Boiling water

[AFriendlyFace]

OK, here's something I've always wondered about but no one's ever been able to explain it in such a way that made sense to me.

 

Why is it that as soon as you take boiling water off the stove it quits boiling?

 

It seems to me that it would take a bit for the water to cool down. I don't understand how simply removing the heating mechanism can instantly cool it enough to lower it from the boiling point.

 

I'm further confused about this because as I have always understood it one of the qualities of water is that it is relatively good at diffusing/absorbing temperature changes. I'm probably not saying that right, but as I understand it, it's better than most liquids at not being overly susceptible to temperature changes so it like holds its temperature longer or something. Is that right?

 

I have two personal theories on why water might instantly stop boiling. If I'm understanding the above quality of water correctly then is it that I simply don't have enough water to resist sudden temperature changes? If I could boil a whole swimming pool (or water tower, or lake, or ocean, or whatever) and then remove it from the massive flame would it continue to boil for a bit longer because there would be so much more of it that it wouldn't be as immediately impacted?

 

My other 'theory' is that it has something to do with pressure. With the pot of water resting on the heating source is that also supplying pressure of some kind, and thus when the pot is lifted off the boiler not only is the temperature reduced but so is the pressure? This makes sense to me in that if I simply turn off the stove without moving the pot of water it does gradually cool down as I would expect.

 

The only other idea I have is that since boiling point is the highest temperature water (in the liquid state) can reach as soon as it's reduced at all it's simply not hot enough to boil anymore. In other words it can never get high enough past boiling point to have room to play with if it's taken away from the burner. This doesn't make sense to me though because it's somewhat counter intuitive to how I think. It still seems like it would take a little while. Also it seems to fly in the face of the fact that water does seem to have different levels of boiling. It has a gentle bubbling/simmering, a medium sort of boil, and then the all out vigourous boil one is supposed to wait for. I would expect it to progress backward through the stages as well.

 

Anyway, I've asked a few people about this, but no one's ever explained it in a way that made sense and stuck with me, so I'm hoping someone does now

 

-Kevin

[Graeme]

I don't know but I'll make a good guess

 

The answer is that it doesn't go immediately off the boil. What you are seeing is the cessation of a difference in temperature because the heating source is hotter than the boiling water. This converts some of the water to steam, and that's what you think is 'boiling' -- the steam forcing it's way up from the bottom of the container to the top, and escaping. With the removal of the heating source -- which has been fighting against the water's natural tendency to equalise it's temperature throughout the liquid -- the bottom of the contain quickly achieves an equilibrium of temperature. The water itself is still at the boiling point (though it'll start to cool), but there is no outside source to force the creation of steam, and hence the water no longer 'boils'.

[MikeL]

I wouldn't know. Frogs tend to avoid boiling water.

[Aeroplane]

OK, here's something I've always wondered about but no one's ever been able to explain it in such a way that made sense to me.

 

Why is it that as soon as you take boiling water off the stove it quits boiling?

 

The only other idea I have is that since boiling point is the highest temperature water (in the liquid state) can reach as soon as it's reduced at all it's simply not hot enough to boil anymore. In other words it can never get high enough past boiling point to have room to play with if it's taken away from the burner. This doesn't make sense to me though because it's somewhat counter intuitive to how I think. It still seems like it would take a little while. Also it seems to fly in the face of the fact that water does seem to have different levels of boiling. It has a gentle bubbling/simmering, a medium sort of boil, and then the all out vigourous boil one is supposed to wait for. I would expect it to progress backward through the stages as well.

 

Anyway, I've asked a few people about this, but no one's ever explained it in a way that made sense and stuck with me, so I'm hoping someone does now

 

-Kevin

 

 

Your only other idea is correct. The water will be *Exactly* at the boiling point, so when no more energy is being supplied to make Energy Loss = Energy Gain (energy loss from steam, heat, etc and energy gain from the heating) it instantly has no more energy to stay at a rolling boil. Thats the simple explanation.

 

To get a bit more complicated we need to look at what is actually happening when we boil water. Have you ever noticed that a rolling boil is not an 'on/off' process? It arrives there gradually doesnt it? The process of heating is the process of adding more energy to the molecules of whatever you are heating, in this case water.

The steam coming from the water is actually the water molecules with the highest energy because they have the energy to 'escape' the water more easily.

 

Ok, so think about the water now you know that there are different levels of molecule energy within it - its a 'bell-curve' - most water molecules will be at a certain average point.

