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  • Have you ever poured yourself a nice cool drink of water before bed, and woken up to

  • a glass of, BLECH?!

  • In just a couple of hours, your refreshing water turned tepid and kinda... 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 flavorbut 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 youve 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 youre 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 Karsten 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!

Have you ever poured yourself a nice cool drink of water before bed, and woken up to

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