Learn

The Science of Arson

Arson

Utilization of fire is essential to our survival. However, fire is also one of the most destructive natural forces, with the power to reduce buildings or forests to ash in just a few hours. While some fires are caused by accidents or natural forces, fires may also be set intentionally—a crime known as arson. Due to the somewhat unpredictable nature of fire, arson investigation requires experts with extensive training.

A living room after a house fire. Everything is destroyed and covered in smoke residue, including the walls and celing.
The aftermath of a house fire.

Chemistry of Fire

Before we discuss arson cases, what are the components needed for a fire to exist? Fire is both a physical and chemical process. For a fire to occur three things are required:

  1. A Fuel Source - Some sort of flammable substance must be present.
  2. Oxygen - Enough oxygen must be present to sustain the combustion. Oxygen makes up about 20% of the air we breathe.
  3. Heat - Enough heat must be present to ignite the fuel source. The minimum temperature required to ignite a liquid fuel source is called flash point.

Fire Triangle

These three components - fuel, oxygen, and heat - can be illustrated by the fire triangle.

A fire triangle. Oxygen is represented by the left side of the triangle, heat by the right side, and fuel by the bottom side.

For a fire to start, all three of these components within the fire triangle must be present. If one of these components is removed, the fire will go out.

Knowledge Check #1

What are the three things required for a fire to occur?

  1. Heat, Humans, and Error
  2. Fuel, Oxygen, and Heat
  3. Humans, Matches, Fuel

Answer: b. Fuel, Oxygen, and Heat

Knowledge Check #2

There are three things required for a fire to occur. What happens if one of those things is removed?

  1. The fire will get hotter.
  2. The fire will get less intense.
  3. The fire will go out.

Answer: c. The fire will go out.

Knowledge Check #3

What percentage of the atmosphere is oxygen?

  1. 10%
  2. 20%
  3. 40%
  4. 60%

Answer: b. 20%

Chemical Reaction

Fire is a chemical reaction, which is when molecules are broken down and rearranged as they combine to produce new substances. More specifically, burning is a type of reaction known as combustion, or the rapid combination of a substance with oxygen, accompanied by heat and light.

Chemical Equation for Combustion

Most fuel sources contain carbon and hydrogen in some ratio (CxHy), which combines with oxygen to form carbon dioxide (CO2) and water (H2O). A simplified chemical equation for combustion may be expressed as:

Fuel (CxHy) + oxygen (02) + spark → water (H2O) + carbon dioxide (CO2) + heat

The substances on the left side of the arrow are known as the reactants or starting materials. The substances on the right side of the arrow are the products.

Exothermic

Because heat is a product of combustion reactions, combustion reactions are exothermic, meaning that they are processes that release heat.

A laboratory flask filled with liquid. The liquid is labeled as hotter than its surroundings. Arrows and text that reads 'heat' surround the flask, indicating that heat is being released.

Knowledge Check #4

Water is a _________ in combustion reactions.

  1. product
  2. reactant

Answer: a. product

Knowledge Check #5

Due to the high temperatures typically present in a combustion reaction, water is usually produced as a ____________, known as water vapor.

  1. solid
  2. liquid
  3. gas

Answer: c. gas

What Starts a Combustion Reaction?

For a combustion reaction to occur, the reactants (or fuel) must reach a certain temperature. The temperature required varies depending on:

  1. The Fuel Source - Some materials ignite at lower temperatures and are more prone to causing fires.
  2. The Presence or Absence of a Spark or Ignition Source - Without a spark or ignition source, very high temperatures are typically required for a fire to start. This temperature is known as the auto-ignition temperature.
    • For wood, the auto-ignition temperature is 572°F

Flash Point

In the presence of a spark or ignition source, liquid fuels will ignite at lower temperatures called the flash point.

Flash points are typically much lower than auto-ignition temperatures. Materials with flash points below 140°F are considered flammable, while while those with flash points above 140°F are classified as combustible.

A thermometer with 140 degrees Farenheight labeled. Above 140 degrees the flash point temperature is labeled combustible, and below 140 degrees is it labeled flammable.

Flash Point and Auto-Ignition of Fuels

Examine the flash point and auto-ignition temperatures of common fuels.

Fuel Flash Point (°F) Auto-Ignition Temperature (°F)
Ethanol 61.9 685
Gasoline -45 536
Diesel 126 493
Jet Fuel 100 410
Kerosene 100-162 428
Vegetable Oil 621 795

Knowledge Check #6

Which substances from the table above would be considered flammable?