 

When the heat is still being applied, the escaping high energy molecules are being replaced at the same rate they are being lost if it is at a rolling boil.

But if the heat is removed, the high energy molecules are still escaping, and they are not being replaced. So the highest energy molecules have been lost, and there are less molecules at an extremely high energy - this leaves the molecules at 'average' and 'low' energies, this isnt enough to keep the water boiling. Partly because the vigorous boiling you see is caused by the high energy molecules escaping from the bottom of the pan and disturbing the water, and partly because the temperature has dropped towards the previous average enough to take it away from the boiling point.

 

Pressure is also an important issue, but only in the context of what ive just explained above - a substance boils when its 'vapour pressure' equals atmospheric pressure. Vapour pressure is just a chemical term meaning how much pressure the substance exerts on it's environment, it goes up when that substance is heated.

So from this you can see that if atmospheric pressure goes down, so does boiling point.

Keeping the lid on the pan probably lowers the pressure slightly, because it is holding in 'low pressure' (low density) steam and keeping high pressure atmosphere out, but the most important effect is mitigating the loss of those highly energetic molecules I was talking about earlier - its harder for them to escape if the air above them is crowded with other escapees.

 

I hope this is ok .

[Aeroplane]

OK, here's something I've always wondered about but no one's ever been able to explain it in such a way that made sense to me.

 

Why is it that as soon as you take boiling water off the stove it quits boiling?

 

It seems to me that it would take a bit for the water to cool down. I don't understand how simply removing the heating mechanism can instantly cool it enough to lower it from the boiling point.

 

I'm further confused about this because as I have always understood it one of the qualities of water is that it is relatively good at diffusing/absorbing temperature changes. I'm probably not saying that right, but as I understand it, it's better than most liquids at not being overly susceptible to temperature changes so it like holds its temperature longer or something. Is that right?

 

I have two personal theories on why water might instantly stop boiling. If I'm understanding the above quality of water correctly then is it that I simply don't have enough water to resist sudden temperature changes? If I could boil a whole swimming pool (or water tower, or lake, or ocean, or whatever) and then remove it from the massive flame would it continue to boil for a bit longer because there would be so much more of it that it wouldn't be as immediately impacted?

 

My other 'theory' is that it has something to do with pressure. With the pot of water resting on the heating source is that also supplying pressure of some kind, and thus when the pot is lifted off the boiler not only is the temperature reduced but so is the pressure? This makes sense to me in that if I simply turn off the stove without moving the pot of water it does gradually cool down as I would expect.

 

Ah and to look at some of your other ideas.

 

Water is resistant to being heated because it can hold a lot of energy so it takes a lot of energy to heat, that is probably what got you that idea .

To illlustrate the example: it takes 4.2 joules to raise 1 g of water by 1 degree, so if you want to boil 2 litres (I will put this in a term the 'international' audience can understand later ), starting at 25 degrees C and ending at 100, that will be 2000g of water you want to raise by 75 at a price of 4.2 per gram. 2000x75x4.2 = 630000 J! or 150000 calories. Thats 13.3% of your recommended (UK) daily allowance of energy you can eat. Its about 40g of fruit and nut dairy milk chocolate .

 

If you boiled a whole ocean (we'll overlook the ecological impact) my explanation above would still be the case, it would just be losing a lot more high energy molecules and the average would still drop too far without any heating to replace them. Possibly it would stay heated a little longer because there is more of it to insulate the rest, but high energy molecules would still be escaping from the surface .

 

What type of cooker do you have? If it is electric, then the reason the pot does that when you just turn off the heat is because the electric heat does not vanish instantly - turn on a ring, leave it to heat up, turn it off and hold your hand over it. It should still be hot. This means that the ring is still helping to produce the high energy water molecules that cause the water to boil. Though this time it has a limited supply of energy, which is why it does not last forever and only prolongs the boiling a little while.

 

If it is a gas or other 'instant off' type heat.. I am not entirely sure. Perhaps the metal you are standing the pan on is heated a little (though this would be a very minor effect I think). I think the most likely explanation is that by not touching and moving the pan, you are not 'disturbing' the 'not quite high enough' energy water molecules and arent therefore knocking *just* enough energy into them to make them decide to move and thus arent contributing towards the drop towards average.

 

 

[Myk]

I'll see if I can try a different approach here.

