can someone explain the science behind why getting fire wet puts it out?
In: Chemistry
Water is an exceptionally good coolant – it takes a *ton* of energy to raise the temperature of water. When you add water to a burning material, the energy produced by the fire is being absorbed by the water rather than doing things that sustain the fire, like volatilising small molecules from the fuel (since combustion is primarily a vapour phase process).
Think about lighting a wood fire – you need to supply a fair bit of energy to the wood before it will start burning. This is because what needs to happen is for the large molecules in the wood to break down into small volatile ones that can actually react with oxygen. This still need to happen when the fire has started, but the energy comes from fire itself. Take that energy away from the fuel, fire goes out.
First we need to understand fire.
Fire is the result of a chemical reaction that releases a large amount of heat. Typically this is a reaction with oxygen, called combustion and some sort of fuel, usually when you see something burning, it’s an organic compoud (think wood, gasoline, etc) that has a lot of hydrogen and carbon in it.
To start a chemical reaction you need a little bit of energy to get things started. Think about trying to tip something over, like a refrigerator. Takes some energy to get it to the tipping point, but you get a LOT of energy out once it does fall.
So oxygen next to the the wood, and it has enough energy to go fast enough to hit the molecule and rattle it. The oxygen then rips the molecule apart, snapping together and re-arranging the atoms. This is sort of how a two magnets will zip together and slam together.
The re-arranged atoms fly off at high speeds, and can rattle nearby molecules, allowing oxygen a chance to rip the next one to parts, causing a chain reaction.
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Ok, so why does water stop fire?
First, water is *created* by these combustion reactions. The oxygen combines with the hydrogen in the molecule, and well, that’s water (two hydrogens, 1 oxygen make water, h20) So the water *can’t burn* again. This means it physically blocks the oxygen from reaching the fuel molecules that it can burn. This slows things down.
Second, water does a great job at absorbing heat, which is the collisions between these molecules. Basically it’s a soft cushion on the atomic scale. So that also slows things down too.
Fire needs oxygen to breathe. Try breathing underwater – it doesn’t work for you and doesn’t work for fire.
Fire. Fast particles. Water. Slow particles. Want slow particles. Slow fast particles with slow particles.
Fire requires things to be a certain temperature before they will burn. The desk I am sitting at right now is made of wood. It is not burning, because it is at room temperature. Wood will only burn if it is hot enough, generally a few hundred degrees. Water turns to steam at a lower temperature. I can wrap a wooden spoon in foil, so the water can’t get in, and put it in a pot of boiling water until it reaches the temperature of the boiling water. I can then take it out and remove the foil. The wooden spoon will be hot, as hot as boiling water, but will not be on fire because although it is hot, it is not hot enough.
When water turns to steam, that process takes a lot of energy. That energy comes from the things around it, and makes them get colder. Fire releases energy, but if I put enough water into the fire, the energy used to turn the water to steam will be more than the energy being released by the fire, so the burning stuff will get colder. If I can get the fuel cold enough, by putting enough water on it, its temperature will drop below the temperature that it burns at, so the fire will stop.
It effectively smothers the flames, depriving the fire of the oxygen it needs.
Remember, kids, always smother a grease / oil fire or use an appropriate fire extinguisher. If you pour water on it it will just spread.
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