Why Does Alka-Seltzer Acidify Water but Act as an Antacid?

[Abu]

Hi everyone. I was testing the pH of water that had alka seltzer placed in it. I used Bromothymol blue indicator to see the pH. The color changed to yellow, which suggests that the water was now acidic. I assume this is because of the CO2 bubbles that form which can acidify the H2O. Am I correct?

I also do not understand why the alka seltzer is considered an antacid when it showed that it acidified the water (using the aforementioned pH indicator).

How does drinking an acidic solution neutralize stomach acids? It seems counter intuitive to me.

Any help would be appreciated, thanks very much.

 

[Charles Link]

See https://science.howstuffworks.com/innovation/science-questions/question116.htm The sodium bicarbonate acts as a buffer, making the stomach acid more neutral, according to additional info that I found in a google. $\\$ See also: https://www.alkaseltzer.com/science-experiments/stomach/ This second "link" uses the same indicator you did. Further explanation might be needed, because when vinegar originally turned it yellow, it went back to blue once the alka-seltzer was introduced...