(Intro)

Michael: Have you ever poured yourself a nice, cool drink of water before bed and woken up to a glass of blehhhh? In just a couple of hours, your refreshing water turned tepid and kind of musty. But why?

Well, we have a couple guesses about why water goes stale, and it's all about the chemistry. Your drinking water isn't just a pure collection of H₂O molecules. It has a lot of other ions and molecules in it too, which can change over time and affect how the water tastes.

So let's first start with carbon dioxide. It's the stuff you exhale, the stuff in soda that makes it fizzy, and that pesky gas that's making the oceans more acidic. And your cup of water is to some extent like a tiny ocean. Water is an excellent solvent, which means it's really good at dissolving lots of other substances, including gases, until it reaches equilibrium with the atmosphere. So as water sits in your room, it gradually absorbs some carbon dioxide from the air. The water molecules and carbon dioxide molecules can then react to each other to form carbonic acid, which lowers the pH of the water, making it slightly acidic. And a little less tasty.

So gas entering the water can affect its flavor. But gas leaving the water can too. Before tap water makes its way to your faucet, water treatment centers sometimes add compounds containing chlorine, up to 4 milligrams per liter, in order to purify it. Chlorine is really good at killing  any bacteria or viruses that might be floating around, which is why we also chlorinate swimming pools. But if you've ever accidentally swallowed any pool water, you know it's kinda gross. So then why does a cool glass of tap water taste good? Well, if you're used to drinking slightly chlorinated water, you might associate a tiny bit of chlorination with a cleaner, crisper taste, and when you let a glass of water sit out, the chlorine will dissipate back into the air as a gas, which could change the taste of your water and make it seem less refreshing.

These chemical factors aside, the main reason for the stale taste could be the most obvious: temperature. Basically, cold temperatures suppress taste. Warmer water has faster-moving molecules, which in turn amplify the flavors you can detect with your taste buds. So when you drink a cold glass of water, you might not be tasting all of the subtle flavors it already has inside, kind of like the subtle flavors in a glass of wine.

The good news? Leaving your glass of water out overnight won't hurt you. Even if it tastes a little gross. So: drink up.

Thanks to Patreon Patron Carston from Switzerland for asking this question. Thanks to all of our Patrons, who keep these answers coming. If you'd like to submit a question to be answered, just go to patreon.com/scishow, and don't forget to go to youtube.com/scishow and subscribe.

(Outro)
 
He noticed that the ice cream mixture made with hot milk froze faster than the mixture made with cold milk.