eli5 : Why does water put out fire ?


eli5 : Why does water put out fire ?

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Water has a massive heat capacity, it “can store a lot of heat energy”, so it cools things down really fast (or heats them up fast, depending on it’s temperature.

It also of course suffocates parts of the fire, the layer of water prevents oxygen getting in, which is the active agent of (most) fires, so all in all it dies pretty fast

Fire needs free of oxygen or oxygen able to be freed to burn and it needs to maintain its temp. Water smothers the fire, starving it of oxygen and cool the material burning. This is why a grease fire is not put out by water, as the grease can maintain it’s heat past the ignition point since it doesn’t mix w water.

Fire requires oxygen (other gases can fuel fires but for simplicity let’s say oxygen). The water completely smothers the fire and stops it getting oxygen gas, so it goes out. At the same time the water also cools down the material that was on fire and makes it less likely to catch fire again.

Water is the easiest and most available but many other things can have the same effect, lots of fire extinguishers use foam or powder, for situations where water would be unsuitable.

Fire needs three things: fuel (the material which burns), activation energy (heat) and oxygen. This is called the fire triangle.

When you put water on fire, it reduces the heat of the burning material, absorbing the heat energy, and changing to steam, hindering the reaction, and reduces the ability of oxygen to reach the reaction.

This being said though, for certain types of fire you do NOT put water on it or it will cause the fire to worsen or spread, such as with oil fires (water turns to steam carrying flammable oil outwards) or some chemical fires.

You may have learned at one point that fire needs three things to keep going: fuel, oxygen, and heat.

Water disrupts the second two. First, it smothers the fire and blocks oxygen gas from getting to it. This is also how putting out a fire with sand or a blanket works.

Second, water takes a lot of energy to heat up. When you dump a lot of liquid water onto a fire, the water absorbs a lot of heat as its temperature rises and even more as it boils and turns to vapor. This can pull enough heat out of the fire that there isn’t enough energy left to keep the combustion reaction going.