Chemistry of Fire

1. Chemistry of Fire

2. Fire Investigation Terms • Fire - Produced when a substance undergoes rapid oxidation involving heat and light. • Fire Triangle – Shows the three elements needed to produce and sustain a fire. • Flash Point – The lowest temperature to which a substance must be heated in order for the substance to give off vapors which will burn when exposed to a flame or ignition source. • Point of Origin – The location where the fire started. • Burn patterns –Noticeable patterns created by the fire as it burns. • Accelerants – Substances, such as gasoline, paint thinner, and alcohol, that accelerate the burning process. • Arson – A fire started deliberately.

3. The Chemistry of Fire • Chemically, fire is a type of oxidation, which is the combination of oxygen with other substances to produce new substances. • To start fire, the minimum temperature needed to spontaneously ignite fuel, known as ignition temperature, must be reached. • The heat evolved when a substance burns is known as heat of combustion.

4. The Chemistry of Fire • An additional factor, besides the liberation of energy, needed to explain fire is the rate or speed at which the oxidation reaction takes place. • A fuel will achieve a reaction rate with oxygen sufficient to produce a flame only when it is in the gaseous state.

5. The Chemistry of Fire • A liquid burns when the temperature is high enough to vaporize it (flash point), while a solid must be hot enough to decompose into gaseous products (pyrolysis). • Glowing combustion or smoldering is burning at the fuel-air interface, such as a cigarette. • Spontaneous combustion, which is rare, is the result of a natural heat-producing process in poorly ventilated containers or areas.

6. The Combustion ReactionFlaming and Glowing Combustion • Combustion is a rapid oxidation reaction, the combination of fuel and oxygen to form carbon dioxide, water, and heat • Chemical reactions that give off heat are called exothermic reactions • Incomplete combustion reactions produce poisonous carbon monoxide

7. The Combustion ReactionFlaming and Glowing Combustion 1. Necessary Components for Combustion: • Combustion requires a number of components as shown by the fire triangle, tetrahedron, and pentagon • The fire triangle shows the essential components as: fuel, oxygen, & heat • The fire tetrahedron adds free radical reactions (chain reaction) • The fire pentagon adds an ignition source

8. The Combustion ReactionFlaming and Glowing Combustion • There are two ways to interrupt a combustion reaction: adding water to absorb heat or adding fire retardants to interrupt the chain reaction process • There are two major types of combustion: • Flaming combustion - both the fuel and oxygen are in the gaseous phase • Glowing combustion - the fuel is solid and only oxygen is in the gaseous phase

9. The Combustion ReactionFlaming and Glowing Combustion 2. Nature of Fuels: • Common fuels can be classified as solids, liquids or gases • Gases – fuels include hydrogen gas, natural gas, methane, and propane • Liquids – fuels include gasoline, fuel oil, kerosene, and ethanol • Solids – fuels include wood, coal, charcoal

10. The Combustion ReactionFlaming and Glowing Combustion 3. Characteristics of Fuels: • Flash point – is the lowest temperature at which a liquid produces enough vapor to be ignited by a small flame • Fire point (self - ignition temperature) – is the temperature at which there is enough heat to cause combustion even in the absence of a source of ignition • Flammable Range – is a measure of the percentage of fuel that, when mixed with air, is needed to sustain combustion

11. The Combustion ReactionFlaming and Glowing Combustion • Relative Vapor Density – a property of compounds relating vapor density to molecular weight. Most materials when vaporized are much heavier than air • Pyrolysis of Solid Fuels – the process by which solid materials are decomposed by heat, forming smaller molecules that can support flaming combustion