– How exactly does water put out a fire? Is it a smothering thing, or a chemical reaction?
In: Chemistry
The way I see it, it removes the heat element. Every fire safety training I’ve ever received, the first thing in the sessions was the fire triangle: Oxygen, Fuel, Heat. Water cools it down, thus removing one of the three crucial elements of the fire triangle. Unless of course it’s fat or electrical fire, in which case you’ve either made it worse or are now likely dead
Water both cools and smothers a fire depending on how you apply it. If saturated or sprayed in a heavy fog pattern it will cool the fire by removing heat both latent (liquid to steam) and sensible (cold water to hot water) by doing this it removes energy from the fire . Going from liquid to steam takes a lot of energy. It can smother a fire as well when going from liquid to steam water can expand by about 1600 times in volume this will displace the air in the space and break the fire triangle (air,heat fuel).
The smothering helps, but mostly it’s just cooling down the burning material. Heating up the water cools the fuel down a lot, but when water evaporates it pulls a lot more heat out of the fuel.
Edit: Reworded some things. Glad to know my chemical engineering degree’s still useful to people even after moving out of the field.
Its a thermodynamic reaction. Basically fire needs enough energy to propagate. When it burns the splitting of molecules creates enough energy for other particles to burn as well.
Water takes a lot of energy to change temperature. So when you put water on fire it basically absorbs a ton of energy that would otherwise go to making more fire. And because it takes so much more energy it lowers the overall temperature. Once the temperature is low enough the fire doesn’t have enough energy to spread anymore.
In a fire the carbon and oxygen molecules are all really really excited and bumping into each other, and combining into CO2, which makes more bumping, in turn setting off more molecules bumping and clicking together.
Water comes in and ruins the party. So much energy is spent breaking the water droplets apart into steam, it robs the other molecules of their bumping and jittering and now they can’t click together to make CO2 and no more chain reaction, no more fire.
Interestingly, if we are talking about wood burning, where does wood come from? Where does it’s MASS come from?
Does it grow out of the ground? Does the tree get its mass from material pulled up by roots? No. Trees grow out of the air. They take up CO2, then photosynthesis uses a photon from the sun to knock the C loose from the O2. The tree spits out the useless waste product (oxygen) and then strings the carbon into chains (along with some oxygen and hydrogen) to make cellulose (wood)
So when you burn wood, you are reversing the chemical reaction and releasing the same amount of energy that went in to breaking the CO2 in the first place, chiefly, the photons, as heat and light. So, in a way, wood is *stored sunlight*.
A fire requires 3 things:
1. fuel
2. Oxygen
3. Heat
It takes a *lot* of energy to actually heat up water. Also, water is colder than most (all?) fires. The water cools the fire below what it needs to burn, then keeps the area cold because it takes a lot of energy to heat water up again, and evaporating water cools the area.
Fire needs 3 things to survive: Heat, Fuel, and Oxygen. We call that the fire triangle. Remove any one of those from the fire and it cannot sustain itself. Water works well on most fires because it rapidly cools the fuels, so it really attacks 2 sides of the triangle. But there are fires that water is not good for, like electrical fires, and metal fires.
Fire needs three things to exist. Heat, Fuel and oxygen. To put out a fire you need to take at least one of the three things out of an equation. There are also different types of fire. A type A (Alpha) fire are things that leave an ash. Wood is an example. A type B (Bravo) fire is a burning liquid like oil. Type C (Charlie) is flammable gases like propane. Type D (delta) are burning metals and a Type E (Echo) is an electrical fire.
For your type A fires water is an excellent way to remove the heat and maybe a bit of smothering to break the triangle. However, using water on a type B, C, D, or even E may very well make things worse. Put water on an oil fire and you have an explosion because it displaces the oil which allows it to burn faster. (More surface area). Dry smothering is the best way to go with that. Sand, dirt, etc. Water can be used on a gas fire to cool it, but a very specific cooling mist pattern over a wide area must be used. Better just to shut off the fuel source if possible. Water should probably never be used on metal fires unless you KNOW exactly what kind of metal you are dealing with. Some metals will explode in contact with water. Electrical is kind of obvious because of the chance of electrocution. Shut off the power then treat as a type A fire.
Certified but retired firefighter here.
Fires have to have four things to exist, we call this the fire tetrahedron.
1. Fuel
2. Oxidizer
3. Heat
4. Chemical Reaction
It’s the 4th one that most people don’t know or remember and is missing in the fire triangle.
Water works in different ways for different fires. A simple wood fire the water cools the wood (heat) and penetrates into it creating a barrier between the wood (fuel) and the air (oxidizer). Water is very effective here.
It is important to remember that when water is used lots of steam is created. This is because water expands over 1700 times when converted to steam.
Fire fighters plan for different scenarios where things like the fuel sources or oxidizer change for example. Additives to the water (detergents) help create foam and increase the penetration of the water so it soaks in. Ever have water sit on the surface of your clothes or couch, we want it to soak in immediately.
For more learning look at chemical fire suppression agents like fm220 and fm200. They work by preventing the chemical reaction. A common misconception is that they remove oxygen from the area. Additionally,Take a look at steam converting a room.
Fire science was so interesting to me and many of you may find it interesting as well. Contact your local fire department and see if they have a citizens fire academy type class. You might get to experience several aspects of fire fighter life including:
Protective equipment – bunker gear, SCBA
Fire apparatuses – tankers, engines, ladder.
Fire fighting tools – nozzles, hoses, hand tools, foams, saws, ladders.
Fire science – How and why it exists. How to “kill” it.
Rescue – Cut up automobiles! Rappel!
Communications – radios and 911 dispatch
Fire command – scene control, public relations, incident command
Safety – man down alarms, strobes
Emergency Medical – cpr, triage, etc
Firefighting history – fire insurance, fire fighting around the world.
Reading the answers, and here I am, feeling like an idiot.
***I thought it was the lack of oxygen***
Latest Answers