Sparkling water is carbon dioxide dissolved in water at high pressure. Remove the bottle cap and the pressure goes away so the carbon dioxide starts fizzing out.
The thing with dissolving carbon dioxide is that there’s a chemical reaction: water H2O + carbon dioxide CO2 -> H2CO3 carbonic acid. Sparkling water can get flat (lose most of its CO2) but there’s still some carbonic acid left in there.
And acids taste sour or bitter. Examples of acids: lemon juice (citric acid), vinegar (acetic acid), carbonic acid, etc.
So the trace amounts of carbonic acid that are left in the (no longer) sparkling water give it a slightly sour taste, which is detectably different than “regular” water. You’d also detect a drop of lemon juice or vinegar added to a glass of water, it would also be slightly sour / slightly bitter.
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