What is the practical result when comparing the taste persistence of plain seltzer water versus flavored soda after both have been left open for 30 minutes?
The flavored soda likely remains significantly more sour due to the persistent non-volatile acids remaining after $ ext{CO}_2$ escapes.
When both plain seltzer water and flavored soda are left open, the dissolved carbon dioxide escapes, causing the carbonic acid ($ ext{H}_2 ext{CO}_3$) to dissipate, making the beverage flat. However, the flavored soda contains other acids, such as citric or phosphoric acid, which are non-volatile; they do not easily turn back into gas and escape with the $ ext{CO}_2$. Consequently, after the fizz is gone, the flavored soda retains these strong acids, causing it to taste markedly more sour or biting compared to the seltzer water, whose initial tartness was almost entirely dependent on the transient carbonic acid.
