Is it bad to reboil water?


by Zdenka
Tags: reboil, water
Zdenka
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#1
Feb28-09, 11:32 AM
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Alright I'm getting so angry now! Every time my granny is here, she doesn't let me reboil the water in our kettle... she said reboiling water causes cancer! I've never heard it's bad, I thought reboiling water actually kills more bacteria that might form when the water becomes warm.. so you need to reboil it. I'm too lazy to refill the kettle with cold water and boil it again.. reboiling is much more convenient! I mean it's water after all, so how can it be bad? Please tell me I'm right!!
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Astronuc
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Feb28-09, 11:41 AM
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No, it does not hurt or cause more harm to reboil water, and it certainly does not increase the risk of cancer. There should be no more metal cations in reboiled water than in the original water.
arildno
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Feb28-09, 11:54 AM
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Perhaps you should scare your grandma back by saying something like:
"Oh you are right, granny, but for the wrong reason: We should NOT re-boil water out of concern for potentially catastrophic molecular hysteresis effects: The boiled and re-boiled water molecules might finally snap, and when the oxygen and hydrogen atoms meet again, KA-BLAAM!!"


Zdenka
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#4
Feb28-09, 01:11 PM
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Is it bad to reboil water?


WHEWWW!! thanks for backing me up guys, I knew I was right on this one! heheh ;-P

Btw I was googling and somebody said that if you keep on reboiling the water, all the oxygen atoms will disappear and you'll end up with a dangerous amounts hydrogen.. I don't think that's true, though!!
Greg Bernhardt
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Feb28-09, 01:17 PM
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Quote Quote by Zdenka View Post
Btw I was googling and somebody said that if you keep on reboiling the water, all the oxygen atoms will disappear and you'll end up with a dangerous amounts hydrogen.. I don't think that's true, though!!
If you keep reboiling the water, at some point there will no longer be anything in the pot :)

Boiling water doesn't break the H2O bond.
Borek
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Feb28-09, 01:33 PM
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A lot will depend on the material kettle is made off.
Andre
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Feb28-09, 01:48 PM
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Boiling water removes nuclei (like air) that facilitate the bubble forming. One the water is devoid of those nuclei, the water may get superheated, followed by explosive boiling.

So there is something with boiling water over and over again, but not cancer.
Zdenka
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Feb28-09, 02:20 PM
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thanks guys! There's nothing like a Physics forum to put my grandma in her place.
Gokul43201
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Feb28-09, 02:24 PM
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Quote Quote by Andre View Post
Boiling water removes nuclei (like air) that facilitate the bubble forming. One the water is devoid of those nuclei, the water may get superheated, followed by explosive boiling.
Even this possibility is extremely unlikely. In a metal kettle, boiling is almost certainly dominated by surface nucleation, rather than bulk nucleation (which would be more likely for a really clean and smooth glass container). So, the concentration of dissolved air should not noticeably alter the bubble nucleation rate.

Furthermore, if you assume the depth of the water in the kettle is about 4 inches (about 10cm, to pick a round number), and looking up the diffusivity for O2 and N2 even at room temperature (about 2-3*10^-9m2/s), and approximating the re-dissolution of air into the kettle as a 1-d diffusion problem, I get a time constant of less than 105s. So, in some hours since the last boiling, the kettle is pretty close to being at equilibrium with air again. So even if the bubble nucleation is dominated by the dissolved air, it appears that there will not be much difference in the amount of dissolved air after a day from the previous boiling.
nottheone
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#10
Feb28-09, 04:10 PM
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It will concentrate anything that is in the water though (that isn't volatile enough to come out in the steam), doubly since it is the second boiling. Studiously always take it right off the second it starts to boil and practically no effect. But who doesn't forget a boiling pot once in a while? That may or may not be a bad thing depending on what's in the water and what if anything is leaching from the kettle. Is it that much a hardship to refill it? Do you know exactly what's in your water? I wouldn't do it, grandma is right (and no one knows everything that causes cancer or who is susceptible and to what, got cancer in your family? Why take the chance). This is one of those things like greenhouse gases, stop doing it and you won't be worse off, don't stop and you roll the dice.
Borek
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Feb28-09, 05:21 PM
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Quote Quote by nottheone View Post
It will concentrate anything that is in the water though (that isn't volatile enough to come out in the steam), doubly since it is the second boiling.
While that's not completely off, nobody boils water long enough for substantial increase of concentration of whatever is in the water. Electric keetles switch off seconds after the water starts to boil. Say it is 10 seconds at 2 kW. That's 20 kJ. At ΔHvap around 40 kJ/mol you are loosing 9 grams of water, out of - say - two cups, or around 500 grams. This is volume change of less than 2%, and similar concentration change.

