Flammables
A combustible material is a material that can burn (i.e. sustain a flame) in air under certain conditions. A material is flammable if it ignites easily at room temperature. In other words, a combustible material takes more effort to ignite and a flammable material ignites almost immediately when exposed to a flame.
The degree of flammability in air depends largely on how easy the material vaporizes. This is related to its composition-specific vapor pressure, which is temperature dependent. The quantity of vapor produced can be enhanced by increasing the surface area of the material, forming a mist or dust. Wood is an example. Finely divided wood dust can undergo explosive flames and produce a blast wave. A piece of paper (made from pulp) catches on fire quite easily. A heavy oak desk is much harder to ignite, even though the wood fibers are the same in all three materials.
Common sense, and scientific consensus until the mid-1700s, suggests that material disappears when burned, as only the ash is left. Further scientific research has found that conservation of mass holds for chemical reactions. The burning of a solid material may appear to lose mass if the mass of combustion gases (such as carbon dioxide and water vapor) is not taken into account. The original mass of flammable material and the mass of the oxygen consumed (from the surrounding air) equals the mass of the flame products (ash, water, carbon dioxide, and other gases). Some metals gain mass when they burn to support the idea (because those chemical reactions capture oxygen atoms into solid compounds rather than gaseous water).
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