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There have been some claims saying that drinking reboiled water is dangerous because it produces arsenic, nitrates, etc. Is this true?

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    $\begingroup$ The claims are not true, they are just rubbish pseudoscience misinformation. Google "reboiled water" and skip the websites that claim to be "natural" "healthy" "chemical-free" etc. Those people don't know what they're talking about but just like to fear monger. $\endgroup$ – orthocresol May 2 '16 at 11:55
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    $\begingroup$ A different concern in reboiling water is that the water has been degassed. Such water can "bump" rather violently when it starts to boil. $\endgroup$ – MaxW May 2 '16 at 18:49
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    $\begingroup$ Drinking boiling water is a bad idea in general - you'll burn your mouth! Wait for it to cool first and you should be fine. $\endgroup$ – Darrel Hoffman May 2 '16 at 18:57
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    $\begingroup$ Boiling water produces arsenic, nitrates, etc? Can it produce gold? $\endgroup$ – paparazzo May 3 '16 at 6:00
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    $\begingroup$ @Paparazzi: sure, about 1 microgram of gold per kilogram of seawater apparently. $\endgroup$ – Steve Jessop May 3 '16 at 11:14
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Reboiling water will do precisely three things:

  1. Some volatile contaminants in the water that survived the first boiling might be driven out by the second boiling.

  2. Some additional evaporation will occur, causing anything dissolved in the water (salts, contaminants) to be slightly more concentrated.

  3. Some additional material from the surface of the vessel you're boiling the water in might dissolve into the water.

It's virtually impossible to imagine any scenario in which this would matter, unless you were taking a huge amount of barely drinkable water and boiling away most of it or using a vessel whose lining could produce toxins in your water. Needless to say, you really shouldn't be drinking barely drinkable water at all, nor should you be boiling water even once in a vessel whose lining can produce toxins. If you're doing these kinds of things, the additional risk from reboiling would be miniscule.

It will definitely not summon aresenic or lead from nowhere.

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    $\begingroup$ I used to go camping a lot, and often times we boiled water to make it safe to drink. In many cases, we would "re-boil" the water because it was easier to do that then keep track of (or separate) "good" water from "bad" water. #2 is your "largest risk" and to be honest, you would have to boil down hundreds of gallons of "bad" water to a few ml worth to be at any kind of risk. That being said, boiling water is usually done to remove bacteria, it won't remove dissolved things (like lead). So don't go drinking a contaminated water, regardless of the number of boilings. $\endgroup$ – coteyr May 4 '16 at 20:57
  • $\begingroup$ @coteyr and if you were that worried about it, get a distilling apparatus! You might lose some minerals, but it doesn't get much safer! $\endgroup$ – Tanenthor May 5 '16 at 10:51
  • $\begingroup$ @Tanenthor There are also micro filters that do not require boiling and remove almost all contaminants that are not dissolved. $\endgroup$ – Tomáš Zato Aug 15 at 12:18
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I am glad you asked this question. There is an element of truth here. If the water already contains contaminants such as arsenic and nitrates, and/or if the pot that the water is being heated in is contaminated, then boiling (or equally well, reboiling) the water could result in higher levels of these contaminants.

On the other hand, if you are starting with pure (i.e. distilled) water in an inert vessel, no amount of boiling is going to cause these contaminants to magically appear.

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    $\begingroup$ "Inert vessel" - so no Philosopher's Stoneware? $\endgroup$ – R.M. May 2 '16 at 14:15
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    $\begingroup$ The very same thing could be same about anything you're putting into any container. There's nothing specific to water, boiled water, or reboiled water. $\endgroup$ – Aaron May 2 '16 at 16:02
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    $\begingroup$ And if the pot is contaminated, it's contaminating each new batch of water, potentially more than any added contamination to first-boiled water. $\endgroup$ – user2338816 May 2 '16 at 23:44
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    $\begingroup$ To put it another way, in terms of these toxins drinking reboiled water is as dangerous as drinking more water. How much more water depends on how long it was boiled for. In practice you aren't going to reduce the volume of water by even 10% just by bringing it to the boil twice under a lid, but you could simmer it. In theory, yeah, you could boil millions of gallons of water down to a nasty dose of arsenic if you could also figure out a way to separate it from the fistfuls of non-toxic salts you're precipitating (limescale). $\endgroup$ – Steve Jessop May 3 '16 at 11:12
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    $\begingroup$ Does the concentration even matter if I plan to drink the whole amount of water anyway? $\endgroup$ – AndreKR May 3 '16 at 18:45
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There has been some claims that drinking reboiling water is dangerous claiming it produced arsenic, nitrates etc. it this true?

It doesn't produce anything. Re-boiling water doesn't change the water into anything dangerous.

