That “stale” taste in your overnight glass? Blame carbon dioxide. When water sits out, CO₂ from the air dissolves into it, forming a tiny bit of carbonic acid. That’s what drops the pH slightly, making it taste flatter and slightly more acidic.
Think of it like a soda that went flat—without the fizz, you notice the underlying minerals. Tap water has dissolved minerals like calcium and magnesium, and once the chlorine or oxygen dissipates, those minerals become the star of the show.
Fun fact: In classical Rome, Pliny the Elder recommended boiling rainwater and storing it in silver vessels to keep it “sweet.” He was onto something, even if his methods were a bit extreme.
When Does Water Actually Go Dangerous?
Here’s where the bad part comes in. If you leave an open glass of water out for more than a day or two, dust, bacteria, and even tiny insects can find their way in. Your mouth introduces microbes every time you take a sip, and those little guys multiply happily in stagnant water.
The risk is low for most healthy people—think of it like a microscopic house party. But if you have a compromised immune system, that party could turn into a riot. Always cover your water if you plan to reuse it later.
Bottled water has a printed expiration date, but that’s mostly about the bottle, not the water. Plastic degrades over time, leaching chemicals like BPA. The water might still be safe to drink, but it won’t taste like a mountain spring.
