Page 35 - Curriculum Visions Dynamic Book

combustion: the special case of oxidisation of a substance where a considerable amount of heat and usually light are given out. Combustion is often referred to as “burning”.
noncombustible: a substance that will not burn.
spontaneous combustion: the effect of a very reactive material
beginning to oxidise very quickly and bursting into flame.
Fire extinguishers and flame retardants
Because combustion is caused by the rapid oxidation of a material, most ways of treating a fire involve two stages: to reduce the temperature, thus slowing down the rate of reaction, and to deny the fuel a further supply of oxygen.
A simple fire blanket thrown over a fire helps
to starve a fire of the oxygen needed for continued burning. Water acts to reduce the temperature and also to cover the fuel so that very little air can reach it. Carbon dioxide gas cannot burn; using carbon dioxide fire extinguishers covers the fire and thus keeps the oxygen out. Powder extinguishers are made of a noncombustible powder that simply blankets the burning material and stops oxygen getting to the fuel.
A flame-retardant is a chemical used on clothing and furniture fabrics that prevents a material from burning easily. The most common chemicals are based on phosphoric or sulphuric acid. When the chemical begins to burn, the compound decomposes, leaving an acid behind that can react with the fabric. The effect is to reduce the organic material of the fabric to carbon. The carbonised (charred) surface produces little fuel with which the oxygen in the
air can react. The same chemical reaction releases carbon dioxide, again preventing oxygen from getting to the material. Borax is another material sometimes used as a flame-retardant coating. Borax melts at fairly low temperatures, smothering the fabric and keeping oxygen from the inflammable fabric.
The only problem with these materials is that they release toxic gases as they react, thereby making it necessary for firefighters to use breathing apparatus.
EQUATION: Fire extinguishing
Aluminium sulphate + sodium carbonate + water ➪ aluminium hydroxide + carbon dioxide + sodium sulphate
Al2(SO4)3(s)+3Na2CO3(aq)+ 3H2O ➪2Al(OH)3(s)+3CO2(g)+3Na2SO4(aq)
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