AN OLD wives tale says that reboiling water is bad for you, as it will drive out all the oxygen.
Is this true?
Verdict:
PARTIALLY TRUE
Firstly, if reboiling water drove out all the oxygen, and a molecule of water is two hydrogen atoms bonded to one oxygen, this would mean you would be left with a kettle full of hydrogen so you would have just made a bomb.
Now, of course this is not what happens as the act of boiling water does not separate the water molecules into its constituent parts, as you would need a lot more heat to do that. What it does do is release most of the air that was trapped in it which can change its flavour but is not dangerous in any way.
Now as air is 78% nitrogen, 21% oxygen and 1% everything else, it is true that you are boiling off the oxygen but your body doesn’t absorb oxygen through your stomach.
All that air that is trapped inside of you would just build up before eventually getting released as a burp, or possibly finding its way out the other way.
Saying that, there is also some truth to the statement that reboiling water could be potentially dangerous.
Aside from air being trapped in water, there are also dissolved minerals to contend with.
According to a survey of tap water quality in Kuala Lumpur, our tap water has minute amounts of magnesium, sulphate and chloride.
It also has scarier sounding heavy metals such as copper, chrome, nickel, cadmium, iron and even lead.
Saying that, the amounts are all within the acceptable levels and our water is generally of good quality and safe to drink.
Of course Kuala Lumpur is not all of Malaysia, but water processing (and our water sources) is similar enough throughout the country that the mineral composition should be relatively the same.
When you boil water, however, some of it escapes as steam, reducing the amount of water in your kettle thus concentrating the dissolved minerals.
If you reboil it again, you get less water and a higher mineral concentration.
Add more water and reboil it, you add to the amount dissolved minerals, thus increasing its concentration.
Done enough times you could potentially increase the concentration to levels where it can be potentially dangerous.
Now this would require a lot of water reboiled over a long span of time, so reboiling it a couple of times would not represent a health risk.
For example, if you keep water in a kettle, boil it, and add water when the level gets low, you aren’t likely to endanger your health.
Also if you are boiling distilled water or have a good water filter attached to your tap that you service regularly, the issue of high mineral concentration will not even arise.
References:
1. Goldberg, David E. (1988). 3,000 Solved Problems in Chemistry (1st ed.). McGraw-Hill
2. https://www.researchgate.net/publication/249018418_A_Survey_of_Tap_Water_Quality_in_Kuala_Lumpur
3. https://climate.nasa.gov/news/2491/10-interesting-things-about-air/
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