Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: John Archer on 26/03/2011 15:30:02
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John Archer asked the Naked Scientists:
Which boils faster, hot or cold water?
What do you think?
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Hot does, it has to hot to boil.
But if the ATM is 50,662.5pa or 1 ATM. it'll melt so it wont matter.
http://www.thenakedscientists.com/forum/index.php?topic=1585.msg349990
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Which boils faster, hot or cold water?
Hot.
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Hot of course. The temp is already high and you need less energy to raise it further.
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Is it a trick question?:)
Cold water can't boil 'faster' if you by that mean comparing one pot of hot, and one pot of cold water, simultaneously warming the water in them to their boiling point. But cold water will warm up faster than warm, as that is a question of the heat-transfer from the stove to the fluid. The hotter something is, the more energy it craves to heat it further. This one may be surprising (http://math.ucr.edu/home/baez/physics/General/hot_water.html)though.
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The hotter something is, the more energy it craves to heat it further.
Yoron, Are you sure that's what you meant to say? If so, I'd like to see some proof.
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Now you got me confused Geezer?
The hotter something is the more energy you need to heat it further? Assume 50 C, what do you need to heat it to 51 C? A source that at least have a heat/energy of 51 C? Assuming a perfect conduction that is. Maybe you read it as me meaning that the 'energy window' is bigger the more energy you want to transfer? Well, it is as i see it? If you look at the amount of energy being transfered per time unit, but it depends on what substance it is and its properties too, there are some temperatures where a fluid utilize the conduction better that otherwise.
"At low driving temperatures, no boiling occurs and the heat transfer rate is controlled by the usual single-phase mechanisms. As the surface temperature is increased, local boiling occurs and vapor bubbles nucleate, grow into the surrounding cooler fluid, and collapse. This is sub-cooled nucleate boiling, and is a very efficient heat transfer mechanism. At high bubble generation rates, the bubbles begin to interfere and the heat flux no longer increases rapidly with surface temperature (this is the departure from nucleate boiling, or DNB). At higher temperatures still, a maximum in the heat flux is reached (the critical heat flux, or CHF).
The regime of falling heat transfer that follows is not easy to study, but is believed to be characterized by alternate periods of nucleate and film boiling. Nucleate boiling slows the heat transfer due to gas bubbles on the heater's surface; as mentioned, gas-phase thermal conductivity is much lower than liquid-phase thermal conductivity, so the outcome is a kind of "gas thermal barrier". At higher temperatures still, the hydrodynamically-quieter regime of film boiling is reached. Heat fluxes across the stable vapor layers are low, but rise slowly with temperature. Any contact between fluid and the surface that may be seen probably leads to the extremely rapid nucleation of a fresh vapor layer ("spontaneous nucleation")."
Or, how did you read it? Also. It takes 1 calorie of heat energy to raise 1 milliliter of water by 1 degree °C or 4.184J/g °C. And depending on the temperature difference between the heating source and the sink, cold water will have a larger temperature difference and so, initially, heat somewhat faster, slowing as the water warms. As I understands it, that is? So yes, I think my statement is correct?
To heat something you have two big ones on Earth. Pressure from the atmosphere (that compress the water molecules together into a liquid) and the bonding energy between each water molecule. So the lower the pressure the faster water boils. Also I assume gravity as such to play a role here even though you might assume that implicitly included.
When it comes to what chaos math like to describe as 'emergences', when substances gets new and different properties (like ice into water into steam) it's a little different as I understands it. Take a look here Heating and Cooling Curves. (http://www.kentchemistry.com/links/Matter/HeatingCurve.htm)
Do you have a different description?
You could cheat if you like, and magically heat cold water faster than warm, you just need to add something that reduce the heat uptake of the water before it starts boiling. Take a look here Nifty tricks. (http://askville.amazon.com/make-cold-water-boil-faster-warm-hot/AnswerViewer.do?requestId=7888741) There you will find a way mentioned to make cold water (ice) boil faster than hot too. Although I don't think it is right? maybe??
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I hope I covered it now :)
It becomes a complicated subject if you look at the specific fluids/substances involved. But generally water will boil faster the warmer it is, as you start heat it. As far as I know.
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At least for polimolecular gases (which is more easy to understand for me [:)]) it's possible to say that, infact, specific heat increases with temperature: the more the T, the more energy levels are activated: first the translationals, then the rotationals too, then the vibrationals too.
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This one may be surprising (http://math.ucr.edu/home/baez/physics/General/hot_water.html)though.
I think that's a horse that has been beaten dead on the forum already, as Geezer and Lightarrow probably recall. There's at least debate on how real that effect is. :p
But for boiling water, I think there's a much easier effect to see. Namely, it's not too hard to superheat water beyond it's boiling point. Water boils easier when it can form bubbles of water vapor, and to form bubbles, it has to be in contact with a rough surface or have particles floating within it. (You'll notice when boiling water on a stovetop, that small bubbles form on the bottom and sides before it begins bubbling.) If you're heating fairly pure water quickly and in a smooth container, you can heat it above 100 C without it boiling. Of course, since it wants to boil, as soon as you give it somewhere to form bubbles, it boils violently, and can be quite dangerous.
I've experienced this a few times when using a microwave to heat water in a mug for tea. Several times when I've added the tea bag, the water has boiled over suddenly, because the water it was superheated and could easily form bubbles on the tea bag.
But this is a trick that would only work if you used a very smooth mug for the hot water and a very rough mug for the cold water and if they were very close in temperature to begin with.
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Now you got me confused Geezer?
Sorry Yoron [:D] Maybe it's a terminology thing.
What I mean is that it always takes the same amount of energy to raise the temperature by a certain amount. The specific heat of a substance is its thermal energy capacity per unit mass.
So, I would say its "craving" for heat is constant regardless of its temperature.
(Although it looks like Lightarrow will point out that is not always the case.)
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Now you got me confused Geezer?
Sorry Yoron [:D] Maybe it's a terminology thing.
What I mean is that it always takes the same amount of energy to raise the temperature by a certain amount. The specific heat of a substance is its thermal energy capacity per unit mass.
So, I would say its "craving" for heat is constant regardless of its temperature.
(Although it looks like Lightarrow will point out that is not always the case.)
There's also heat transfer to take into account. In theory, the hotter the water is, the less efficient your burner will be at actually transferring energy to it, since heat transfer is generally more efficient for larger temperature differences.
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There's also heat transfer to take into account. In theory, the hotter the water is, the less efficient your burner will be at actually transferring energy to it, since heat transfer is generally more efficient for larger temperature differences.
I fully agree with that. The rate of energy transfer is greatly affected by temperature difference, and that could have a major impact on efficiency. Maybe that is what Yoron was getting at.
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Ok, so, if you have cold water at 1°C and you put it in a pot on the flame, it will take less time to heat it to 41°C then the time required to heat hot water at 41°C from 41°C to 81°C.
But if you want to boil the cold water, you still have to reach 41°C, then you have to take it from 41°C to 100°C, so the time required to boil cold water will always, obviously, be greater then the time required to boil hot water...
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Ok, so, if you have cold water at 1°C and you put it in a pot on the flame, it will take less time to heat it to 41°C then the time required to heat hot water at 41°C from 41°C to 81°C.
But if you want to boil the cold water, you still have to reach 41°C, then you have to take it from 41°C to 100°C, so the time required to boil cold water will always, obviously, be greater then the time required to boil hot water...
I don't think that was ever in doubt [;D]
(And if someone can produce an experiment that demonstrates otherwise, it will be the scientific breakthrough of the century!)