 

When you apply energy to a substance, that energy either goes to raising the temperature of that substance or to changing it's state (solid, liquid, gas) and that is called a phase change. So if your pot of water is at 100 C (the boiling point at sea level) it still takes more energy to convert the water to the gas phase. Water held at the boiling point without that extra 'kick' technically won't evaporate.

 

What you (and most people) are referring to as boiling is more appropriately called bubbling. It's bubbles of water vapour that have formed at the bottom of the pot from nucleating reactions. That is also the hottest place the water is touching while on the burner. Now when you take it off the burner it is still boiling, you just don't have the bubbles rising from the bottom anymore because it is no longer hotter than the rest of the pot of water. Without more heat you are not able to make water change states from liquid to gas. However steam is still rising from the pot, that is your boiling, your evaporating of the water because it is still right at the boiling point and as the molecules collide at the right angle it is just enough to cause some I those molecules of water to convert into gas and fly away as steam.

 

Pressure is a whole nother matter and bears nothing to do with why water stops bubbling when removed from the burner. As stated above it's atmosphereic pressure and it changes the boiling point. Pressure applied directly to a substance would prevent it from making the phase transition...for instance water at 150 C but still a liquid. Same idea behind making diamonds, lots of heat and especially pressure and it gets 'crushed' into a solid jewel.

 

Hope that sort of makes more sense to ya.

 

Greg

Edited February 10, 2009 by Myk

[AFriendlyFace]

Thanks everyone!

 

Graeme's answer, which seems like a less technical version of Greg's answer, was the easiest for me to understand, but I think I'm actually grasping what Xeran and Greg are saying as well.

 

That was very helpful, and made more sense to me than most of the answers I've received on this in the past

 

Water is resistant to being heated because it can hold a lot of energy so it takes a lot of energy to heat, that is probably what got you that idea .

To illlustrate the example: it takes 4.2 joules to raise 1 g of water by 1 degree, so if you want to boil 2 litres (I will put this in a term the 'international' audience can understand later ), starting at 25 degrees C and ending at 100, that will be 2000g of water you want to raise by 75 at a price of 4.2 per gram. 2000x75x4.2 = 630000 J! or 150000 calories. Thats 13.3% of your recommended (UK) daily allowance of energy you can eat. Its about 40g of fruit and nut dairy milk chocolate .

Thanks, that was what I was talking about. I was just too rusty on it to explain it better.

 

If you boiled a whole ocean (we'll overlook the ecological impact) my explanation above would still be the case, it would just be losing a lot more high energy molecules and the average would still drop too far without any heating to replace them. Possibly it would stay heated a little longer because there is more of it to insulate the rest, but high energy molecules would still be escaping from the surface .

That makes sense.

 

What type of cooker do you have? If it is electric, then the reason the pot does that when you just turn off the heat is because the electric heat does not vanish instantly - turn on a ring, leave it to heat up, turn it off and hold your hand over it. It should still be hot. This means that the ring is still helping to produce the high energy water molecules that cause the water to boil. Though this time it has a limited supply of energy, which is why it does not last forever and only prolongs the boiling a little while.

It is electric, and I was of course thinking that the fact that it was still sitting on the hot burner was probably doing it. I just didn't understand why lifting it off the burner would make such a sudden difference, but I think I do now.

 

 

 

Pressure is a whole nother matter and bears nothing to do with why water stops bubbling when removed from the burner. As stated above it's atmosphereic pressure and it changes the boiling point. Pressure applied directly to a substance would prevent it from making the phase transition...for instance water at 150 C but still a liquid. Same idea behind making diamonds, lots of heat and especially pressure and it gets 'crushed' into a solid jewel.

Ohhh

 

 

 

Thanks guys

 

-Kevin

[Graeme]

When you apply energy to a substance, that energy either goes to raising the temperature of that substance or to changing it's state (solid, liquid, gas) and that is called a phase change. So if your pot of water is at 100 C (the boiling point at sea level) it still takes more energy to convert the water to the gas phase. Water held at the boiling point without that extra 'kick' technically won't evaporate.

That goes to show how long it has been since I did High School physics. I had forgotten about the energy required for a phase change, though I remembered it as soon as Greg reminded me.

[JamesSavik]

Simplest answer:

 

Water can not exist in the liquid state at temps above 100C.

 

Boiling is water becoming water vapor and escaping as steam.

 

It takes a constant input of heat to keep water boiling because the water loses energy as the steam escapes.

