explosions. introduction caused by a chemical reaction. the reaction releases a large amount of gas...
TRANSCRIPT
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
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.