Remember conservation of mass. Mass can't just suddenly show up or disappear. It has to come from somewhere, and it has to go somewhere.
The mass of this mixture/solution is decreasing as time goes on. That
means that some mass is going somewhere, out of the dish and away
off the scale.
-- React to form a soluble compound ?
Maybe, but that doesn't take mass away
-- They don't react ?
Maybe, but that doesn't take mass away
-- React to form a precipitate ?
Maybe, so big deal. It sinks to the bottom. It doesn't take mass away.
-- React to form a gas ?
If that happens, then the gas carries mass away !
That must be it !
