Why will water start to taste bad when you leave it in a glass for too long, but will taste fine when you dispense it out of a tap where it’s been sitting for months?

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If you leave water in a ceramic, glass, or cup it will start to taste bad after a day or two, even if it’s covered with a lid. However, when we pour water out of a tap into a glass it will taste fine, even though it’s been sitting for days/months in a water reserve tank. Why is this the case?

In: Other

Normally, water has a bunch of dissolved gases in it, as well as dissolved minerals that can react with oxygen. Inside the pipe/tank, those gases have nowhere to go and there’s no oxygen to react with, so the water doesn’t really “age”. As soon as you sit it out in the open the dissolved gas mix changes and any reactions that need oxygen to proceed can get going. There’s enough of that going on in most water to change the taste enough to notice.

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Water evaporates from the glass over time, potentially leaving a higher concentration of everything else (minerals, chlorine, etc…)

When exposed to air and light, the water in the glass can … well, grow stuff.

The water in the glass can absorb stuff from the air… dust, scents, etc…

So first off the water in your tap isn’t sitting around for months. It came pretty recently from a water treatment facility of some kind and stayed mostly sealed away from air and light and odors and dust until it got to your tap.

Your accustomed to the taste of chlorinated water. When you leave a glass sitting out it de gasses and also the chlorine evaporates out. I actually prefer it.

As others said, mostly loss of dissolved gases.

Our water company aerates the water as part of the treatment from open reservoir to pipe. Basically a open tank/pond with water forced up, like a decorative fountain.