Why does water on a damp object stop fire?


Why does both cold water and boiling water stop a fire from being lit on an object? I understand that it does, but what exactly stops it?

In: Chemistry

If I remember correctly, heat will obviously turn water to steam. At the exact inflection point where this happens, there’s a huge energy requirement (that’s why steam burns are worse than boiling water burns).
so the heat energy is spent boiling the water.

A few things happen when you dump water on a fire. The first is the water is a lot colder than whatever is burning (burning wood, for example, can reach temperatures of 2000°F), so the water cools it off just by absorbing heat from the burning material, and lower temperatures mean less combustion. The water also boils off, which takes a lot of energy, cooling the material further. On top of that, the water vapor that results from boiling dispaces oxygen, which is necessary for combustion, so the rate of combustion is reduced further. At a certain point, temperatures are low enough and enough oxygen has been displaced that combustion is no longer possible, so the fire goes out.

Fire needs three things: Heat, fuel, and oxidizer.

Water can attack all of these things, though some fuels may actually prevent it from doing this (see: magnesium).

First, heat. Water starts off way colder than your fire. Fires tend to run many hundreds of degrees celsius, and your water is rarely over 30. By cooling the fire directly, it can quickly put it out. This is boosted by the boiling of water. It takes more heat to convert a bucket of already-boiling-hot water into steam than it does to heat it to boiling hot in the first place. As the water is turned to vapor, it cools its surroundings significantly more than it would just by touch. This boiling effect makes water much more effective at extinguishing fires than other room-temperature materials.

Second, oxidizer. Most fires use oxygen from the air as their oxidizer, and by covering the fuel, the water separates it from oxygen. Additionally, as it boils, the water expands a thousandfold in size, pushing almost all usable air away from the fire. With no air, the fuel will cool down and the fire can no longer be sustained, even when air returns.

Lastly, the fuel itself. Water doesn’t really destroy the fuel, but as I mentioned, it can thoroughly separate it from air. Especially powdered fuels.


We can also begin to understand from this how watts can go *wrong* in putting out a fire. While removing heat from the fire is always good, it may not be able to remove nearly enough heat to offset the harm it does.

For instance, in an oil fire, water sinks to the bottom of the oil before boiling. This rapid expansion of steam, instead of pushing air away from the fuel, blasts the fuel outwards in a small explosion, causing it to mix with air even faster and create a much larger fiery explosion than you’d get just from the boiling.

Another example is magnesium. Magnesium metal will rip oxygen off of almost anything to burn. This includes things like water and carbon dioxide. While it produces less heat while ripping the oxygen off of water than it would from free oxygen, that water is a lot denser – the oxygen is much more tightly packed than it is in air, and so the fire can consume it much more quickly (and explosively). On top of that, you’re left with hydrogen gas, which then floats away and reacts with free oxygen in the air, producing more heat.

Wood needs to be a certain temperature in order to be gaseous enough to burn, solids and liquids dont burn so it needs to be HOT. If it gets too cold it won’t have fuel and the chain reaction ends.