However, if the water is already contaminated, then lengthy boiling will evaporate some of the water, and the concentration of the chemicals already in the water will increase due to the lower concentration of water.

If you follow the standard recommendation and boil the water for a full minute, then depending on heat input and environmental conditions you could be evaporating as much as a cup of water a minute. Let's assume the worst case - dry air, very hot stove, lots of surface area on pot, and only two cups of water to start with. If you boil it for a full minute, you might lose half your volume, increasing the concentration of whatever is in the water twice.

So how can you avoid this without capturing the water from the steam?

Use a lid - a lot of steam collect on there and drip back into the pan. Boil a lot of water at once, rather than boiling two cups for a minute, boil 4, 8, or sixteen cups. Boiling a small amount requires little heat, but often people will turn the burner all the way up and leave it there, boiling it off far faster. By boiling a larger amount, it's matched better to the stove and you'll only be adding the heat needed to get it to a rolling boil, rather than adding more heat than necessary.

Once the pot gets to a boil, turn the heat down - it needs less heat to stay boiling than it does to start boiling.

Lastly, once the minute is up, place the pot on a cool surface to cool the water down more quickly. It can take 30 minutes to cool to drinking temperature depending on the environment, and during that time it's evaporating faster than normal. Cooling it quickly will also save water.

Is it safe to drink reboiled water?

It's no safer than drinking boiled water, but each time it's boiled the contaminants become more concentrated. Limit boiling to only when necessary - if the water has been contaminated since the last boiling. Generally you won't need to re-boil water, but if you're unsure then you're simply going to have to weigh the risks of biological contamination against the risks of inorganic contamination.

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There is a very unusual case in which reboiling water makes water (marginally) more dangerous to drink.

Suppose that you have a toxin or pathogen in the water that is neither volatilized nor denatured at 100 degrees Celsius. By boiling the fluid, you reduce the amount of $\ce{H2O}$, while the amount of the dangerous substance remains constant. Then, a fixed amount of boiled fluid will have a higher concentration of the dangerous substance than the same amount of unboiled fluid.

This may seem like a bizarre hypothetical, but this exact situation arose during the 2014 Toledo water crisis, which stemmed from a Microcystis bloom in Lake Erie. Microcystis, a variety of cyanobacterium, produces substances called microcystins, which are not destroyed or removed by boiling. The relevant authorities issued instructions not to boil the water during the crisis. (Of course, it is not the case that boiling converts potable water into unpotable water. Rather, it converts unpotable water into very unpotable water.)

That said, the other answers are correct in stating that reboiling water is almost always a safe practice. Even in water that contains a dangerous substance that is not removed or denatured by boiling, boiling the water is unlikely to make it significantly more dangerous to drink, since in practice, one does not typically remove a large fraction of a fluid's $\ce{H2O}$ content when boiling it.

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First need to separate produce from concentrate.

If two or more molecules reacts to form different molecule(s) then you have produced. Or a single molecule could split. Or a single could could be changed like trans to a cis double bond. Cannot produce an element (e.g. O) - it is present or not. Not a lot of this going on at the boiling temperature of water. The non bacterial components of water are pretty stable.

Boiling water removes volatile components. Arsenic is a chemical element - it cannot be produced. Arsenic is not volatile alone or as part of another molecule. What is going to boil off is H20, N2, CO2, CO ..... The non-volatile components will be concentrated.

Nitrate is NO3 so I guess it is possible to produce it as N and O are present in water but that is highly unlikely. And even if so boiling a second time would not change that. I don't think a nitrate or anything it is a part of is volatile so it would not boil off. It would just concentrate.

You cannot produce arsenic as it is chemical element. It is in the water or not. It would just be concentrated.

What does happen at the boiling temperature of water is the bacteria is killed. If you killed the bacteria the first time then it should not grow back. There is no purpose to boiling a second time if boiling did what it was supposed to do the first time (kill bacteria).

Yes potentially bad stuff could be concentrated but boiling a second time is not a factor. Boil once for 10 minutes is equivalent to boiling twice for 5 minutes in regards to concentration.

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  • $\begingroup$ You can produce arsenic from compounds which contain it. Either way it still isn't dangerous. $\endgroup$ – bon May 3 '16 at 19:38
  • $\begingroup$ @bon What? If it is present then yes it is dangerous. Arsenic is not just dangerous in element form - in fact that is not a common form. Did you vote me down? Regardless you are not going to break down mineral containing arsenic at the boiling temperature of water. $\endgroup$ – paparazzo May 3 '16 at 20:07
  • $\begingroup$ @bon Either way it still isn't dangerous? Not according the medical community and wiki. en.wikipedia.org/wiki/Arsenic_poisoning They even made up symptoms, diagnosis, and treatment? $\endgroup$ – paparazzo May 4 '16 at 0:03

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