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I have to comment on the latest bit of woo going around, about the dangers of re-boiling water. Well, I already did comment, but since it brings up an objection I haven’t seen anyone else raise I figure this audience would be interested in reading about it.


This woo fails on so many levels. Let’s say there’s X grams of non-volatile stuff dissolved in Y kilos of water, and ignore saturation. Try the following experiment:

1. Fill your favorite kettle with water, and check the water level.
2. Set it to boil.
3. Once done, come back and look at the water level.

The ratio of the water level before it was boiled to the level after determines how much the concentration has risen. So if half of the water is gone after boiling, you’ve doubled the concentration of non-volatiles. You might get that scenario with a stove kettle you’ve forgotten about; if you then top it back up with regular water and pay more attention, the concentration is now 50% higher than normal (that’s 0.5 * 2 + 0.5 * 1).

But I can’t see any drop in water level after my electric kettle automatically shuts off. The increased concentration is negligible, while at the same time I’ve boiled off the volatiles and killed off almost all the organisms that’d make me sick. This is a net win, and several more rounds of boiling via my electric kettle will only improve the situation.

This story is trivially refuted by information you already have on hand.

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