But why? – Boiling purifies it. Thus, giving it a second boil should make it all more sterile, right?
But, no. In fact, it is the exact opposite.
Moreover, let’s take a closer look at what happens when water boils.
At first glance, you see it bubble and steam, but there is also something happening at the chemical level. By cooking the same water multiple times, the oxygen content changes and potentially harmful substances accumulate. However, you can actually taste this with tea: Tea brewed with fresh water tastes very different to water that has been re-used.
However, it is true that boiling water is a great way to kill bacteria. But, when you boil water over and over again, harmful gasses and poisonous substances like nitrates, arsenic, and fluoride can form as a result of the changed chemical structure.
Furthermore, some studies have shown a connection between this fluoride and the harmful effects it has on the nervous system and brain.
Always remember that the minerals found in water don’t evaporate. Thus, if you were to boil water over and over again until it had all evaporate, you’d find a crust of minerals is still there. After this process, it really isn’t just a drinking water anymore and the minerals that are usually good for you, like calcium, can create problems and cause kidney stones to form.
This is the reason why you should stop boiling the same water twice. Definitely, it depends on how clean your tap water may be to begin with. However, smaller amounts for your tea isn’t really a big problem, but the accumulated effect over a long period of time is really what might be better to avoid.
So, if you’ve boiled too much water than what is needed, it's better to water it to your plants once it's cooled instead – it isn’t dangerous for them!