If you put keetle on the gas stove and you forget to switch it off, that's another story, but how often you do that?
Blenton
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#12
Feb28-09, 06:57 PM
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Now if one were to reduce say 10 cups of water to 1 in a kettle, would the increased concentration have any health problems with it?
Greg Bernhardt
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Feb28-09, 07:04 PM
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Quote Quote by Borek View Post
If you put keetle on the gas stove and you forget to switch it off, that's another story, but how often you do that?
ha i did that in college, left a small pot of water on the burner for 3 hours!!! i rushed into the kitchen in a panic when I remembered and grabbed the metal handle thus burning the crap out of my hand. The bottom of the pot was a strange greenish yellow. I threw it out!
Zdenka
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Feb28-09, 07:13 PM
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Quote Quote by nottheone View Post
But who doesn't forget a boiling pot once in a while? That may or may not be a bad thing depending on what's in the water and what if anything is leaching from the kettle.
Ours is an electric kettle though, it just turns off automatically once the water reaches boiling point. I don't think anyone uses a pot over the stove, nowadays! Except maybe grandmas, hahah

The reason I reboil water is that it saves time and energy (literally) of having to refill the kettle with COLD water again and reboil it.. I save on electricity! Sometimes you just want to have another cup of coffee during the afternoon, but the kettle of water you boiled in the morning has become warm already.
TheStatutoryApe
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Feb28-09, 11:13 PM
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Quote Quote by Borek View Post
While that's not completely off, nobody boils water long enough for substantial increase of concentration of whatever is in the water. Electric keetles switch off seconds after the water starts to boil. Say it is 10 seconds at 2 kW. That's 20 kJ. At ?Hvap around 40 kJ/mol you are loosing 9 grams of water, out of - say - two cups, or around 500 grams. This is volume change of less than 2%, and similar concentration change.

If you put keetle on the gas stove and you forget to switch it off, that's another story, but how often you do that?
considering its his grandmother its possible she grew up with water that wasn't nearly as pure. maybe a slight increase in concentration of the impurities was enough to greatly change the taste. also your earlier note of what the kettle was made of may apply here more so. I understand there are people who are very snooty about their tea aswell. just as bad as wine snobs.
Loren Booda
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#16
Mar1-09, 01:00 AM
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If you want to concentrate the solutes in water, a most simple way is to boil the water off.

I had a kettle with some kind of coating inside that peeled off. That kettle has piped its last.
physics girl phd
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#17
Mar1-09, 08:46 AM
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Quote Quote by TheStatutoryApe View Post
considering its his grandmother its possible she grew up with water that wasn't nearly as pure. maybe a slight increase in concentration of the impurities was enough to greatly change the taste.
This is decidedly true with the well-water I grew up with (if you let "fresh" water from the spigot sit in your glass for a half hour, you'd see iron oxide deposit out of it). We had to scour the inside of our teapot monthly (and worry towards the end of the month that iron oxide would flake off the inside of the pot and appear in your tea ). This was before the days of "Brita" pitchers and the like (even now my mother replaces the filter three times more than specified AND replaces the pitcher every year or less). I NEVER drank that water straight up... it always needed lemon or a touch of Kool-aid powder something.
mgb_phys
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Mar1-09, 11:18 AM
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If you are really fussy about your tea you should use fresh water because it has more air dissolved, heating removes the air.
Of course this might just be along the lines of people who buy speaker cable with directional copper - but if people can make a fuss about the proper way to drink wine then I suppose you can do the same with tea.


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