[KJames]

Simplest answer:

 

Water can not exist in the liquid state at temps above 100C.

 

Boiling is water becoming water vapor and escaping as steam.

 

It takes a constant input of heat to keep water boiling because the water loses energy as the steam escapes.

 

Technically, boiling is any substance reaching its vaporization tempurature and converting from liquid to vapor.

 

Actually, a boil is what happens while a heat source is applied to any container holding any liquid for a period of time, causing the liquid's tempurature to increase until the energy stored in the liquid exceeds its capacity for retention at that air tempurature (I cite that if you heat sugar it melts, and boils much hotter than water due to its crystalline structure). As long as more energy is being put into the liquid than can be transferred out by the constraints of the laws of physics it will "boil" due to the violent molecular change which is occurring. Once a heat source is removed, the liquid still evaporates at a rate faster than normal but is no longer at a "boil", this is why it still creates vapor, until the liquid's tempurature approaches that of the air around it and evaporation returns to a normal rate for that air tempurature, or the liquid cools and returns to its solid form (in the case of my sugar example the semi-solid is also possible, although still classified as liquid, if you have ever seen sugar being spun).

[Bondwriter]

There was a question on a trivia show lately about the temperature at which water evaporated. The answer was: any temperature. Actually, even ice evaporates, as I could notice when we had ten days of below zero/thirty-two degrees lately.

[rknapp]

I don't know if anyone mentioned it but another thing to keep in mind is that any substance that is in the process of a phase change will remain at that temperature threshold until the change is complete and the quality factor reaches a value of 1. In the case of water, it will remain at ~373*K until the saturated liquid-vapor becomes entirely superheated vapor, at which point the temperature will begin rising again. If you remove a pot containing saturated liquid-vapor from the heating source, it will drop below 373*K relatively quick and cease "boiling", as it will then be below the boiling point.

 

Likewise if you put ice cubes in a container and leave it at room temperature, the temperature of the ice will rise to ~273*K (0*C, 32*F). At this point it will melt, and the temperature of the resulting liquid H2O will remain at 273*K until there is no long any H2O in the solid phase. It will then continue rising until it reaches room temperature (or ambient temperature).

 

Pressure plays a significant role in the phase of any substance ONLY if the substance is in a sealed container. A pot of boiling water is open to the atmosphere, so the pressure of the boiling water will always be 101 kPa (1 atmosphere). Notice that "pressure cookers" are completely sealed when in operation. Raise the pressure and the temperature will also rise (PV=nRT, PV=mRT).

 

I hate thermodynamics.

[Myk]

There was a question on a trivia show lately about the temperature at which water evaporated. The answer was: any temperature. Actually, even ice evaporates, as I could notice when we had ten days of below zero/thirty-two degrees lately.

 

Any temperature.....I suppose technically that is true, but extremely unlikely. You would have to alter the pressure (atmospheric) a lot, and in some cases so much so it would no longer be called atmospheric pressure, cause the surounding material would need to be solid or absent to make extremes in pressure. Thats why when you say the boiling point of water is 100 C, that is referring to sea level, which is a standard 1 ATM of pressure.

 

Ice converting directly into steam is called sublimation. It is natural for dry ice, which is frozen (I should say solid) CO2.

 

Good point about the temp not changing during phase changes Robbie, tis very true, if not slightly complicated at times .

 

G

Edited February 11, 2009 by Myk

[AFriendlyFace]

Ice converting directly into steam is called sublimation. It is natural for dry ice, which i frozen (I should say solid) CO2.

For a second there I thought you were trying to take the credit for dry ice - or at least relate some dry ice experience, then I realized you probably meant 'is' instead of 'i'.

 

On another note I just realized we have a thread going right now about eggs and boiling water. Seems an odd coincidence

[Myk]

For a second there I thought you were trying to take the credit for dry ice - or at least relate some dry ice experience, then I realized you probably meant 'is' instead of 'i'.

 

On another note I just realized we have a thread going right now about eggs and boiling water. Seems an odd coincidence

lol, yeah umm...thats what happens when I don't proof read

[Graeme]

On another note I just realized we have a thread going right now about eggs and boiling water. Seems an odd coincidence

I'm more amused by the fact that both are also quite active threads

[Jack Frost]

There was a question on a trivia show lately about the temperature at which water evaporated. The answer was: any temperature. Actually, even ice evaporates, as I could notice when we had ten days of below zero/thirty-two degrees lately.

