Why, when you leave a glass of water standing does it taste horrible after a while?
Can anyone explain the reaction that takes place?
Multiplying bacteria, perhaps?
[Homer]
Mmmmmmm… bacteria!
[/Homer]
Barry
Loss of aeration?
Another explanation might be that it picks up flavor from the vessel it is in.
I think aeration is a pretty good answer though.
Perfectly pure water actually tastes mighty dull. Drinking water contains trace minerals. Real “mountain stream” water is absolutely delicious; freshly distilled water is bland to the point of being “blah.”
re a glass of drinking water that’s just standing for a while, could it just be that it’s warm?
Trinopus
Really, pure water doesn’t taste dull, since it’s tasteless! 
Dust settles in it, methinks.
Well, yes and no… There’s a subtlety here…
Part of it is that we expect water to have a flavor, and if it doesn’t, we’re surprised.
(Ever drink a glass of orange juice when you, for whatever reason, thought it was going to be a glass of Coca-Cola?)
But another part of it is, simply because of the way we’re made, a completely tasteless liquid isn’t aesthetically “neutral,” as you might think, but is actually (slightly) unpleasant.
Trinopus
Hmmmm… one thing I’ve noticed is that water straight from the tap tastes fine at first, but takes on a metallic flavour after a while. I always keep a big glass of water by my bed, and have often woken up to take a mouthful of the nastiest tasting water known to mankind.
On the other hand, if I pass the water through my Brita filter first, it still tastes fresh the next morning. From this, I can only assume it’s removing something which is normally present in the water, and deteriorates over time. Perhaps some kind of mineral or metallic element?
I, too, keep a big glass of water on my nightstand, although I usually just have a gulp when going to bed and and drain it on awakening.
BUT my water seems to ‘effervesce’ overnight (there are large distinct bubbles on the inner glass) and it tastes better to me than it did the night before.
Three more observations (obvious ones, maybe):
- water always tastes better out of a glass container (to me, anyway);
- water fresh from the tap always tastes better in the winter (because it’s colder, I suppose), and;
- the water in London is nasty, nasty, nasty! When I lived there, I used bottled water for everything except bathing and washing up.
I now drink a little over a gallon of water a day - fresh from the tap, no filtering - and find it delicious (particularly with ice and a small slice of lime).
My two cents…
-bbb-
I think it’s most likely due to a person’s perception of how the water should taste which stems from how it usually tastes to them.
You’ll get used to the taste of your water even if it’s a little nasty if you drink it long enough… with some exceptions for really bad water. And beleive it or not, if it changes without your “approval” so to speak, (water quality changing at the source unbeknownst to you vs you putting in a filter on purpose) you’ll usually not like it - even if it’s better.
Where I am the water is hard, and when I went to the coast it tasted very different and weird… for a couple months. When I came back home this water tasted weird, again until I got used to it.
The temperature of water can affect it’s taste, the same as with other drinks. If water’s really cold, subtle little chemical traces of odd tastes will slip by unnoticed as your sense of taste is deadend a bit by the cold temp. Warm it up a bit (by sitting it out all night) and it may have more “flavour” when you take the next sip. Especially so if the bacteria from your mouth have 8 hours to swim around in it after you took one sip (no one’s under the illusion that they have a 100% sterile mouth or that not even a ul of spit gets into the glass when you stick your lip in it right?)
What’s in your mouth can also mix with things in the water and alter the taste a bit. The same water tastes different to me just after having brushed my teeth and used mouthwash than it does having a stale fermented “coffee & plaque” film all over my mouth in the morning.
If the dissolved gas composition changes after sitting out all night, that may affect taste too. Again though as with all these things, it’s not so much that the water taste has objectively deteriorated, it’s more that you’re not really expecting it to taste that way. If you’re used to chugging 2L of ice cold chlorinated water after eating salad or jogging 5 miles, you’ll most likely notice a difference in taste when you drink a cup of essentially dechlorinated warm water (with a few bacteria multiplied in it) exposed to air washed over your own morning breath; and you may interpret that as a bad taste.
I’d say it’s more due to the above than to a radical chemical atleration of tap water brought on by letting it sit for a few hours :).