1. Explosions

2. 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.

3. 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)

4. Combined Gas Law IFANDTHEN T increasesV is constantP increases T increasesP is constantV increases V increaseT is constantP decreases V increasesP is constantT increases P increasesT is constantV decreases P increasesV is constantT increases

5. 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.

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. Wire Mesh Sifting Box

13. 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

14. 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

15. 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.