There is a commonly shared idea that you shouldn’t boil the same water twice. When the water is boiled, the bacteria contained in it cannot survive. So, if the bacteria are already dead, why the water that has been boiled twice is considered to be dangerous? Although this opinion is extremely odd, there might be some sense behind it. However, the re-boiled water is not threatening your health.
Boiling the Same Water Twice Is Far From Dangerous
The tap water is nowhere near pure. There are a lot of gases, such as oxygen and carbon dioxide, and minerals, such as magnesium and calcium, dissolved within the tap water. When the water is boiled, all these substances evaporate. It doesn’t really matter if the water is boiled more than once, especially since the longer you boil it the more substances will evaporate.
Most of the minerals that are dissolved in the water still remain at the bottom of the water in a concentrated form, even after boiling it for a long time. If you boil the water twice, the amount of these minerals will get more concentrated. However, most of these minerals are not dangerous at all so, it is completely irrelevant whether they are a little more concentrated.
The only reason why you should avoid boiling the same water twice is that the water contains some dangerous chemicals, such as uranium and barium. When you boil tab water, the water at the bottom of the kettle cannot be replaced. By boiling the water more than once, you keep adding more of these chemicals which remain in a concentrated form. Nevertheless, the level of these chemicals is extremely low. So, drinking water that has been boiled more than once cannot affect your health at all.