There might be situations when the player wants to just remove one chemical without removing all of them, so they would use the "opposite" chemical.
Sure, but those are very rare situations compared to the amount of times you accidentally add an opposing chemical and mess up your attack. Not only that, I want to discourage players from holding onto one mixture for too long (mixture turtling if you will) and forcing them to rebuild their mixture to get rid of a chemical should help with that. Still, I'll think about it.
Ideally all combos should be made with one or two taps with your fingers in correct positions
Just to be sure, when I say "combinations" I mean permutations of individual chemicals (mixtures), not products from reactions. It's simply infeasible to have all unique mixtures in reach of such few inputs, attempting to achieve that would require heavy neutering of the mechanic. You don't need to create complex combos all the time, most mixtures for dealing damage take very few inputs (especially in the next build since you're able to fill your tank with a chemical by holding down its key).
