I came up against a gelatinous cube and beat it by pouring a base over it, it was dead in a turn

Anyone else using chemicals like this?

Your character would also need to know that a particular chemical reaction would happen.  An Alchemist Artificer or Wizard with alchemy tool kit proficiency would reasonably know basic chemistry.  A similarly smart character with smithing proficiency might know basic metallurgy.  Masonry proficiency might allow some knowledge of geology and concrete mixtures.

But an average intelligence character without a relevant proficiency is unlikely to know much, if any, scientific knowledge.  Medieval peasants and illiterate nobles alike were ignorant of much of what we learned in high school science classes.

Also alchemy isn't chemistry.  The acids featured in these games aren't usually real life chemical acids either but rather a sort of fantasy spoof on them, same with venoms.  Still there's no reason an alchemist wouldn't know something that could neutralize the cubes acid with a successful  int based alchemy supplies roll against an appropriate dc.