Why does Alka-Seltzer dissolve in water?

Question

What causes it to dissolve?

Answer 1

It contains sodium citrate and sodium carbonate. Sodium compounds are almost always soluble in water.   When the sodium citrate dissolves in water it produces citric acid which reacts with the carbonate ion to make carbon dioxide gas, which accounts for the bubbles.  

Alka seltzer also contains some aspirin, actually the sodium salt of acetylsalicylic acid, which dissolves in water.  Different versions also contain some other medications.

Answer 2

The aspirin (acetylsalicylic acid) is not an active ingredient in the effervescent action of Alka-Seltzer, but the baking soda (sodium bicarbonate) and citric acid are active.

C6H8O7 + 3 NaHCO3 → 3 H2O + 3 CO2 + Na3C6H5O7

citric acid + baking soda → water + carbon dioxide + sodium citrate

The carbon dioxide is released as a gas.

I might also add that you should never, ever, ever put one on your tongue.  I did and it was a taste I won't soon forget.