Why is dropping an alka seltzer in water a chemical change?
1 reason is because chemical and physical effects have taken change where molicules have been removed or changed and the effects are irreversable.
There are too many things going on in that process to characterize it as strictly chemical or strictly physical; a little of both is happening. The most visible evidence of a reaction is actually not the Alka-Seltzer (which is not a single compound) reacting with water at all, it's one component of the Alka-Seltzer reacting with another component of the Alka-Seltzer. The water is just an intermediary.