  1. ethanol
  2. ethanol, gasoline, jet fuel
  3. ethanol, gasoline, diesel, jet fuel, kerosene
  4. All of the fuel sources on the chart are flammable.

Answer: c. ethanol, gasoline, diesel, jet fuel, kerosene

Knowledge Check #7

It is sunny spring day with a temperature of 65°F. Which of these substances from the table above would ignite if a spark, such as from a match, was present?

  1. ethanol and gasoline
  2. diesel, jet fuel, and kerosene
  3. All these substances will ignite if a spark is present.

Answer: a. ethanol and gasoline

Knowledge Check #8

True or False: Flash points are typically higher than auto-ignition temperatures.

  1. True
  2. False

Answer: b. False

Types of Combustion

Depending on the availability of oxygen, combustion may be complete or incomplete.

  • In complete combustion, oxygen is plentiful, and the fuel source reacts completely, producing primarily carbon dioxide and water. The burning of fuel in your car is one example of complete combustion.
  • Incomplete combustion takes place when there is not enough oxygen present to complete the reaction. In this case, instead of carbon dioxide being produced, carbon and carbon monoxide an odorless, colorless, poisonous gas are also produced, contaminating the smoke.

Complete and Incomplete Combustion

Open Complete & Incomplete Combustion in a new window

Note: The presentation may take a moment to load.

Knowledge Check #9

Do the following statements best describe complete or incomplete combustion? Match the statements to each category:

Categories:

  • Complete Combustion
  • Incomplete Combustion

Statements:

  • Products are toxic and pollutants
  • Plentiful supply of air
  • Produces carbon dioxide and water
  • Limited supply of air
  • Produces carbon or carbon monoxide and water
  • Product contributes to global warming

Answer:

  • Complete Combustion:
    • Product contributes to global warming
    • Plentiful supply of air
    • Produces carbon dioxide and water
  • Incomplete Combustion:
    • Produces carbon or carbon monoxide and water
    • Limited supply of air
    • Products are toxic and pollutants

Combustion Reactions

Combustion reactions can also be classified according to how they began and the speed at which they propagate spread .

  • Smoldering - A slow, flameless, low-temperature combustion of a solid fuel, such as wood, coal, or cotton. Smoldering is a very localized burn that produced thick, tarry smoke. Smoldering fires can turn into flames if enough oxygen is present.

    A pile of smoldering charcoal.
    Smoldering charcoal is an example of a flameless combustion reaction.
  • Rapid Combustion - Gas burns quickly, producing heat and light in the form of a flame. This is the type of combustion we most associate with fires.

    A close-up of a gas stove burner.
    Turning on a gas stove ignites the burner quickly, so it's an example of rapid combustion.
    A close-up of striking a match.
    Striking a match to light it is another example of rapid combustion.
  • Spontaneous Combustion - Materials burst into flames without the application of a spark. This generally occurs because of reactions occurring within the material that release sufficient heat to cause ignition (the auto-ignition temperature).

    A forest fire.
    Forest fires are examples of spontaneous combustion, as the dry grasses and leaves start burning in the summer heat. Other examples are spontaneous fires that start in coal mines when the coal dust interacts with oxygen in the air.
  • Explosive Combustion - A very fast combustion reaction in which a large of amount of gaseous product is quickly produced, causing rapid expansion and a loud sound.

    Fireworks.
    Fireworks are examples of explosive combustion. Sparks light the fuse, exploding the firework and producing a lot of energy in the form of heat, light, and sound.

Knowledge Check #10

Is it possible for a person to spontaneously combust? There have been several cases throughout history where a person seemed to burn from the inside out. Open Know about the facts and theories of spontaneous human combustion and either watch the video or read the transcript.

True or False: Matilda Rooney spontaneously combust.

  1. True
  2. False

Answer: b. False

Knowledge Check #11

Match the following examples to the type of combustion reaction:

Types of Combustion Reactions:

  • Rapid Combustion
  • Smoldering
  • Explosive Combustion
  • Spontaneous Combustion

Examples:

  • a cigarette butt is left on a couch cushion
  • a candle burning
  • an oil-soaked rag suddenly ignites
  • fireworks

Answers:

  • Rapid Combustion - a candle burning
  • Smoldering - a cigarette butt is left on a couch cushion
  • Explosive Combustion - fireworks
  • Spontaneous Combustion - an oil-soaked rag suddenly ignites