Page 38 - Curriculum Visions Dynamic Book

Aluminium compounds as foaming agents
Aluminium compounds can be used both
as a source of carbon dioxide gas and a source of foam. Carbon dioxide gas can,
in turn, be used to put out fires. For this reason such combinations were used to make liquid-type fire extinguishers for many years.
In the extinguisher, the two reagents (the liquids that will react) are kept apart until the extinguisher is to be used. Then, a knob on the extinguisher is struck, breaking the seal between the liquids and causing them to react.
The reaction produces a gelatinous precipitate of aluminium hydroxide and carbon dioxide gas. The gas cannot easily escape through this sticky liquid, and instead forms bubbles inside it. The result
is a foam containing carbon dioxide that immediately squirts from the extinguisher nozzle. This has the effect of blanketing the fire with materials that will not burn, thus preventing oxygen from feeding the flames.
 Imagine this flask as the inside of a fire extinguisher. Two colourless liquids are kept separate and do not react. The lower one is concentrated sodium carbonate solution, the upper one concentrated aluminium sulphate solution.
EQUATION: Fire extinguishing
Aluminium sulphate + sodium carbonate + water ➪ aluminium hydroxide + carbon dioxide + sodium sulphate Al2(SO4)3(aq) + 3Na2CO3(aq) + 3H2O(l) ➪ 2Al(OH)3(s) + 3CO2(g) + 3Na2SO4(aq)
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