If you hear these things when you're young enough, or if you hear them a lot over a long period of time, it's hard not to simply accept them as true. Sure, we'd all like to think we've got strong enough BS detectors that nothing gets by us, but that's not how we work. Every now and then, something slips through the cracks, and we don't even realize it has until something comes along to fly in the face of what "everybody knows." That's why it can take years before you find out that waking a sleepwalking person won't harm them, and you're not going to actually catch a cold because it's cold outside.
As these myths slowly die out, new ones can come along to replace them. A surprisingly persistent one has to do with whether it's safe to boil the same pot of water a second time. As with many myths, it takes a kernel of truth and stretches it a bit too far. Let's get to the bottom of it.
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We can all agree that boiling your water is a good idea.
When you're out in the wilderness and you find some natural water, it's good practice to boil it before you drink it to kill off any unwanted microbes that might be out for a swim.
But what if you're all ready to make some tea and you get a phone call?
When you come back, you find that water you boiled has now gone cold. Can you boil it again?
If you ask some people, it's the worst thing you could do.
When boiling water a second time, the trace amounts of minerals in the water like arsenic are supposed to become concentrated to deadly proportions. This, so the story goes, makes the water toxic and puts you at risk of a list of diseases as long as your arm.
It sounds scary, but there are a few problems with this claim.
For one, those minerals have to actually be there in the first place to concentrate. If you boil distilled water twice, nothing even close to that could possibly happen.
But what about tap water?
Sure, boiling water enough for, say, three ounces to evaporate will bring you a measurable difference in concentration for any dissolved chemicals in the water.
But there are a couple of reasons why even this concentration is nothing to worry about.
But there are a couple of reasons why even this concentration is nothing to worry about.
Suppose you boil a quart of tap water.
You then use about 6.7 ounces of that water to make a cup of coffee or tea. When you drink it, let's say you also take in about 0.2 milligrams of fluoride. This is well within acceptable levels of fluoride according to the American Public Health Service.
Now, let's say you boil the water enough to evaporate three ounces of water. By the way, this takes way longer than you would need to boil your water a second time.
Now, let's say you boil the water enough to evaporate three ounces of water. By the way, this takes way longer than you would need to boil your water a second time.
So after all that, how much fluoride would be in a cup of tea made from this water?
All told, you end up with 0.22 milligrams of the stuff. Not only is this still well below potentially harmful levels, but it's not even significant compared to how much you had to start with.
Therefore, you'd only reasonably get dangerous levels of concentration in your water if it wasn't safe to drink to begin with.
Therefore, you'd only reasonably get dangerous levels of concentration in your water if it wasn't safe to drink to begin with.
The only reason not to reboil your water is it might make your tea taste worse.
This is because boiling water drives oxygen out of it, which often makes it taste flatter.
Main image via Wonderopolis
Collage image via Science ABC | Roman Sigaev / Fotolia
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