Hello Everyone,
Why does water put out fire? Besides, it’s wet. What exactly happen? Does water cover the combustible material and for a bond between it and the fire?
There’s a couple of things going on, I believe.
First, water cools the fuel to where fire is no longer possible.
Second, the steam displaces the air and thus deprives the fire of oxygen.
Mainly cooling. Of course with enough water it also blocks of the supply of oxygen. Burning substances that have their own supply of oxygen often aren’t extinguished by water, and burning aluminum can strip the oxygen out of water molecules to keep burning.
Slight tangent:
What elemental sodium does when placed in water … is that properly called “burning”? Or is that reaction better described in other terms?
So water removes the heat, so it’s too cool for fire.
The heat of the fire evaporates the water into steam and that reaction removes enough energy from the fire that it can’t sustain itself.
It’s an oxidation reaction, or combination with oxygen, which is what’s happening in a combustion reaction - but it’s also what’s happening with a rusty car, which lacks a bit in comparison with the idea of burning. The flame produced from sodium and water is actually the combustion of the released (and very hot) hydrogen from water with atmospheric oxygen. Meanwhile, you can burn sodium directly and it looks like this. I’d stick with calling it an oxidation reaction.
Water has two actions. cools the fire and if enough water is used it can deprive the fire of oxygen.
A slightly more involved description. Generally solids don’t burn in the obvious way, they get hot enough to begin releasing vapours that do burn. If you lower the temperature of the burning item sufficiently, the vapor release stops, and therefore the fire does not continue. Hence some things can’t be controlled with water… Their vapor release point is lower than the possible cooling effect of water. For a heavily engaged fire, a sensible temperature for that to occur is 90ish deg C
I have a strange perspective on fire. I have always viewed fire as some sort of living organic being, trying to survive at all costs, just like any human being or animal would. Very fierce in its survival mechanism.
True, although solids do burn directly. When you see glowing embers without flame, that’s a solid burning. Steel wool burns similarly.
You have the redox reaction of
2Na(s) + 2H2O → 2NaOH(aq) + H2(g)
So, sodium is so reactive that it can use water as oxydizing agent, releasing reduced hydrogen as a biproduct. The reaction is so exotermic that it ignites the released H2, which burns with air oxygen to produce water
You can actually go further for a “fire tetrahedron”, heat, fuel, oxygen, and a sustained reaction. Some firefighting agents like halon, work by chemically disrupting the reaction with no significant cooling or blanketing effect.
You can gain insight (and a lot of entertainment) into what burning means by learning about interesting compounds such as Chlorine Trifluoride.
Sand Won’t Save You This Time
The link and excerpt above are from chemist Derek Lowe’s “Things I won’t Work With” blog - if you google that you can find many more fun tales. FOOF is another favorite. And an interest in burning leads naturally to exploding, so check out Azidoazide Azides.
There’s also a great book about the history of rocket fuels called “Ignition!”, by John Clark - you can find a pdf online for free, e.g. here:
That’s right, but there’s an interesting reason why water is so effective here, and it’s something called the latent heat of evaporation.
Picture this: if you have a small container of 100 grams of water at room temperature, you may recall from school that it takes 1 calorie of heat to change the temperature of 1 g of water by 1 degree C. So every 100 calories of heat you add changes the temperature of our container by 1 degree.
After we’ve added about 7800 calories, our 100 g of water will be heated up to the boiling point. Now the question is: how much more heat will it take to change this 100 g of water at 100 C to 100 g of water vapor at 100 C?
The answer is surprisingly large: it will take 54,000 calories. It only took 7800 to heat it up from room temperature to boiling, but it takes 54,000 more to change it to vapor, without even increasing its temperature any.
So for a fire to keep burning, it has to be above the temperature where water vaporizes, but to get to that point, it sucks a HUGE amount of the heat out of the fire for every gram of water that’s on it.
By the way, that latent heat thing is also why steam burns you so badly - for every gram of water vapor that condenses on you, your skin gets 540 calories. And it’s why a cold drink gets warmed up so much by water condensing onto the glass on a humid day.
I only took a couple of 100-level chemistry courses, so it’s difficult for me to wrap my head around the notion that a nolecule with no oxygen in it could be such a potent oxidizer.
Yeah, oxidizer is a bad name, although there’s historical reasons for it. It should be called de-electronizer.
Actually the easiest way to understand this is to look at the periodic table.(But ignore the noble gases) As you go up and right things become better oxidizers and as you go up and left you get things that reduce more easily. Since fluorine is on the upper right (except for helium) it’s a better oxidizer than anything including oxygen. Hydrogen and sodium are in the upper left so they’re really good at reducing.
Does this imply that hydrogen + fluorine compounds would be about as unstable/corrosive/toxic/exothermic as compounds can be?
What difference, if any, is there between burning and corrosion except the timeframe?