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Why Does Tap Water Go Stale Overnight?

A few months ago I experienced a frustrating problem over and over again that I’m sure you’ll relate to. I would leave a glass of water out on my nightstand when I went to bed, and every morning when I woke up I noticed the taste was a bit off. It simply didn’t taste as when my water was poured the night before. Why was this happening to my tap water? Having been “had” by the stale morning water on more than one occasion, I finally did some research to see if there was any professional consensus among scientists and water experts as to what causes this strange phenomenon. Here’s what I found:

It turns out, though many people are familiar with the “stale water” conundrum, few scientific studies or experiments have been done on the subject. Why is that? The answer is reassuring: According to what I read, tap water that’s left out for a day or two is completely safe to drink. The slight chalky aftertaste that develops is a natural occurrence because of the initial water treatment along with a few other factors. General consensus: still safe, but not nearly as tasty. (That said, if I were to leave that glass out for a few weeks, airborne microbes and pathogens would start to collect and it would be time to toss it. But who leaves water out that long?)

glass of water on nightstand

Now, for those that are just naturally curious and need hard answers, like me, there are indeed a few concrete reasons why tap water will change in taste over time. The first is a little obvious – the temperature rises a bit, “opening up” the taste and smell of the water. Tap water is kept cooled in your plumbing system, so it will become several degrees warmer as it sits on your end table overnight. An online water sommelier (yes, I swear that’s a real thing) attributes the taste you experience in the morning to the faster molecules in the water lifting the smell of the water to your nose as you drink it.

Once you take temperature into account, the explanation goes into chemistry territory, so bear with me: Most water (in America especially) is treated with trace amounts of chlorine to keep it clean. Chlorine quickly evaporates, so by the time you drink that bedside water in the morning, the chlorine is long gone. The change in taste is actually the lack of chlorine in the water. Regions with less chlorine treatment would most likely notice less of a difference.

Also, as your cup of water sits out in the open, a bit of carbon dioxide will dissolve into it. This makes carbonic acid, which apparently isn’t harmful even though it has the word “acid” in it, but it does lower the PH of the water slightly. This can cause the difference in taste in the morning as well.

There you have it: some scientific jargon to basically explain that it will always taste slightly different after a few hours. After realizing I wasn’t too much of a fan of the chlorine and chemicals used to treat my tap water making it less than delicious the next day, I installed a full house filtration system from Pelican Water. Filtered water doesn’t contain chlorine, pathogens, or other highly suspect microbes that cause that eventual chalky taste. Now that I’ve been used to drinking refreshing filtered water for nearly a month, I can’t imagine going back to straight tap water. That’s the Pelican Water difference.

Disclaimer: The information on this website has not been reviewed by the FDA. Products offered for sale herein are not intended to treat, cure, or prevent any disease or health condition. No medical claims are being made or implied. Contaminants mentioned are not necessarily in your water.
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