MadSci Network: Chemistry

Re: What is the difference between a combustible material & a flammable one?

Date: Thu Mar 5 14:13:27 1998
Posted By: John Letourneau, Lab Technician, Canadian Forestry Service
Area of science: Chemistry
ID: 888527582.Ch

The heat given off by combustibles is not only referred to as the Heat of 
Combustion but is also known as the Enthalpy of Combustion.  There is a 
book which is called The CRC Handbook of Chemistry and Physics which is 
revised and published annually.  This book contains a multitude of physical 
and chemical data on various chemicals.  Since I have access to a copy, I  
know that it has a table titled "Enthalpy of Combustion of Hydrocarbons" 
that contains combustion data for about 150 common chemicals.  This book 
should be available in your local library or a university library if you 
have access.

The heat of combustion of materials is determined by a method known as 
calorimetry.  Although the concept is pretty simple, the field has  
progressed significantly and has become very complex.  A simple calorimeter 
consists of a container that contains a substance of known properties (for 
an easy example we'll use water).  To find out the Heat of Combustion for a 
certain material the material is burned in such a way that all of the heat 
is transferred to the substance in the container (water in our case).  
Since we are burning a substance and transferring the heat to the water, 
the water should warm up.  From the increase in the water temperature, the 
heat produced by the combustion can be calculated. There are a number of 
important things that we have to know before we can make the calculation.  
These include;

- the exact amount of material combusted
- the exact amount of water in your calorimeter
- the temperature of the water before and after the combustion takes place
- the heat capacity of the substance in the calorimeter (water in our case)

Due to the properties of different substances, not all materials warm up at 
the same rate.  The amount of energy needed to warm substances is known as 
the heat capacity.  For water, it is known that it takes 0.9988 calories 
(~1 cal) to increase the temperature of 1 mL of water by 1 degree Celsius. 

The formula for figuring out the Heat of Combustion of the original 
substance would be;

Heat of combustion = (T2-T1)*Cf*V

T1 = the initial temperature (deg. C)
T2 = the final temperature
Cf = the heat capacity (in units of cal/deg mL)
V = the volume of the substance being heated

As you might imagine, the science of calorimetry has advanced far beyond 
the level of heating a container full of water.  Nevertheless this is how 
the original experiments were done and offers a good place to start when 
trying to understand the basic principles.

As for the difference between the terms combustible and flammable, I 
believe it is the following (although I haven't been able to find out for 
- Flammable is a substance which under normal conditions has the ability to 
catch fire with a minimal ignition source (such as a spark).  An example of 
this might be a substance such as propane. 
- Combustible materials would be any material that will burn.  In this 
category we could also place propane and the like but it would also include 
materials that need more vigorous conditions to burn and are not likely to 
catch fire with a simple spark.  An example of a combustible material of 
this sort would be wood or paper.  In my opinion therefore, all flammable 
materials are combustible, but all combustible materials are not 
necessarily flammable.

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