How does water extinguish fire?
In: Chemistry
Water serves two purposes in extinguishing fires. First is it’s heat capacity: a lot of heat can be absorbed from a fire (or literally the material which has not yet burned). This heat quench is a combination of the heat necessary to bring water to boil, and the heat of boiling. Enough water can cool fires.
Secondly, for many materials, the fire itself is composed of a hot plasma of highly reactive chemical species—some charged and some as radicals (unpaired, unbound electrons) water features both freely dissociating protons which can grab electrons, and electron pairs on oxygen. These can grab radicals out of the plasma and prevent them from ripping apart other molecules.
It should be said though, that water is not good at stopping fires of things that are immiscible with water (grease fires on cooktops is the simplest example) or where application of water could cause more damage to the thing we wish to protect (pretty much any electronic system of sufficient value)
A fire needs a few key things to sustain itself: fuel, energy, and oxygen (or other oxidizing agent)
Dropping fire on a fire deprives the fire of heat (energy) and oxygen.
Latest Answers