Forensic Chemistry- Chemistry of Fire

Fire

oxidation produces __**heat**__ and __**light**__ (flame)

**Combustion reactions**

* **oxygen combining with another substance and releasing energy (heat and light)**
* **One of the products will be an oxide**

fire

is a type of ***oxidation***

__**Ignition Temperature:**__

the min. the temperature needed to ignite fuel (start a fire)

Common Ignitors

match, electrical discharges, sparks, chemicals

__**The heat of Combustion:**__

The heat evolved when a substance burns (how much energy is given off)

__**Exothermic**__

**chemical rxn that gives off heat**

An additional factor

is the ***rate*** or ***speed*** at which the oxidation reaction takes place

__**Rate of the Reaction**__

The speed at which a reaction occurs:


1. __**State**__
2. __**Temperature**__

__***SOLID***__***:***

solid→gas, temp. must be REALLY high

__**pyrolysis:**__

**solid decomposes into gas (products combine with O2  to make fire**

__***LIQUID***__***:***

liquid→gas, only burns if it reaches “__**flash point**__” temp., then ignited by an outside source (match, sparks, chemicals, etc.)

ignition temp

* always higher than flash point temp.
* Eg gasoline FP = -50ºF vs IT = 495ºF

__***GAS***__:

will produce a flame only if enough oxygen

__**Oxidizing Agents**__:

* chemicals that supply oxygen for detonation (explosions)
* Ex- black powder (75%KNO3, 15%C, 10%S)

Nitroglycerin (O2 and fuel in the same molecule)

***Glowing combustion*** (***smouldering***):

* burning at the fuel-air interface, such as a cigarette

***Spontaneous combustion***:

the result of a natural heat-producing process in poorly ventilated areas (rare)

When will Combustion occur?

Combustion will only occur when the fuel O2 mix is “just right”

__**Flammable Range**__**:**

* **gas fuel-air mix (the range that fuel**

concentrations in air are capable of burning)

* ex-gasoline 1.3-6.0%

***Lean***

low fuel concentration vs. high air conc.

***Rich***

high fuel concentration vs. low air conc.

What type of reaction is fire?

Fire is a “chain reaction”:

**fuel + air + energy 🡪 heat 🡪 burn 🡪 heat**

( cont. until oxygen or fuel runs out)

Increasing T

Increasing rate of reaction = fires spread QUICKLY!

3 requirements for a fire combustion

* Three Requirements of Combustion:


1. **Fuel**
2. **Sufficient oxygen**

**(oxidizing agent)**


1. **Initiation heat, heat to sustain rxn**