The Burning Process: Stages

The burning process can be viewed on either the molecular scale or the macro scale.

The macro scale refers to things we can easily see. (As scientists we mean things we can observe - is there a difference between seeing and observing? Think about it!)

We can't really see things occur on the molecular scale, but we can use models (or pictures or animations) to help us visualize what's happening. Let's look at the stages of a fire on the molecular scale.

The Molecular Scale

Stage I. Heating an external source supplies heat causing the temperature of the substance to increase. The extent of temperature change depends on the specific heat of the material.
Stage II. Transition physical, mechanical, and thermal properties change. This may include melting or vaporization of the substance and may involve softening in the case of polymers.
Stage III. Degradation thermally unstable bonds begin to break. Materials such as polymers may melt before (or as) they burn.
Stage IV. Decomposition at still higher temperatures the majority of the bonds reach failure point, causing the release of gaseous molecules which differ depending on the material that's burning.
Stage V. Oxidation in the presence of oxygen at high temperatures, oxidation of the gaseous fragments proceeds rapidly producing heat, flame, and combustion products (mostly carbon dioxide and water).

Check out the animation below and see how many of the stages you observe.

Drag your mouse over the firewood until the molecules (they're cellulose) appear and click to add heat.




The macro scale describes changes that we can observe with the naked (rated PG-13) eye. The stages of a fire on the macroscale are described below.

The Macro Scale

Stage I. Initial fire an initial fire can be generated from a variety of ignition sources.
Stage II. Fire build-up the initial fire generates more heat that causes further decomposition and vaporization of additional material that burns, producing dense smoke and toxic gases. The rate of fire build-up depends on the ease of ignition, measured by the ignition temperature.
Stage III. Flashover when most of the combustible material reaches a temperature above the ignition temperature, the material bursts into flame in a near-explosive manner. This is really cool.
Stage IV. Fully-developed fire all combustible material is contributing to the fire, with extensive heat, smoke, and toxic gas generation. Containing the fire becomes more important than extinguishing it.
Stage V. Fire
propagation
a developed fire becomes the ignition source for adjacent materials, leading to the development of new fires (that is, the fire spreads from one room to another).

Recall the fire triangle and think about how it applies to both the molecular and macro scales.

Now let's continue on to see what you've learned.


Source:

Hilado, C. J. Flammability Handbook for Plastics, 5th Ed. Technomic Publishing: Lancaster, PA,1998; p. 29-37.


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