Chemistry of Fire

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.

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.

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.

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.

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

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

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

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

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

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