Explosions

Introduction• Caused by a chemical reaction. • The reaction releases a large amount of gas and a

large amount of energy very quickly• Explosion sends a pressure wave through the

surrounding materials. • Reaction is either an oxidation or decomposition

reaction. – Oxidation – combining 2 substances to make a new

one– Decomposition – single compound is broken down

into 2 or more simpler products.

Properties of Gases• Made of tiny particles in constant motion– Moving more quickly than in liquid or solid– Motion gives the particles energy = kinetic energy

• Kinetic Molecular Theory -- behavior of gases is predictable– Made of many particles moving rapidly– Particles are smaller than the distance between

them• Therefore, most of the volume of a gas is empty space

– When a particle hits a solid, it does NOT lose energy– No force of attraction between gas particles or

particles of a solid (container)

Combined Gas LawIF AND THEN

T increases V is constant P increases

T increases P is constant V increases

V increase T is constant P decreases

V increases P is constant T increases

P increases T is constant V decreases

P increases V is constant T increases

Many explosive devices depend on the relationships between T, V, and P. Some heat the devices and hold the volume constant, others increase the amount of gas in a container while keeping the volume constant. In both, pressure increases.

Video of Kinetic MolecularTheory

Characteristics of an Explosion

• Explosion does not require oxygen like fire does• Energy and gases are released as the products of

a chemical reaction. • The expanding gases produce a pressure wave– A wave is a disturbance in matter that carries energy– A pressure wave is a longitudinal wave (accordion)• Compressions • Rarefactions

Characteristics of an Explosion• Energy released in an explosion forces

particles in the air to move back and forth. In some explosions, the gases are held in a container. – The walls will stretch until they burst and

fragment. – The debris, flying in all directions is what is

dangerous.• Shrapnel – flying debris from an explosion

Types of Explosives

• Most are solids • Most fit into one of two categories based on

their rate of reaction– Low explosives

• React more slowly • If unconfined, they burn rather than explode• Less dramatic explosions than high explosives

– High explosives• React more quickly • Explode regardless if they are confined or not

Low Explosives• Produce a combustion reaction called deflagration –

rapid, intense burning• Produces a pressure wave that travels less than the

speed of sound• Often used as propellants -- Produces enough gas

and energy to push a bullet or other object away from the original explosion

• Small amount of energy is needed to ignite it– Spark or burning fuse

• Black powder and smokeless gunpowder are common examples

• Fireworks• Natural gas mixing with oxygen

High Explosives• Materials that detonate – an explosion that results in a violent

disruption to the surrounding area. • TNT• Decomposition reaction• Produces a shock wave because the gas particles are moving

more quickly than the speed of sound• Two categories:

– Primary High Explosives• Very sensitive to heat, pressure and movement• Used as primers in shotgun shells or bullets, or used to detonate other

explosives• Nitroglycerin only

– Secondary High Explosives• Can be handled safely in small amounts• Can cause violent explosions• TNT and dynamite • Silica and nitroglycerin

Collecting Explosive Evidence• Like arson, explosion evidence is often destroyed in

an explosion AND fire usually follows an explosion – Evidence can be destroyed

• Can be dangerous– Shards of glass or sharp pieces present– Toxic or flammable vapors remain in the air– Risk of second explosive device

• Large crater marks the point of origin– Collect soil samples and debris from inside the crater– May sift through debris with wire mesh

• Usually packaged in cans or glass jars– Contain gas residue– Protection from sharp-edged debris

Wire Mesh Sifting Box

Microscopic Examination

• Examine debris for unexploded material– Shape and color of some

low explosives make them easy to identify (see picture)

• Examine pieces of the explosive device– Wrapper from dynamite– Metal from a pipe– Pieces of wire

Run Chemical Tests to Analyze

• Wash debris with Acetone

• Use a presumptive color spot test to screen for the presence of various explosives. – Mix a small amount of

concentrated acetone and explosive solution with a specific reagent and observe the reaction• This can give them a

preliminary identification of the explosive

Additional Test Run to Confirm Preliminary Identity

• Chromatography– Mixture of sample is

placed on cellulose or silica gel plates and rinsed with a solvent.

– Compare resulting chromatogram to known chromatograms of various explosives

• Infrared spectroscopy– Beam of infrared light is

passed through a sample. – The spectrometer

measures how much energy was absorbed by the sample.

– Based on the absorption, the scientist can identify the components of the sample.