Reboiling water is when you boil it, allow it to cool below the boiling point, and then boil it again. You often find yourself in a situation when you unintentionally or (un)consciously reboiling water in the kettle. If you have perfectly pure distilled deionized water, nothing will happen if you reboil it. However, ordinary water contains dissolved gases and minerals.
The main reason why you should never re-boil the same water again lies in the fact that the composition of the water at boiling point gets altered. That can be very dangerous and you know little about it.
The chemistry of the water changes when you boil it because this drives off the volatile compounds and dissolved gases. There are many cases where this is desirable. However, if you boil the water too long or reboil it, you risk concentrating certain undesirable chemicals that may be in your water. Examples of chemicals that become more concentrated include nitrates, arsenic, and fluoride.
Generally, boiling water, allowing it to cool and then reboiling it does not present much of a health risk.The moment water is put for re-boiling, the dangerous components that should escape from the water will stay inside. Suddenly, you are interested in knowing more about those harmful substances, am I right? Most common, re-boiled water contains fluoride, nitrates and arsenic. It is known that healthy minerals are retained in boiled water. By re-boiling it the healthy components are creating a dangerous situation to which we are all exposed because they become amassed in the re-boiled water.