Ice would still melt at -2C if the sun is out and strong enough. The resulting liquid would dry up due to low humidity of the cold air.

 

But it's a different story when it's -15C and sunny. Ice would be so frozen by then.

[rknapp]

I'm more amused by the fact that both are also quite active threads

I'm having visions of that joke thread that was posted a while back where the arbitrary thread topic was changing a light bulb, and it described each type of poster that every forum has. Hmmm, where the people who give tips on the best ways to boil water, or to peel an egg? Where's the guy who says boiling water and peeling eggs is for losers? Where's the guy who has a unique contraption that boils eggs and peels them at the same time but refuses to post pictures about where? Where are the posters that flame the above-mentioned posters?

[KJames]

I don't know if anyone mentioned it but another thing to keep in mind is that any substance that is in the process of a phase change will remain at that temperature threshold until the change is complete and the quality factor reaches a value of 1. In the case of water, it will remain at ~373*K until the saturated liquid-vapor becomes entirely superheated vapor, at which point the temperature will begin rising again. If you remove a pot containing saturated liquid-vapor from the heating source, it will drop below 373*K relatively quick and cease "boiling", as it will then be below the boiling point.

 

Likewise if you put ice cubes in a container and leave it at room temperature, the temperature of the ice will rise to ~273*K (0*C, 32*F). At this point it will melt, and the temperature of the resulting liquid H2O will remain at 273*K until there is no long any H2O in the solid phase. It will then continue rising until it reaches room temperature (or ambient temperature).

 

Pressure plays a significant role in the phase of any substance ONLY if the substance is in a sealed container. A pot of boiling water is open to the atmosphere, so the pressure of the boiling water will always be 101 kPa (1 atmosphere). Notice that "pressure cookers" are completely sealed when in operation. Raise the pressure and the temperature will also rise (PV=nRT, PV=mRT).

 

I hate thermodynamics.

 

 

For someone who hates thermodynamics, R', that's an excellent explanation.......

 

 

Q: How many thermodynamic engineers does it take to boil a pot of water?

 

 

A: None, they only describe it happening--and they don't know where their spouse puts the pots in the kitchen.

[KJames]

I'm having visions of that joke thread that was posted a while back where the arbitrary thread topic was changing a light bulb, and it described each type of poster that every forum has. Hmmm, where the people who give tips on the best ways to boil water, or to peel an egg? Where's the guy who says boiling water and peeling eggs is for losers? Where's the guy who has a unique contraption that boils eggs and peels them at the same time but refuses to post pictures about where? Where are the posters that flame the above-mentioned posters?

 

 

And I'll leave it to you, RKnapp, to come up the the Rube Goldberg inspired contraption that will hard-boil, then peel, eggs.

[rknapp]

And I'll leave it to you, RKnapp, to come up the the Rube Goldberg inspired contraption that will hard-boil, then peel, eggs.

 

Oh but I already have! It works perfectly, giving me a great hard-boiled egg with no traces of shell or inner membrane, and you can't tell it was peeled by machine! I can't take any pictures though because I left my expensive Lyka (sp?) camera at my hot italian boyfriend's mansion in Naples, and my Leerjet is in the shop having its Rolls Royce engines tuned for supersonic speed!

 

(actually that statement wasn't entirely false... my hot boyfriend is part italian!)

[KJames]

Oh but I already have! It works perfectly, giving me a great hard-boiled egg with no traces of shell or inner membrane, and you can't tell it was peeled by machine! I can't take any pictures though because I left my expensive Lyka (sp?) camera at my hot italian boyfriend's mansion in Naples, and my Leerjet is in the shop having its Rolls Royce engines tuned for supersonic speed!

 

(actually that statement wasn't entirely false... my hot boyfriend is part italian!)

 

 

Yeah, I could tell from all the pictures you posted... )

[Bondwriter]

(...) its Rolls Royce engines tuned for supersonic speed!

(...)

Over here in the Caymans, it's a Ferrari or nothing. Rolls Royces are frowned upon. Sooo dreadfully nouveau riche!

[Myk]

Over here in the Caymans, it's a Ferrari or nothing. Rolls Royces are frowned upon. Sooo dreadfully nouveau riche!

 

Please...there is what 2 ferraries on the whole island??

[tomon]

How come nobody went for the really really simple answer:

 

It's Magic!