressure wave that can disrupt the surroundings.

In a deflagration the reaction front moves through the material by heat transfer at less than the speed of sound in it. It produces a rapid push of hot gases, as in black powder or smokeless propellant, which is why low explosives deflagrate.

The first option describes detonation, in which a supersonic shock wave carries the reaction and produces a violent shattering effect, the behaviour of high explosives. Removing a safety fuse and the expansion of gases are not definitions of deflagration.

Answer: D

The N-nitro compound is nitroguanidine.

Explosives are classed by the atom to which the nitro group is attached. In nitroguanidine the NO₂ group is bonded to a nitrogen atom (N-NO₂), making it a nitramine, like RDX and HMX.

Nitrocellulose and nitroglycerine are nitrate esters, with the group on oxygen (O-NO₂), and picric acid (trinitrophenol) is a C-nitro compound, with the groups on the benzene ring.